What is Phantom Limb Pain?
Phantom limb pain is pain that is felt in the amputated limb that is no longer there (Hanyu-Deutmeyer AA, 2022). Phantom limb pain differs from residual limb pain which is felt in the stump itself.
Phantom limb pain is a prevalent negative effect after limb amputation, affecting around 64% of amputees. (Limakatso, 2020)
Several circumstances may increase the risk for phantom limb pain (PLP), among them: residual limb pain, traumatic amputation, proximal amputation site, phantom sensations, and specific coping strategies. (Limakatso, 2020) Chronic PLP may derive from both peripheral as well as central mechanisms. (Hanyu-Deutmeyer AA, 2022)
In this 2022 study, Fuchs et al. (Fuchs, 2022) have set out to explore whether unilateral upper limb amputees with and without PLP have sensory changes only in the residual limb, pointing to peripheral processes, or in other body sites as well, which could point to central processes. They also intended to explore whether sensory changes have associations with the presence of PLP, or its intensity.
In total, 37 unilateral upper limb amputees with and without PLP were recruited, along with 19 healthy controls. PLP was assessed using the Phantom Pain Severity subscale of the Multidimensional Pain Inventory. Patients with a zero score on pain severity were considered to be without PLP (N=14), and those scoring above were considered with PLP (N=23).
Quantitative sensory testing modalities
Quantitative sensory testing (QST) was performed using Medoc’s PATHWAY for thermal testing of heat pain thresholds and warm detection thresholds, Medoc’s AlgoMed digital algometer was used for pressure pain threshold testing. Two-point discrimination threshold was assessed using 28 calibrated compasses with a 3mm step size.
Testing on different body sites
All patients were tested on both corners of the mouth on the hairy skin, on the contralateral thenar, on the residual limb: 5 cm from the edge of the stump and outside of any scar tissue, and the homologous site on the contralateral arm. Controls were matched to patients in terms of testing locations.
Difference between body sites
Comparing sensory thresholds (warm detection, heat pain and pressure pain) between groups did not reveal any significant differences in any of the body sites in the initial analysis.
Pressure pain threshold (PPT) measurements did not differ significantly between groups group but differed significantly between body-sites. However, both corners of the mouth did not differ. Post-hoc comparison showed that PPT were significantly lower at the ipsilateral arm vs. the contralateral arm.
Heat Pain Thresholds and Pressure Pain Thresholds were both significantly lower at the residual limb as compared to the contralateral arm in patients irrespective of the PLP intensity.
What measure predicts Phantom Limb Pain intensity?
Further investigation of PLP intensity revealed that patients suffering from higher levels of PLP had lower HPT in all body sites except for at the ipsilateral residual limb. The multiple regression analysis found that PLP intensity could for a large part be predicted by sensory thresholds at the contralateral mouth corner; an increase was found in warm detection thresholds, combined with a decrease in heat pain thresholds.
Difference between Phantom Limb Pain and Residual Limb Pain
A linear Mixed Model for statistical prediction of PLP and RLP showed that Phantom limb pain intensity was significantly and inversely correlated to HPT at the mouth, while residual limb pain was significantly and inversely correlated with heat pain threshold at the residual limb. This difference possibly points to separate mechanisms each contributing to altered heat pain thresholds based on peripheral vs. central mechanisms.
The fact that HPT was significantly lower at both corners of the mouth cannot be explained by S1 cortex plastic reorganization since that would have led to unique unilateral changes. This rather suggests central changes other than S1 reorganization.
Phantom limb pain seems to be connected to central pain processing; however, the exact mechanisms of sensitization need to be explored further.