Multiple sclerosis and neuromyelitis optica. Case report and speculation.

Among the indigenous people of tropical and subtropical countries neuromyelitis optica is more common than multiple sclerosis (MS), which is exceptionally rare. Alternatively, one might conclude that when MS does occur in these countries it usually assumes the spinal-optic form. The evidence on which these statements are based is reviewed, and 6 cases from Natal are presented. These observations could have a bearing on the pathogenesis and epidemiology of MS.