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Objective Fibromyalgia is defined by idiopathic, chronic, widespread musculoskeletal pain. In adults with fibromyalgia, meta-analysis of lower-leg skin biopsy demonstrated 45% pooled prevalence of abnormally low epidermal neurite density (END). END <5th centile of the normal distribution is the consensus diagnostic threshold for small-fiber neuropathy. However, the clinical significance of END findings in fibromyalgia is unknown. The prevalence of small fiber pathology has not yet been studied in juvenile fibromyalgia. Methods We screened 21 patients aged 13-20y with fibromyalgia diagnosed by pediatric rheumatologists. Fifteen meeting the American College of Rheumatology criteria (modified for juvenile fibromyalgia) underwent lower-leg measurements of END and completed validated questionnaires assessing pain, functional disability, and dysautonomia symptoms. The primary outcome was proportion of fibromyalgia patients with END <5th centile of age/gender/race-based laboratory norms. Cases were systematically matched by ethnicity, race, sex, and age to a group of previously biopsied healthy adolescents with selection blinded to biopsy results. All 23 controls matching demographic criteria were included. Results Among biopsied JFM patients, 53% (8/15) had END <5th centile versus 4% (1/23) of healthy controls (p<0.001). Mean patient END was 273/mm2 skin surface (95% confidence interval: 198-389) versus 413 (95% CI: 359-467; p<0.001). As expected, fibromyalgia patients reported more functional disability, dysautonomia, and pain than healthy controls. Conclusion Abnormal END reduction is common in adolescents with fibromyalgia, with similar prevalence to adults with fibromyalgia. More studies are needed to fully characterize the significance of low END in fibromyalgia and to elucidate the clinical implications of these findings.

Metrical indecomposability is a condition for the validity of the classical ergodic theorem. Its counterpart in the quantum-statistical theorem is believed to be non-degeneracy of the energy eigenvalues. It is shown here, however, that the coarse-grained quantum-statistical theorems (due to von Neumann and to Pauli & Fierz) remain valid in the presence of energy degeneracy. This requires, therefore, a reassessment of the significance of these theorems; this is discussed, and it is concluded that the von Neumann approach is unsatisfactory in that it fails to yield a microscopic criterion for either ergodicity or validity of the H -theorem.