Hyperexcitability in fibromyalgia (FM)

OBJECTIVE: 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.

METHODS: 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.


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


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