Following varying degrees of injury, the regenerating and sprouting peripheral nerve axons may discharge spontaneously, causing a typical constant aching or burning pain. With chronic stimulation by the C-fiber nociceptors from nearby healing areas, afferent terminals in the dorsal root ganglion may develop sensitivity to sympathetic stimuli such as noradrenalin, resulting in the modulation of signals from mechanoreceptors and perception of light touch as a painful stimulus, i.e., allodynia. These main processes seem to be driven by complex interactions among different ion channels, neurotransmitters, and receptor activities. Among them, abnormal accumulation of sodium channels at the site of injury has received intense attention in recent years as a mam cause .

Sodium channel antagonists such as lidocaine bind to abnormal sodium channels which have an unusually high affinity for such drugs, and this diminishes the ectopic discharges within the superficial sensory afferents. Because abnormal im­pulses are generated in the damaged or sensitized cutaneous sensory nerves in patients with PHN, direct application of lidocaine via topical administra­tion has a distinct advantage over systemic therapy. Delivery of the drug directly to the site of pain generation minimizes the systemic drug exposure, and so it reduces adverse effects.
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A previous initial study on topical lidocaine was conducted using 5% lidocaine gel. The treatment was found to yield significant analgesia that had a rapid onset within 30 minutes following gel appli­cation and a long duration of up to 24 hours. Pain relief was reported only when the topical lidocaine was administered on the painful area and not when the lidocaine gel was applied on the non-painful contralateral skin site. Furthermore, the blood level of lidocaine was negligible (< 0.6 ug/mL) and no systemic side effects were reported. Hence, it was thought that pain relief was obtained by the local direct action of the lidocaine gel, and it was not due to systemic absorption. Lidocaine gel is messy, and so it requires a dressing that when removed, results in abrasion in more than one-third of paitents. This can be especially troublesome to elderly patients who have fragile skin or patients who may already have local skin sensitivity in the affected areas. Due to these limitations, the 5% lidocaine gel was refor­mulated into a patch (Lidoderm patch™), con­sisting of a soft, stretchable, non-woven polyethylene backing with a drug-containing adhesive layer.

In controlled clinical trials conducted with this subsequently developed 5% lidocaine patch, this treatment provided significantly lower pain intensity and greater pain relief as compared with a vehicle patch. The present investigation confirmed these previous controlled studies, that cutaneous lidocaine application (lidocaine patch) on the PHN-affected dermatomes relieves spontaneous pain and allodynia. Besides the pain-relieving abilities, the other two main benefits of the topical lidocaine patch are the lack of any significant systemic side effects and the lack of any significant interactions with other drugs. In multiple clinical trials, the patch was well tolerated by patients who were also taking other pain medication, and there were no indications of systemic adverse effects that may have resulted from drug-to-drug interactions. In our study, the lidocaine patch was also well tolerated with very few systemic adverse effects by all subjects. No significant skin reaction was observed at any patch application site on any person. Viagra Professional