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Schizencephaly

Schizencephaly is a rare cortical malformation that manifests as a grey matter-lined cleft extending from the ependyma to the pia mater. Some authors have not used the term, preferring to group these patients under the blanket term porencephaly. For the purpose of this article, and generally speaking, schizencephaly is reserved for clefts lined by grey matter (polymicrogyria) thought to represent a true malformation, whereas porencephaly implies an encephaloclastic event (e.g. ischaemia).

Epidemiology

Schizencephaly is rare with an estimated incidence of ~ 1.5 : 100,000 live births. It is almost always sporadic, although a few familial cases have been described 6. There is no known gender predilection 6

Interestingly schizencephaly appears to be overrepresented in abandoned or adopted children, raising the possibility that in untero insults (e.g. cocaine and other 'street drugs') may be a risk factor 6

Clinical presentation

The abnormality is increasingly detected at antenatal ultrasound, especially when it is of the open-lip variety. 

In general developmental delay, motor disturbance correlate with the degree of anatomical abnormality, especially when clefts are present bilaterality. Seizures are relatively common, although the severity and prevalence of seizures do not appear to be dependent on the anatomical extent of involvement.

Pathology

Although exact pathogenesis is uncertain, it is thought most likely to be the result of abnormal neuronal migration. Some authors propose an early in utero vascular insult as the cause. Whether this is from an in-utero insult or the expression of genetic factors is unclear. Some familial cases have been reported, in which case hereozygous germline mutations of the  homeobox gene EMX2 are often encountered 6.

A cleft extends from the ependymal surface of brain to the pia mater, and the two layers meet in the cleft : the so-called pial-ependymal seam 5. The grey matter that lines the cleft is abnormal, usually representing polymicrogyria.

Radiographic features

Schizencephaly can sometimes be bilateral, and is divided into two morphological types:

  • open lip
    • the cleft walls are separated and filled with CSF
    • most common form in bilateral cases 6
  • closed lip
    • the cleft walls are in apposition
    • most common form in unilateral cases

Most often the cleft involves the posterior frontal or parietal lobes (70%), and although large clefts can extend to involve the temporal or occipital lobes, isolated involvement of these lobes is uncommon 6

It is frequently associated (50 - 90%)6 with other cerebral anomalies :

Ultrasound

Increasingly, schizencephaly is being diagnosed antenatally or post partum with cranial ultrasound. Closed lip schizencephaly is difficult to identify, whereas open lip forms can, if large, be readily seen (see article on fetal schizencephaly). 

MRI

MRI is the imaging modality of choice, and enables identification of the pial-ependymal cleft visualisation of cortical dyplasia and heterotropic gray matter.

  • closed lip (type I) : seen as nipple-like out-pouching at the ependymal surface
  • open lip (type II) : heterotopic gray matter lined CSF cleft seen extending from ventricular to cortical surface

Differential diagnosis

  • focal cortical dysplasia
    • sometimes may have a cleft on the cortical surface that does not extend completely to the ventricular surface
  • heterotopic grey matter
    • closed lip schizencephaly can mimic a band of grey matter heterotopia. Assessing the ventricular outline will often demonstrate a slight cleft whereas periventricular grey matter will usually bulge into the ventricle.
  • porencephaly
    • a zone of encephalomalacia that extends from the cortical surface to the ventricular surface but is lined by gliotic white matter, not grey matter
    • it is worth noting that some authors would refer to schizencephaly as 'true porencephaly'6 .

See also

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