This article includes discussion of schizencephaly and developmental porencephaly. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Schizencephalies are brain dysgeneses characterized by cerebral clefts lined by the polymicrogyric cortex extending from the pial surface to the lateral ventricles. The severity of the clinical picture, made up by motor deficits, intellectual disability, and epilepsy, is related to type, location, and size of the clefts and the presence of associated brain abnormalities. Epidemiologic and genetic studies suggest both acquired and genetic causes. In this article, the author describes the various types of schizencephaly with an update on recent findings.
• Polymicrogyria (ie, patches of minute gyri are present in open and closed lip schizencephaly).
• Polymicrogyria may be due to genetic or environmental causes.
• Careful history taking may reveal acquired damage in utero.
• Genetic studies may reveal genetic defects, closely mimicking acquired lesions.
Historical note and terminology
Schizencephalies are structural abnormalities of the brain characterized by congenital clefts unilaterally or bilaterally spanning the cerebral hemisphere from the pial surface to the lateral ventricles and lined by polymicrogyric cortex. There are 2 types as defined by Yakovlev and Wadsworth: the closed lips and the open lips types (Yakovlev and Wadsworth 1946). In the closed lips type, the defects that connect the arachnoid and ventricular spaces are lined by pial-ependymal seams. The seams merge tightly like a zipper, isolating the arachnoid space from the lateral ventricle. The dysplastic neocortex beneath the seam manifests polymicrogyria, detectable on MRI, adding evidence of the fetal origin of the lesion. The open lips type of schizencephaly presents an open connection between arachnoid and ventricular spaces. Macroscopically, this type appears identical to the full-thickness hemisphere defect better known as porencephaly. Porencephaly denotes a transmantle defect without grey matter lining the borders (lips) of the defect, although this distinction between porencephaly and schizencephaly is not always kept. Schizencephaly may be unilateral with a patch of superficial polymicrogyria in the opposite hemisphere
Schizencephalies originate during the fetal period in which neocortical neurons have not yet established their definite place in vertical and horizontal organization. Disruptions affecting the evolution of the neocortex during this period may result in disordered vertical and horizontal register of the cortical layers, presenting as polymicrogyria. Polymicrogyria arises during the neuronal migration period, extending to the stabilization period during which connectivity of the neocortex is achieved. It presents on MRI as very small fused gyri. Polymicrogyria may be caused by genetic and acquired disorders. Its sequential relationship to the mantle defects in schizencephaly has not been solved definitely. In a classification of cortical developmental disorders, Barkovich and colleagues put schizencephaly in a separate subgroup of polymicrogyrias (Barkovich et al 2012).
Porencephalies are generally thought to arise from a destructive process. Porencephaly and schizencephaly have overlapping morphological features, but the term schizencephaly, first coined by Yakovlev and Wadsworth, is reserved for cases in which polymicrogyric cortical tissue overlies the defects (Yakovlev and Wadsworth 1946a; Yakovlev and Wadsworth 1946b). These authors emphasized the malformative nature and early ontogenetic origin of schizencephalies. They also distinguished type 1 (or closed-lip) schizencephaly, in which the walls of the cleft are in contact with each other, and type 2 (or open-lip) schizencephaly, characterized by separated lips and cerebrospinal fluid-filled clefts. Although type 1 is well defined, type 2, also in the original publication, is much more diverse in shape, extent, and associated abnormal findings. In some patients bilateral schizencephaly may be closed on 1 side and open on the other side.
Schizencephaly has been increasingly recognized in vivo due to the progressive refinement of neuroimaging techniques (Barkovich and Kjos 1992; Hayashi et al 2002; Barkovich 2005; Barkovich et al 2012). Although first generation MR scanners did not achieve sufficient resolution to differentiate polymicrogyria from pachygyria, recent improvements have made this possible, and indeed imperative for a definite diagnosis.
Sometimes full-thickness defects without polymicrogyria bordering the defect but with associated cortical malformation distant from the defect are classified as schizencephaly (Kopyta et al 2014). In this way the definition, especially of type 2, sometimes tends to become overstretched.
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