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The epidemiology of TM depends on its underlying cause. The annual incidence of idiopathic complete TM is uncertain, but conservative estimates based on studies in US, European, and Israeli populations range from 1 to 4 cases per million, with a bimodal distribution of age presentation, peaking at ages 10 to 19 and 30 to 39 years. A population-based study estimated a mean annual incidence rate of 0.82 per 100,000 person-years, or 1.2 per 100,000 person-years for adults. Acute partial TM commonly heralds or accompanies multiple sclerosis (MS), which affects up to 0.3% of persons of Northern European ancestry. The pattern of longitudinally extensive TM is commonly associated with neuromyelitis optica spectrum disorders, which may account for approximately 1% of all CNS demyelinating disease in North American and European populations, a somewhat greater proportion of African-Americans and Hispanics, up to 30% in the Japanese population, and virtually all cases in ethnic or racial groups in which typical MS is virtually absent, such as aboriginal peoples of North America and black Africans. Onset may occur at any age. MS-associated cases typically occur between ages 20 and 50 years (median about 29 years) whereas the median onset age for neuromyelitis optica-related cases is about a decade later. There is a female predominance in all subtypes. These syndromes also affect children, although the incidence is unknown. The treatment of TM in children is usually the same as in adults.