Phantom pain poses a great issue due to its high prevalence and the fact that it is difficult to manage clinically. Despite being thoroughly investigated, it continues to be a mysterious phenomenon. What is needed is to take a look at the general tendencies of phantom pain from a larger perspective and based on empirical evidence, speculate in some of the mechanisms involved, and from there on synthesize a potential treatment based on the accumulated knowledge. The aim of this study was to answer the question: How can the mechanisms involved in phantom pain be explained? First, empirical evidence for peripheral and central mechanisms were listed, as these were argued to play a role in the constitution of phantom pain. Among peripheral aspects are: Phantom pain is known to correlate with stump pain; Phantom pain can be modulated or exacerbated by ectopic discharge from a neuroma; Skin temperature at the stump is known to correlate with phantom pain. Among central aspects are: Sensitization of neurons located in the dorsal horns can affect phantom pain; Cortical reorganization correlates strongly with phantom pain; Phantom pain often seems to share characteristics with preamputation pain; Regional anesthesia is only able to reduce pain in some instances; Top-down processes like attention, depression or catastrophizing can exacerbate phantom pain. It is argued that since no stimuli from the periphery is necessary for phantom pain to occur, the central mechanisms must be of special importance.
The theory of predictive coding ascribes a key role to the top-down component. It considers phantom sensation as an expression of a “filling-in” mechanism, but cannot explain why the sensation is painful or the mechanisms of how the phantom emerges. The neuromatrix theory describes how components from the peripheral and central nervous system together constitutes phantom pain. It considers signals from the periphery to have a modulating role in painful experiences and not to be the cause, but it does not account for the proportions in which different components contribute to the sensation of pain, or why phantom pain emerges.
From the understanding that several mechanisms together constitute phantom pain and with a particular emphasis on the effects of top-down processes, a new hypothesis was formed. It was hypothesized that sensory and motor training of the phantom limb during hypnosis could reduce phantom pain. A study was designed to test this hypothesis. In the control group the therapy consisted of hypnosis with relaxation being the main theme suggested. The therapy was set to be administered four times over the period of four weeks. Participants will be allocated to the groups with hypnotizability (SHSS:C) and pain intensity as primary and secondary stratification parameters respectively. Measures of pain intensity, unpleasantness, and frequency will be made before and after the intervention period as well as in a follow-up one month later. Additionally, Beck Depression Inventory (BDI-II) and Pain Catastrophizing Scale (PCS) are administered to uncover affective aspects of phantom pain. Measures of pain intensity and unpleasantness will also be made before and after each intervention to determine immediate effects of the therapy as well as the three consecutive days following therapy to obtain a measure more independent of the immediate effects.
To test the feasibility of the paradigm pilot data were obtained. Two participants were included “John” and “Peter” and only the intervention group was tested. John showed a substantial reduction in pain frequency in the measures of number of attacks on days with attacks (from 10 to 1), and in the duration of each attack (from 120 seconds to 60). A decrease was also observed in PCS (from 25-18) with the largest decrease in the subscale of helplessness. A small immediate effect was observed in pain intensity and unpleasantness in the first two sessions but not during the final two. A small immediate effect was observed in all session with Peter. However, no effects were seen in any other measures. It was suggested that the fact the participants were on pain medicine, might have caused a floor effect, due to the fact that they already at the start of the therapy rated pain low, why this should be abstained. To increase the effects on the general measure, it was argued that the hypnosis sessions could be supplemented with daily training tasks at home.
Conclusions regarding the effect of the therapy cannot be made, as this was a pilot study, however indications provided by the pilot data illustrate the paradigm as being feasible and can potentially be enhanced by the suggested elements. The full scale study is necessary to provide conclusions regarding the effect of the therapy.