Lissencephaly types III and IV present with microcephaly. In type III lissencephaly the cortex is thin. Lissencephaly type IV is also called radial microbrain. The cortex is thick. Lissencephaly type IV occurs in neonates with cytomegalovirus infection.
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Polymicrogyria
Polymicrogyria is a disorder of neuronal organization. The neurons reach the cortex but do not organize well, leading to the formation of multiple small gyri. Polymicrogyria is diagnosed by MRI. Polymicrogyria may be an isolated finding or may be associated with other brain abnormalities. Polymicrogyria may be associated with degenerative central nervous system disorders (Zellweger disease or neonatal adrenoleukodystrophy), central nervous system infections (cytomegalovirus infection), or developmental disorders (Aicardi syndrome). Neonates with polymicrogyria and clinical suspicion of cytomegalovirus infection or periventricular cysts, calcifications (often periventricular), or cerebellar hypoplasia should have their cerebrospinal fluid studied for the possibility of cytomegalovirus infection. Treatment of cytomegalovirus infection with Ganciclovir stops the excretion of virus in the urine. The possibility that treatment with ganciclovir may arrest further progression of brain damage has been raised. Seizures in patients with polymicrogyria are treated with antiepileptic drugs. Surgical treatment should be considered.

Hemimegalencephaly
Hemimegalencephaly refers to enlargement of one cerebral hemisphere (Figure 49.1). Magnetic resonance imaging of the brain is the study of choice to establish the diagnosis. Unilateral hemimegalencephaly occurs because of hamartomatous overgrowth of one hemisphere. The cortex is dysplastic and the white matter is abnormal. The lateral ventricle in the enlarged hemisphere is enlarged in proportion to the lateral ventricle of the smaller hemisphere. The anterior aspect of the lateral ventricle on the larger hemisphere points superiorly and anteriorly.

Figure 49.1.— Hemimegalencephaly. CT of the brain with contrast demonstrates enlarged left lateral ventricle and pachygyric left posterior cerebral cortex.