appears in a few weeks, but severe residual symptoms and even disability may persist. The disease can be monophasic, i.e. a single episode with permanent remission afterwards. However, at least 85% of patients have a relapsing form of the disease with repeated attacks of transverse myelitis and/or optic neuritis. In patients with the monophasic form, the transverse myelitis and optic neuritis occur simultaneously or within days of each other. On the other hand, patients with the relapsing form are more likely to have weeks or months between the initial attacks, and to have better motor recovery after the initial transverse myelitis event. Relapses usually occur early, with about 55% of patients having a relapse in the first year and 90% in the first five years. It is possible that the relapsing form is related to the anti-AQP4+ seropositive status and the monophasic form related to its absence. Unlike MS, NMO rarely has a secondary progressive phase in which patients have increasing neurologic decline between attacks without remission. Instead, disabilities arise from the acute attacks. Approximately 20% of patients with monophasic NMO have permanent visual loss, and 30% have permanent paralysis in one or both legs. Among patients with relapsing NMO, 50% have blindness or paralysis within five years. In some patients (33% in one study), transverse myelitis in the cervical spinal cord resulted in respiratory failure and subsequent death. However, the spectrum of NMO has widened, due to improved diagnostic criteria; and the options for treatment have improved. As a result, researchers believe these estimates will be lowered. Epidemiology Prevalence varies by region, ranging from 0.5 to 10 cases per 100,000 people. Unlike MS, prevalence has not been found to be related to latitude. NMO is more common in women than men, with women comprising over two-thirds of patients and more than 80% of those with the relapsing form of the disease. A retrospective study found that prevalence of #Neuromyelitis optica spectrum disorders (NMOSD) was 1.5% among a random sample of neurological patients, with a MS:NMOSD ratio of 42:7. Among 13 NMOSD patients, 77% had long spinal cord lesions, 38% had severe optic neuritis, and 23% had brain or brainstem lesions. Only 56% had clinically definite NMO at follow-up. NMO is more common in Asians than Caucasians. In fact, Asian optic-spinal multiple sclerosis (OSMS) (which constitutes 30% of the cases of MS in Japan) has been suggested to be identical