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Biochemical mechanisms in pathogenesis of infantile epileptic spasm syndrome

Open AccessPublished:January 06, 2023DOI:https://doi.org/10.1016/j.seizure.2023.01.004

      Highlights

      • The molecular mechanisms leading to IESS and developmental delay remain obscure.
      • The current literature and data on Finnish IESS patients are reviewed.
      • IESS is characterized by an imbalance of inhibitory, (NGF, IGF-1, ACTH, GABA), and excitatory (glutamate, nitrites), factors, (2) an abnormality of the hypothalamic-pituitary-adrenal axis, (3) inflammation, (4) altered proliferation, migration, apoptosis, synaptogenesis and myelination of the brain.
      • An interaction between the HPA axis, nerve growth factors and immune system was already shown by the Nobel Prize winner Rita Levi-Montalcini.
      • An altered maturational process limited to a certain sensitive period of infancy could possibly explain why so many, seemingly independent etiological factors lead to the same clinical syndrome and developmental delay. Understanding these factors may lead to novel therapies.

      Abstract

      The molecular mechanisms leading to infantile epileptic spasm syndrome (IESS) remain obscure. The only common factor seems to be that the spasms are restricted to a limited period of infancy, during a certain maturational state. Here the current literature regarding the biochemical mechanisms of brain maturation in IESS is reviewed, and various hypotheses of the pathophysiology are put together. They include: (1) imbalance of inhibitory (NGF, IGF-1, ACTH, GABA) and excitatory factors (glutamate, nitrites) which distinguishes the different etiological subgroups, (2) abnormality of the hypothalamic pituitary adrenal (HPA) axis linking insults and early life stress, (3) inflammation (4) yet poorly known genetic and epigenetic factors, and (5) glucocorticoid and vigabatrin action on brain development, pinpointing at molecular targets of the pathophysiology from another angle.
      An altered maturational process may explain why so many, seemingly independent etiological factors lead to the same clinical syndrome and frequently to developmental delay. Understanding these factors can provide ideas for novel therapies.

      Keywords

      1. Introduction

      West syndrome (infantile spasms) is a triad of epileptic spasms, hypsarrhythmia, and developmental stagnation or regression. It has been known already for 180 years (West 19841) [
      • West W.J.
      On a peculiar form of infantile convulsions.
      ].
      In the newest ILEA classification in 2022 [
      • Zuberi S.
      • Wirrell E.
      • Yozawitz E.
      • Wilmhurst J.
      • Specchio N.
      • Riney K.
      • et al.
      ILEA classification and definition of epilepsy syndromes with onset in neonates and infants: Position statement by the ILEA TASK force on nosology and definitions.
      ]. Infantile epileptic spasms syndrome (IESS) is a term that should be used and proposed to encompass both West syndrome as well infants presenting with epileptic spasms who do not fulfill the criteria of West syndrome (lacking one of these three criteria). Mandatory diagnostic criteria are epileptic spasms often occurring in clusters and interictal EEG of hypsarrhythmia, multifocal or focal epileptiform discharges. It is a strictly age-dependent encephalopathy of early infancy (before the age of two years).
      Search of pathogenetic factors of the syndrome both in human and animal studies has been described in excellent reviews [
      • Baram T.
      Pathophysiology of massive infantile spasms: perspective and putative role of the brain adrenal axis.
      ,
      • Frost J.
      • Hrachovy R.
      Pathogenesis of infantile spasms: a model on developmental desynchronization.
      ,
      • Paciorkowski A.
      • Tio L.
      • Dobyns W.
      A genetic and biologic classification of infantile spasms.
      ,
      • Galanopoulou A.
      • Moshe S.
      Pathogenesis and new candidate treatments for infantile spasms and early life encephalopathies; a view from preclinical studies.
      ,
      • Orsini A.
      • Foiadelli T.
      • Costagliola G.
      • Michev A.
      • Cosolini R.
      • Consolini R.
      • et al.
      The role of inflammatory mediators and epileptic encephalopathies and therapeutic implications.
      ]. There are eight animal infantile spasms models [
      • Galanopoulou A.
      • Moshe S.
      Pathogenesis and new candidate treatments for infantile spasms and early life encephalopathies; a view from preclinical studies.
      ,
      • Stafström C.
      • Holmes G.
      Infantile spasms: criteria for an animal model.
      ,
      • Galanopoulou A.
      Basic mechanisms of catastrophic epilepsy – overview from animal models.
      ,
      • Dulla G.
      Utilizing animal models of infantile spasms.
      ].
      However, the pathogenesis has remained unknown. The main question is why and how intellectual disability is such a common outcome in IESS patients even after cessation of the spasms and hypsarrhythmia.
      The syndrome has previously been classified into symptomatic (80%), cryptogenic (15–20%) and idiopathic (15%) groups, depending on known etiology. The newer ILAE classification divides IESS etiology into structural-metabolic, genetic, infectious-immune and unknown groups [
      • Berg A.T.
      • Berkovic S.F.
      • Brodie M.J.
      • Buchhalter J.
      • Cross J.H.
      • van Emde Boas W.
      • et al.
      Revised classification and terminology, 2005–2009.
      ,
      • Scheffer I.
      • Berkovic S.
      • Capovilla G.
      • Connolly M.
      • French J.
      • Guilhoto L.
      • et al.
      ILAE classification of the epilepsies: position paper of the ILEA commission for classification and termiology.
      ].
      In the present study, the cryptogenic (unknown) group of IESS is characterized by normal prior development and no evident etiological factors after careful etiological investigations. In symptomatic IESS patients (with known etiology), there is a pre-existing encephalopathy, abnormal neurological signs, and lesions identified by brain imaging.
      IESS can result from lesional brain damage, involving both cortical and/or subcortical mechanisms.
      The peculiar feature of this epileptic encephalopathy is its extraordinary response to ACTH first reported by Sorel and Dusaucy-Bauloye [
      • Sorel L.
      • Dusaucy-Bauloye A.
      Findings in 21 cases of Gibbs' hypsarrhythmia; spectacular effectiveness of ACTH.
      ]. Successful treatment may lead to dramatic improvements of cognition and function. The first-line treatments currently are ACTH and prednisolone except in tuberous sclerosis vigabatrin. The mechanisms of ACTH action may include (1) induction of steroid release and (2) a direct steroid-independent action on the melanotropin receptor (See later: HPA axis).
      The only common factor of extremely heterogenous etiologic backgrounds seems to be a well-defined maturational status during a limited period in infancy: IESS has an age-dependent onset before 2 years (typically at 4–8 months) and some spasms remit spontaneously.

      2. Maturation of the brain

      What are the most crucial changes in the brain in the pathogenesis of IESS? At the age when IESS usually appears, the brain is at a very sensitive and active stage of development [
      • Volpe J.
      • Volpe J.
      Neuronal proliferation, migration, organization, and myelination.
      ].
      The maximal growth of the dendrites (the main sites of synaptic transmission) starts in the 30th gestational week and goes on until the age of 1 year or older [
      • Volpe J.
      • Volpe J.
      Neuronal proliferation, migration, organization, and myelination.
      ]. Myelin is necessary for proper brain function in the developing brain and throughout life. New myelin is made by oligodendrocytes that are derived from precursors, oligodendrocyte progenitor cells [
      • Barateiro A.
      • Brites D.
      • Fernandes A.
      Oligodendrocyte development and myelination in neurodevelopment: molecular mechanisms in health and disease.
      ,
      • Kuhn S.
      • Gritti L.
      • Crooks D.
      • Dombrowski Y.
      Oligodendrocytes in development, myelin generation and beyond.
      ].
      In general, many studies on cognitive delay in children focus on possible destructive or degenerative structural lesions in the brain. However, arrested or altered development may play a bigger role. ACTH and glucocorticoids accelerate normal developmental events in different organs. Glucocorticoid administration accelerates lung maturation when used to treat idiopathic respiratory distress syndrome in newborns [
      • Bolt R.
      • van Weissenbruch M.
      • Lafeber H.
      • Delemarre-van de Waal H.
      Glucocorticoids and lung development in the fetus and preterm infant.
      ]. In animal models ACTH accelerates myelin protein turnover [
      • Palo J.
      • Savolainen H.
      The effects of high doses of synthethic ACTH on rat brain.
      ] and dendritic sprouting in young rats [
      • Holmes G.
      • Weber D.
      Effects of ACTH on seizure susceptibility in the developing brain.
      ,
      • Mochetti I.
      • Spiga G.
      • Hayes V.
      • Congelo A.
      Glucocorticoids differentially increase nerve growth factor and basic fibroblast growth factor in the rat brain.
      ]. The autopsies of children with IESS, histochemistry using Golgi-COX technique [
      • Huttenlocher R.
      Dendritic development in neocortex of children with mental defect and infantile spasms.
      ] showed defective cortical dendritic development, resembling an immature neonatal brain.
      In animals, ACTH has a so-called ”catch-up” effect on dendrites by stimulating the synthesis of nerve growth factors whereby RNA and DNA syntheses are increased 20 and various enzymes in the central nervous system (CNS) are induced [
      • Pranzelli M.
      On the molecular mechanism of adrenocorticotrophic hormone in the CNS: neurotransmitters and receptors.
      ]. The proportion of patients with IESS exhibiting delayed myelination is relatively high up to 73% [
      • Schropp C.
      • Staudt M.
      • Staudt F.
      • Bise K.
      • Obletter N.
      • Breit A.
      • et al.
      Delayed myelination in children with West syndrome: an MRI-study.
      ]. A follow-up magnetic resonance imaging (MRI) brain scan is recommended, especially if the first one was performed before the age of 2 years, due to immature myelination patterns seen in infants [
      • Muthaffar O.
      Brain magnetic resonance imaging findings in infantile spasms.
      ]. The onset of spasms also often coincides with the functional maturation of cerebral cortex [
      • Juhász C.
      • Chugani H.
      • Muzik O.
      • Chugani D.
      Neuroradiological assessment of brain structure and function and its implication in the pathogenesis of West syndrome.
      ] and correlates very closely with developmental milestones [
      • Muthaffar O.
      Brain magnetic resonance imaging findings in infantile spasms.
      ].
      Small-for-gestational-age infants were more susceptible to IESS than preterm infants approriate for gestational age [
      • Riikonen R.
      Decreasing perinatal mortality: unchanged infantile spasms morbidity.
      ]. The inverse association with gestational age suggests that the preterm must reach a certain developmental stage before spasms can be manifested [
      • Berg A.
      • Chakravorty S.
      • Koh S.
      • Grispan Z.
      • Shelhaas R.
      • Saneto R.
      • et al.
      Why West? Comparisons of clinical, genetic and molecular features of infants with and without spasms.
      ].

      3. Molecular mechanisms

      3.1 Inhibitory factors: Nerve growth factors, GABA, ACTH and the hypothalamic-pituitary-adrenal (HPA) axis)

      3.1.1 Nerve growth factors in brain maturation

      Neurotrophic factors are endogenous substances that control cell proliferation and differentiation in the nervous system. Trophic effects are essential during development for brain maturation.
      A variety of growth factors and their receptors are present in the CNS: brain-derived neurotrophic factor (BDNF), nerve growth factor (β-NGF, NGF), glial cell line-derived neurotrophic factor (GDNF), insulin-like growth factor (IGF)-1 and -2. Neurotrophic factors are involved in the selection of optimal neuronal connections.

      3.1.2 A Nerve growth factor (NGF)

      NGF (β-NGF) is perhaps the prototypical growth factor in that it was one of the first to be described. It is an important regulator of nerve cell survival and differentation during early development. NGF exerts its biological action by binding to the specific receptor tropomyosin kinase receptor A (TrkA), which is a typical tyrosine kinase receptor. NGF acts especially on the cholinergic neurons in basal forebrain [
      • Mobley W.
      • Rutkowski J.
      • Tennekoon G.
      • Gemeski J.
      • Buchanan K.
      • Johnston M.
      Nerve growth factor increases choline acetyltransferase activity in developing basal forebrain neurons.
      ]. A number of different stimuli can induce nerve cells to release stored and newly synthethized NGF. Since it was first discovered by the Nobel Laureates, Rita Levi-Montalcini, and Stanley Cohen in early fifties, numerous biological processes involving NGF have been identified, one of them being the regulation of the immune system. NGF synthesis is increased by inflammatory processes mediated in part by interleukins [
      • Spranger M.
      • Lindholm D.
      • Bandtlov C.
      • Heumann R.
      • Ghahn H.
      • Näher-Noe H.
      • et al.
      Regulation of nerve growth factor (NGF) synthesis in the rat central nervous system: Comparison between the effects of interleukin-1 and various growth factors in astrocyte cultures and in vivo.
      ]. An interaction of (HPA) axis, nerve growth factors and immune system was later reported and the wide spectrum of biological functions of NGF revealed [
      • Scaccianoce S.
      • Cigliana G.
      • Muscolo L.
      • Porcu A.
      • Navarra D.
      • Perez-Polo J.
      • et al.
      Hypothalamic involvement in the activation of the pituitary-adrenocortical axis by nerve growth factor.
      ,
      • Levi-Monltalcini R.
      • Dal Toso R.
      • della Valle F.
      • Skaper S.
      • Leon A.
      Update of the NGF saga.
      ].
      In this review the above-mentioned links between nerve growth factors, the HPA axis and immune system are in the focus. Interactions between them are involved in the pathogenesis of IESS. Special attention is paid to the age-dependency of those factors, and to therapeutic considerations in IESS. I will concentrate on human studies even though they are few. Animal studies have been covered in previously published excellent reviews (See earlier).
      • -
        Neonatal period
      In neonates with hypoxic-ischemic injury (one of the main etiological factors leading to IESS, especially in developing countries), CSF BDNF was increased, and β-NGF (NGF) decreased [
      • Korhonen L.
      • Riikonen R.
      • Nawa H.
      • Lindholm D.
      Brain derived neurotrophic factor is increased in cerebrospinal fluid of children suffering from asphyxia.
      ,
      • Riikonen R.
      • Korhonen L.
      • Lindholm D.
      Cerebrospinal nerve growth factor—a marker of asphyxia?.
      ]. Increased BDNF concentrations might counteract neuronal damage. In turn, CSF NGF concentrations might serve as a marker of asphyxia. Furhermore, we showed increased CSF excitatory amino acids (glutamate and aspartate) concentrations [
      • Riikonen R.
      • Kero P.
      • Simell O.
      Excitatory amino acids in cerebrospinal fluid in neonatal asphyxia.
      ], as was also shown by Hagberg et al. [
      • Hagberg H.
      • Thornberg E.
      • Blennow M.
      • Kjellmer I.
      • Lagercranz H.
      • Thringer K.
      • et al.
      Excitatory amino acids in the cerebrospinal fluid of asphyxiated infants: relationship to hypoxic-ischemic encephalopathy.
      ]. The epileptic process is considered to consist of three phases: initial insult, latency period (epileptogenesis), and recurrent seizures (symptomatic epilepsy). This latency period includes a cascade of events: neuronal death, inflammation, synaptic network reorganization and, altered maturation. It can be considered as one target of preventive therapy.
      • -
        Infancy and NGF
      We measured CSF NGF in patients with IESS [
      • Riikonen R.
      • Söderström S.
      • Vanhala S.
      • Ebendal T.
      • Lindholm D.
      West`s syndrome: cerebrospinal fluid nerve growth factor and effect of ACTH.
      ]. Patients with symptomatic etiology had a poor response to ACTH therapy and low NGF levels. The delayed development of these infants and and their poor response possibly reflects massive neuronal death. We have shown that NGF gene expression is modulated by ACTH in patients, in accordance with earlier observations of glucocorticoid-induced NGF expression in animal models [
      • Lindholm D.
      • Castrén E.
      • Berzaghi M.
      • Blöchl A.
      • Thoenen H.
      Activity-dependent and hormonal regulation of neurotrophin mRNA levels in brain–implications for neuronal plasticity.
      ].
      Inflammation has been found around epileptic tubers in tuberous sclerosis (TS) [

      Boer K., Jansen F., Nellist M., Redeker S., van den Ouweland A., Spliet W., et al. Inflammatory processes in cortical tubers and subependymal giant cell tumors of tuberous sclerosis complex. Epilepsy Res 2008; 78: 7–21. 1016/j.eplepsyres.2007.10.002.i.10.

      ,
      • Prabowo A.
      • Anink J.
      • Lammens M.
      • Nellist M.
      • Ans M.W.
      • van den Ouweland A.
      • et al.
      Fetal brain lesions in tuberous sclerosis complex:TORC1 activation and inflammation.
      ]. We have also shown high CSF NGF concentrations in patients with IESS and TS compared to normal controls, to IESS patients with unknown etiology, and particularily to those with another type of known etiology [
      • Riikonen R.
      • Kokki H.
      CSF nerve growth factor (β-NGF) is increased but IGF-1 in normal in children with tuberous sclerosis and infantile spasms.
      ]. However, in patients with postinfectious etiology of IESS the CSF NGF concentrations were extremely high [
      • Riikonen R.
      • Kokki H.
      CSF nerve growth factor (β-NGF) is increased but IGF-1 in normal in children with tuberous sclerosis and infantile spasms.
      ]. This might be due to a prolonged hyperactivity of the neurons in these conditions. An mTor-mediated inflammatory processs may be involved epileptogenesis of TS [
      • Zhang B.
      • Zou J.
      • Rensing N.
      • Yang M.
      • Wong M.
      Inflammatory mechanisms contribute to the neurological manifestations of tuberous sclerosis complex.
      ].
      • -
        Therapeutic proposal
      Over-production of NGF might serve as a potential target for intervention, in postinfectious IESS, and in TS [
      • Levi-Monltalcini R.
      • Dal Toso R.
      • della Valle F.
      • Skaper S.
      • Leon A.
      Update of the NGF saga.
      ,
      • Riikonen R.
      • Kokki H.
      CSF nerve growth factor (β-NGF) is increased but IGF-1 in normal in children with tuberous sclerosis and infantile spasms.
      ,
      • Aloe L.
      • Rocco M.L.
      • Bianchi P.
      • Luigi Manni L.
      Nerve growth factor: from the early discoveries to the potential clinical use.
      ].
      1:1: b IGF-1
      IGF-1 is a key factor that regulates neurogenesis and synaptogenesis from embryonic to adult stages in the development of the brain [
      • Leventhal P.
      • Russel J.
      • Feldman E.
      IGFs and the nervous system.
      ]. Several aspects of neuronal development, structure, and function are regulated by growth hormone (GH)/IGF-1. IGF is especially important at an early age.
      Early stress is associated with dendritic atrophy and synaptic reorganization [
      • Brunson K.
      • Kramár E.
      • Lin B.
      • Chen Y.
      • Colgin L.L.
      • Yanagihara T.
      • et al.
      Mechanisms of late-onset cognitive decline after early-life stress.
      ]. IGF-1 induces m-Tor to prevent cell death during cellular stress [
      • Popowski M.
      • Ferguson H.A.
      • Sion A.M.
      • Koller E.
      • Knudsen E.
      van den Berg CL Stress and IGF-I differentially control cell fate through mammalian target of rapamycin (mTOR) and retinoblastoma protein (pRB).
      ]. IGF-1 and IGF-2 can rescue neurons from apoptosis. IGF also reduces the neuro-inflammation acting on astrocytes and microglia [
      • Labandeira-Garcia J.
      • Costa-Besada M.
      • Labendeira C.
      • Villar-Cheda B.
      • Rodriguuez-Rerez
      Insulin-like growth factor-1 and neuroinflammation. Mini Review article.
      ]. Both synaptic connectivity and neuro-inflammatory processes may involve the P3-Akt-mTOR signaling pathway.
      • a
        Infantile epileptic spasms and IGF-1, a biomarker of disease severity
      The majority of structural IESS etiology are characterized by various forms of pre-, peri- or postnatal brain damage. Prenatal stress in animals has been shown in animals to decrease IGF-1 [
      • Szczesny E.
      • Basta-Kaim A.
      • Slusarczyk J.
      • Trojan E.
      • Glonbik K.
      • Regulska M.
      • et al.
      The impact of prenatal stress on insulin-like growth factor-1 and pro-inflammatory cytokine expression in the brain of adult male rats: The possible role of suppression of cytokine signalling proteins.
      ].
      In our study, in patients with IESS, low CSF IGF-1 concentrations correlated with the severity and length of early stress, cortical damage, poor response to therapy and poor cognitive outcome. CSF IGF-1 concentrations of the infants with spasms with unknown etiology, in the absence of early pre- and perinatal insults and stress, with normal imaging studies, good response to ACTH therapy, were similar to those of age-matched controls. The majority of these patients had good cognitive outcome [
      • Riikonen R.S
      • Jääskeläinen J.
      • Turpeinen U.
      Insulin-like growth factor-1 is associated with cognitive outcome in infantile spasms.
      ].
      Children with known etiology had markedly low CSF IGF-1 concentrations, and significantly lower CSF ACTH concentrations when compared with children with unknown etiology. Low CSF IGF-1 and ACTH concentrations were associated with a history of early insults or stress, cerebral atrophy, poor response to therapy, and later worsening of cognitive outcome. The following hypothesis was postulated: In IESS the brain cannot produce steroids, which stimulate secretion of IGF-1, an essential growth factor for survival of synapses.
      This higlights the potential usefulness of CSF IGF-1 as a biomarker for disease severity [
      • Riikonen R.S
      • Jääskeläinen J.
      • Turpeinen U.
      Insulin-like growth factor-1 is associated with cognitive outcome in infantile spasms.
      ,
      • Walker L.
      • Janigro D.
      • Heinemann U.
      • Riikonen R.
      • Bernard C.
      • Patel M.
      WONOEP appraisal: molecular and cellular biomarkers for epilepsy. Critical and invited commentary.
      ,
      • Riikonen R.
      Insulin-like growth factors in the pathogenesis and treatment of neurological disorders in children.
      ].
      It is also notable that in premature children developmental delay was associated with perinatal IGF-1 deficiency showing that IGF-1 is important for early cognition [
      • Hellström A.
      • Ley D.
      • Hansen-Pupp I.
      • Hallberg B.
      • Löfqvist C.
      • van Marter L.
      • et al.
      Insulin-like growth factor 1 has multisystem effects on foetal and preterm infant development.
      ].
      • a
        Brain growth
      Homozygous Igf1-/- mice show microcephaly and hypomyelination in the whole brain [
      • Beck K.L.
      • Powel-Braxton L.
      • Widmer H.R.
      • Valverde J.
      • Hefti F.
      IGF-1 gene distruption results in reduced brain size, CNS hypomyelination, and loss of hippocampal granule and striatal paralbumin-containing neurons.
      ]. An association has also been found between the decline of brain growth, poor response to ACTH and lower intellectual outcome in infants with IESS [
      • Rener-Primec Z.
      • Lozar-Krivec J.
      • Krivec U.
      • Neubauer D.
      Head growth in infants with infantile spasms may be temporarily reduced.
      ]. The cognitive decline is associated with dendritic atrophy and synaptic reorganization [
      • Brunson K.
      • Kramár E.
      • Lin B.
      • Chen Y.
      • Colgin L.L.
      • Yanagihara T.
      • et al.
      Mechanisms of late-onset cognitive decline after early-life stress.
      ].
      • a
        IESS and autism
      A clinical association between symptomatic IESS (with known etiology) and infantile autism has been found by Samundsen et al. [
      • Samundsen E.
      • Ludvigsson P.
      • Rafnsson V.
      Risk of autism spectrum disorders after infantile spasms. A population-based study nested in a cohort with seizures in the first year of life.
      ]. Both conditions show low CSF IGF-1 concentrations [
      • Vanhala R.
      • Turpeinen U.
      • Riikonen R.
      Low levels of insulin-like growth factor-1 in cerebrospinal fluid in children with autism.
      ,
      • Riikonen R.
      • Makkonen I.
      • Vanhala R.
      • Turpeinen U.
      • Kuikka J.
      • Kokki H.
      Cerebrospinal fluid insulin-like growth factors IGF-1 and IGF-2 in infantile autism.
      ]. Furthermore, abnormalities in ventral forebrain development and pathways of synaptic function were detected both in IESS (with copy number variants) and in autism (Paciorkowski et al. [
      • Paciorkowski A.
      • Thio L.
      • Rosenfeld J.
      Copy number variants and infantile spasms: evidence for abnormalities in ventral forebrain development and pathways of synaptic function.
      ]) suggesting common biologic mechanisms.
      • -
        Therapeutic proposal
      In an animal model of IESS the sodium channel blocker, tetrodotoxin, (TTX) was recently used to assess the therapeutic effects of IGF-1 [
      • Ballester-Rosado C.
      • Le J.
      • Lam T.
      • Mohila C.
      • Lam S.
      • Anderson A.
      • et al.
      A role for insulin-like growth factor 1 in the generation of epileptic spasms in a murine model.
      ]. TTX causes injury at the infusion site in the brain and results in spasms that are virtually identical to those seen in IESS patients. Long-term video EEG recordings with quantitative immunohistochemical and biochemical analyses were used to show IGF-1’s role in spasm generation. Immunohistochemical methods revealed widespread loss of IGF-1 from cortical neurons but an increase in reactive astrocytes in TTX-induced lesions, very similar to the changes observed in neocortex of patients with IESS. The infants in that study had suffered from perinatal strokes and required surgery to control their seizures. Their study showed that (1-3) IGF-1 (Trofinetide, a novel synthetic analog of the amino‐terminal tripeptide of IGF-1, which acts through IGF-1 receptors), was effective and was a promising therapeutic candidate [
      • Ballester-Rosado C.
      • Le J.
      • Lam T.
      • Mohila C.
      • Lam S.
      • Anderson A.
      • et al.
      A role for insulin-like growth factor 1 in the generation of epileptic spasms in a murine model.
      ]. Trofinetide rescued inhibitory neurons and abolished spasms and hypsarrhythmia. (1-3) IGF-1 is smaller than the complete IGF-1 hormone, allowing it to cross the blood brain barrier more easily.
      Alterations in neocortical IGF-1 expression in neonatal stroke patients and animals reported in that study were remarkably similar to the report of reduced levels of CSF IGF-1 in children with spasms of known etiology [
      • Riikonen R.S
      • Jääskeläinen J.
      • Turpeinen U.
      Insulin-like growth factor-1 is associated with cognitive outcome in infantile spasms.
      ].
      In the CURE Infantile Spasms Consortium (CURE) study, the retinal toxicity of vigabatrin was greatly reduced when administered together with this tripeptide [
      • Lubbers L.
      • Sloka S.
      • Iyengar S.
      The CURE Infantile Spasms Consortium, CURE Staff
      A team science approach to discover novel targets for infantile spasms (IS).
      ]. Additional animal studies are needed to know whether trofinetide (1-3) IGF-1 is effective in certain models of IESS, and not in others [
      • Lubbers L.
      • Sloka S.
      • Iyengar S.
      The CURE Infantile Spasms Consortium, CURE Staff
      A team science approach to discover novel targets for infantile spasms (IS).
      ].
      Tropea et al. [
      • Tropea D.
      • Giacometti E.
      • Nathan R.
      • Wilson N.
      • Beard C.
      • McCurry M.
      • et al.
      Partial reversal of Rett Syndrome-like symptoms in MeCP2 mutant mice.
      ] demonstrated that the loss of function of the MECP2 (methyl-CpG-binding protein 2) of the knockout mouse with Rett syndrome like symptoms can be rescued by an active peptide fragment of IGF-1. Trofinetide has recently been shown to be safe and efficacious in some neurodevelopmental disorders [
      • Glaze D.
      • Neul J.
      • Kaufmann W.
      • Berry-Kravis E.
      • Condon S.
      • Stoms G.
      • et al.
      Double-blind, randomized, placebo-controlled study of trofinetide in pediatric Rett syndrome.
      ,
      • Berry-Kravis E.
      • Horrigan J.
      • Tartaglia N.
      • Glass L.
      • Jones N.
      A double-blind, randomized, placebo-controlled clinical study of therapeutic Trofinetide in the treatment of Fragile X syndrome.
      ,

      Kolevzon A., Breen M., Siper P., Halpern D., Rieger H., Weisman J., et al. Clinical trial of insulin-like growth factor-1 in Phelan-McDermic syndrome. Molecular Autism 2022;13. doi 10.1186/s13229-022-00493-7. 17 (2022). Letter to the Editor./Open Access.

      ].
      ACTH, glucocorticoids and ketogenic diet can be effective therapies for IS, all of which affect IGF-1 levels [
      • Cheng C.M.
      • Kelley B.
      • Wang J.
      • Strauss D.
      • Eagles D.A.
      • Bondy CA.
      A ketogenic diet increases brain insulin-like growth factor receptor and glucose transporter gene expression.
      ,
      • Agha A.
      • Monson JP.
      Modulation of glucocorticoid metabolism by the growth hormone—IGF-1axis.
      ]. The results implicate IGF-1 in the pathogenesis of IESS and IGF-1 analogues as potential therapeutics for IESS. In patients having known etiology of spasms, the normalisation of IGF-1 function might contribute to normal development.

      3.1.3 GABA and neurosteroids

      In general, gamma-amino-butyric acid (GABA) functions as an inhibitory neurotransmitter in the mature brain. It has a complex biology due to its various receptor isoforms and the developmental expression of the subunits by programmed molecular regulation. The most important is the GABAA receptor, which functions as a chloride ion channel, and initially serves excitatory function, but after triggering of the GABAA switch obtains an inhibitory role in neuronal networks. The association between GABA-signalling and IESS has been acknowledged in a variety of sources. The therapeutic effects of the GABA transaminase inhibitor, vigabatrin, are well-known. Pathology specimens indicate decreased expression [
      • Agha A.
      • Monson JP.
      Modulation of glucocorticoid metabolism by the growth hormone—IGF-1axis.
      ,
      • Crino P.
      Focal brain malformations: seizures, signaling, sequencing.
      ] and altered neurosteroid sensitivity of GABAA receptors in IESS [
      • Jansen L.
      • Peugh L.
      • Ojemann J.
      GABAa receptors in catastrophic infantile epilepsy.
      ]. ACTH increases the levels of endogenous neurosteroids, which are particularly potent enhancers of GABAA receptor function [
      • Jansen L.
      • Peugh L.
      • Ojemann J.
      GABAa receptors in catastrophic infantile epilepsy.
      ,
      • Torres J.M.
      • Ruiz E.
      • Ortega E.
      Effects of CRH and ACTH administration on plasma and brain neurosteroid levels.
      ,
      • Smith S.
      • Shen H.
      • Hua Gong Q.
      • Zhou X.
      Neurosteroid regulation of GABAA receptors Focus on the α 4 and δ subunits.
      ].
      Neurosteroids, also known as neuroactive steroids, including tetrahydrodeoxycorticosterone (THDOC: 3 α21-dihydroxy-5 α-pregnan-20-one) and allopregnanole, are endogenous, or exogenous steroids, that rapidly alter neuronal excitability through interaction with ligand-gated ion channels and other cell surface receptors [
      • Reddy DS.
      Neurosteroids: endogenous role in the human brain and therapeutic potentials.
      ]. Progesterone and its metabolites increase GABA-ergic transmission [
      • Lan N.C.
      • Gee KW.
      Neuroactive steroid actions at the GABAA receptor.
      ]. A progesterone metabolite, 3-alfa-ol-20-one, is especially potent in this respect. The steroid recognition site of this metabolite is functionally coupled to an expressed GABAA receptor. Endogeneous neurosteroids have anticonvulsant actions [
      • Gunn B.
      • Cunningham L.
      • Scott G.
      • Mitchell S.
      • Lambert J.
      GABAA receptor-acting neurosteroids: a role in the development and regulation of the stress response.
      ]. They may decrease glutaminergic activity and neurotransmitter release [
      • Smith C.
      • Gibbs T.
      • Farb D.
      Pregnenolone sulfate as a modulator of synaptic plasticity.
      ].
      Patients with IESS and poor response to ACTH had a low dehydroepiandrosterone (DHEA)/androstenedione ratio during therapy compared to responsive patients supporting the hypothesis that neurosteroids have anticonvulsant actions [
      • Riikonen R.
      • Perheentupa J.
      Serum steroids and success of corticotropin therapy.
      ]. The therapeutic effect of ACTH may also be mediated through other steroids than corticosteroids.
      Airaksinen et al. [
      • Airaksinen E.
      • Tuomisto l
      • Riikonen R.
      The concentrations of GABA, 5-HIAA and HVA in the cerebrospinal fluid of children with the infantile spasms and effects of ACTH treatment.
      ] have shown that patients with known IESS etiology had a lower concentration of CSF GABA than IESS patients with unknown etiology or control patients.
      • -
        Therapeutic proposal
      Ganaxalone (3α-hydroxy-3β-methyl-5α-pregnan-20-one) is a novel steroid drug related to allopregnanolone and has anticonvulsant effects, modulating GABAA receptors. The utility of pharmacological enhancement of GABAA-mediated inhibition could be applied to the control of refractory IESS [
      • Kerrigan J.
      • Shields W.
      • Nelson T.
      • Bluestone D.
      • Dodson W.
      • Bourgeois B.
      • et al.
      Ganaxolone for treating intractable infantile spasms: a multicenter, open-label, add-on trial.
      ].

      4. Excitatory neurotransmitters

      4.1 Glutamate

      In an animal model, rats were prenatally treated with betamethasone to mimic a stressor that sensitises the HPA axis to postnatal N-methyl-D-aspartic acid (NMDA), resulting in seizures resembling IESS that were sensitive to ACTH [
      • Velisek L.
      • Jehle K.
      • Asche k
      • Veliskova J.
      Model of infantile spasms induced by N-methyl-D-aspartic acid in prenatally impaired brain.
      ]. After a hypoxic-ischemic event in human neonates CSF glutamate and aspartate were increased [
      • Riikonen R.
      • Kero P.
      • Simell O.
      Excitatory amino acids in cerebrospinal fluid in neonatal asphyxia.
      ]. The CSF glutamate level is increased in children with spasms of known etiology [
      • Spink D.
      • Snead I.I.I.C.
      • Swann J.
      • Martin D.
      Free amino acids in cerebrospinal fluid from patients with infantile spasms.
      ]. The excitatory role of glutamate combined with a lack of counteracting control by inhibitory interneurons or the thalamus results in an imbalance of neurotransmission.
      The excitatory aminoacid acid (EAA) system is transiently enhanced early in life. This enhancement plays a critical role in plasticity in early learning and morphogenesis. The hypersensitivity renders the brain vulnerable to pathologic excitation by the EAA system [
      • Hattori H.
      • Wasterlain C.
      Excitatory amino acids in the developimg brain: ontogeny, plasticity, and excitotoxicity. Review article.
      ]. The age-dependency of IESS might be explained by the maturational differences and susceptibility of the brain to excitotoxic injury mediated by EAA receptors [
      • Hattori H.
      • Wasterlain C.
      Excitatory amino acids in the developimg brain: ontogeny, plasticity, and excitotoxicity. Review article.
      ].
      Neuronal death caused by excessive glutamate is counteracted by growth factors in the immature brain [
      • Mattson M.
      • Murran M.
      • Kater S.
      Fibroblast growth factor and glutamate: opposing roles in the generation and degeneration of hippocampal neuroarchitechtures.
      ]. If the expression of nerve growth factors necessary for further maturation is insufficient or lacking, and cannot counteract glutamate, the cellular program written in the genetic code cannot proceed and, as a result, there is a stagnation in brain development. The age-dependency of IESS may result from the inability to keep up with a deadline of the sensitive period due to the imbalance of neurotransmitters or delayed GABA switch. This imbalance may proceed to apoptosis or cell death.
      • -
        Therapeutic proposal
      Dysregulation of astrocyte glutamic transporters can contribute to the development of epilepsy [
      • Peterson A.
      • Garcia T.
      • Cullion K.
      • Tiwari-Woodruff S.
      • Pedapati E.
      • Devin K.
      • et al.
      Targeted overexpression of glutamate transporter-1 reduces seizures and attenuates pathological changes in a mouse model of epilepsy.
      ]. Novel targets for antiepileptic drugs may evolve from these studies [
      • Peterson A.
      • Garcia T.
      • Cullion K.
      • Tiwari-Woodruff S.
      • Pedapati E.
      • Devin K.
      • et al.
      Targeted overexpression of glutamate transporter-1 reduces seizures and attenuates pathological changes in a mouse model of epilepsy.
      ].

      4.2 Nitric oxide metabolites, nitrates and nitrites

      Nitric oxide (NO) has been implicated in the mediation of a neuronal excitotoxic cascade. CSF levels of NO metabolites were measured in 31 Finnish patients with IESS and 12 control patients. The CSF concentrations were measured using nitrate reductase and Griess reaction method. The levels were higher in patients than in the controls. The patients with spasms of known etiology had significantly higher levels of NO metabolites than controls or the patients with an unknown etiology [
      • Vanhatalo S.
      • Riikonen R.
      Nitric oxide metabolites, nitrates and nitrites in the cerebrospinal fluid in children with West syndrome.
      ]. This suggests that NO is involved in the excitotoxic mechanisms of IESS and may in part explain the neuronal injury and often times compromised neurodevelopment of these children.
      In conclusion, imbalances between expression of inhibitory (GABA, nerve growth factors and ACTH) and excitatory (glutamate and nitric oxide) neurotransmitters associated with IESS may be part of the basic pathological process, or may only reflect underlying brain damage. There is no clinical evidence yet for either of the hypotheses. However, abnormalities in these factors may help to find identify precision therapies as shown with IGF-1.

      5. ACTH as an inhibitory factor and downregulator of CRH, a proconvulsant

      5.1 HPA axis and early stress

      Early life stress and an early brain-damaging insult (i.e., hypoxia, stroke, infection) may trigger a cascade of molecular and cellular changes leading to epilepsy [
      • Joels M.
      • Krugers H.
      • Karst H.
      Stress-induced changes in hippocampal function.
      ].
      Stress is a common precipitant of seizures and can provoke seizures by several mechanisms including changes in neurotransmission and hormonal levels in the brain.
      Considerable evidence supports a role of HPA in the pathogenesis of IESS, including decreased CSF ACTH levels in patients with IESS shown in many studies [
      • Nalin A.
      • Faccinetti F.
      • Galli V.
      • Petraglia F.
      • Storchi R.
      • Gennazzani A.
      Reduced ACTH content in the cerebrospinal fluid of children affected by cryptogenic infantile spasms with hypsarrhythmia.
      ,
      • Baram T.
      • Mitchell W.
      • Hanson R.
      • Snead 3rd, O.A
      • Horton E.
      Cerebrospinal fluid and cortisol are reduced in infantile spasms.
      ,
      • Riikonen R.
      How do cryptogenic and symptomatic infantile spasms differ? A review biochemical studies in Finnish patients.
      ,
      • Nagamitsu S.
      • Matsishi T.
      • YamashitaY S.T.
      • Iwanaga R.
      • Murakamu Y.
      • et al.
      Decreased cerebrospinal fluid levels of beta-endorphin and ACTH in children with infantile spasms.
      ]. The unusual, rapid and favorable treatment response to ACTH is a crucial clue to understanding the pathophysiology of IESS.
      Baram et al. [
      • Baram T.Z.
      • Mitchell W.G.
      • Snead III, O.C.
      • Horton E.J.
      • Saito M.
      Brain-adrenal axis hormones are altered in the CSF of infants with massive infantile spasms.
      ,
      • Brunson K.
      • Khan B.S.
      • Eghbal-Ahmadi M.
      • Baram TZ.
      Corticotropin (ACTH) acts directly on amygdala neurons to down-regulate corticotropin-releasing hormone gene expression.
      ] proposed that early stress (insult or injury) triggers IESS by increasing corticotrophin-releasing hormone (CRH) expression and activity. Endogenous ACTH activates melanotropin receptors and promotes CRH gene expression in the amygdala [
      • Brunson K.
      • Khan B.S.
      • Eghbal-Ahmadi M.
      • Baram TZ.
      Corticotropin (ACTH) acts directly on amygdala neurons to down-regulate corticotropin-releasing hormone gene expression.
      ]. CRH is a powerful convulsant in the immature brain and but weak later in childhood and in the adult brain. Exogenous (therapeutic) ACTH down-regulates CRH via melanocortin receptors [
      • Brunson K.
      • Khan B.S.
      • Eghbal-Ahmadi M.
      • Baram TZ.
      Corticotropin (ACTH) acts directly on amygdala neurons to down-regulate corticotropin-releasing hormone gene expression.
      ,
      • Brunson K.
      • Avishai-Eliner S.
      • Baram T.
      ACTH treatment of infantile spasms: mechanisms of its effects in modulation of neuronal excitability.
      ].
      Our study consisted of 30 children with IESS [
      • Riikonen R.S
      • Jääskeläinen J.
      • Turpeinen U.
      Insulin-like growth factor-1 is associated with cognitive outcome in infantile spasms.
      ]. Patients with spasms with known etiology had high early stress index, markedly low pretreatment CSF ACTH concentrations, and low CSF IGF-1 concentrations, when compared with children having an unknown etiology of the spasms.
      Although CRH acutely stimulates ACTH secretion, chronically elevated brain CRH desensitizes the CRH receptors and eventually decreases ACTH release by negative feed-back mechanism [
      • Baram T.
      Pathophysiology of massive infantile spasms: perspective and putative role of the brain adrenal axis.
      ,
      • Brunson K.
      • Avishai-Eliner S.
      • Baram T.
      ACTH treatment of infantile spasms: mechanisms of its effects in modulation of neuronal excitability.
      ,
      • Hauger R.L.
      • Irwin M.R.
      • Lorang M.
      • Aguilera G.
      • Brown MR.
      High intracerebral levels of CRH result in CRH receptor downregulation in the amygdala and neuroimmune desensitation.
      ,
      • Jezova D.
      Control of ACTH secretion by excitatory amino acids functional significance and clinical implication.
      ].
      Because synthesis of IGF-1 needs a continuous influx of steroids, low ACTH in our study correlated with reduced IGF-1 concentrations, impairment of synaptic transmission, and consequently cognitive decline, and to epileptic encephalopathy.
      Vasopressin
      The convulsant role of CRH shown by Baram [
      • Baram T.
      Pathophysiology of massive infantile spasms: perspective and putative role of the brain adrenal axis.
      ] was supported by vasopressin test also in a small number of patients with IESS. Corticotropin release was higher after vasopressin in children with known etiology (+70) than with unknown etiology (-16) [
      • Riikonen R.
      How do cryptogenic and symptomatic infantile spasms differ? A review biochemical studies in Finnish patients.
      ]. The magnitude of the response depens on the level of endogenous CRH and thus it can be used as an in vivo bioassay of endogenous CRH [
      • Salata R.
      • Jarrett D.
      • Verbalis J.
      • Robinson G.
      Vasopressin stimulation of adrenocorticotropin releasing factor (CRF) which provides evidence for CRF mediation of diurnal rhythm of ACTH.
      ].
      The CRH-excess theory could explain why so many etiologically different disorders lead to the same clinical picture manifested as IESS [
      • Brunson K.
      • Eghbal-Ahmadi M.
      • Baram T.
      How do the many etiologies of West syndrome lead to excitability and seizures? The corticotropin releasing hormone gene excess hypothesis.
      ]. This is an attractive theory. Unfortunately, however, CRH levels in the CSF of IESS patients are not elevated [
      • Baram T.
      • Mitchell W.
      • Brunson K.
      • Haden E.
      Infantile spasms; hypothesis driven therapy and pilot human infant experiment using corticotropin-releasing hormone receptor antagonists.
      ], and, a clinical trial of a competitive antagonist of CRH did not show any improvements of the EEG pattern or seizure status [
      • Baram T.
      • Mitchell W.
      • Brunson K.
      • Haden E.
      Infantile spasms; hypothesis driven therapy and pilot human infant experiment using corticotropin-releasing hormone receptor antagonists.
      ].

      6. Inflammation, a two-way interaction with seizures

      Sorel and Dusaucy-Bauloye [
      • Sorel L.
      • Dusaucy-Bauloye A.
      Findings in 21 cases of Gibbs' hypsarrhythmia; spectacular effectiveness of ACTH.
      ] started experimentally to treat infantile spasms with ACTH, suspecting that the underlying mechanism might be immunological. The selective effectiveness of ACTH in IESS suggests existence of underlying inflammation.
      However, direct evidence for a role of inflammation in IESS is minimal. Immune and inflammatory processes could be detected by blood or CSF levels of cytokines prior to treatment only in a few studies [
      • Shandra O.
      • Moshe S.
      • Galanopoulou A.
      Inflammation in epileptic encephalopathies.
      ]. Some of the elevated pro-inflammatory cytokine levels in IESS patients were observed to be reduced by ACTH treatment [
      • Shandra O.
      • Moshe S.
      • Galanopoulou A.
      Inflammation in epileptic encephalopathies.
      ]. Some animal models support the role of inflammatory pathways in epileptogenesis of IESS [
      • Shandra O.
      • Moshe S.
      • Galanopoulou A.
      Inflammation in epileptic encephalopathies.
      ,
      • Barker-Haliski M.L.
      • Löscher W.
      • White H.S.
      • Galanopoulou AS.
      Neuroinflammation in epileptogenesis: Insights and translational perspectives from new model of epilepsy.
      ]. Inflammatory pathways interact with the neuroendocrine pathway [
      • Shandra O.
      • Moshe S.
      • Galanopoulou A.
      Inflammation in epileptic encephalopathies.
      ].
      There is a two-way interaction between inflammation and seizures. According to animal models, inflammation promotes neuronal hyperexcitability, altered synaptic transmission, excitotoxicity and seizures. Seizure activity leads to the production of inflammatory molecules that, in turn, can cross the blood-brain barrier (BBB), and affect seizure severity and, recurrence.It has been shown that e.g. brain injury, such as ischemic injury, traumatic brain injury, chronic neurodegenerative diseases, CNS infections and perinatal strokes cause tissue inflammation that seems to contribute to both cell death and long-term hyperexcitability [
      • Vezzani A.
      • French J.
      • Bartfai T.
      • Baram T.
      The role of inflammation in epilepsy.
      ]. It permits BBB extravasation of peripheral immune cells or molecules that would otherwise remain in the circulation. Seizure activity leads to the production of inflammatory molecules that, in turn, affect seizure severity.

      6.1 Infantile spasms

      Increased permeability of the BBB was found in a group of 50 patients with IESS and it was more pronounced in the group with known etiology for the spasms than with unknown etiology [
      • Siemes H.
      • Siegert M.
      • Aksu F.
      • Emrich R.
      • Hanefeld F.
      • Scheffner D.
      CSF protein profile in infantile spasms. Influence of etiology and ACTH or dexamethasone treatment.
      ].
      Some of the genes linked to IESS are components of neuroinflammatory pathways. Dysregulation of the mammalian target of rapamycin (mTOR) pathway is implicated in the disease pathology of tuberous sclerosis complex 1 and 2 (TSC1 and TSC2), and cortical dysplasia. In all these conditions, pronounced activation of microglia and astrocytes takes place. There is a constant activation of inflammatory pathways in cortical tubers and subependymal giant cell tumors of patients with TS [

      Boer K., Jansen F., Nellist M., Redeker S., van den Ouweland A., Spliet W., et al. Inflammatory processes in cortical tubers and subependymal giant cell tumors of tuberous sclerosis complex. Epilepsy Res 2008; 78: 7–21. 1016/j.eplepsyres.2007.10.002.i.10.

      ,
      • Prabowo A.
      • Anink J.
      • Lammens M.
      • Nellist M.
      • Ans M.W.
      • van den Ouweland A.
      • et al.
      Fetal brain lesions in tuberous sclerosis complex:TORC1 activation and inflammation.
      ] by the cascade m-TOR pathway [
      • Galanopoulou A.
      • Gorter J.
      • Cepeda C.
      Finding a better drug for epilepsy: the mTOR pathway as an antiepileptogenic target.
      ].
      ACTH therapy (an anti-inflammatory drug therapy) has an excellent response in IESS of patients with TS [
      • Riikonen R.
      Could prevention of infantile spasms have been possible in a historical cohort of 31 tuberous sclerosis patients?.
      ].
      Infections were considered to be etiological factors in 29 Finnish patients (10%) with IESS [
      • Riikonen R.
      Infantile spasms: infectious diseases.
      ]. The outcome of the patients with infectious etiology appeared to be particularly poor (90% cognitively delayed) compared with the overall outcome of patients [
      • Riikonen R.
      Could prevention of infantile spasms have been possible in a historical cohort of 31 tuberous sclerosis patients?.
      ]. All four patients studied a few months after the acute episode still showed high CSF NGF levels. We suggested that CSF NGF can be used as an immunological marker of an ongoing CNS process.
      Further evidence for the role of inflammation in IESS comes from a recent PET (positron emission tomography) study showing focal areas of increased C-Pk11195, a compound which selectively binds to the translocator protein (TSPO) [
      • Chugani H.
      • Kumar A.
      Neuroinflammation in children with infantile spasms: a prospective study before and after treatment with Acthar Gel (Repository Corticotropin injection).
      ]. Neuroinflammation is mediated by activated microglia, which express TSPO [
      • Werry E.
      • Bright F.
      • Piquet O.
      • Iyyner L.
      • Halliday G.
      • Hodges J.
      • et al.
      Recent developments in TSPO PET Imaging as a biomarker of neuroinflammation in neurodegenerative disorders.
      ]. This study of Chugani et al. (2021) [
      • Chugani H.
      • Kumar A.
      Neuroinflammation in children with infantile spasms: a prospective study before and after treatment with Acthar Gel (Repository Corticotropin injection).
      ] has, however, short-comings. Most patients included in the study were >1 year of age and had failed multiple treatments. Hence the observed inflammation may reflect refractory epilepsy.
      Neuroinflammation is an important mechanism in some etiological subgroups of IESS (acquired epilepsy and TS). Some new data also link neuroinflammation to cognitive deficits [
      • Mazarati A.
      • Lewis A.
      • Pittman O.
      Neurobehavioural comorbidities of epilepsy: role of inflammation.
      ].

      6.2 Therapeutic proposal

      Novel anti-inflammatory approaches on cellular and molecular targets should be designed to modify the pathophysiolocal processes [
      • Orsini A.
      • Foiadelli T.
      • Costagliola G.
      • Michev A.
      • Cosolini R.
      • Consolini R.
      • et al.
      The role of inflammatory mediators and epileptic encephalopathies and therapeutic implications.
      ,
      • Vezzani A.
      • Balosso S.
      • Ravizza T.
      Neuroinflammatory pathways as treatment targets and biomarkers in epilepsy.
      ].

      7. Major interactions between 1. neurotrophic factors, 2. HPA axis and 3. inflammation in the pathogenesis of IESS. See Fig. 1

      Some hypotheses are presented below.
      • 1
        Neurotrophic factors play a key role in neuroprotection and developmental maturation of the brain. An altered maturational process may provide the necessary framework for IESS.
        • a)
          NGF: NGF synthesis is influenced by hormones and the immune systems [
          • Spranger M.
          • Lindholm D.
          • Bandtlov C.
          • Heumann R.
          • Ghahn H.
          • Näher-Noe H.
          • et al.
          Regulation of nerve growth factor (NGF) synthesis in the rat central nervous system: Comparison between the effects of interleukin-1 and various growth factors in astrocyte cultures and in vivo.
          ,
          • Riikonen R.
          • Söderström S.
          • Vanhala S.
          • Ebendal T.
          • Lindholm D.
          West`s syndrome: cerebrospinal fluid nerve growth factor and effect of ACTH.
          ]. Some genetic conditions and signalling cascades linked to IESS are components of neuroinflammatory pathways (e.g. TSC1, TSC2, and cortical dysplasia). In TS, mTor-mediated inflammatory processes may be involved pathogenesis [
          • Zhang B.
          • Zou J.
          • Rensing N.
          • Yang M.
          • Wong M.
          Inflammatory mechanisms contribute to the neurological manifestations of tuberous sclerosis complex.
          ].
        • b)
          IGF-1: IGF-1 modulates neuroinflammation [
          • Labandeira-Garcia J.
          • Costa-Besada M.
          • Labendeira C.
          • Villar-Cheda B.
          • Rodriguuez-Rerez
          Insulin-like growth factor-1 and neuroinflammation. Mini Review article.
          ]. Patients with IESS and with normal CSF-IGF-1, also have a normal level of ACTH in CSF, and an intact HPA axis. In contrast, patients with IESS and a high index of early stress, brain damage, and low CSF IGF-1 have low CSF ACTH. IGF-1 is a biomarker of ACTH treatment response, progression of epilepsy and later cognitive outcome of IESS patients [
          • Riikonen R.S
          • Jääskeläinen J.
          • Turpeinen U.
          Insulin-like growth factor-1 is associated with cognitive outcome in infantile spasms.
          ].
      • 2
        HPA axis has multiple endocrinological, cognitive and immunologic functions. It has been shown that in patients, NGF gene expression is modulated by ACTH [
        • Lindholm D.
        • Castrén E.
        • Berzaghi M.
        • Blöchl A.
        • Thoenen H.
        Activity-dependent and hormonal regulation of neurotrophin mRNA levels in brain–implications for neuronal plasticity.
        ]. The HPA axis has an essential role in the response to stress and in anti-inflammatory reactions. ACTH downregulates CRH which can cause seizures during early development but not later. Inflammatory mediators can trigger the HPA axis.
      • 3
        Inflammatory pathways interact with the neuroendocrine pathway [
        • Shandra O.
        • Moshe S.
        • Galanopoulou A.
        Inflammation in epileptic encephalopathies.
        ]. ACTH therapy has a unique effect in IESS including immune-suppression. Inflammation plays a role in some etiological subgroups of IESS patients.
      The present data clearly suggest that these three systems interact in the pathogenesis IESS. Further basic and clinical studies are needed to understand in detail what happens at molecular and cellular level.

      8. What are the mechanisms of glucocorticoids and vigabatrin on early brain maturation and cognition

      8.1 ACTH and glucocorticoids as key therapeutic options in IESS treatment

      Although glucocorticoids are necessary for normal development, excess exposure has deleterious effects, inhibiting fetal growth and altering the trajectory of brain maturation. Glucocorticoid stimulation induced by mild stress has positive effects on hippocampal memory processing and synaptic plasticity, but severe stress induces apoptosis. Repeated high-dose glucocorticoid treatment mimics chronic stress [

      Mc Ewen B. Neuronal and systemic effects of chronic stress. Chronic Stress 2017 Jan-Dec 1: 2470547017692328. 10.1177/2470547017692328.

      ]. As stress increases, learning first improves and then declines to levels below baseline. An inverted U-shaped function curve depicts the dependency of memory on stress level [
      • Joels M.
      Corticosteroid effect in the brain: U-shape it.
      ]. Prolonged stress and chronically elevated HPA activity result in microscopic and macroscopic changes (e.g. reduced dendritic branching) in hippocampus [
      • Joels M.
      • Karst H.
      • Alfarez D.
      • Heine V.
      • Qin Y.
      • van Riel E.
      • et al.
      Effects of chronic stress on structure and cell function in rat hippocampus and hypothalamus.
      ]. Glucocorticoids induce increased apoptosis and subsequent cognitive decline [
      • Sapolski G.
      Glucocorticoids, stress and excitotoxic neuronal death.
      ]. Steroid treatment during critical periods of brain development may impair myelination and cell division, resulting in long- term behavioural effects [
      • Weichsel M.
      The therapeutic use of glucocorticoids in the perinatal period. Potential neurological hazards.
      ]. This has been shown after treatment of premature newborns with respiratory failure postnatally with glucocorticoids (dexamethasone): a dramatic impairment of cognitive abilities at school age can be observed [
      • Weichsel M.
      The therapeutic use of glucocorticoids in the perinatal period. Potential neurological hazards.
      ,
      • Yeh T.F.
      • Lin Y.J.
      • Lin H.C.
      • Huang C.C.
      • Hsieh W.S.
      • Lin H.C.
      • Tsai CH.
      Outcomes at school age after postnatal dexamethasone therapy for lung disease of prematurity.
      ,
      • Poggi Davis E.
      • Sandman C.
      • Buss C.
      • Wing D.
      • Head K.
      Fetal glucocorticoid exposure is associated with preadolescent brain development.
      ].

      8.2 Vigabatrin

      In the UKISS study on IESS patients, at 4 year age the cognitive development and epilepsy outcome were better with hormonal treatments than after vigabatrin in patients having unknown etiology [
      • Darke K.
      • Edwards S.
      • Hancock E.
      • Johnson A.
      • Kennedy C.
      • Lux A.
      • et al.
      Developmental and epilepsy outcomes at age of 4 years in the UKISS trial comparing hormonal treatments to vigabatrin for infantile spasms: a multicenter randomized trial.
      ]. Clinical toxicity of VGB is manifested as visual field damages (in about a third of the patients) [
      • Riikonen R.
      • Rener-Primec Z.
      • Carmant L.
      • Dorofeeva M.
      • Hollody K.
      • Szabo I.
      • et al.
      Does vigabatrin cause visual field defects: an international multicentre study.
      ] and structural brain abnormalities in MRI. Reversible symmetrical involvement of the basal ganglia, thalamus and brainstem have been reported related to the use of this medicine [
      • Dracopoulos A.
      • Widjaja E.
      • Raybaud C.
      • Westall C.
      • Snead 3rd, O.C.
      Vigabatrin-associated reversible MRI signal changes in patients with infantile spasms.
      ,
      • Valenzuala R.
      • Crespo A.
      • Princich J.
      • et al.
      Vigabatrin-associated brain abnormalities on MRI and other neurological symptoms in patients with West syndrome.
      ].
      At plasma concentrations relevant in human clinical use, vigabatrin can cause apoptotic neurodegeneration in the developing animal brain but not later when the brain development has been completed [
      • Bittigau P.
      • Sifringer M.
      • Ikonomidou C.
      Antiepileptic drugs and apoptosis in the developing brain.
      ,
      • Pitkänen A.
      • Kubova H.
      Antiepileptic drugs in neuroprotection.
      ]. Of concern is the axonal degeneration, which is irreversible [
      • Sidhu R.
      • Biggio M.
      • Tuor U.
      • Seshia S.
      Low-dose vigabatrin (γ-vinyl GABA)-induced damage in the immature brain.
      ]. Neuronal death is associated with reduced expression of neurotrophins. 121 These findings were considered to present one possible mechanism of cognitive impairment and reduced head circumference associated with pre- or postnatal vigabatrin exposure [
      • Bittigau P.
      • Sifringer M.
      • Genz K.
      • Reith E.
      • Pospischi D.
      • Govindarajalu S.
      • et al.
      Antiepileptic drugs and apoptotic neurodegeneration in the developing brain.
      ].
      The adverse effects would favor the use of small doses and short duration of therapy but persisting seizures might be more harmful than the adverse effects. Detailed safety assessments of these drugs in infants are needed.

      9. Novel genetic approaches

      Identification of various genetic causes of IESS has helped in constructing animal models [
      • Stafström C.
      • Holmes G.
      Infantile spasms: criteria for an animal model.
      ,
      • Dulla G.
      Utilizing animal models of infantile spasms.
      ]. Due to the large number and heterogeneity of individual genes, however, it has not been possible to build up a unifying theory. Modern genetics and bioinformatics offer new tools to simultaneously analyse huge amounts of genetic data.
      Exome sequencing data was used in the epi-4K-study to understand protein-protein networks in IESS [
      • Michaud J.L.
      • Lachance M.
      • Hamdan F.F.
      • Carmant L.
      • Lortie A.
      • Diadori P.
      • et al.
      The genetic landscape of infantile spasms.
      ]. Gene ontology, gene set enrichment and molecular pathway analysis are new approaches [
      • Paciorkowski A.
      • Tio L.
      • Dobyns W.
      A genetic and biologic classification of infantile spasms.
      ,
      • Ashburner M.
      • Ball C.
      • Botstein D.
      • Butler H.
      • Cherry M.
      Gene Ontology: tool for the unification of biology.
      ]. The two best described pathways of the pathogenesis of IESS are abnormalities in the gene regulatory network of GABAergic forebrain development, and abnormalities in molecules expressed at the synapse [
      • Paciorkowski A.
      • Thio L.
      • Rosenfeld J.
      Copy number variants and infantile spasms: evidence for abnormalities in ventral forebrain development and pathways of synaptic function.
      ]. Gene ontology and pathway enrichment analysis of clinical laboratory-confirmed pathogenic variant-harboring genes was performed in a multicenter cohort of 509 infants with newly diagnosed epilepsy [
      • Berg A.
      • Chakravorty S.
      • Koh S.
      • Grispan Z.
      • Shelhaas R.
      • Saneto R.
      • et al.
      Why West? Comparisons of clinical, genetic and molecular features of infants with and without spasms.
      ]. This more recent analysis of cellular compartment gene sets demonstrates that genes expressed in the neuronal cell body, such as Golgi and endoplasmic reticulum, were preferentially associated with IESS. By contrast, axonal and synaptic regions, dendrites and plasma membrane are preferentially implicated in the non-spasm group [
      • Berg A.
      • Chakravorty S.
      • Koh S.
      • Grispan Z.
      • Shelhaas R.
      • Saneto R.
      • et al.
      Why West? Comparisons of clinical, genetic and molecular features of infants with and without spasms.
      ].
      Clear differences existed between the biological pathways leading to spasms versus other seizure types occurring at the same age [
      • Berg A.
      • Chakravorty S.
      • Koh S.
      • Grispan Z.
      • Shelhaas R.
      • Saneto R.
      • et al.
      Why West? Comparisons of clinical, genetic and molecular features of infants with and without spasms.
      ].
      Pathway analysis showed a bigger impact of genes having broad neurodevelopmental and regulatory pathways in infantile epileptic spasms than in other infantile seizures [
      • Berg A.
      • Chakravorty S.
      • Koh S.
      • Grispan Z.
      • Shelhaas R.
      • Saneto R.
      • et al.
      Why West? Comparisons of clinical, genetic and molecular features of infants with and without spasms.
      ].
      There is still much to be learned about gene transcription, histone modification, methylation, alternative splicing, translation, ubiquination, transposable elements and other control mechanisms of gene expression in the brain of a developing child. So far, the study of regulatory microRNAs (miRNAs) is an example of emerging approaches [
      • Berg A.
      • Chakravorty S.
      • Koh S.
      • Grispan Z.
      • Shelhaas R.
      • Saneto R.
      • et al.
      Why West? Comparisons of clinical, genetic and molecular features of infants with and without spasms.
      ].

      10. Discussion

      Advances have been made in the search for the pathophysiological mechanisms of IESS, but the mechanisms are still unknown. Many results come from animal experiments, and studies in humans are few. Diverse mechanisms lead to the same clinical picture. IESS appear to be a genetically heterogenous condition that can result from abnormalities in important developmental pathways in the brain. Synaptic functional pathways play a critical role in IESS pathogenesis [
      • Paciorkowski A.
      • Thio L.
      • Rosenfeld J.
      Copy number variants and infantile spasms: evidence for abnormalities in ventral forebrain development and pathways of synaptic function.
      ] and IGF-1 is a key factor that regulates synaptogenesis in the developing brain [
      • Leventhal P.
      • Russel J.
      • Feldman E.
      IGFs and the nervous system.
      ].
      In this review the biochemical, molecular factors which might play an important role in the pathogenesis of IESS have been reviewed based on the literature. Our own studies on Finnish IESS patients are reviewed, too, as part of a long-lasting research dealing with many aspects of IESS. The role of nerve growth factors was shown; CSF β-NGF was low in unresponsive cases and very high in postinfectious IESS. IGF-1 seems to be a biomarker of disease severity, and low IGF-1 (affecting synaptic transmission) together with low ACTH levels lead to cognitive decline. There is an imbalance between inhibitory (NGF, IGF1-1, ACTH, GABA) and excitatory (glutamate and nitric oxide) factors in IESS. Patients with known etiology differed from the groups with unknown etiology.
      Early stress leads to changes of neurotransmitters and hormonal and immunological parameters. Pre-and perinatal factors are the cause of IESS in 78% of cases. High CRH can cause structural changes of the brain. Abnormal function of the HPA axis can lead to IESS.
      The physiological role of the neurotrophin nerve growth factor (NGF) has been characterized. Important interactions between HPA axis, nerve growth factors and immune system exist [
      • Spranger M.
      • Lindholm D.
      • Bandtlov C.
      • Heumann R.
      • Ghahn H.
      • Näher-Noe H.
      • et al.
      Regulation of nerve growth factor (NGF) synthesis in the rat central nervous system: Comparison between the effects of interleukin-1 and various growth factors in astrocyte cultures and in vivo.
      ,
      • Aloe L.
      • Rocco M.L.
      • Bianchi P.
      • Luigi Manni L.
      Nerve growth factor: from the early discoveries to the potential clinical use.
      ]. In the pathogenesis of IESS all of them seem to be involved (See Fig. 1).
      Fig 1
      Fig. 1Interactions between the main mechanisms and hypotheses of the pathogenesis of IESS as presented in the text 1. Nerve growth factors, 2. Hypothalamic pituitary adrenal (HPA) axis and 3. Inflammation.
      When injury occurs in the developing brain, normal maturational processes are disturbed.There are many factors which can cause cognitive decline in patients with IESS.
      Limitations and strengths of the study
      • 1)
        The results of the Finnish studies are region-specific and should be confirmed in other centers.
      • 2)
        We have mainly excluded detailed discussion on animal models and have concentrated on studies in human infants.
      • 3)
        The data from Finland are unique because CSF samples were available. Neurotransmitters and neuropeptides were examined in a large number of patients which increases the value of our results. Human studies are rare so far. We encourage other countries to include CSF sampling as it is the practice in Finland, especially on a research basis and for refractory patients of IESS.
      • 4)
        The therapeutic suggestions proposed are based on pathophysiological mechanisms.

      11. Conclusions

      An altered maturational process could explain why so many, seemingly independent etiological factors lead to the same clinical syndrome and frequently to developmental delay.
      What kind of research is needed
      • 1)
        Basic science: We need further studies on the maturation of the brain. Because undisturbed synaptic function is crucial for the cognition, studies on synaptic funtion are important (including ultrastructural autopsy studies). The role of nerve growth factors in preclinical therapeutic trials (e.g. trofinetide) should be explored.
      • 2)
        Basic science: What is the genetic blueprint that governs developmental steps and gene expression in the human brain at around 6 months of age, the sensitive period?
      • 3)
        Applied science: We need to understand the mechanisms of different therapies. What is the optimal dosage and duration of the therapy from the point of view of survival of the neurons? What are the mechanisms behind a ketogenic diet, cannabinoids, deep brain stimulation, ganaxalone and other steroids, anti-inflammatory drugs, and, trofinetide (1-3) IGF-1?
      • 4)
        Clinical studies: We need further studies on patients with unknown etiology (multicenter case collecting and analyses, brain autopsies?)
      • 5)
        Clinical trials: Guidelines of standards of care IESS therapy: What are the optimal doses ACTH/prednisolone/vigabatrin and the ideal duration of the therapy? What are the adverse effects of these treatments on memory?
      • 6)
        Clinical trials: Because there is a latency period from a stress event to onset of spasms, prevention some cases by antiepileptic medication (e.g. TS, hypoxic-ischemic injury) should be tested. We are awere that such studies are ongoing.
      • 7)
        Basic science and clinical trials: Understanding in detail the interaction between the three systems: neurotrophic factors, HPA axis and immune system in pathogenesis of IESS.

      Ethical approval

      I confirm that I have read the Journal`s position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.
      The study adhered to the tenets of the Helsinki Declaration. The Ethical Committee of The Children`s Hospital, Helsinki and Children`s Hospital, Kuopio approved the study. The study had institutional approval. Patients or their caregivers gave consent to the research and to publication of the results. The Finnish article`s main data and materials can be accessed in Helsinki and Kuopio Universities. The investigations were made at the Hospital for Children and Adolescents, Helsinki University, Helsinki, and Children`s Hospital of Kuopio, Finland, where the original data is kept.

      Declaration of Competing Interest

      I have no conflict of interest to disclose.

      Acknowledgments

      I thank Dr. Mauno Airaksinen, Professor of Pharmacology (Emeritus) and Dr. Jaana Lähdetie, M.D., Ph.D., Pediatric Neurologist, for help in writing this paper. There has been no funding for this work.

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