Other Associations

Achondroplasia

Agenesis of the corpus callosum widely recognized of which are Beckwith-Wiedemann syndrome and the linear sebaceous nevus syndrome. The abnormally large cerebral hemisphere shows regions of broad and fused gyri. Cross sections of the hemisphere reveal regions of thickened cerebral cortex and frequently heterotopia in the cerebral white matter. Microscopically, the hemisphere shows marked disorganization with regions resembling pachygyria. In other regions, the neurons of the malformed cortex are abnormally spaced and disorganized in orientation. The clinical presentation of hemimegalencephaly is variable. [62] One clue to the possible existence of hemimegalencephaly is the identification of body asymmetries, especially of the skull and face, or the recognition of other features of a syndrome associated with hemihypertrophy. Most patients with hemihypertrophy present during the first year of life with frequent, focal seizures or infantile spasms. Some infants show asymmetries of motor development that can be subtle or present as frank hemiparesis. The EEG is variable, showing a fast rhythmical pattern in the alpha to beta range, burst-suppression patterns, spike- wave or repetitive triphasic complexes, or both. Some authors assign a poor prognosis to repetitive triphasic complexes. Mental retardation is frequently associated with hemimegalencephaly and appears to be correlated to some extent with the severity of the epilepsy. Patients with normal development and intelligence or well-controlled partial seizures have been reported. The association of hemimegalencephaly with Wilms' tumor of the kidney is important to recognize. Given the difficulties of medical seizure control in the majority of cases, and the adverse effects that ongoing seizures potentially have for developmental outcome, early modified hemispherectomy has been repeatedly advocated and successfully performed in these children. y

Microcephaly is defined by the head circumference being two standard deviations below that predicted by the body size. This definition does not provide any indication of the nature of the brain abnormality. The etiology of microcephaly is complex (..T.a.ble 2.8-4 ) and includes conditions that restrict brain growth (such as the craniosynostosis), destroy formed brain substance before completion of brain growth (examples include hypoxic-ischemic insults and some infections), or intrinsically impair brain growth. Intrinsically impaired brain growth can be the result of a metabolic insult (exogenous or inborn), or a defect of brain formation itself (as is assumed to be the case in the so-called primary genetic microcephalies, the chromosome breakage syndromes, and also in the majority of syndromes associated with microcephaly). It should be kept in mind that a small head circumference at birth does not rule out metabolic conditions, nor does a normal head circumference at birth rule out primary genetic causes of microcephaly. Microcephaly is a manifestation of many syndromes and is very common in syndromes that have mental retardation and cortical migration abnormalities as a component. Jones'^ has compiled 39 syndromes that have microcephaly as a frequent component and an additional 21

syndromes in which microcephaly is occasionally present. One hundred and seventy six syndromes have been

Figure 28-9 Hemimegalencephaly. An axial T2-weighted MRI scan demonstrates the moderately larger right hemisphere compared with the left. The cerebral cortex is poorly defined on the right side, and the T2-imaging highlights the abnormal signal intensity of the cerebral white matter on the same side.

_TABLE 28-4 -- VARIOUS CONDITIONS ASSOCIATED WITH MICROCEPHALY_

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