Central Nervous System Malformations in Trisomy 9

Trisomy 21, 18 and 13 are the most common varieties of autosomal trisomy recognized at birth; most of the others lead to spontaneous abortions in the first trimester. Trisomy 9, a rare trisomy, is compatible with life, but, unlike trisomy 21,18 and 13, the range of manifestations has not been well catalogued. Central nervous system abnormalities have been reported in the majority of cases, usually including a dilated fourth ventricle and malformed cerebellum. The posterior fossa malformation closely resembles the descriptions of the Dandy-Walker malformation leading some to suggest this designation, while others have suggested that the features are unique to trisomy 9. Two cases of trisomy 9 are presented in this report which extend the range of neuropathologic manifestations in this cytogenetic disorder. The first infant had cortical migration abnormalities, anomalous hippocampal formation, simplified inferior olivary nuclei, germinal matrix cysts, mild ventriculomegaly, syringomyelia, and a large myelomeningocele without a Chiari type II malformation. The fourth ventricle was normal in size and the cerebellum unremarkable. The second infant had a cystically dilated fourth ventricle and widely separated cerebellar hemispheres with an intact cerebellar vermis, the features of which we felt were compatible with the Dandy-Walker malformation. In addition, agenesis of the corpus callosum, anomalous hippocampal formation, subpial glial nodules and mild ventriculomegaly were present. These cases extend the range of malformations that may be associated with trisomy 9, and raise the differential diagnosis of trisomy 9 when these malformations are identified. Furthermore, the malformation of the posterior fossa cystic enlargement of the fourth ventricle and lateral separation of the cerebellar hemispheres is compared with published descriptions of the Dandy-Walker malformation. This malformation in cases of trisomy 9 fulfill the criteria for the Dandy-Walker malformation as described in infants with presumably normal karyotypes.