Sympathetic asymmetry in post amputation pain

Striking numbers of amputees are plagued with chronic Post Amputation Pain (PAP), which can present as either Residual Limb Pain (RLP) or Phantom Limb Pain (PLP). Understanding the pathophysiology of RLP and PLP is central to establishing an acceptable standard of care. Disinhibition or activation of the sympathetic nervous system has been postulated to play a substantial role in patients’ report of pain. This study aimed to understand the contribution of sympathetic mechanisms in PAP by directly examining the vasomotor sympathetic characteristics of phantom and residual limb pain. We hypothesized that compared to amputees without PAP (i.e. PLP and/or RLP), amputees with such pain will exhibit the following characteristics of sympathetically maintained pain in their residual limbs: decreased relative temperature (by Infra-Red Telethermography; IRT), discoloration, asymmetric sweating (vasomotor), and swelling or edema (sudomotor). Patients who reported PAP (i.e. PLP and/or RLP), PLP, or RLP were compared with patients who reported no pain (i.e. neither PLP or RLP). There was no significant difference in edema (p=0.17, p=0.31, p=0.54) or discoloration (p=1.00, p=0.75, p=0.74) of the residual limb between any cohort of patients. No patients showed signs of asymmetric sweating. Difference in temperature by IRT between the residual and unaffected limbs was similar between amputees reporting PAP, PLP, or RLP and not reporting any pain (p=0.97, p=0.80, p=0.60). These findings suggest no significant evidence of sympathetic activation or receptor up-regulation to ‘maintain’ PAP. A large definitive trial, including invasive manipulation of the SNS and more quantitative measurement of SNS function, should be conducted to ultimately resolve this question.