Hypoalgesia in the referred pain areas after bilateral injections of hypertonic saline into the trapezius muscles of men and women: a potential experimental model of gender-specific differences.

OBJECTIVE: The study was to assess the local pain intensity, referred pain patterns, and changes in the pressure pain thresholds of the local and referred pain areas following bilateral injections of hypertonic saline into trapezius muscles and to delineate gender differences in pain profile and the possible modulation of pressure pain thresholds.
METHODS: Fifteen healthy men and women each participated. Pain intensity and areas were assessed after bilateral injections. The pressure pain thresholds in the trapezius and posterolateral neck muscles (referred pain areas) were measured before and 7.5 and 15 minutes postinjection.
RESULTS: The time to maximal pain intensity was significantly shorter after the second injection than after the first injection. Patients exhibited a bilaterally symmetrical distribution of pain areas that included the neck to shoulder angle and the posterolateral neck. Bilateral injections significantly elevated pressure pain thresholds in the right and left posterolateral neck muscles 7.5 and 15 minutes postinjection. In men but not women, the pressure pain thresholds measured in the posterolateral neck muscles 7.5 and 15 minutes after the second injection were higher than those after the first injection and also higher than those after the second injection in women, men tolerated more pressure than women at all time points. DISCUSSION: These findings indicate that both excitatory and inhibitory mechanisms modulate pain response characteristics in bilateral neck-shoulder pain conditions and suggest that there may be a more potent inhibitory control mechanism in men than women. This experimental model may be potentially used to detect gender differences in descending inhibition.