Pre- and postoperative predictors of phantom limb pain.

Phantom limb (PLP) as well as residual limb pain (RLP) are still a very significant problem after amputation and their causes are only partially known. Here we tested whether the predisposition for the frequency, duration and intensity of PLP and RLP is shared with other prior chronic pains and/or the presence of postamputation subacute pain. A week preoperatively we collected data using validated questionnaires, a pain diary and interviews on past chronic pain conditions, acute pain, depression, anxiety, pain interference, life control, social support and affective distress and pain ratings one day before the amputation in 52 patients scheduled for limb amputation. In the week postamputation and again three and 12 months thereafter, we collected data on postoperative wound pain, PLP, RLP and non-painful phantom sensation (PLS). Phantom and residual limb pain indices were calculated per patient, integrating the intensity, frequency and duration of past chronic pain, PLP, RLP and PLS into a single value to index the overall burden of pain. We report that acute and chronic pain long before but not on the day before the amputation and subacute pain postamputation significantly predicted up to half of the variance in the incidence and severity of PLP, RLP and PLS 12 months postamputation. Moreover, the severity of these painful sensations at 12 months postamputation was partially predicted by preamputation scores of depression and anxiety. These findings corroborate the hypothesis that chronic pain and non-painful sensations following limb amputation are strongly related to longstanding preoperative chronic pain and to subacute postoperative pain as well as to psychological factors before the amputation that may be inherited and/or acquired (learning- and memory-related). Furthermore, we also confirm that subacute pain postamputation comprises an additional risk factor for long-term painful sensations postamputation. Our results suggest that timely interventions might prevent the development of chronic pain.

Implicit operant learning of pain sensitization.

Operant conditioning mechanisms have been demonstrated to be important in the development of chronic pain behavior, but it is not clear whether and how this extends to pain perception itself. The fear-avoidance theory suggests that hypersensitivity may be induced by anticipatory pain avoidance learned through negative reinforcement by acute reductions of pain and fear. But the precise mechanism of the assumed 'sensory decalibration' has not been specified. The present study with healthy subjects investigated whether operant learning of enhanced short-term sensitization may provide the 'proximal' mechanism and whether gradual learning of hypersensitivity can take place without subjects' awareness. We used an experimental model of implicit learning based on a behavioral adjustment method of sensitization measurement developed and validated previously, combining it with standard methods of operant response shaping of increased sensitization or habituation. Results indicated that operant discrimination training with reinforcement of short-term sensitization in the seconds range can produce gross up or down changes in sensitivity within an hour without subjects' awareness of reinforcement contingencies. Consequently, implicit learning of enhanced pain sensitization may be a suitable model to investigate operant plasticity of pain perception in addition to basic sensory and neuronal mechanisms and to link these with the clinical construct of pain-fear avoidance.