SYMBOLMany of the patients we see in our pain clinic have localized neuropathic pain. Is there an effective analgesic that can be applied directly to the painful area?
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Four percent and 5% lidocaine patches can be cut to fit over the area of pain. Because there has been minimal clinical research on the 4% lidocaine patches, which are nonprescription items, they aren't widely marketed. For this reason, the following discussion focuses on the 5% lidocaine patch, which requires a prescription.
The 5% lidocaine patch is the only topical anesthetic agent to receive FDA approval for the treatment of a neuropathic pain condition, specifically, postherpetic neuralgia (PHN). It measures 10 cm x 14 cm and has a polyethylene adhesive backing. As many as three patches can be applied simultaneously to intact skin for up to 12 hours; the patches must then be removed for at least 12 hours. When a patient is debilitated or has impaired renal function, it's recommended that a smaller area be treated.1 Although some patients wear the patch during the day, others prefer to wear it at night. Patients sometimes apply cold packs to the affected area when the patch is off. (Clinical data and anecdotal evidence support the use of cold for pain relief.)
MECHANISM OF ACTION
A local anesthetic, lidocaine blocks the conduction of impulses and stabilizes neuronal membranes, thereby relieving pain. When the patch is applied directly to the painful area, the drug penetrates the skin to act locally on the damaged or dysfunctional nerves and soft tissue underlying the site.2
After application of the 5% patch, it's estimated that 3% of the lidocaine dose is absorbed. This amount produces analgesia without anesthesia, so there's no skin numbness or loss of touch or sensation of temperature. In one study, patients generally reported pain reduction within 30 minutes, with peak pain relief occurring two to four hours after application.3 Because at least 95% of the lidocaine remains in the used patch, instruct patients to wash their hands after handling it and to dispose of used patches in such a way that children and pets can't reach them.1
The benefit of a local mechanism of action is that, with appropriate use, there is minimal systemic absorption of lidocaine, and adverse effects such as central nervous system depression or excitation are averted.2-5 Local absorption also results in fewer drug interactions, an important consideration because many people with chronic pain also require opioids, nonopioids, or adjuvant analgesics (such as gabapentin).
Patches deliver drugs in different ways. The lidocaine patch delivers the drug topically, which should be distinguished from transdermal delivery. Transdermal drug delivery, such as that effected by the fentanyl patch, is systemic and creates a greater potential for systemic side effects and drug interactions. Another significant difference between lidocaine and transdermal patches is that the former can be cut without destroying the drug delivery system.
The 5% lidocaine patch has been studied in patients with PHN, the most common complication of herpes zoster (shingles),2 a condition causing often intractable pain that's seen predominantly in the elderly. Patients with PHN frequently experience allodynia, an intense skin sensitivity to stimuli that do not ordinarily cause pain. Some patients with allodynia report that the mere touch of clothing on the affected area produces excruciating pain.
One randomized, double-blind, placebo-controlled study evaluated 35 patients (mean age, 75) with established PHN and allodynia of the torso or upper extremities.3 As many as three patches were applied to cover the area of greatest pain as fully as possible for 12 hours. The patients wearing the lidocaine patches had significantly reduced pain intensity at all time points (from 30 minutes to 12 hours after patch application) compared with those either receiving no treatment or wearing a placebo patch. There was minimal systemic absorption of lidocaine and, consequently, no systemic adverse effects. In addition to providing pain relief, the patches were found to be easy to apply and remove. They also protected the skin from contact with painful stimuli, and the majority of patients found this protection to be "soothing."
A later crossover clinical trial of 32 patients (mean age, 77) with PHN yielded comparable results.5 Most of the subjects in the study either had been unable to obtain pain relief or had experienced intolerable side effects when using other pain-relieving agents, such as antidepressants and anticonvulsants. Subjects were randomized to receive either a lidocaine patch or a placebo patch for the first 14-day study period; they then received the alternate treatment for the second 14-day study period. Twenty-five patients (78.1%) preferred the lidocaine patch phase of the study, three (9.4%) preferred the placebo phase, and four (12.5%) expressed no preference. Twenty-nine subjects (90%) reported moderate or better pain relief with the lidocaine patch, and seven of these patients reported "complete relief." No serious adverse events were reported.
The effectiveness and tolerability of the lidocaine patch in the treatment of other neuropathic pain conditions, including postthoracotomy and postmastectomy pain, have been studied.2,6 Sixteen patients with refractory peripheral neuropathic pain who had experienced inadequate pain relief or intolerable side effects with other analgesics participated in an open-label prospective study. Thirteen (81%) reported moderate or better pain relief with the lidocaine patch; only one patient reported a side effect (mild skin irritation).
PRECAUTIONS AND ADVERSE REACTIONS
The lidocaine patch is contraindicated in persons who are sensitive to amide-type local anesthetics, such as bupivacaine and ropivacaine, and the product has not been studied for use in children or pregnant or nursing women. It should be used with caution in patients receiving antiarrhythmic drugs such as tocainide and mexiletine because of possible additive toxic effects.1
The most common localized adverse reactions to the lidocaine patch are mild or transient erythema and edema, which resolve within a few minutes to hours after patch removal. Lidocaine overdose from cutaneous absorption is very rare; however, wearing the patch for more than the recommended 12 hours during a 24-hour period or wearing more than three patches at a time can result in increased absorption and high blood concentrations of lidocaine. Instruct patients to remove the patch if a metallic taste or oral numbness occurs. Management of overdose includes immediate discontinuation of the local anesthetic, close monitoring of cardiovascular status, and treatment of symtoms.1