Antispasmodic Agents Muscle Relaxants

Baclofen (Lioresal®): GABA receptor antagonist, descending pain modulation best for pain associated with spasticity through central pain modulating mechanisms. Oral tablets of 5 and 10 mg dosed TID; may titrate every 3 days to effect with maximum dose: 80 mg/day. Can cause sedation, dizziness, weakness, hypotension, nausea, respiratory depression, constipation; discontinue by slow taper; withdrawal syndrome consists of hallucinations, seizures; need to monitor liver function since it may increase alkaline phosphatase and AST levels; dose adjustment is necessary in patients with renal impairment. Baclofen is available for intrathecal administration to control severe spasticity. Initial intrathecal trial of 50-100 |xg to measure reduction in spasticity. Must be done in a monitored setting with resuscitation equipment available. Pyridostigmine (Antilirium®R ) 1 mg IV is the antidote.

Dantrolene: Oral administration is 25 mg QD x 7 days, then 25 mg TID x 7 days, then 50 mgTID x 7 days, then 100 mgTID. Blackbox warning about symptomatic fatal or nonfatal hepatitis; discontinue drug if no benefit is observed after 45 days.

Diazepam (Valium®R ): Adults: 2-10 mg TID-QID, elimination half-life 100 h: avoid in elderly, avoid in patients with renal or hepatic impairment. It has a significant abuse potential; dizziness, drowsiness, confusion; children >6 months: 1-2.5 mg half-life 20-50 h; active metabolites extend half-life up to 100 h. It must be withdrawn slowly from high doses to avoid seizures (4-6 weeks).

Tizanidine (Zanaflex®R ): a2-Adrenergic agonist, initial dose may have to be as low as 0.25 mg because of potential significant sedation. Titrate up to 2-4 mg every 6-8 h until relief or excessive side effects occur; hypotension, sedation, asthenia, and dry mouth (dose related) are frequent; need to monitor for elevated liver function enzyme levels and hepatotoxicity. Maximum dose recommended 36 mg/day.

Cyclobenzaprine (Flexeril®), 5 mg TID; may increase to 10 mg TID; elimination halflife ~18 h in young subjects, ~33 h in elderly, and ~46 h in patients with hepatic impairment.

Anticholinergic effects (drowsiness, urinary retention, dry mouth): avoid in elderly; QT prolongation: avoid in patients with arrhythmias, cardiac conduction disturbances, heart block, heart failure, or recent myocardial infraction; may raise intraocular pressure: avoid in patients with glaucoma.

Carisoprodol (Soma®®) 350 mg tablets; maximum recommended dose is TID or QID. Not recommended in children <12 years; drowsiness; can cause psychological and physical dependence (metabolized to meprobamate, a barbiturate tranquilizer), withdrawal symptoms can therefore occur with discontinuation; excessive use, overdose, or withdrawal may precipitate seizures; reports describe idiosyncratic or allergy-type reactions after first dose (mental status changes, transient quadriplegia, fever, angioneurotic edema, asthmatic episodes).

Chlorzoxazone (Parafon forte); Adults: 250-750 mg TID-QID. Dizziness and drowsiness may occur with rare cases of hepatotoxicity, gastrointestinal irritation, and rare cases of gastrointestinal bleeding; it may also cause red or orange urine; should be avoided in patients with liver impairment. Children: 125-500 mg TID-QID or 20 mg/kg/day in three or four divided doses; same side effects and toxicities can occur as in adults.

Metaxalone (Skelaxin®): 800 mg TID-QID. Not recommended in children <12 years; do not use in patients with renal or hepatic failure or a history of anemia; dizziness or drowsiness; rare cases of leukopenia or hemolytic anemia may occur. Reported to be less sedating than other muscle relaxants.

Methocarbamol (Robaxin®): 1,500 mg QID for the first 2-3 days, then 750 mg QID. Available orally in 500, 750, and 1,000 mg tablets. Robaxin is also available as an injectable. Do not use injection in patients with renal failure; may cause brown-to-black or green discoloration of urine; may impair mental status; may exacerbate symptoms of myasthenia gravis.

Orphenadrine (Norflex®): (available generic only) 100 mg BID; orally, combination products are given TID-QID: it should be avoided in the elderly; it may raise intraocular pressure and should therefore be avoided in patients with glaucoma; it can be associated with gastrointestinal disturbances; elimination half-life 13-20 h which may be extended when use is prolonged; it should be avoided in patients with cardiospasm or myasthenia gravis; and it is contraindicated in duodenal or pyloric obstruction or stenosing peptic ulcers. The anticholinergic effects (drowsiness, urinary retention, dry mouth) can be prominent.

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