Local Anesthetics

Lidocaine is reported to have significant analgesic effects that are distinct from those produced by morphine (Wu et al. 2002). In this randomized double-blind, active-placebo-controlled, crossover trial, the authors demonstrated that stump pain was diminished both by morphine and by lidocaine while phantom pain was diminished only by morphine. These observations suggest that the mechanisms and pharmacological sensitivity of phantom and stump pains differ. Stump pain may be predominantly peripherally mediated via a mechanism involving sodium channels, while phantom pain may involve both peripheral and central mechanisms (Wu et al. 2002). Despite the observed efficacy, the drugs tested did not eliminate pain completely, suggesting that these patients may require multimodal therapy, and that future analgesic studies in this area should be expanded to include neuraxial opioids, anticonvulsants, and antidepressants to the currently tested drugs.

Abram and Yaksh in another animal model demonstrated that systemic local anesthetics can affect the behavioral responses to noxious stimulation by two distinct mechanisms (Abram and Yaksh 1994). While they are capable of blocking nociceptor-induced spinal sen-sitization, they do so incompletely and only at blood levels that are close to those associated with symptoms of toxicity. They also appear to have no effect on previously established spinal hypersensitivity. Therefore, it appears likely that the predominant effect of systemic lidocaine on neuropathic pain is through suppression of spontaneous impulse generation arising from injured nerve segments or associated dorsal root ganglia (Abram and Yaksh 1994).

Lidocaine is available as a 5% transdermal patch which is applied for 12 h per day and is approved for use in postherpetic neuralgia after the skin lesions have healed and the skin is intact. It can produce 30-40% reduction in pain in some patients. Although there are no controlled studies, some physicians are prescribing these patches for pain other than postherpetic neuralgia and the patients do get relief (e.g., low back pain and wrist pain). Galer reported in March 2005 at the American Pain Society annual meeting that his data strongly suggest that the patch (currently approved in the United States for treating postherpetic neuralgia) was as effective as celecoxib for reducing daily pain intensity in patients with osteoarthritis of the knee (Galer 2005).

Mexilitine [Mexitil® (Boehr Ingelheim Pharmaceuticals, Ridgefield, CT) 150 mg advancing to qid dosing] is a cardiac antiarrhythmic drug which is a lidocaine analogue available in oral form (Management of Chronic Pain Syndromes 2005). For cardiac arrhythmias, both mexilitine and lidocaine decrease ventricular irritability, stabilize the Purkinje fiber system, and decrease circuit reentry arrhythmias. It is this sodium channel interaction that likely reduced the neural activity in studies of neuroma-generated nerve pain (Chabal et al. 1989).

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