Editor's comment: Small-fiber polyneuropathy involves body-wide damage to the small unmyelinated nerve fibers. (Unmyelinated means the nerve fibers do not have a myelin sheath – the fatty substance that covers and protect nerves.)
Objective evidence that small-fiber polyneuropathy underlies some illnesses currently labeled as fibromyalgia.
Fibromyalgia is a common, disabling, syndrome that includes chronic widespread pain plus other diverse symptoms. No specific objective abnormalities have been identified, precluding definitive testing, disease-modifying treatments, and identification of causes.
In contrast, small-fiber polyneuropathy (SFPN), despite causing similar symptoms, is definitionally a disease caused by dysfunction and degeneration of peripheral small-fiber neurons. SFPN has established etiologies, some diagnosable and definitively treatable, e.g., diabetes.
To evaluate the hypothesis that some patients labeled with "fibromyalgia" have unrecognized SFPN causing their illness symptoms, we analyzed SFPN-associated symptoms, signs, and pathological and physiological markers in 27 fibromyalgia patients and 30 matched normal controls. Fibromyalgia subjects had to satisfy American College of Rheumatology criteria plus document actual fibromyalgia diagnoses.
Study instruments comprised the Michigan Neuropathy Screening Instrument (MNSI), the Utah Early Neuropathy Scale (UENS), distal-leg neurodiagnostic skin biopsies, plus autonomic-function testing (AFT).
41% of skin biopsies from fibromyalgia subjects vs. 3% of biopsies from control subjects were diagnostic for SFPN, and
MNSI and UENS scores were higher among fibromyalgia than control subjects (all P ≤ 0.001).
Abnormal AFTs were equally prevalent suggesting that fibromyalgia-associated SFPN is primarily somatic [of or relating to the body].
Blood tests from all 13 fibromyalgia subjects with SFPN-diagnostic skin biopsies provided insights into etiologies.
All glucose tolerance tests were normal, but eight subjects had dysimmune markers, 2 had hepatitis C serologies, and one family had apparent genetic causality.
These findings suggest that some pain patients labeled with "fibromyalgia" have unrecognized small-fiber polyneuropathy, a distinct disease that can be objectively tested for and sometimes definitively treated.
Source: Pain, June 5, 2013. By Anne Louise Oaklander, Zeva Daniela Herzog, Heather Downs and Max M. Klein. Department of Neurology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, 02114; Department of Pathology (Neuropathology), Massachusetts General Hospital, Boston, Massachusetts, 02114.