Porencephaly is a cavity that may develop prenatally or postnatally in a cerebral hemisphere.
Cavities often communicate with a ventricle, but they may also be enclosed (ie, noncommunicating) fluid-filled cysts. Increased intracranial pressure and progressive hydrocephalus (see Hydrocephalus) can occur with porencephaly, especially with noncommunicating forms, but is uncommon.
Causes of porencephaly include
Neurologic examination is usually abnormal, with manifestations including either low or increased muscle tone, developmental delays, hemiparesis, or impairment of visual attention. However, a few children develop only minor neurologic signs and have normal intelligence. Diagnosis is confirmed by cranial CT, MRI, or ultrasonography. Prognosis is variable. Treatment is supportive.
Hydranencephaly is an extreme form of porencephaly in which the cerebral hemispheres are almost totally absent. Usually, the cerebellum and brain stem are formed normally, and the basal ganglia are intact. The meninges, bones, and skin over the cranial vault are normal. Often hydranencephaly is diagnosed by prenatal ultrasonography.
Neurologic examination is usually abnormal, and the infant does not develop normally; children often have seizures and intellectual disability. Externally, the head may appear normal, but when transilluminated, light shines completely through.
CT or ultrasonography confirms the diagnosis.
Treatment is supportive, with shunting if head growth is excessive.
Schizencephaly, which some experts classify as a form of porencephaly, involves the presence of abnormal slits, or clefts, in the cerebral hemispheres. These clefts extend from the cortical surface to the ventricles and, unlike in other porencephalies, are lined with heterotopic gray matter. This gray matter bears some of the structural features of polymicrogyria (see Malformed Cerebral Hemispheres : Polymicrogyria), ie, there are miniature folds and abnormal lamination, resembling abnormally formed gyri. If the walls of the cleft are tightly opposed, so that MRI does not show a clear channel of CSF from the ventricle to the subarachnoid space, the defect is called closed-lip schizencephaly; if a CSF channel is visible, the defect is called open-lip schizencephaly. Open-lip schizencephaly may lead to hydrocephalus.
Unlike other porencephalies, many of which are thought to result from brain injury, schizencephaly represents a defect in neuronal migration and is thus more often a genetically determined malformation. Affected infants often have developmental delay and, depending on the location of the defect, may have focal neurologic findings such as hemiparetic weakness or spasticity. Seizures are common in both types of schizencephaly.
Treatment is supportive.