Phantom limb pain intensity is associated with generalized hyperalgesia

After limb amputation, most amputees suffer from phantom limb pain (PLP). The mechanisms underlying this condition are complex and insufficiently understood. Altered somatosensory sensitivity (either heightened or lowered) might contribute to PLP. Recent studies have tested this assumption but mainly focused on the residual limb. However, altered somatosensation in PLP might be generalized. In this study, we applied quantitative sensory testing to 37 unilateral upper-limb amputees (23 with PLP, 14 without PLP) and 19 healthy controls. We assessed thresholds to heat pain (HPT), pressure pain (PPT), warmth detection (WDT), and two-point discrimination (2PDT) at the residual limb, a homologous point and the thenar of the intact limb, and both corners of the mouth. We did not find significant differences in any of the thresholds between the groups. However, higher PLP intensity was significantly related to lower HPT at all measured body sites except for the residual limb. At the residual limb, lower HPT were observed in more distal amputations and in amputees showing a higher degree of prosthesis use. Although WDT did by itself not significantly correlate with PLP intensity at any of the body sites, multiple regression analysis showed the highest multiple correlations with PLP intensity for a combination of high WDT and low HPT at the corners of the mouth. In this model, the combination of HPT and WDT shared 58% of the variance with PLP intensity. Other factors of potential importance, especially residual limb pain, were not significantly associated to any sensory threshold. We conclude that the intensity, but not the presence of PLP is positively associated with higher heat pain sensitivity. Since this association was observed at various, distributed body sites, we suggest that central mechanisms might be underlying such generalized hyperalgesia.