Both peripheral nerve injury and neuroma pain are the result of changes in sodium channel expression. Lidocaine selectively inhibits the spontaneous ectopic activity by binding to sodium channels. Subanesthetics concentrations of lidocaine are able to produce a differential block of the ectopic discharges, but not propagation of impulses, suppressing differentially the associated neuropathic pain symptoms. The aim of this study was to investigate the differences between the analgesic effects of lidocaine 0.5% and a control group of lidocaine 0.1% on several neuroma related pain modalities.
Sixteen patients with neuropathic pain due to painful neuromas caused by nerve injury participated in this randomized, double-blind experiment. The patterns of sensory changes were compared before and after injection of 1 ml lidocaine 0.5% and 0.1% close to the neuroma, the sessions being 1–2 weeks apart. Spontaneous and evoked pains were assessed using a visual analogue scale (VAS), quantitative and qualitative sensory testing. The primary end-point measure was defined as the change in pain score measured from baseline until 60 min after injection. Assessments of spontaneous pain and evoked pain were done post injection at 15 s, 30 s, 1 min, and at 5-min intervals for the first 30-min post injection and then every 10-min to 1 hr post injection. The assessments of pain were performed between the limbs in the following order: spontaneous pain, then assessment of dynamic mechanical allodynia and then hyperalgesia.
Lidocaine dose-dependently reduced spontaneous and evoked pain scores by more than 80% with maximum effects between 1 and 5 min for evoked pain and between 3 and 15 min for spontaneous pain. While evoked pain normalized rapidly reaching about 50% of the control level 20 min after the injection, spontaneous pain levels continue to be lower in comparison with baseline values for more than 60 min. When comparing the time course of analgesia between spontaneous and evoked pain, lidocaine-induced a greater reduction of evoked pain, but with shorter duration than spontaneous pain. The differences between evoked pain and spontaneous pain were statistically significant in both groups (lidocaine 0.5% group; p = 0.02 and lidocaine 0.1% group; p = 0.01). Reproducibility was high for all assessed variables. Surprisingly, both lidocaine concentrations produced a sensory loss within the area with hyperalgesia and allodynia: hypoesthesia occurred earlier and lasted longer with lidocaine 0.5% (between 30 s and 5 min) in comparison with lidocaine 0.1% (p = 0.018).
Differential analgesic effects of subanesthetic concentrations of local lidocaine on evoked and spontaneous pain in human neuroma suggest that different mechanisms underlie these two key clinical symptoms. Spontaneous pain and evoked pain need an ongoing peripheral drive and any possible CNS amplification change is temporally closely related to this peripheral input.
Painful neuroma represents a clinical model of peripheral neuropathic pain that could lead to a significant step forward in the understanding of pain pathophysiology providing the opportunity to study spontaneous and evoked pain and the underlying mechanisms of neuropathic pain. The proposed model of neuropathic pain allows testing new substances by administration of analgesics directly where the pain is generated.
2015. Vol. 8, 37-44 p.