Schizencephaly is an extremely rare developmental birth defect characterized by abnormal slits, or clefts, in the cerebral hemispheres of the brain. Babies with clefts in both hemispheres (called bilateral clefts) commonly have developmental delays, delays in speech and language skills, and problems with brain-spinal cord communication. Individuals with clefts in only one hemisphere (called unilateral clefts) are often paralyzed on one side of the body, but may have average to near-average intelligence. Individuals with schizencephaly may also have an abnormally small head, mental retardation, partial or complete paralysis, or poor muscle tone. Most will experience seizures. Some individuals may have an excessive accumulation of fluid in the brain called hydrocephalus.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.