Baclofen is a GABA receptor antagonist and is believed to work through descending pain modulation in the central nervous system. It works best for pain associated with spasticity. It is available for oral as well as intrathecal (Lioresal®, Novartis, East Hanover, NJ) administration. Intrathecally, it is administered via a continuous implanted pump and is very effective in cases of severe spasticity such as cerebral palsy or multiple sclerosis (Bowery et al. 1980, Albright et al. 1993).
The oral tablets of 5 or 10 mg are titrated slowly to effect increasing every 3 days until therapeutic benefit is reached or overwhelming side effects occur. Sedation, dizziness, weakness, hypotension, nausea, respiratory depression, and constipation may occur; the drug must be discontinued by slow taper. The maximum dose is 80 mg/day orally, and the withdrawal syndrome consists of hallucinations or even seizures. Care must be taken in renal failure patients and elevations of alkaline phosphatase and aspirate aminotransferase (AST) levels may occur. Baclofen (Lioresal®®) - 5 mg tid to 15 mg tid.
Dantrolene is a powerful muscle relaxant which is used in the treatment of malignant hyperthermia as an intravenous agent. Dantrolene, unlike other antispasm muscle relaxants, acts peripherally instead of centrally by inhibiting the release of calcium ions from the sarcoplasmic reticulum (Max and Gilron 2001). Orally, it is titrated as 10 or 25 mg qd x 7 days, then 25 mg tid x 7 days, then 50 mg tid x 7 days. It should be discontinued if no benefit is observed after 45 days. It does have a black box warning about possible nonfatal or even fatal hepatic failure.
Diazepam (Valium®®, Roche Laboratories, Nutley, NJ) has been used for many years as a muscle relaxant, often prescribed after whiplash injuries. In adults, 2-10 mg tid-qid can be prescribed. However it carries all of the problems of the benzodiazepines such as abuse potential as a "tranquilizer" or anxiolytic agent. Patients may experience dizziness, drowsiness, and confusion possibly with memory difficulty at higher doses. It has active metabolites which can significantly extend the half-life up to 100 h. Again, it should be avoided in patients with renal insufficiency or hepatic impairment.
Tizanidine (Zanaflex®, Acorda Therapeutics, Hawthorne, NY) is an a-2 adrenergic agonist like clonidine and works through central modulation. Side effects like hypotension, sedation, asthenia, and dry mouth (dose related) can be significant and so very low doses should be started initially. This may be even less than 0.5 mg. The dose can be gradually increased to a maximum of 36 mg/day. It may cause elevated liver function studies or even hepatotoxicity. Dosages need to start small, 1-2 mg qhs, further titrated to 4-8 mg qhs and 2-4 mg bid (Mclain 2002).
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