Epidemic neuromyasthenia and chronic fatigue syndrome: epidemiological importance of a cluster definition.

Outbreaks of illness variously identified by a number of terms, including epidemic neuromyasthenia, myalgic encephalomyelitis, Iceland disease, and atypical poliomyelitis, have been reported from many countries during the past 45 years. Since the first well-described outbreak occurring in 1934, > 60 outbreaks have been reported, but few of these have been described in considerable detail. These outbreaks are usually cited in historical reports of chronic fatigue syndrome (CFS) since each of these outbreaks appears to contain a number of cases meeting the current case definition of CFS. There has been inadequate attention given to the fact that epidemic neuromyasthenia and related clusters characterized by various complaints, including fatigue, do not have an accepted epidemiological or clinical definition, and only rarely have descriptions of these clusters included a specific case definition. When such case definitions have been applied, the occurrence of cases meeting the current case definition for CFS appears to be both variable and infrequent. This report utilizes examples of several well-documented outbreaks to emphasize specific aspects that should be considered in the investigation of future clusters.