Adaptation to warming but not cooling at slow rates of stimulus change in thermal threshold measurements.

The relationship between thermal detection threshold and rate of temperature change of the thermal stimulus when slow (<1 degrees C s(-1)) rates of change are employed was investigated. Using both the reaction time (RT) inclusive Method of Limits and RT exclusive Method of Levels healthy volunteers had warming (WDT) and cooling detection thresholds (CDT) measured at four different rates of temperature change (0.3, 0.5, 0.7 and 1.0 degrees C s(-1)) from the thenar and/or mental regions using a contact thermode. With the Method of Limits, CDT increased linearly with rate of temperature change suggesting increments were due to RT artefacts. This was further supported by threshold assessment with the Method of Levels which showed CDT were unaffected by the rate of change in the RT exclusive method (P > 0.1). In contrast, WDT did not increase linearly with rate of stimulus temperature change when the Method of Limits was used and threshold assessment with the Method of Levels showed WDT assessed using a 0.3 degrees C s(-1) ramp rate were significantly higher than those measured with a 1 degrees C s(-1) rate of change (P < 0.05). This study indicates that adaptation to a warming stimulus can occur at faster rates of stimulus change than previously anticipated and identifies differences in warming and cooling pathways in sensitivity to adaptation.

Differential effects of neuropathic analgesics on wind-up-like pain and somatosensory function in healthy volunteers.

OBJECTIVES: To investigate the effects of gabapentin, carbamazepine, and amitriptyline on temporal summation, simple nociceptive pain, and innocuous touch sensation in healthy volunteers. METHODS: A placebo controlled four-way crossover double-blind randomized protocol was followed. Seventeen healthy subjects, male and female, aged 18 to 24, took part. Punctate pain, temporal summation pain to repeat punctate stimulation, and vibration detection threshold were assessed in triplicate. Study drugs were given as bedtime and early morning doses with assessments carried out midmorning. RESULTS: Gabapentin and carbamazepine significantly reduced the intensity of temporal summation pain (P < 0.001 and P < 0.01 respectively), whereas amitriptyline significantly increased temporal summation pain (P < 0.001). None of the drugs affected pain produced by a single punctate stimulus (P > 0.05). Carbamazepine increased vibration detection thresholds (P < 0.05), but neither gabapentin nor amitriptyline had any detectable effect on vibration. DISCUSSION: We have shown that gabapentin, carbamazepine, and amitriptyline, three pharmacologically different drugs, have distinct and quantifiable effects on somatosensory pathways in healthy volunteers. These findings provide a link between pharmacology of the study drugs and clinical effectiveness. The effects of gabapentin and carbamazepine on temporal summation pain show that these drugs can block centrally amplified wind-up pain in the absence of a neuropathic disorder.

A novel treatment of postherpetic neuralgia using peppermint oil.

BACKGROUND: Postherpetic neuralgia remains a difficult problem to treat. A number of therapies have been shown to be effective, but some patients have intractable pain. PATIENT: The case of a 76-year-old woman whose pain had been resistant to standard therapies is described. The pattern of quantitative sensory testing results for this patient led the authors to believe that she had an "irritable nociceptor" type of pathophysiology. INTERVENTION: The patient was instructed to apply neat peppermint oil (containing 10% menthol) to her skin, resulting in an almost immediate improvement in her pain. This pain relief persisted for 4-6 hours after application of the oil. RESULTS: The patient was successfully treated with topical peppermint oil. During 2 months of follow-up she has had only a minor side effect, with continuing analgesia. The authors believe this is the first evidence of peppermint oil (or menthol) having a strong analgesic effect on neuropathic pain. The possible mechanisms of action of peppermint oil are discussed.