Topical
lidocaine (5%) leads to sufficient
pain relief in only 29%-80% of treated patients, presumably by small-fiber block. The reasons for nonresponse are unclear; it may be due to different underlying
pain mechanisms or partly insufficient
anesthetic effect. Using quantitative sensory testing (QST) following the protocol of the DFNS (German Research Network on
Neuropathic Pain), this study aims to assess the type and extent of somatosensory changes after
lidocaine application in healthy volunteers. Twenty-six healthy volunteers underwent QST on the volar forearm, including thermal and mechanical detection and pain thresholds, twice before (for baseline retest reliability) and once after 6-hour simultaneous application with
lidocaine patch 5% and contralateral placebo in a double-blinded manner. Pre and post differences of QST parameters were analyzed by paired t-test (Bonferroni-corrected alpha 0.0023). QST profiles did not change between the 2 baseline measurements and after the placebo application.
Lidocaine application led to a significant change of only the small-fiber-associated thresholds (increase of thermal detection and mechanical pain thresholds, decrease of mechanical
pain sensitivity). Tactile detection thresholds representing Aβ function remained unchanged. Interindividually, the extent of the small-fiber block varied widely (eg, thermal detection thresholds: in 54% of the subjects there were only minimal changes; in only 8% were there changes of >60% of the maximal achievable value). Topical
lidocaine (5%) induces thermal
hypoesthesia and pinprick hypoalgesia, suggesting an isolated but only partial block of Aδ and C fibers of unpredictable extent. Further studies must analyze the influencing factors and determine whether patients with poor
analgesic effect, in particular, are those with insufficient small-fiber block.