This question motivated Levy et al. to investigate the endogenous inhibition efficiency of the trigeminal and extra-trigeminal areas of the face, neck, and arm in healthy volunteers. Two TSA devices were used to allow thermal test and conditioning stimulation within these areas. CPM was tested in three different zones; 1) forehead (V1) and cheek (V3), 2) cheek and neck (C4), and 3) neck and arm (C7). Additionally, spatial summation of pain (SSP) and temporal summation (TS) of pain were tested on all four areas.
Interestingly, the forehead was found to be least sensitive to heat pain, requiring the highest temperature to reach a VAS 5-6 pain rating. The only configuration that yielded a significant CPM response, was the neck and arm set-up, which did not involve any of the trigeminal sites. Spatial summation of pain was elicited in all four sites, and post-hoc analysis showed less spatial summation of pain in the forehead as compared to the other regions. Temporal summation of pain did not differ significantly between sites.
This study showed inefficient pain modulation in trigeminal, as compared to extra-trigeminal areas. These findings could point to a possible factor in the etiology of pain syndromes involving the trigeminal area.
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