Results of isoproterenol tilt table testing in monozygotic twins discordant for chronic fatigue syndrome.

BACKGROUND The pathogenesis of chronic fatigue syndrome (CFS) is unknown. Neurally mediated hypotension (NMH) has been suggested as a common comorbid condition or a potential underlying cause. METHODS We conducted a cotwin control study of 21 monozygotic twins who were discordant for CFS. One twin met the 1994 Centers for Disease Control and Prevention criteria for CFS, and the other twin was healthy and denied chronic fatigue. The twins were selected from a volunteer twin registry in which at least 1 member reported persistent fatigue. As part of a 7-day clinical evaluation, all 21 twin pairs were evaluated with a 3-stage tilt table test with isoproterenol hydrochloride for the assessment of NMH. The presence of NMH was defined as syncope or presyncope associated with a decrease of 25 mm Hg in blood pressure and no associated increase in heart rate. RESULTS A positive tilt table test result was observed in 4 twins with CFS (19%) and in 4 healthy twins (19%). This difference was not statistically significant (matched pair odds ratio, 1.0; 95% confidence interval, 0.2-5.4; P>.90). Compared with the healthy twins, the twins with CFS reported more severe symptoms of CFS and NMH both in the week before and during the tilt table test. CONCLUSIONS These results do not support a major role for NMH in CFS. They highlight the importance of selecting well-matched control subjects, as well as the unique value of the monozygotic cotwin control design in the study of this illness. Arch Intern Med. 2000;160:3461-3468.