Possible deficiencies of pain modulation in fibromyalgia (FM)

OBJECTIVE: To examine possible deficiencies in endogenous pain
modulating mechanisms in fibromyalgia patients compared with
matched pain-free control subjects.

DESIGN/SUBJECTS/METHODOLOGY: Pain reduction was investigated
in 25 female patients with fibromyalgia and 26 age-matched
healthy women using the diffuse noxious inhibitory controls
(DNIC) paradigm. Tonic thermal stimuli at painful and
nonpainful intensities, tailored to individual heat pain
thresholds, were employed to induce pain inhibition. The
anticipated effect was assessed by measuring the electrical
pain threshold and detection threshold, using a double
staircase method. Only nontender control points were
stimulated (thermode on the foot, electrodes on the inner
forearm).

RESULTS: The patients with fibromyalgia had
significantly lower heat pain thresholds than the healthy
subjects, but similar electrical detection and pain
thresholds. The repeatedly applied electrical stimuli resulted
in a degree of perceptual adaptation that was similar between
the two groups. However, concurrent tonic thermal stimuli, at
both painful and nonpainful levels, significantly increased
the electrical pain threshold in the healthy subjects but not
in the fibromyalgia patients. The electrical detection
threshold was not affected in either group.

CONCLUSIONS: Pain
modulation, produced by a concurrent tonic stimulus in healthy
persons, was not seen in the fibromyalgia group. The patients
either had deficient pain modulation or were unable to
tolerate a tonic stimulus intense enough to engage a
modulatory process. It remains to be established whether the
pain reduction found in the healthy subjects was the
conventional DNIC effect, another effect (e.g., distraction),
or a combination of both.

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