X-linked neuronal migration disorders: Gender differences and insights for genetic screening.

Cortical development depends on neuronal migration of both excitatory and inhibitory interneurons. Neuronal migration disorders (NMDs) are conditions characterised by anatomical cortical defects leading to varying degrees of neurocognitive impairment, developmental delay and seizures. Refractory epilepsy affects 15 million people worldwide, and it is thought that cortical developmental disorders are responsible for 25% of childhood cases. However, little is known about the epidemiology of these disorders, nor are their aetiologies fully understood, though many are associated with sporadic genetic mutations. In this review, we aim to highlight X-linked NMDs including lissencephaly, periventricular nodular heterotopia and polymicrogyria because of their mostly familial inheritance pattern. We focus on the most prominent genes responsible: including DCX, ARX, FLNA, FMR1, L1CAM, SRPX2, DDX3X, NSHDL, CUL4B and OFD1, outlining what is known about their prevalence among NMDs, and the underlying pathophysiology. X-linked disorders are important to recognise clinically, as females often have milder phenotypes. Consequently, there is a greater chance they survive to reproductive age and risk passing the mutations down. Effective genetic screening is important to prevent and treat these conditions, and for this, we need to know gene mutations and have a clear understanding of the function of the genes involved. This review summarises the knowledge base and provides clear direction for future work by both scientists and clinicians alike.

Magnetic resonance imaging features of isolated periventricular heterotopia in pediatric epilepsy: a comparative study

Objective: Periventricular nodular heterotopia is a neurodevelopmental disorder in which neurons fail to migrate to the cortical surface, forming discrete areas of grey matter adjacent to the lateral ventricles. Given that periventricular nodular heterotopia is seen as an incidental finding in patients without epilepsy, causality between periventricular nodular heterotopia and epilepsy cannot be assumed. Furthermore, the structural characteristics of periventricular nodular heterotopia in patients with epilepsy are poorly defined and can be misleading. In this article, we investigate whether structural radiological characteristics of heterotopia can predict epileptogenicity in pediatric patients. Methods: Pediatric patients with periventricular nodular heterotopia, but no other epilepsy-associated cortical abnormalities on magnetic resonance imaging, were identified and divided into two groups: with epilepsy and without epilepsy. Radiological characteristics of laterality, regionalization, largest dimension and number of nodules were compared between the two groups. Results: Only periventricular nodular heterotopia spreading across several regions was associated with a statistically higher chance of epilepsy. Other features including laterality, individual region, number and largest dimension did not reliably predict epileptogenicity. Significance: Most radiological characteristics of periventricular nodular heterotopia are similar in patients with and without epilepsy. The involvement of multiple periventricular regions with heterotopia was the only feature that inferred a higher risk of epilepsy. Periventricular nodular heterotopia requires a comprehensive work-up and should be interpreted in the context of each individual patient and not assumed to be directly causative of epilepsy, nor unrelated to it. Therefore, further studies using additional structural and functional imaging modalities are needed to determine the radiological features of epileptogenic periventricular nodular heterotopia.

Epilepsy surgery in the first months of life: a large type IIb focal cortical dysplasia causing neonatal drug-resistant epilepsy.

Focal cortical dysplasia is a common cause of medically refractory epilepsy in infancy and childhood. We report a neonate with seizures occurring within the first day of life. Continuous video-EEG monitoring led to detection of left motor seizures and a right frontal EEG seizure pattern. Brain MRI revealed a lesion within the right frontal lobe without contrast enhancement. The patient was referred for epilepsy surgery due to drug resistance to vitamin B6 and four antiepileptic drugs. Lesionectomy was performed at the age of two and a half months, and histopathological evaluation confirmed the diagnosis of focal cortical dysplasia type IIb (FCD IIb). The patient is free of unprovoked seizures without medication (Engel Class I) and is normally developed at 36 months after surgery. The case study demonstrates that FCD IIb may cause seizures within the first day of life and that epilepsy surgery can be successfully performed in medically intractable patients with a clearly identifiable seizure onset zone within the first three months of life. Although radical surgery such as hemispherectomy and multi-lobar resections are over-represented in early infancy, this case also illustrates a favourable outcome with a more limited resection in this age group.

Basic mechanisms of epileptogenesis in pediatric cortical dysplasia.

Cortical dysplasia (CD) is a neurodevelopmental disorder due to aberrant cell proliferation and differentiation. Advances in neuroimaging have proven effective in early identification of the more severe lesions and timely surgical removal to treat epilepsy. However, the exact mechanisms of epileptogenesis are not well understood. This review examines possible mechanisms based on anatomical and electrophysiological studies. CD can be classified as CD type I consisting of architectural abnormalities, CD type II with the presence of dysmorphic cytomegalic neurons and balloon cells, and CD type III which occurs in association with other pathologies. Use of freshly resected brain tissue has allowed a better understanding of basic mechanisms of epileptogenesis and has delineated the role of abnormal cells and synaptic activity. In CD type II, it was demonstrated that balloon cells do not initiate epileptic activity, whereas dysmorphic cytomegalic and immature neurons play an important role in generation and propagation of epileptic discharges. An unexpected finding in pediatric CD was that GABA synaptic activity is not reduced, and in fact, it may facilitate the occurrence of epileptic activity. This could be because neuronal circuits display morphological and functional signs of dysmaturity. In consequence, drugs that increase GABA function may prove ineffective in pediatric CD. In contrast, drugs that counteract depolarizing actions of GABA or drugs that inhibit the mammalian target of rapamycin (mTOR) pathway could be more effective.

Dynamic changes of interictal post-spike slow waves toward seizure onset in focal cortical dysplasia type II.

OBJECTIVE: A post-spike slow wave (PSS) as part of a spike and slow wave is presumably related to inhibition of epileptic activity. In this study, we evaluated dynamic changes of PSS power toward seizure onset in patients with focal cortical dysplasia (FCD) type II. METHODS: We collected data from 10 pediatric patients with FCD type II, who underwent invasive monitoring with subdural grids. The PSS were averaged based on spike-triggering in 30s epochs during both interictal and preictal periods. We quantitatively measured and compared PSS power and distribution between interictal and preictal periods, both within and outside the seizure onset zone (SOZ). RESULTS: PSS power was significantly higher in all areas during preictal period compared with interictal period. During preictal period, PSS power within SOZ was significantly higher than outside SOZ. From interictal to preictal period, the number of electrodes with high power PSS significantly increased within SOZ and decreased outside SOZ. CONCLUSIONS: Toward seizure onset, PSS power increased in all areas, predominantly within SOZ, and became confined into SOZ in a subset of FCD type II patients. SIGNIFICANCE: Preictal PSS power increase and confinement into SOZ accompany transition to seizures.

Rufinamide for the treatment of refractory epilepsy secondary to neuronal migration disorders.

OBJECTIVE: To evaluate the efficacy and tolerability of add-on rufinamide in children with refractory epilepsy symptomatic of neuronal migration disorders. MATERIALS AND METHODS: We recruited 69 patients in a prospective, open-label, add-on treatment study from six Italian and one German centers for pediatric and adolescent epilepsy care according to the following criteria: age 3 or above; focal or generalized seizures refractory to at least three previous antiepileptic drugs (AEDs), alone or in combination, secondary to neuronal migration disorders; two or more seizures per month in the last 6 months; use of another AED, but no more than three, at baseline. Informed consent from parents and/or caregivers was obtained at the time of enrollment. RESULTS: We enrolled 69 patients with a mean age of 15 years (range 3-43). Forty-three patients (62%) had a 50-99% seizure reduction, and two (3%) became seizure-free. Seizure frequency was unchanged in 18 (26%) and worsened in 6 (8.7%). Twenty-nine patients (42%) reported adverse side effects, whilst taking rufinamide. Irritability was the most common side effect (11 patients), followed by decreased appetite (10), mood shift (6), vomiting (5), drowsiness (4), and decreased attention (2). Blood levels of concomitant anticonvulsive drugs were transiently abnormal in 5 patients. CONCLUSION: In our population of severely refractory epilepsy due to neuronal migration disorders, rufinamide appeared to be effective and generally well tolerated.

Case report: Neuronal migration disorder associated with chromosome 15q13.3 duplication in a boy with autism and seizures.

Neuronal migration disorders are a group of disorders that cause structural brain abnormalities and varying degrees of neurocognitive impairment, resulting from abnormal neuronal migration during brain development. There are several mutations that have been associated with these disorders. Here the case of a 4-year-old autistic boy is presented, who was found to have evidence of a neuronal migration disorder on magnetic resonance imaging (MRI) during a workup for seizures. Genetic testing did not reveal any of the gene mutations known to be associated with neuronal migration disorders but did reveal a microduplication at chromosome 15q13.3, a locus that has been previously associated with autism, cognitive impairment, and seizures. Although the concurrent presence of the genetic and structural abnormalities does not necessarily imply causality, the simultaneous independent occurrence of both conditions is certainly unusual. It is possible that there may be an association between this duplication syndrome and aberrant neuronal migration.

Diffusion tensor imaging of subependymal heterotopia.

A magnetic resonance (MR) diffusion tensor imaging (DTI) study was performed in a newborn with bilateral subependymal heterotopia (SE). White matter fractional anisotropy (FA), axial diffusivity (AD) and radial diffusivity (RD) were compared to values obtained in four newborns with moderate perinatal asphyxia and normal MRI findings. The reduction of FA and increase of AD and RD in the newborn with SE were the in vivo late expression of alterations in the intermediate zone, with an underlying arrest of neuronal migration.

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.

The significance of fetal ventriculomegaly: etiology, short- and long-term outcomes.

Fetal cerebral ventriculomegaly (VM) is diagnosed when the width of one or both ventricles, measured at the level of the glomus of the choroid plexus (atrium), is > or = 10 mm. VM can result from different processes: abnormal turnover of the cerebrospinal fluid (CSF), neuronal migration disorders, and destructive processes. In a high percentage of cases, it is associated with structural malformations of the central nervous system (CNS), but also of other organs and systems. The rate of associated malformations is higher (> or =60%) in severe VM (>15 mm) and lower (10-50%) in cases of borderline VM (10-15 mm). When malformations are not present, aneuploidies are found in 3-15% of borderline VM; the percentage is lower in severe VM. The neurodevelopmental outcome of isolated VM is normal in > 90% of cases if the measurement of ventricular width is between 10 and 12 mm; it is less favorable when the measurement is > 12 mm.

A structural basis for reading fluency: white matter defects in a genetic brain malformation.

BACKGROUND: Multiple lines of evidence have suggested that developmental dyslexia may be associated with abnormalities of neuronal migration or axonal connectivity. In patients with periventricular nodular heterotopia--a rare genetic brain malformation characterized by misplaced nodules of gray matter along the lateral ventricles--a specific and unexpected reading disability is present, despite normal intelligence. We sought to investigate the cognitive and structural brain bases of this phenomenon. METHODS: Ten adult subjects with heterotopia, 10 with dyslexia, and 10 normal controls were evaluated, using a battery of neuropsychometric measures. White matter integrity and fiber tract organization were examined in six heterotopia subjects, using diffusion tensor imaging methods. RESULTS: Subjects with heterotopia and those with developmental dyslexia shared a common behavioral profile, with specific deficits in reading fluency. Individuals with dyslexia seemed to have a more prominent phonological impairment than heterotopia subjects. Periventricular nodular heterotopia was associated with specific, focal disruptions in white matter microstructure and organization in the vicinity of gray matter nodules. The degree of white matter integrity correlated with reading fluency in this population. CONCLUSIONS: We demonstrate that a genetic disorder of gray matter heterotopia shares behavioral characteristics with developmental dyslexia, and that focal white matter defects in this disorder may serve as the structural brain basis of this phenomenon. Our findings represent a potential model for the use of developmental brain malformations in the investigation of abnormal cognitive function.