What medications can be used for treatment of PHN? What is the mechanism of action of these medications?
Because of the various possible etiologies of PHN pain, multiple types of medications have been tried, all with vari-able degrees of success. Overall, medication management for PHN is not standardized and the treating physician must try different combinations of medications to see which will work for any individual patient. At present, the only medica-tion approved by the US Food and Drug Administration for PHN pain is 5% lidocaine patch (Lidoderm, Endo Pharmaceuticals, Chadds Ford, Pennsylvania). This patch is placed over the painful area for 12 hour periods, and then left off for 12 hours. Up to three patches may be placed to cover the painful area. Although the exact mechanism of action is unknown, it is believed to be secondary to decreas-ing the ectopic firing of peripheral nerve endings.
The most commonly used medications for the treat-ment of PHN pain include the tricyclic antidepressant agents (TCAs) and the antiepileptic drugs. Multiple studies looking at the TCAs, especially amitriptyline, have shown a benefit to their use. The mechanism of action relates to their ability to block reuptake of serotonin and norepi-nephrine in the central nervous system, thus enhancing the action of the central nervous system’s descending inhibitory pathways. These medications are started at a low dose, usually in the evening, and titrated to effect or side-effects, which may include sedation, dry mouth, or urinary retention. As patients with PHN oftentimes have trouble in sleeping, a nighttime dose of a TCA, with its sedative prop-erties, is often a useful medication.
The antiepileptic drugs are often used for pain for PHN. This is a varied class of drugs, encompassing multiple med-ications with various mechanisms of action. Gabapentin is a commonly used antiepileptic drug for PHN pain. This drug, a γ-aminobutyric acid (GABA) analog, has an unknown mechanism of action, which may be related to the increased metabolism of glutamate, an excitatory neu-rotransmitter. Overall, this medication is very well toler-ated, the most common side-effect being sedation, which is usually seen in the elderly. Older antiepileptic drugs such as phenytoin and carbamazepine are prescribed for neuro-pathic pain; however, their use is decreasing because of their poor side-effect profile and multiple drug–drug interac-tions. Some of the newer antiepileptic medications (lamot-rigine, oxcarbazepine) have been used for PHN pain, but there are no large studies to document their efficacy.
Nonsteroidal anti-inflammatory drugs (NSAIDs) and opiates are often prescribed for the pain of PHN. The NSAIDs work by decreasing prostaglandin production at the peripheral nerve receptors, thus desensitizing them, and the opiates work by binding to the opiate receptors in the brain and spinal cord, stimulating the descending inhibitory pathway. The use of opiates for PHN pain is slowly becoming accepted, as the literature increasingly shows that these drugs, when given appropriately, are safe and effective. A recent study showed the efficacy of long-acting oxycodone for treatment of PHN pain. NSAIDs may be given to prevent the pain from a peripheral mechanism, but there is little literature to support their use.
Other medications that have been described include cap-saicin cream, baclofen, intravenous lidocaine, and mexiletine. Again, there is little in the literature to support the use of any one of these medications, and different combinations need to be tried until the patient is comfortable.