Type II Arnold-Chiari malformation with normal spine in trisomy 18.

A variety of anomalies of the central nervous system are observed in trisomy 18. The present case describes an infant having a type II Arnold-Chiari malformation without spina bifida. One previous case of an Arnold-Chiari malformation was reported in trisomy 18 but that infant also had a lumbar meningomyelocoele. Abnormalities of cerebral gyration, hydrocephalus, and agenesis of the corpus callosum were also found in the present case.