Disabling fatigue may have genetic roots

By Anne Harding

NEW YORK (Reuters Health) - Disabling fatigue in children is not simply a symptom of depression, but appears to be an illness in its own right -- one that may be inherited, a UK study of twins hints.

Unexplained disabling fatigue "can't be thought of as just depression in another form," Dr. Tom A. Fowler of Cardiff University in Wales, the study's lead author, told Reuters Health.

Given that depression often accompanies fatigue and vice versa, Fowler and his team performed a twin study to investigate environmental and genetic influences on both conditions.

Twin studies allow researchers to distinguish between the effects of environment and genes. While identical twins have the same environment and genes, fraternal twins share an environment but only half of one another's genetic material. So identical twins would be more likely to share conditions with a heavy genetic influence.

The researchers looked at 2,398 pairs of twins between the ages of 8 and 17. Fatigue was considered disabling if it interfered with at least one aspect of school, leisure activities or relationships with family or peers, and if it required the child to rest for at least an hour daily. Fatigue was classified as short term if it lasted longer than one week, and long-term if it lasted more than a month.

Both short- and long-term fatigue appeared to have genetic roots, the researchers found, while the environmental influence for short-term fatigue was stronger than the familial influence.

Importantly, they say, the analysis found that while fatigue frequently accompanied depression, it showed a different pattern of genetic and environmental influences.

"This suggests that fatigued states in children should be considered as valid entities in their own right and not as variants of depression," the researchers write. Individuals with fatigue may become depressed in response to this "disabling disorder," they add.

The cause of chronic fatigue in children, which may be related to chronic fatigue syndrome (CFS) in adults, remains unclear, Fowler notes.

In a previous study, he and his colleagues found a prevalence of long-lasting fatigue among children similar to that of CFS in adults. Among their sample of children aged 8 to 17, 2.34 percent had experienced fatigue lasting longer than three months, while 1.29 percent had symptoms that met the definition of adult CFS.

SOURCE: British Journal of Psychiatry, September 2006.