a1 Department of Medicine, University of Washington School of Medicine, Seattle,Washington. firstname.lastname@example.org
a2 Department of Psychiatry, University of California, San Diego & VA San Diego Healthcare System, San Diego, California.
a3 Department of Epidemiology, University of Washington & Vietnam Era Twin Registry, Seattle,Washington.
a4 Department of Medicine, University of Washington School of Medicine, Seattle,Washington.
a5 Department of Genetics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.
Prolonged fatigue equal to or greater than 1 month duration and chronic fatigue equal to or greater than 6 months duration are both commonly seen in clinical practice, yet little is known about the etiology or epidemiology of either symptom. Chronic fatigue syndrome (CFS), while rarer, presents similar challenges in determining cause and epidemiology. Twin studies can be useful in elucidating genetic and environmental influences on fatigue and CFS. The goal of this article was to use biometrical structural equation twin modeling to examine genetic and environmental influences on fatigue, and to investigate whether these influences varied by gender. A total of 1042 monozygotic (MZ) twin pairs and 828 dizygotic (DZ) twin pairs who had completed the University of Washington Twin Registry survey were assessed for three fatigue-related variables: prolonged fatigue, chronic fatigue, and CFS. Structural equation twin modeling was used to determine the relative contributions of additive genetic effects, shared environmental effects, and individual-specific environmental effects to the 3 fatigue conditions. In women, tetrachoric correlations were similar for MZ and DZ pairs for prolonged and chronic fatigue, but not for CFS. In men, however, the correlations for prolonged and chronic fatigue were higher in MZ pairs than in DZ pairs. About half the variance for both prolonged and chronic fatigue in males was due to genetic effects, and half due to individual-specific environmental effects. For females, most variance was due to individual environmental effects.
(Received May 28 2007)
(Accepted July 31 2007)
c1 Address for correspondence: Dr. Ellen Schur, Harborview Medical Center, Box 359780, 325 Ninth Avenue, Seattle, WA 98104.