The influence of offset analgesia on the onset and offset of pain in patients with fibromyalgia.

Offset analgesia (OA) is a form of endogenous pain inhibition characterized by a disproportionately large reduction in pain perception after a small decrease in temperature during noxious thermal stimulation. In this study, the presence of OA was evaluated in patients with fibromyalgia and compared with healthy age-matched and sex-matched controls. Offset analgesia was induced by noxious thermal stimulation on the arm, causing a visual analog score (VAS) of about 50 mm, followed by a 1°C temperature decrease. The OA response is defined by the reduction in VAS induced by the 1°C stimulus decrease. To assess whether the OA response could be enhanced, 2 tests were applied: repetition of the OA paradigm and 1°C temperature downward steps after an initial OA test (downward step test). To assess whether OA affects onset of pain, OA steps at increasing temperatures (upward OA step test) were applied. Patients with fibromyalgia had reduced OA responses with a reduction in VAS of 65.3 ± 4.5% (mean ± SD) vs controls 97.8 ± 4.6% (P < 0.001). Decreased OA responses were not enhanced or restored by repeating the OA paradigm or by the downward step test. Defective engagement of OA had a significant effect on pain onset, as observed from the upward OA step test, with less tolerability to noxious pain stimulation in patients with fibromyalgia. In conclusion, patients with fibromyalgia showed less pain inhibition as measured by the OA paradigm, which influenced both the onset and offset of pain.