Clustering of symptoms associated with fibromyalgia in a Finnish Twin Cohort.

The objective was to assess the prevalence and heritability of symptoms associated with fibromyalgia in a population-based working-age twin sample. The study was based on the 12,502 like-sexed twins of the Finnish Twin Cohort and 49 diagnosed fibromyalgia patients who answered the same questionnaire in 1990-1992. Questions that were considered to best match symptoms of fibromyalgia were validated between the twins and the fibromyalgia patients. Latent class analysis was used to classify the subjects into more homogeneous groups with respect to the symptom items. The pairwise distribution of symptom classes in relation to zygosity and gender was modelled using quantitative genetic models to estimate heritability. Responses to all fibromyalgia-related items were obtained from 10,608 twins. A similar proportion of men (12%) and women (13%) was placed in the third latent class, which best represented possible fibromyalgia patients. Subjects in this class had a similar symptom profile as the diagnosed fibromyalgia patient group, but they were less severely affected. The two other latent classes represented subjects that were virtually symptom free and subjects with some symptoms. The heritability of liability to symptom class membership was estimated to be 51% (95% CI 45-56%). The prevalence of symptoms associated with fibromyalgia in our population-based sample unselected for disease status was comparable to the prevalence of widespread pain reported in population based studies. The symptoms known to be associated with fibromyalgia seem to have a strong genetic background.