Actions of Anticonvulsants in Pain Therapy

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Similarities between the pathophysiological phenomena observed in some epilepsy models and in neuropathic pain models justify the rational use of anticonvulsant drugs in the symptomatic management of neuropathic pain disorders. The availability of newer anticonvulsants tested in higher quality clinical trials has marked a new era in the treatment of neuropathic pain.

Today, gabapentin (Neurontin®) and pregabalin (Lyrica®) have become the first-line anticonvulsants used in pain management. Considerable research has defined the mechanisms by which these agents produce antinociception. The drugs bind to the A-2D subunit of the presynaptic voltage-gated calcium channel on C-nociceptor fibers entering the spinal cord, preventing calcium entry into the cell, thus preventing the fusion of the neurotrans-mitter releasing vesicles to the cell membrane which is necessary for the release of the neurotransmitters into the synapse. Side effects include weight gain, peripheral edema, and short-term memory loss. Package insert specifies dosing for renal failure.

Gabapentin (Neurontin®®): 100, 300,400, 600, 800 mg tablets; 100, 300,400 mg capsules. Functions as an A-2D subunit blocker on voltage-gated calcium channels in the spinal cord as well as a tetrodotoxin-resistant sodium channel (TTXr) blocker. Considered first-line therapy for neuropathic pain. Daily dose 900-4,000 mg/day in TID or QID dosing. Elderly may only tolerate 100 mg QD-TID.

Pregabalin (Lyrica®®): 25, 50, 75, 100, 150, 200, 225, 300 mg capsules. Functions as an A-2D subunit blocker on voltage-gated calcium channels in the spinal cord. Approved for use in the treatment of postherpetic neuralgia (PHN), diabetic neuropathy, and fibromyalgia, but is used in many neuropathic pain syndromes. Start 50-75 mg dosing TID or QID titrated up to 400 mg/day.

Topiramate (Topamax®): 25, 50,100,200 mg tablets; 15,25 mg capsules. Blocks voltage-dependent sodium channels, augments gamma aminobutyric acid (GABA) at some subtypes of the GABA-A receptor, antagonizes the AMPA/kainate subtype of the glutamate receptor, and inhibits the carbonic anhydrase enzyme. Dosing 75-600 mg QD or BID, Food and Drug Administration (FDA) approved for use in migraine prophylaxis. Does not cause weight gain (may cause mild to moderate weight loss - thought to be due to carbonic anhydrase appetite suppression) and is often used as a substitute in women with neuropathic pain who are concerned about weight gain. Inhibition of carbonic anhydrase can also result in nephrolithiasis (CaPO4 stones). May cause closed-angle glaucoma in susceptible individuals - discontinue if blurred vision and eye pain develop.

Lamotrigine (Lamictal®): Na+ channel blocker, 50-400 mg QD or in divided doses. Need to monitor for side effects (liver function). Found to be effective in central poststroke pain syndrome.

Carbamazepine (Tegretol®®): Older agent, only proven effectiveness is in trigeminal neuralgia. Functions as a Na+ channel blocker. Many side effects, including potential hepatic and/or bone marrow damage. Need to monitor liver function and blood count. Particular caution must be taken in patients with bone marrow depression. Can result in hyponatremia so serum Na+ must be monitored when titrating. Dosing is usually initiated at 100 mg BID as tolerated. Effective dose range for pain control varies from 200 to 1,200 mg/ day.

Oxcarbazepine (Trileptal®): A structural derivative of carbamazepine, with a ketone substitution on the dibenzazepine ring. Reduced impact on the liver with metabolism. Avoids the risk of anemia or agranulocytosis occasionally associated with carbamazepine. Can result in hyponatremia so serum Na+ must be monitored when titrating. Dosing is usually initiated at 300 mg BID as tolerated. Recommended daily dose is 1,200 mg/day.

Phenytoin (Dilantin®®1): older agent, first-generation sodium channel blocking anticonvulsant. Used for neuropathic pain (PO dose 100 mg TID). Need to monitor therapeutic level. Major precautions: porphyria, liver disease, myocardial insufficiency, cardiac arrhythmias, hypotension. Intravenous (IV) phenytoin 15 mg/kg infused over 2 h can provide up to 7 days of pain reduction in a crisis (McCleane 1999). Gingival hyperplasia can be disturbing to patients.

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