Chromosomal disorders: background and neuroradiology.

Chromosomal aberrations, and their associated syndromes of dysmorphism and malformations of multiple organ systems, have been long recognized as significant causes of severe mental impairment. With a few major exceptions, these clinical syndromes have only recently been catalogued as systematically ( 1, 2) as have been single-gene disorders. The neuropathologic features of many of these syndromes have been characterized only broadly, if at all. Not surprisingly, there is no detailed compendium of the neuroradiologic features of the malformations of the central nervous system (CNS) associated with various chromosomal aberrations. Certain chromosomal aberrations, such as trisomy 21 and the associated Down syndrome, are encountered with sufficient frequency to permit systematic study of their effects on the CNS. These studies show that disruption of the nervous system is nearly a universal feature of these syndromes, particularly those involving autosomes. Neuroradiologic investigations of these conditions are becoming increasingly important in their characterization for two reasons: 1) In some cases, admittedly few, CNS effects are mediated in part by surgically remediable structural abnormalities, such as hydrocephalus or atlantoaxial instability. These structural abnor-