Neuronal migrational disorders: diffuse cortical dysplasia or the "double cortex" syndrome

Functional MRI Study of Verbal Fluency in a Patient with Subcortical Laminar Heterotopia

Rationale:

Double cortex syndrome is a malformation in which there is a band of subcortical heterotopic grey matter separated from the cortex by white matter. The functional activity of the heterotopic neurons is unclear.

Patient:

A 13-year-old female was evaluated for seizures. The EEG showed bifrontal spike wave disturbance. Band heterotopia, in association with mild reduction of sulcation of the cerebral hemispheres, was found on MRI. Psychological assessment indicated the presence of variable cognitive abilities, with verbal IQ [82] generally better than nonverbal IQ [59], and specific difficulties in language comprehension and mathematics.

Method:

Functional MRI was used to localize the areas of language and motor activation. The language activation paradigm was a visual verb generation task with a visual fixation baseline. The motor paradigm consisted of alternating blocks of sequential finger tapping and rest. Coronal functional and anatomical images were obtained.

Results:

The motor paradigm produced activation of the primary motor cortex, the band heterotopia and the supplementary motor cortex. The language paradigm produced activation of the left inferior frontal gyrus and left supplementary motor area, but not of the band heterotopia.

Conclusions:

The activation of heterotopic grey matter during a motor task demonstrates a hemodynamic association with motor activity and suggests that this tissue may be functional. Such association was not seen with the language task. We speculate that later maturing functions such as language are restricted in their development to the normal situated superficial cortex in our patient.

Fiber tractography assessment in double cortex syndrome

INTRODUCTION

Subcortical band heterotopia (SBH) or double cortex syndrome is a malformation of cortical development that may be related to intractable epilepsy and severe mental retardation or to mild epilepsy and slight mental delay or normal cognitive functions. Several studies have been performed using neuroradiological or neurophysiological techniques, like SPECT, PET, MRS, fMRI, and MEG, in attempt to better characterize this neuronal migration disorder. Recently, also diffusion tensor imaging (DTI) and fiber tracking (FT) have been used to investigate on white matter anomalies in SBH, adding more information about such gray matter anomaly.

METHODS

We report on three cases of SBH, evaluated with MRI, DTI, and FT.

CONCLUSIONS

The data gathered from DTI and TF allow us to hypothesize a new functional role for heterotopic gray matter.

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.

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.

[Heterotopic gray matter. Report of four pediatric cases]

Severe infant epilepsy is included within difficult etiologic diagnosis. Gray matter heterotopias are an uncommon cause. The authors report four observations of gray matter heteropias concerning three-, six-, seven- and nine-year-old girls, presenting no particular antecedents. No consanguinity was noted. The first occurrence of epilepsy ranged from the age of nine months to the age of four years. A mild mental retardation was found in three cases, and mental regression in one case. A status epilepticus was noted in three children. Magnetic resonance imaging scans showed subependymal heterotopias in one case and diffuse cortical heterotopias in three cases associated to a partial agenesis of corpus calloseum in one case and pachygyria in two cases.

Normotopic and heterotopic cortical representations of mystacial vibrissae in rats with subcortical band heterotopia

The tish rat is a neurological mutant exhibiting bilateral cortical heterotopia similar to those found in certain epileptic patients. Previous work has shown that thalamocortical fibers originating in the ventroposteromedial nucleus, which in normal animals segregate as 'barrel' representations for individual whiskers, terminate in both normotopic and heterotopic areas of the tish cortex (Schottler et al., 1998). Thalamocortical innervation terminates as barrels in layer IV and diffusely in layer VI of the normotopic area. Discrete patches of terminals are also observed in the underlying heterotopic area suggesting that representations of individual vibrissa may be present in the heterotopic somatosensory areas. The present study examines this issue by investigating the organization of the vibrissal somatosensory system in the tish cortex. Staining for cytochrome oxidase or Nissl substance reveals a normal complement of vibrissal barrels in the normotopic area of the tish cortex. Dense patches of cytochrome oxidase staining are also found in the underlying lateral portions of the heterotopic area (i.e. the same area that is innervated by the ventroposteromedial nucleus). Injections of retrograde tracers into vibrissal areas of either the normotopic or heterotopic area produce topographically organized labeling of neurons restricted to one or a small number of barreloids within the ventroposteromedial nucleus of the thalamus. Physical stimulation of a single whisker (D3 or E3) elicits enhanced uptake of [(14)C]2-deoxyglucose in restricted zones of both the normotopic and heterotopic areas, demonstrating that single whisker stimulation can increase functional activity in both normotopic and heterotopic neurons. These findings indicate that the barrels are intact in the normotopic area and are most consistent with the hypothesis that at least some of the individual vibrissae are 'dually' represented in normotopic and heterotopic positions in the primary somatosensory areas of the tish cortex.

Functional neuroradiologic investigations in band heterotopia

Band heterotopias are an example of genetic generalized neuronal migration disorders that may be present in patients with mild epilepsy and normal or slightly impaired intellect, as well as in patients with intractable epilepsy and mental retardation. The case of a 17-year-old left-handed female patient with epilepsy and normal cognitive development is reported in whom single-photon emission computed tomography (SPECT), proton magnetic resonance spectroscopy, and functional magnetic resonance imaging (fMRI) were performed. MRI revealed the presence of bilateral asymmetric band heterotopia. SPECT revealed a left frontoparietal and occipital hypoperfusion, demonstrating a good correlation with the electroencephalogram abnormalities. Because of the appearance of new types of seizures, the patient underwent a second MRI investigation together with a proton magnetic resonance spectroscopy (MRS) study. MRI confirmed bilateral band heterotopia characterized by greater thickness in the left hemisphere at the frontal and occipital level. MRI and SPECT findings were in agreement with left occipital electroencephalogram abnormalities and with occipital seizure type. Qualitative results of proton MRS revealed normal spectra profiles in the examined left frontal and occipital heterotopic area and in the normal overlying cortex. Later, fMRI was performed. The finger-tapping test of the right hand yielded the activation of both normal left sensory-motor cortex and the facing band heterotopia. In the right hemisphere, only the activation of the sensory-motor neocortex was observed; no involvement of the right misplaced brain tissue was present. This functional behavior could be considered the consequence of poor neuronal representation. On the contrary, the involvement of both band heterotopia and normal cortex observed in the left hemisphere could be the result of many synaptic interconnections. Functional investigations may have an important role in defining the activity of band heterotopia per se and in relation to the overlying neocortex.

Distribution and initiation of seizure activity in a rat brain with subcortical band heterotopia

PURPOSE

Misplaced (heterotopic) cortical neurons are a common feature of developmental epilepsies. To better understand seizure disorders associated with cortical heterotopia, the sites of aberrant discharge activity were investigated in vivo and in vitro in a seizure-prone mutant rat (tish) exhibiting subcortical band heterotopia.

METHODS

Depth electrode recordings and postmortem assessment of regional c-fos mRNA levels were used to characterize the distribution of aberrant discharge activity during spontaneous seizures in vivo. Electrophysiologic recordings of spontaneous and evoked activity also were performed by using in vitro brain slices from the tish rat treated with proconvulsant drugs (penicillin and 4-aminopyridine).

RESULTS

Depth electrode recordings demonstrate that seizure activity begins almost simultaneously in the normotopic and heterotopic areas of the tish neocortex. Spontaneous seizures induce c-fos mRNA in normotopic and heterotopic neocortical areas, and limbic regions. The threshold concentrations of proconvulsant drugs for inducing epileptiform spiking were similar in the normotopic and heterotopic areas of tish brain slices. Manipulations that blocked communication between the normotopic and heterotopic areas of the cortex inhibited spiking in the heterotopic, but not the normotopic, area of the cortex.

CONCLUSIONS

These findings indicate that aberrant discharge activity occurs in normotopic and heterotopic areas of the neocortex, and in certain limbic regions during spontaneous seizures in the tish rat. Normotopic neurons are more prone to exhibit epileptiform activity than are heterotopic neurons in the tish cortex, and heterotopic neurons are recruited into spiking by activity initiated in normotopic neurons. The findings indicate that seizures in the tish brain primarily involve telencephalic structures, and suggest that normotopic neurons are responsible for initiating seizures in the dysplastic neocortex.

Heterotopic neurogenesis in a rat with cortical heterotopia

Early cellular development was studied in the neocortex of the tish rat. This neurological mutant is seizure-prone and displays cortical heterotopia similar to those observed in certain epileptic patients. The present study demonstrates that a single cortical preplate is formed in a typical superficial position of the developing tish neocortex. In contrast, two cortical plates are formed: one in a normotopic position and a second in a heterotopic position in the intermediate zone. As the normotopic cortical plate is formed, it characteristically separates the subplate cells from the superficial Cajal-Retzius cells. In contrast, the heterotopic cortical plate is not intercalated between the preplate cells because of its deeper position in the developing cortex. Cellular proliferation occurs in two zones of the developing tish cortex. One proliferative zone is located in a typical position in the ventricular/subventricular zone. A second proliferative zone is located in a heterotopic position in the superficial intermediate zone, i.e., between the two cortical plates. This misplaced proliferative zone may contribute cells to both the normotopic and heterotopic cortical plates. Taken together, these findings indicate that misplaced cortical plate cells, but not preplate cells, comprise the heterotopia of the tish cortex. Heterotopic neurogenesis is an early developmental event that is initiated before the migration of most cortical plate cells. It is concluded that misplaced cellular proliferation, in addition to disturbed neuronal migration, can play a key role in the formation of large cortical heterotopia.

SPECT and MRI findings in a case of extensive neuronal migration disorder

We report a 20-year-old male with epilepsy, mild mental retardation, growth asymmetry, and MRI and SPECT features of unilateral subcortical ectopic cortex. The neurological examination showed mild growth asymmetry, hemiparesis and hemihypoesthesia and pyramidal signs on the left side. EEG showed focal abnormality in the right frontotemporal region. MRI revealed pachygyria and severe heterotopia associated with some abnormalities of ventricles and cerebellum on the right. Cortical responses were absent on stimulation of the left median and tibial nerves. Central motor conduction time from cortex to left upper extremity was prolonged in magnetic stimulation test. SPECT using 99 mTc-HMPAO revealed increased perfusion of the right subcortical region as compared with those of overlying cortical mantle and opposite hemisphere. To our knowledge, there has been no report documenting such a large and extensive subcortical ectopic cortex which appears as a mass distorting and shifting the middle structure in an adult, such as in our case.

Neuronal migrational disorders in children with epilepsy: MRI, interictal SPECT and EEG comparisons

Single-photon emission computed tomography (SPECT) is being increasingly used in the investigation of children with epilepsy and may provide insights into congenital malformations. We analyzed the interictal 99Tc-HMPAO-SPECT in a series of seven children with developmental disorders of the neocortex, each of them representing a prototype of cerebral dysgenesis, such as lissencephaly, pachygyria, opercular dysplasia, polymicrogyria, nodular heterotopia and band heterotopia. The patients studied were selected among 22 epileptic children with neuronal migrational disorders (NMDs). Interictal SPECT hypoperfusion was observed in the area homologous to MRI findings in all the examined children. In three patients low perfusion was also present in the opposite hemisphere, probably due to functional involvement or related to an underlying microdysgenesis, not revealed by structural imaging. EEG features were in agreement with low perfusion areas, both anatomically and functionally, in all children. In one patient hypoperfusion area differed from that revealed by MRI and EEG. Ictal SPECT has been considered a useful tool for accurately locating the epileptic focus. Nevertheless, interictal brain perfusion studies, together with proton magnetic resonance spectroscopy, may play an important role in detecting anatomic substrate in developmental disorders of the neocortex.