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.