Spontaneous chronic widespread pain in combination with hyperalgesia to pressure stimuli is the hallmark of fibromyalgia (FM). We tested whether muscular hyperalgesia can exist in a muscle without spontaneous pain, which could indicate a generalized hyperexcitability of the nociceptive system in patients with FM.
Twelve women with FM and 12 age matched female controls participated in this blind study. Patients had no spontaneous pain in the anterior tibial (AT) muscle. The pressure pain threshold was tested on the AT muscle. The pain threshold to electrical single and repeated stimulations of the skin and of the right AT muscle was assessed. Pain was evoked in the left AT muscle by infusion of sterile hypertonic saline (5.7%, 2.8 ml over 480 s). The saline induced muscle pain intensity and duration were assessed by continuous recordings on an electronic visual analog scale (VAS), and the distribution of pain was assessed on drawings. The sequence of electrical sensibility tests and the infusion of hypertonic saline was randomized.
Pressure pain thresholds were lower (p < 0.02) in patients with FM compared to controls. Thresholds for pain evoked by electrical stimulation at the skin were not significantly different in the 2 groups. The pain threshold to repeated intramuscular stimulation was significantly (p = 0.02) lower for the patients with FM compared to the control group, indicating that the temporal nociceptive summation was more pronounced in patients with FM. This is an indication of central sensitization (hyperexcitability). Infusion of hypertonic saline evoked muscle pain with a longer duration (p = 0.01) in patients with FM, and referred pain that spread to a larger area (p = 0.002) than in controls. This is an indication of central hyperexcitability.
There is a state of central hyperexcitability in the nociceptive system in FM. This hyperexcitability can be revealed by excitation of intramuscular nociceptors in a muscle with no spontaneous pain.