The effect of peripheral glycerol on trigeminal neuropathic pain examined by quantitative assessment of abnormal pain and sensory perception

In nine patients with trigeminal neuropathic pain after nerve injury, we ex amined prospectively the effect of peripheral glycerol neurolysis on abnorm al pain and sensory perception. In the painful facial skin area of these pa tients, we found increased temperature and tactile thresholds and the prese nce of abnormal temporal summation of pain. In seven patients: neuropathic pain was peripheral and disappeared after application of local anaesthesia at or proximal to the site of nerve injury. Neuropathic pain was central in two patients, and unresponsive to local anaesthesia applied proximal to th e sire of nerve injury. Sis weeks after injection of glycerol proximal to t he site of nerve injury, no or marginal pain relief was found in 8 patients with peripheral or central trigeminal neuropathic pain. On the other hand, in one of the patients with peripheral trigeminal neuropathic pain, glycer ol was given at the site of nerve injury, and produced total pain relief fo r the whole observation period of 7 months. In this patient, pain relief wa s associated with normalisation of abnormal temporal summation of pain, whi ch was not observed in the 8 patients with no or marginal pain relief. No f urther changes in temperature or tactile thresholds were found in any of th e 9 patients after a single injection of absolute glycerol. Total pain reli ef in one of the patients probably is related to the ability of glycerol to inhibit ongoing ectopic impulse generation at the site of nerve injury. We suggest that glycerol-induced reduction of primary afferent hyperactivity may secondarily result in down-regulation of central neuronal hyperexcitabi lity. The efficacy of application of glycerol at the site of nerve injury i n patients with peripheral trigeminal neuropathic pain may warrant further investigation. However, this prospective study doer not provide evidence th at application of glycerol proximal to the site of nerve injury has a place in the treatment of trigeminal neuropathic pain.