On the pharmacological effects of two lidocaine concentrations tested on spontaneous and evoked pain in human painful neuroma: A new clinical model of neuropathic pain

Abstract

Abstract Aims Spontaneous and evoked pains are key symptoms of patients with neuropathic pain and there is a current discussion on the predictive value of evoked pain read outs for the reduction of spontaneous pain. Here we describe a new clinical model of neuropathic pain that may be useful for evaluation of new drugs. We also report the effects of lidocaine in this model as reference values. Methods In a randomized double-blind experiment, the analgesic effects of local lidocaine were investigated separately for spontaneous pain and for stimulus-evoked allodynia and hyperalgesia in sixteen patients with painful neuromas after traumatic nerve injuries in the upper extremities. The patterns of sensory changes were compared before and after treatment with lidocaine (0.1% or 0.5%, 1 ml), with 1–2 weeks interval, injected close to the neuroma. Spontaneous and evoked pains were assessed using a visual analogue scale (VAS) and quantitative/qualitative sensory testing. Results Lidocaine dose-dependently reduced spontaneous and evoked pain scores by more than 90% 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 minutes after the injection spontaneous pain levels were lower than 25% at this time. Moreover, in 4 patients the reduction of ongoing pain lasted 24 h whereas evoked pain had returned to baseline levels in all the patients after 1 h. Conclusion Differential analgesic effects of local lidocaine on spontaneous and evoked pain suggest that different mechanism underlie these two key clinical symptoms. Thus, clinical trials assessing localized traumatic neuropathic pain should investigate both aspects of pain separately with the proposed model allowing testing of new drugs systemically or locally administered.