CHAPTER 2-143: SKELETAL MUSCLE RELAXANTS

INTRODUCTION

Drugs discussed in this chapter are centrally acting skeletal muscle relaxants that exert their effects indirectly. Dantrolene, a direct acting skeletal muscle relaxer, is described in Section III (Cyproheptadine). The drugs commonly used as skeletal muscle relaxants are listed in Table II–60. Carisoprodol (Soma^®) and baclofen (see also Baclofen) are often abused as recreational drugs.

MECHANISM OF TOXICITY

1. Central nervous system. Most of these drugs cause generalized CNS depression.

   1. Baclofen (see also Baclofen) is an agonist at the GABA(B) receptor and can produce profound CNS and respiratory depression as well as paradoxical muscle hypertonicity and seizure-like activity.

   2. Spastic encephalopathy with increased muscle tone, hyperreflexia and myoclonus is common with carisoprodol overdose.

   3. Cyclobenzaprine and orphenadrine possess anticholinergic properties.

   4. Tizanidine, a centrally acting alpha₂ agonist, has effects similar to those of clonidine.

2. Cardiovascular effects. Hypotension may occur after overdose. Baclofen has caused bradycardia in up to 30% of ingestions. Orphenadrine has sodium channel blocking effects similar to tricyclic antidepressants. Massive orphenadrine ingestions have caused supraventricular and ventricular tachycardia.

3. Pharmacokinetics varies with the drug. Absorption may be delayed because of anticholinergic effects (see also Table II–66).

TOXIC DOSE

The toxic dose varies considerably among drugs, depends largely on individual tolerance, and can be influenced by the presence of other drugs, such as ethanol. For most of these drugs, ingestion of more than 3–5 times the usual therapeutic dose may cause stupor or coma.

1. Baclofen. In adults, CNS depression, delirium, seizures, and hypertension occurred more frequently after ingestion of more than 200 mg. However, in children respiratory arrest was reported in a 22-month-old child who ingested 120 mg (10.9 mg/kg) and an estimated 60 mg of baclofen caused coma, flaccidity, hyporeflexia, bradycardia, and hypotension in a 3-year-old child.

2. Carisoprodol. Death was reported in a 4-year-old child who ingested approximately 3, 500 mg, and a 2-year-old child who ingested two tablets (350 mg each) required intubation.

3. Orphenadrine. A 2-year-old child had seizures and tachycardia after ingesting 400 mg. In a series of 10 fatal cases, the mean amount ingested by 6 adults was 22 mg/kg and by 4 children was 72 mg/kg.

4. The lowest dose of tizanidine associated with coma in an adult was between 60 and 120 mg.

CLINICAL PRESENTATION

Onset of CNS depression usually is seen within 30–120 minutes of ingestion. Lethargy, slurred speech, ataxia, coma, and respiratory ...