Incidence of myalgic encephalomyelitis/chronic fatigue syndrome in a large prospective cohort of U.S. nurses

Background: The incidence of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), the rates of both under-diagnosis and over-diagnosis, and the nature of the onset of the condition have not been assessed in large studies of health professionals. Purpose: To determine the cumulative incidence of ME/CFS in a large population of health professionals, to examine the nature of the onset of the illness, and to estimate the frequency of both over-diagnosis and under-diagnosis of ME/CFS. Methods: We sent an email questionnaire to participants in the Nurses’ Health Study II (NHS II), a large prospective cohort of female nurses. Forty-two thousand three hundred and ninety-four women completed the questionnaire, which included the 1994 Centers for Disease Control and Prevention (CDC) criteria for ME/CFS. Results: One-hundred and two women (240 per 100,000 surveyed) developed an illness that met criteria for ME/CFS between 1989 and 2009. The onset of ME/CFS was gradual in 40.6%, sudden (following flu-like illness or other precipitating events) in 18.8%, followed emotional or physical trauma in 32.3%, and was uncertain in the rest. Under-diagnosis was common: only 15 (15%) of the women who met criteria for ME/CFS reported having been diagnosed. Over-diagnosis also was common: four times as many subjects had been diagnosed with ME/CFS by community doctors as actually met criteria. The distribution of symptoms was not different in comparing cases with a sudden onset to those with a gradual onset. Conclusions: In this large cohort of female nurses, we found a low cumulative incidence of ME/CFS. Over-diagnosis and under-diagnosis were high, even in this medically sophisticated population.