Brain activity for modulation of allodynia by topical lidocaine in post-herpetic neuropathy

Abstract

Topical lidocaine is shown to be effective in alleviating post herpetic neuropathy (PHN) type chronic pain. In this study, we examine brain activity related to tactile allodynia (pain due to brushing of the skin) in PHN, using fMRI. We compare effects of acute and longer-term use of lidocaine. All enrolled PHN patients had pain for at least 3 months. They were trained to use a finger-span device for rating their pain. During a given scanning session, patients 1) rated fluctuations of their pain during the application of a random sequence of mechanical stimuli to the body part having PHN, 2) during mechanical stimulation to a corresponding unaffected body part (stimulus control), and 3) during a visual task (motor-cognitive control). Patients were scanned prior, 6 hours, and 2 weeks post continuous use of topical lidocaine (LidoDerm). Fifteen PHN patients were studied, 11 completed all phases. Allodynia pain was significantly lower after 2 weeks of LidoDerm use. Prior to LidoDerm use, brain activity in relation to mechanical stimulation of allodynia site showed activity in primary, secondary, insula, anterior cingulate, cortices and thalamus. This activity pattern was maintained and further enhanced after acute and longer-term therapy. When brain activity was defined in relation to pain scores, prior to LidoDerm use, it was localized mainly in medial prefrontal cortex, and significantly decreased after 6-hours and 2-weeks of LidoDerm use. The results suggest that LidoDerm therapy is associated with decreases in brain activity corresponding to regions we have previously identified for chronic neuropathic pain. Moreover, our data imply that decreased allodynia is accompanied with increased sensitivity to touch. These results remain preliminary. More rigorous analyses, taking into account the control scans, will be reported. The study demonstrates the use of brain scanning to identify brain areas specifically modulated by therapy. Funded by Endo Pharmaceuticals. Topical lidocaine is shown to be effective in alleviating post herpetic neuropathy (PHN) type chronic pain. In this study, we examine brain activity related to tactile allodynia (pain due to brushing of the skin) in PHN, using fMRI. We compare effects of acute and longer-term use of lidocaine. All enrolled PHN patients had pain for at least 3 months. They were trained to use a finger-span device for rating their pain. During a given scanning session, patients 1) rated fluctuations of their pain during the application of a random sequence of mechanical stimuli to the body part having PHN, 2) during mechanical stimulation to a corresponding unaffected body part (stimulus control), and 3) during a visual task (motor-cognitive control). Patients were scanned prior, 6 hours, and 2 weeks post continuous use of topical lidocaine (LidoDerm). Fifteen PHN patients were studied, 11 completed all phases. Allodynia pain was significantly lower after 2 weeks of LidoDerm use. Prior to LidoDerm use, brain activity in relation to mechanical stimulation of allodynia site showed activity in primary, secondary, insula, anterior cingulate, cortices and thalamus. This activity pattern was maintained and further enhanced after acute and longer-term therapy. When brain activity was defined in relation to pain scores, prior to LidoDerm use, it was localized mainly in medial prefrontal cortex, and significantly decreased after 6-hours and 2-weeks of LidoDerm use. The results suggest that LidoDerm therapy is associated with decreases in brain activity corresponding to regions we have previously identified for chronic neuropathic pain. Moreover, our data imply that decreased allodynia is accompanied with increased sensitivity to touch. These results remain preliminary. More rigorous analyses, taking into account the control scans, will be reported. The study demonstrates the use of brain scanning to identify brain areas specifically modulated by therapy. Funded by Endo Pharmaceuticals.