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 examined prospectively the effect of peripheral glycerol neurolysis on abnormal pain and sensory perception. In the painful facial skin area of these patients, we found increased temperature and tactile thresholds and the presence 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 the site of nerve injury. Six weeks after injection of glycerol proximal to the 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, glycerol was given at the site of nerve injury, and produced total pain relief for the whole observation period of 7 months. In this patient, pain relief was associated with normalisation of abnormal temporal summation of pain, which was not observed in the 8 patients with no or marginal pain relief. No further changes in temperature or tactile thresholds were found in any of the 9 patients after a single injection of absolute glycerol. Total pain relief 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 hyperexcitability. The efficacy of application of glycerol at the site of nerve injury in patients with peripheral trigeminal neuropathic pain may warrant further investigation. However, this prospective study does not provide evidence that application of glycerol proximal to the site of nerve injury has a place in the treatment of trigeminal neuropathic pain.