Capsaicin-induced flare and vasodilatation in patients with post-herpetic neuralgia

Abstract

The aim of the present study was to investigate the role of primary afferent fibres with polymodal nociceptors in the various pain symptoms and signs associated with post-herpetic neuralgia (PHN). Forty-four patients with PHN affecting thoracic dermatomes were examined clinically for evidence of sensory disturbance to touch and pinprick and compared to 14 normal subjects and 9 subjects with evidence of past herpes zoster infection but no pain. The patients were then divided into 3 groups on the basis of their clinical symptoms and signs-those with steady burning discomfort only (n = 12), those with burning discomfort, allodynia and hyperalgesia to pinprick (n = 17), and those with burning discomfort, allodynia and hypalgesia to pinprick (n = 15). Indirect measurement of primary afferent fibre function was performed by measuring the neurogenic axon reflex flare to topical capsaicin using Doppler flowmetry in the 5 clinical groups. The 2 groups with allodynia had significantly decreased neurogenic flare responses compared to PHN subjects without allodynia and the 2 control groups. These results suggest that allodynia in patients with post-herpetic neuralgia may be a consequence of disrupted function of primary afferent fibres.