Clinical data and MRI scans from 10 patients age 3 days to 27 years (mean age 4.6 years) with lissencephaly were reviewed in the Departments of Radiology, Neurology and Pediatrics, University of California, San Francisco, CA. The youngest patients had the most severe lissencephaly but no correlation was found between the severity of the malformation as graded by MRI and the severity of clinical disease. One child age 2 1/2 with frontal lobes most severely affected was walking, speaking in 3 to 4 word sentences, and following instructions appropriate to her age, and physical examination revealed only very mild hypotonia. The anatomical deformity in the brain was not associated with any delay in myelination in patients in this study. Seizures occurred in all but one of the patients greater than four months of age; they began with infantile spasms or focal motor seizures and tonic seizures commonly occurred by the end of the second year. Hypsarrhythmia was present in two patients. 
COMMENT. The normal pattern and rate of myelination and the absence of porencephalies or other destructive lesions in 9 of the 10 patients in this report suggests that the arrest of neuronal migration in lissencephaly results from the absence of molecular components of the glial fibers or intracellular matrix. Lissencephaly is described in two types according to clinical, radiological, and pathological characteristics. In type I lissencephaly, agyria and pachygyria regions have a molecular layer, an outer cellular layer, a cell-sparse layer, and a deep cellular layer composed of heterotopic incompletely migrated neurons. In type II lissencephaly, the thickened cortex is disorganized without evidence of layers, and hydrocephalus and multiple anomalies are usually present.