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Topicals Offer Pain Relief for Postherpetic Neuralgia Patients.

FORT LAUDERDALE, FLA. -- Growing clinical evidence supports the use of topical analgesics in postherpetic neuralgia.

It's becoming clear that irritable nociceptors are at work in postherpetic neuralgia and that a lidocaine patch can safely and effectively relieve the pain, experts said at the annual meeting of the American Pain Society.

Neuropathic pain is a common and debilitating symptom in postherpetic neuralgia and other diseases. "One of the fundamental clinical signs of neuropathic pain is that pain is present within an area of sensory abnormality," Dr. Howard L. Fields said in a symposium sponsored by Endo Pharmaceuticals at the meeting.

But neuropathic pain has two distinct clinical pictures, said Dr. Fields, professor of neurology and physiology at the University of California, San Francisco.

In the deafferentation type of neuropathic pain, the primary sensory neurons are completely damaged, hence the profound sensory deficit and cold skin. Allodynia--pain upon stimulation--may or may not be present, and a local skin block has no effect.

In contrast, the irritable nociceptor type of neuropathic pain presents with minimal sensory deficit, warm skin, and marked allodynia. Significantly, a local skin block provides relief.

The nerves are injured in irritable nociceptor pain, but some primary afferent fibers are still intact. These primary afferents have increased sodium channels in the periphery that are thought to cause spontaneous discharge and sensitization. The primary afferent fibers that generate pain are located in the skin, so researchers have tested topical therapies and found them effective in postherpetic neuralgia, Dr. Fields said.

Dr. Michael Rowbotham, also of the university, reviewed a series of studies that he was involved in. In a double-blinded study of 39 patients, application of 5% lidocaine gel covered with Tegaderm dressing over the painful site relieved pain better than a vehidegel. Lidocaine in the contralateral non-painful site did not affect the painful site, showing that the effect was local (Ann. Neurol. 37[2]:246-53, 1995).

The Tegaderm dressing caused adhesive skin burns when removed, so investigators devised an easy-to-apply, stretchy, 10-by-14-cm medicated patch.

In a double-blinded trial of 35 patients, the Lidoderm patch improved pain ratings over the vehicle patch after 2 hours of application, and did so without making the skin numb. "We're delivering a local anesthetic in concentrations that are subanesthetic," Dr. Rowbotham said.

Blood levels were 50-100 ng/mL, which is one-tenth the minimum antiarrhythmic concentration of systemic lidocaine. The drug never accumulates if the patient applies the patch for only 12 hours a day, he added.

In another study the lidocaine patch reduced postherpetic pain by 29% and relieved allodynia in 100 patients; a vehicle patch reduced pain by 23% and had no effect on allodynia. Nearly 60% of those using lidocaine reported moderate or better pain relief.

A recent study suggested that the lidocaine patch is safe and effective for long-term use. Most of the 32 patients studied had used the patch for more than 3 years, and most said it was the only treatment that significantly relieved their postherpetic pain (Pain 80[3]:533-38, 1999).

The lidocaine patch relieves postherpetic neuralgia just as well as antidepressants, opioids, or gabapentin, he said. But a patch is much easier to use and doesn't produce systemic blood levels, which is a boon for elderly patients.

Topical lidocaine via a patch "should be considered first-line treatment and used in conjunction with other available treatments, Dr. Rowbotham said.

Unfortunately, the patch will not help all patients. The average postherpetic neuralgia patient has both deafferentation and irritable nociceptor mechanisms contributing to their pain, so "topical agents will give significant but incomplete relief," Dr. Fields said.

"If you have loss of primary afferents, then there's nothing for the local anesthetic to act upon and you don't have relief of the pain," he added.
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Title Annotation:analgesia administration
Comment:Topicals Offer Pain Relief for Postherpetic Neuralgia Patients.(analgesia administration)
Author:CHENG, GUANG-SHING
Publication:Family Practice News
Article Type:Brief Article
Geographic Code:1USA
Date:Apr 1, 2000
Words:623
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