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 we re discordant for CFS. One twin met the 1994 Centers for Disease Control an d Prevention criteria for CFS, and the other twin was healthy and denied ch ronic fatigue. The twins were selected from a volunteer twin registry in wh ich at least 1 member reported persistent fatigue. As part of a 7-day clini cal evaluation, all 21 twin pairs were evaluated with a 3-stage tilt table test with isoproterenol hydrochloride for the assessment of NMH. The presen ce of NMH was defined as syncope or presyncope associated with a decrease o f 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 we ll as the unique value of the monozygotic cotwin control design in the stud y of this illness.