Herpes zoster is a result of the reactivation of the latent varicella-zoster virus (VZV), and is characterized by unilateral, radicular pain and a vesicular eruption that is limited to the dermatome innervated by a single spinal or cranial sensory ganglion. Herpes zoster may display a variety of complications, and post-herpetic neuralgia (PHN) is the most common and intractable complication in many patients. PHN has been defined as a pain that persists from 1 to 6 months after the onset of a rash or after the resolution of a rash. The incidence of both herpes zoster and PHN increase with age, so PHN is more common in an aging population. Patients with PHN typically describe their pain as a constant throbbing and/or burning pain, or as an intermittent, sharp, shooting pain, or as both. In addition, the majority of patients complain of allo- dynia, which is the perception of pain in response to such non-painful stimuli such as light touch, changes in temperature and movement. Sometimes the pain is so severe and disabling that patients suffering from prolonged PHN pain may experience impairment of their physical and psychological functioning.
Numerous treatments for PHN have been employed with varying degrees of success, including anticonvulsants (e.g., gabapentin and carbamazepin), antidepressants (e.g., amitriptyline and despira- mine), opioids (e.g., morphine and oxycodone), topical analgesics (e.g., capsaicin) and nerve blocks. However, the usefulness of many of the oral medications is limited by their potential for systemic adverse effects, drug-to-drug interactions and the variable pharmacokinetics; this is particularly true among elderly patients who may have decreased liver or renal function and chronic illnesses that require numerous concomitant medications. At one time topical capsaicin was a recommended treatment for PHN, but the clinical trials using this drug have reported mixed results. Moreover, this substance has been shown to be neurotoxic with resultant neuronal damage. Once PHN is established, performing invasive local anesthetic blocks are unlikely to provide more than temporary relief.
These imitations have led to the search for new and effective agents that can both relieve chronic pain and used safely. One such agent, lidocaine in a 5% concentration patch, has been shown in multiple published trials to act as a targeted peripheral analgesic and to have efficacy in treating PHN. This form of topical lidocaine was approved by the US FDA in 1999 for the relief of pain associated with PHN. Its presumed mechanism of action is via a nonselective blockade of peripheral sodium channels on the sensory afferents at the site of application on the skin. The most significant advantages associated with use of the 5% lidocaine patch is that it can offer a potential benefit with minimal risk for drug-to-drug interactions and systemic toxicity. We tested this new formulation in a randomized, cross-over, vehicle- controlled, three-session study that was designed to evaluate the safety and effects on pain relief for PHN in Koreans.