No major role for the EMX2 gene in schizencephaly

[Congenital malformations of the brain misinterpreted as sequelae of poliomyelitis]

BACKGROUND

Poliomyelitis is an infectious disease of the peripheral motor neurons, which predominantly affects children and causes residual palsies. Because of the oral poliomyelitis vaccination started in Germany in 1960 and 1962 and the following rapid decline of the incidence of this infection, the postpolio syndrome in Germany is a disease of older people.

METHODS

Since 2008, we have offered a poliomyelitis outpatient consultation at the Center of Geriatrics, Protestant Hospital Göttingen-Weende and have treated 33 patients.

RESULTS

The spectrum of persistent deficits after poliomyelitis ranges from palsy of single extremities to severe disability with (temporary) ventilator dependence. Many patients suffer from scoliosis or shortening of limbs of different degrees, which promotes degenerative diseases of the spinal cord and joints with secondary myelopathy, injury of spinal nerve roots or peripheral nerves or respiratory failure. The postpolio syndrome is characterized by an increase of the functional deficits after decades of compensation. The palsies of 2 of the 33 patients were not caused by poliomyelitis but by myelomeningocele and schizencephaly, respectively.

CONCLUSION

The motor deficits acquired in childhood enable the majority of the patients to successfully master their lives. Because of the limited compensatory capacities of postpolio patients, even small increases in the severity of the palsy can cause a severe decline of the functional status and an impairment of the ability to live an independent life. In a substantial proportion of patients with the diagnosis poliomyelitis the symptoms are caused by other diseases.

Hypoxia: A teratogen underlying a range of congenital disruptions, dysplasias, and malformations

In this review, we explore evidence that hypoxia in the developing human fetus can lead not only to the more commonly accepted disruptive-type defects, but also patterns of anomalies that suggest that hypoxia can exert a more classic teratogenic effect, using the brain as one example. We review neuropathology in the context of intrauterine hypoxia, particularly as it relates to carbon monoxide poisoning, in utero strokes, and homozygous alpha-thalassemia. In general, the associated brain injuries resemble those seen with other causes of hypoxic-ischemic injury. Fetal strokes during development usually lead to loss of brain tissue in areas that do not follow a typical embryologic pattern, and therefore are considered disruptions. However, there is also evidence that fetal brain ischemia can cause more classically recognized patterns of abnormal embryonic neuronal migration and organization such as polymicrogyria, cortical dysplasia, or dysgenesis, including select types of focal cortical dysplasia. This study summarizes available literature and evidence to raise clinicians' awareness regarding the association between hypoxia and congenital anomalies, including brain malformations.

International consensus recommendations on the diagnostic work-up for malformations of cortical development

Malformations of cortical development (MCDs) are neurodevelopmental disorders that result from abnormal development of the cerebral cortex in utero. MCDs place a substantial burden on affected individuals, their families and societies worldwide, as these individuals can experience lifelong drug-resistant epilepsy, cerebral palsy, feeding difficulties, intellectual disability and other neurological and behavioural anomalies. The diagnostic pathway for MCDs is complex owing to wide variations in presentation and aetiology, thereby hampering timely and adequate management. In this article, the international MCD network Neuro-MIG provides consensus recommendations to aid both expert and non-expert clinicians in the diagnostic work-up of MCDs with the aim of improving patient management worldwide. We reviewed the literature on clinical presentation, aetiology and diagnostic approaches for the main MCD subtypes and collected data on current practices and recommendations from clinicians and diagnostic laboratories within Neuro-MIG. We reached consensus by 42 professionals from 20 countries, using expert discussions and a Delphi consensus process. We present a diagnostic workflow that can be applied to any individual with MCD and a comprehensive list of MCD-related genes with their associated phenotypes. The workflow is designed to maximize the diagnostic yield and increase the number of patients receiving personalized care and counselling on prognosis and recurrence risk.

Latent Schizencephaly With Psychotic Phenotype or Schizophrenia With Schizencephaly? A Case Report and Review of the Literature

OBJECTIVES

Although schizencephaly belongs to the class of neurodevelopmental disorders, which are a well-known predisposing factor for psychosis, there is a lack of relevant studies and diagnostic guidelines on this relationship.

METHOD

A case report of first-episode psychosis with persistent negative symptoms associated with schizencephaly is described and compared with 7 other cases found in the literature.

RESULTS

We found perinatal pathology, cognitive deficit, and EEG abnormality in a patient with atypical initial symptoms of psychosis such as olfactory hallucinations. Abnormal EEG findings (left frontal spikes and frontal intermittent rhythmic delta activity) called for magnetic resonance imaging, which revealed left parieto-occipital closed-lip schizencephaly. The patient exhibited a partial response to low-dose amisulpride treatment.

CONCLUSION

We conclude that schizencephaly in our patient was at first asymptomatic and later developed into clinically manifest schizophrenia-like disorder. Both magnetic resonance imaging and EEG were essential tools for establishing this diagnosis.

Schizencephaly: A Review of 734 Patients

BACKGROUND

Schizencephaly is a rare congenital cerebral malformation associated with serious neurological manifestations. The number of studies regarding schizencephaly is limited.

METHODS

We conducted a literature review and extracted data from the case reports. Of 199 articles retrieved, 156 articles (734 patients) met our inclusion criteria.

RESULTS

Patient characteristics included microcephaly (41.5% of patients), seizures (74.1%), bilateral cleft (41.4%), open lip (61.3%), septo-optic dysplasia (69.1%), and ventricular dilation (60.5%). The majority of clefts were in the frontal and parietal lobes. When these potential association factors were assessed by univariate logistic regression microcephaly (OR = 21.75, P < 0.001), corpus callosum agenesis (OR = 9, P < 0.001), motor impairments (OR = 6.21, P < 0.001), and bilateral clefts (OR = 6.31, P < 0.001) seems to have the strongest association, but also age at diagnosis <10 years (OR = 1.05, P < 0.001), right (OR = 1.85, P = 0.001) or left (OR = 2.71, P < 0.001) side clefts and septum pellucidum (OR = 3.7, P = 0.002) agenesis were associated with neurocognitive dysfunctions.

CONCLUSIONS

We describe novel findings with practical implications for predicting neurocognitive outcomes in patients with schizencephaly. Most patients had neurological impairments including motor (90.0%) or cognitive (77.5%) dysfunctions. Bilateral clefts, motor impairment, microcephaly, and corpus callosum agenesis were strongly associated with neurocognitive impairment. A lack of large cohorts of patients with schizencephaly prevented comparison of our results; most previous studies are case reports or small case series.

Schizencephaly revisited

Purpose

In this paper, I will report the range of appearances of schizencephaly in children and fetuses by reviewing a 10-year experience from a single centre and detail classification systems for the different forms of schizencephaly. This will lead to re-assessment of possible aetiological and mechanistic causes of schizencephaly.

Methods

All cases of pediatric and fetal schizencephaly were located on the local database between 2007 and 2016 inclusive. The studies were reviewed for the presence, location and type of schizencephaly, as well as the state of the (cavum) septum pellucidum, the location of the fornices and the presence of other brain abnormalities.

Results

The review included 21 children and 11 fetuses with schizencephaly. Schizencephaly (type 1) was found in 9% of children but no fetuses, schizencephaly (type 2) was present in 67% of the pediatric cases and in 45% of fetuses, whilst schizencephaly (type 3) was present in approximately 24% of children and 55% of fetuses. Other brain abnormalities were found in 67% of children and 55% of fetuses.

Conclusion

I have proposed a new system for classifying schizencephaly that takes into account all definitions of the abnormality in the literature. Using that approach, I have described the appearances and associations of pediatric and fetal cases of schizencephaly from a single centre. Review of the current literature appears to favour an acquired destructive aetiology for most cases of schizencephaly, and I have proposed a mechanism to explain the cortical formation abnormalities found consistently in and around areas of schizencephaly.

Porencephaly in a fennec fox (Vulpes zerda)

A postmortem examination revealed a large brain cavity in the right cerebral hemisphere of a 9-year-old male fennec (Vulpes zerda). The cavity was filled with cerebrospinal fluid and extended to the right lateral ventricle. Swelling and displacement of the right hippocampal area were also observed. Histologic examination revealed no evidence of previous infarct lesions, hemorrhage, inflammation or invasive tumor cells. Observation of the defective part suggested a local circulatory disorder during the fetal stage, although the cause was not detected. No neurological symptoms that could enable a provisional diagnosis were observed during the course of his life. This is the first report of asymptomatic porencephaly in a fennec fox.

Successful surgery for refractory seizures associated with bilateral schizencephaly: two case reports and literature review

Schizencephaly is a rare malformation of cortical development resulting from cell migration defects that occur unilaterally or bilaterally. The type of the schizencephalic cleft can be open lip or closed lip. Patients suffering from refractory seizures secondary to schizencephaly should be considered for surgical treatment. In this paper, we retrospectively analyzed two patients with confirmed schizencephaly and intractable seizures. The evaluation methods included a medical history assessment, a neurological examination and magnetic resonance imaging (MRI). Continuous intracranial video-electroencephalogram (vEEG) monitoring with surface electrodes and deep electrodes was evaluated to confirm the epileptogenic zones associated with the schizencephalic lesions. Cortical electrical stimulation was performed to evaluate the neurophysiology of the relevant brain regions. Epileptic focus resection was performed close to the schizencephalic cleft according to the results of intracranial EEG and stimulation while preserving neurological functions. MRI revealed bilateral open lip schizencephaly in one patient and closed lip schizencephaly in the other patient. The epileptogenic zones were localized close to the schizencephalic clefts. The seizure outcome was Engel's class Ia in both patients at 1-year follow-up. No significant neurological deficits were found, and their activities of daily life were significantly improved. We conclude that abnormal cortex near the schizencephalic clefts may display an extrinsic epileptogenicity. Accurate localization of the epileptogenic zones using intracranial EEG and electrical stimulation can lead to a seizure-free outcome in patients with refractory epilepsy associated with schizencephaly.

Prenatal Diagnosis and Postnatal Outcome of Schizencephaly

The aim of this study was to present our experience with 5 cases of fetal schizencephaly in terms of prenatal diagnostic features, and postnatal outcome. The database of prenatal diagnosis unit was searched for antenatally diagnosed cases with schizencephaly. Maternal characteristics, ultrasonography, prenatal-postnatal magnetic resonance imaging (MRI) findings, and postnatal outcome were noted. Of 5 cases, 2 had definitive prenatal diagnoses on ultrasound and 3 cases were diagnosed by fetal MRI. All cases had cerebral cortical migration anomalies including polymicrogyria, subependymal heterotopia, and lissencephaly, and 2 cases had additional extracranial malformations. Three cases showed regression of the cerebral clefts on follow-up postnatal MRIs. Three cases had moderate to severe psychomotor retardation, and 1 case needed repeated ventriculoperitoneal shunt operation due to hydrocephaly. Prenatal diagnosis of schizencephaly with ultrasonography is not straightforward and required further evaluation with fetal MRI. Additional cerebral anomalies worsen the prognosis of schizencephaly.

Genomic variants and variations in malformations of cortical development

Malformations of cortical development (MCDs) are a common cause of neurodevelopmental delay and epilepsy and are caused by disruptions in the normal development of the cerebral cortex. Several causative genes have been identified in patients with MCD. There is increasing evidence of role of de novo mutations, including those occurring post fertilization, in MCD. These somatic mutations may not be detectable by traditional methods of genetic testing performed on blood DNA. Identification of the genetic cause can help in guiding families in future pregnancies. Research has highlighted how elucidation of key molecular pathways can also allow for targeted therapeutic interventions.

Schizencephaly-diagnostics and clinical dilemmas

BACKGROUND

Schizencephaly is an uncommon congenital disorder of cerebral cortical development. The defect is characterized by the presence of a cleft in the brain extending from the surface of the pia mater to the cerebral ventricles. The margins of the cleft are lined with heterotropic, dysplastic gray matter. The causes of schizencephaly are heterogeneous and can include teratogens, prenatal infection, maternal trauma, or EMX2 mutations.

METHOD

In the present paper, the authors described difficulties in employing diagnostic imaging in differentiating between type II (open-lip) schizencephaly and much more common intracranial fluid spaces of a different origin (arachnoid cysts and hydrocephalus).

RESULT

In all the three cases, the treatment consisted in implantation of a shunt system; nevertheless, it should be emphasized that a surgical intervention in the third presented case (type II schizencephaly) aimed at relieving the symptoms of intracranial hypertension-a directly life-threatening condition-since shunting is not a method of treating schizencephaly itself.

CONCLUSIONS

Although proper interpretation of the character of intracranial fluid spaces is of significance for further therapeutic management, yet, the key decision as to the surgical intervention is made based on clinical presentation, predominantly on symptoms of intracranial hypertension.

Malformations of cortical development: clinical features and genetic causes

Malformations of cortical development are common causes of developmental delay and epilepsy. Some patients have early, severe neurological impairment, but others have epilepsy or unexpected deficits that are detectable only by screening. The rapid evolution of molecular biology, genetics, and imaging has resulted in a substantial increase in knowledge about the development of the cerebral cortex and the number and types of malformations reported. Genetic studies have identified several genes that might disrupt each of the main stages of cell proliferation and specification, neuronal migration, and late cortical organisation. Many of these malformations are caused by de-novo dominant or X-linked mutations occurring in sporadic cases. Genetic testing needs accurate assessment of imaging features, and familial distribution, if any, and can be straightforward in some disorders but requires a complex diagnostic algorithm in others. Because of substantial genotypic and phenotypic heterogeneity for most of these genes, a comprehensive analysis of clinical, imaging, and genetic data is needed to properly define these disorders. Exome sequencing and high-field MRI are rapidly modifying the classification of these disorders.

Schizencephaly: association with young maternal age, alcohol use, and lack of prenatal care

Schizencephaly is a rare malformation of cortical development characterized by congenital clefts extending from the pial surface to the lateral ventricle that are lined by heterotopic gray matter. The clinical presentation is variable and can include motor or cognitive impairment and epilepsy. The causes of schizencephaly are heterogeneous and can include teratogens, prenatal infection, or maternal trauma. Reported genetic causes include chromosomal aneuploidy, EMX2 mutations, and possible autosomal recessive familial cases based on recurrence in siblings. In an effort to identify risk factors for schizencephaly, we conducted a survey of 48 parents or primary caretakers of patients with schizencephaly born between 1983 and 2004. We discovered that young maternal age, lack of prenatal care, and alcohol use were all significantly associated with risk of schizencephaly. Our results suggest that there are important nongenetic, intrauterine events that predispose to schizencephaly.

A novel mutation in the β-tubulin gene TUBB2B associated with complex malformation of cortical development and deficits in axonal guidance

Neurological disorders characterized by abnormal neuronal migration, organization, axon guidance, and maintenance have recently been associated with missense and splice-site mutations in the genes encoding α- and β-tubulin isotypes TUBA1A, TUBB2B, TUBB3, and TUBA8. We found a novel heterozygous mutation c.419G > C in exon 4 of the gene encoding TUBB2B in a female with microcephaly, agenesis of the corpus callosum, open-lip schizencephaly of the left parietal lobe, extensive polymicrogyria, basal ganglia and thalami dysmorphisms, and vermis and right third nerve hypoplasia. The missense change results in a glycine to alanine substitution; the mutated residue falls within an invariant glycine-rich region and therefore is likely to result in impaired protein function and possibly microtubule formation. This study expands the spectrum of brain malformations associated with mutations in the β-tubulin gene TUBB2B, supporting its critical role in migration/organization and axon guidance processes. In addition, it suggests a possible genetic aetiology of schizencephaly, thus strengthening the hypothesis that there is a common pathophysiological base in polymicrogyria and schizencephaly.

A developmental and genetic classification for malformations of cortical development: update 2012

Malformations of cerebral cortical development include a wide range of developmental disorders that are common causes of neurodevelopmental delay and epilepsy. In addition, study of these disorders contributes greatly to the understanding of normal brain development and its perturbations. The rapid recent evolution of molecular biology, genetics and imaging has resulted in an explosive increase in our knowledge of cerebral cortex development and in the number and types of malformations of cortical development that have been reported. These advances continue to modify our perception of these malformations. This review addresses recent changes in our perception of these disorders and proposes a modified classification based upon updates in our knowledge of cerebral cortical development.

Association of common genetic variants in GPCPD1 with scaling of visual cortical surface area in humans

Visual cortical surface area varies two- to threefold between human individuals, is highly heritable, and has been correlated with visual acuity and visual perception. However, it is still largely unknown what specific genetic and environmental factors contribute to normal variation in the area of visual cortex. To identify SNPs associated with the proportional surface area of visual cortex, we performed a genome-wide association study followed by replication in two independent cohorts. We identified one SNP (rs6116869) that replicated in both cohorts and had genome-wide significant association (P(combined) = 3.2 × 10(-8)). Furthermore, a metaanalysis of imputed SNPs in this genomic region identified a more significantly associated SNP (rs238295; P = 6.5 × 10(-9)) that was in strong linkage disequilibrium with rs6116869. These SNPs are located within 4 kb of the 5' UTR of GPCPD1, glycerophosphocholine phosphodiesterase GDE1 homolog (Saccharomyces cerevisiae), which in humans, is more highly expressed in occipital cortex compared with the remainder of cortex than 99.9% of genes genome-wide. Based on these findings, we conclude that this common genetic variation contributes to the proportional area of human visual cortex. We suggest that identifying genes that contribute to normal cortical architecture provides a first step to understanding genetic mechanisms that underlie visual perception.

Candidate gene sequencing of LHX2, HESX1, and SOX2 in a large schizencephaly cohort

Schizencephaly is a malformation of cortical development characterized by gray matter-lined clefts in the cerebral cortex and a range of neurological presentations. In some cases, there are features of septo-optic dysplasia concurrently with schizencephaly. The etiologies of both schizencephaly and septo-optic dysplasia are thought to be heterogeneous, but there is evidence that at least some cases have genetic origin. We hypothesized that these disorders may be caused by mutations in three candidate genes: LHX2, a gene with an important cortical patterning role, and HESX1 and SOX2, genes that have been associated with septo-optic dysplasia. We sequenced a large cohort of patients with schizencephaly, some with features of septo-optic dysplasia, for mutations in these genes. No pathogenic mutations were observed, suggesting that other genes or non-genetic factors influencing genes critical to brain development must be responsible for schizencephaly. © 2010 Wiley-Liss, Inc.

Epidemiology and clinical spectrum of schizencephaly in south-eastern Hungary

The epidemiology and clinical spectrum of schizencephaly in south-eastern Hungary have been surveyed in a retrospective population-based study. A total of 10 patients (6 boys and 4 girls) were found with schizencephaly among 185 486 live births in a period of 14 years (July 1, 1992 to June 30, 2006), which means a birth prevalence of 0.54 per 10 000 (95% confidence interval [CI]: 0.20-0.87). The schizencephaly was unilateral in 7 cases (with closed lips in 5 and open lips in 2 patients) and bilateral in 3 children (with closed lips in 2 and open lips in 1). The septum pellucidum was absent in 5 cases; however, optic nerve hypoplasia was not found in these patients. Delayed development and intellectual disability were observed in all patients, except 2 with unilateral closed lip schizencephaly. Epilepsy was diagnosed in 3 patients (2 with unilateral and 1 with bilateral schizencephaly).

Disturbed Wnt Signalling due to a Mutation in CCDC88C Causes an Autosomal Recessive Non-Syndromic Hydrocephalus with Medial Diverticulum

The etiology of non-syndromic hydrocephalus is poorly understood. Via positional cloning in a consanguineous family with autosomal recessive hydrocephalus we have now identified a homozygous splice site mutation in the CCDC88C gene as a novel cause of a complex hydrocephalic brain malformation. The only living patient showed normal psychomotor development at the age of 3 years and 3 months and her deceased aunt, who was assumed to suffer from the same condition, had mild mental retardation. The mutation in the affected patients, a homozygous substitution in the donor splice site of intron 29, resulted in a shorter transcript due to exclusion of exon 29 and loss of functional protein, as shown by Western blotting (p.S1591HfsX7). In normal human tissue panels, we found CCDC88C ubiquitously expressed, but most prominently in the fetal brain, especially in pons and cerebellum, while expression in the adult brain appeared to be restricted to cortex and medulla oblongata. CCDC88C encodes DAPLE (HkRP2), a Hook-related protein with a binding domain for the central Wnt signalling pathway protein Dishevelled. Targeted quantitative RT-PCR and expression profiling of 84 genes from the Wnt signalling pathway in peripheral blood from the index patient and her healthy mother revealed increased mRNA levels of CCDC88C indicating transcriptional upregulation. Due to loss of CCDC88C function β-catenin (CTNNB1) and the downstream target LEF1 showed increased mRNA levels in the patient, but many genes from the Wnt pathway and transcriptional target genes showed reduced expression, which might be explained by a complex negative feedback loop. We have thus identified a further essential component of the Wnt signalling pathway in human brain development.

Neuropsychological functioning in a young adult case of schizencephaly

Schizencephaly is a rare neuromigrational/organizational disorder characterized by the development of cerebral clefts, which are typically associated with neurological sequelae including seizures, motor disturbances, and cognitive dysfunction. Although there are multiple case reports of schizencephaly and associated neurological sequelae, primarily in children, the literature regarding neuropsychological manifestations of schizencephaly is limited. This article reviews the case of a woman diagnosed with bilateral schizencephaly at age 29. Neuropsychological testing revealed intact intelligence and memory functioning. However, impairments were noted in attention, executive functioning, expressive language skills, visual-spatial abilities, and bilateral manual motor skills, all of which were adversely impacting her functional abilities (e.g., ability to be gainfully employed). Given the potential variability in deficits associated with schizencephaly, this case demonstrates the utility of neuropsychological evaluation for understanding cognitive and functional consequences of bilateral schizencephaly.

Heterozygous mutations in SIX3 and SHH are associated with schizencephaly and further expand the clinical spectrum of holoprosencephaly

Schizencephaly (SCH) is a clinically and etiologically heterogeneous cerebral malformation presenting as unilateral or bilateral hemispheric cleft with direct connection between the inner and outer liquor spaces. The SCH cleft is usually lined by gray matter, which appears polymicrogyric implying an associated impairment of neuronal migration. The majority of SCH patients are sporadic, but familial SCH has been described. An initial report of heterozygous mutations in the homeobox gene EMX2 could not be confirmed in 52 patients investigated in this study in agreement with two independent SCH patient cohorts published previously. SCH frequently occurs with additional cerebral malformations like hypoplasia or aplasia of the septum pellucidum or optic nerve, suggesting the involvement of genes important for the establishment of midline forebrain structures. We therefore considered holoprosencephaly (HPE)-associated genes as potential SCH candidates and report for the first time heterozygous mutations in SIX3 and SHH in a total of three unrelated patients and one fetus with SCH; one of them without obvious associated malformations of midline forebrain structures. Three of these mutations have previously been reported in independent patients with HPE. SIX3 acts directly upstream of SHH, and the SHH pathway is a key regulator of ventral forebrain patterning. Our data indicate that in a subset of patients SCH may develop as one aspect of a more complex malformation of the ventral forebrain, directly result from mutations in the SHH pathway and hence be considered as yet another feature of the broad phenotypic spectrum of holoprosencephaly.

New trends in neuronal migration disorders

Neuronal migration disorders are an heterogeneous group of disorders of nervous system development and they are considered to be one of the most significant causes of neurological and developmental disabilities and epileptic seizures in childhood. In the last ten years, molecular biologic and genetic investigations have widely increased our knowledge about the regulation of neuronal migration during development. One of the most frequent disorders is lissencephaly. It is characterized by a paucity of normal gyri and sulci resulting in a "smooth brain". There are two pathologic subtypes: classical and cobblestone. Classical lissencephaly is caused by an arrest of neuronal migration whereas cobblestone lissencephaly caused by overmigration. Heterotopia is another important neuronal migration disorder. It is characterized by a cluster of disorganized neurons in abnormal locations and it is divided into three main groups: periventricular nodular heterotopia, subcortical heterotopia and marginal glioneural heterotopia. Polymicrogyria develops at the final stages of neuronal migration, in the earliest phases of cortical organization; bilateral frontoparietal form is characterized by bilateral, symmetric polymicrogyria in the frontoparietal regions. Bilateral perisylvian polymicrogyria causes a clinical syndrome which manifests itself in the form of mild mental retardation, epilepsy and pseudobulbar palsy. Schizencephaly is another important neuronal migration disorder whose clinical characteristics are extremely variable. This review reports the main clinical and pathophysiological aspects of these disorders paying particular attention to the recent advances in molecular genetics.

Genetic basis in epilepsies caused by malformations of cortical development and in those with structurally normal brain

Epilepsy is the most common neurological disorder affecting young people. The etiologies are multiple and most cases are sporadic. However, some rare families with Mendelian inheritance have provided evidence of genes' important role in epilepsy. Two important but apparently different groups of disorders have been extensively studied: epilepsies associated with malformations of cortical development (MCDs) and epilepsies associated with a structurally normal brain (or with minimal abnormalities only). This review is focused on clinical and molecular aspects of focal cortical dysplasia, polymicrogyria, periventricular nodular heterotopia, subcortical band heterotopia, lissencephaly and schizencephaly as examples of MCDs. Juvenile myoclonic epilepsy, childhood absence epilepsy, some familial forms of focal epilepsy and epilepsies associated with febrile seizures are discussed as examples of epileptic conditions in (apparently) structurally normal brains.

Neuronal migration disorders: clinical, neuroradiologic and genetics aspects

Disorders of neuronal migration are a heterogeneous group of disorders of nervous system development. One of the most frequent disorders is lissencephaly, characterized by a paucity of normal gyri and sulci resulting in a 'smooth brain'. There are two pathologic subtypes: classical and cobblestone. Six different genes could be responsible for this entity (LIS1, DCX, TUBA1A, VLDLR, ARX, RELN), although co-delection of YWHAE gene with LIS1 could result in Miller-Dieker Syndrome. Heterotopia is defined as a cluster of normal neurons in abnormal locations, and divided into three main groups: periventricular nodular heterotopia, subcortical heterotopia and marginal glioneural heterotopia. Genetically, heterotopia is related to Filamin A (FLNA) or ADP-ribosylation factor guanine exchange factor 2 (ARFGEF2) genes mutations. Polymicrogyria is described as an augmentation of small circonvolutions separated by shallow enlarged sulci; bilateral frontoparietal form is characterized by bilateral, symmetric polymicrogyria in the frontoparietal regions. Bilateral perisylvian polymicrogyria results in a clinical syndrome manifested by mild mental retardation, epilepsy and pseudobulbar palsy. Gene mutations linked to this disorder are SRPX2, PAX6, TBR2, KIAA1279, RAB3GAP1 and COL18A1. Schizencephaly, consisting in a cleft of cerebral hemisphere connecting extra-axial subaracnoid spaces and ventricles, is another important disorder of neuronal migration whose clinical characteristics are extremely variable. EMX2 gene could be implicated in its genesis. Focal cortical dysplasia is characterized by three different types of altered cortical laminations, and represents one of most severe cause of epilepsy in children. TSC1 gene could play a role in its etiology.

CONCLUSION

This review reports the main clinical, genetical and neuroradiological aspects of these disorders. It is hoped that accumulating data of the development mechanisms underlying the expanded network formation in the brain will lead to the development of therapeutic options for neuronal migration disorders.