Is disabling fatigue in childhood influenced by genes?

Background. Medically unexplained chronic fatigue in childhood may cause co nsiderable disability and (by definition) its cause remains unclear. A stud y of fatigue in healthy twins has been undertaken to examine whether or not genetic factors play a part. Method. A questionnaire survey of the main carers of an epidemiological pop ulation-based sample of 670 twin pairs who were asked about periods of unex plained and disabling fatigue in their twins. Out of 1340 individuals a per iod of disabling fatigue was reported for 92 (6.9 %). Thirty-three (2.5 %) reported disabling fatigue for more than 1 month. Zygosity could be confide ntly assigned in 98 % of the sample providing 278 monozygotic (MZ) and 378 dizygotic (DZ) pairs. These data were analysed using a structural equation modelling approach. Results. The results showed that disabling fatigue in childhood is highly f amilial with an MZ tetrachoric correlation (rMZ) of 0.81 and a DZ tetrachor ic correlation (rDZ) of 0.59, for fatigue lasting at least a week. The most acceptable model using Akaike's information criteria, was one containing a dditive genetic effects (A) and shared environment (C) plus residual (or no n-shared) environment (E). For fatigue lasting at least a month rMZ was 0.7 5 and rDZ 0.47. The most acceptable model included just A and E. However, t he role of shared environment could not be conclusively rejected. Conclusions. Unexplained disabling fatigue in childhood is substantially fa milial. Both genetic and shared environmental factors are worth further exp loration in a search for the causes.