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Chapter: Modern Medical Toxicology: Neurotoxic Poisons: Anaesthetics and Muscle Relaxants

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Cyclobenzaprine - Muscle Relaxants

Cyclobenzaprine (proheptatriene, dimethylpropylamine) is a synthetic chemical, and is a tricyclic amine similar in structure and action to amitriptyline.

Cyclobenzaprine

Cyclobenzaprine (proheptatriene, dimethylpropylamine) is a synthetic chemical, and is a tricyclic amine similar in structure and action to amitriptyline. It is sometimes used in the treat- ment of acute skeletal muscle spasm. Therapeutic doses range from 30 to 60 mg/day in adults. It should only be used for short periods (up to two or three weeks). The major metabolites of cyclobenzaprine include amitriptyline, norcyclobenzaprine and nortriptyline.

Adverse effects are anticholinergic in nature comprising mydriasis, warm, dry, flushed skin, dry mouth, urinary retention, disorientation, and sometimes convulsions. These symptoms become more pronounced in overdose, and in addition, the following manifestations are seen : urinary retention, tachycardia, delirium, hallucinations, hypotension (rarely hypertension), respiratory depression, and cardiac arrhythmias.

The CNS depressant effects of cyclobenzaprine are enhanced when it is coingested with other CNS depressant drugs (benzodi- azepines, opioids, barbiturates, ethanol, sedative hypnotics, etc.).Concomitant cyclobenzaprine and anticholinergic therapy may result in additive anticholinergic effects. Concomitant mono- amine oxidase (MAO) inhibitor and cyclobenzaprine therapy may result in hyperpyrexia, excitation, and seizures.

Treatment is on similar lines as for tricyclic overdose. Gastric lavage is said to be beneficial upto several hours after ingestion. Serious symptoms may necessitate the use of physostigmine (1 to 4 mg IV slowly, over 5 to 10 minutes). If the heart rate exceeds 160 beats/minute and/or the patient demonstrates evidence of haemodynamic instability with central anticholinergic symptoms, treatment with phys- ostigmine may be necessary.

Teratogenicity has been reported following first trimester maternal exposure to cyclobenzaprine. A syndrome of imper- forate oropharynx with costovertebral and auricular anomalies was reported in a child of 32-week gestation following first trimester maternal ingestion of cyclobenzaprine and zomepirac.

Forensic Issues (Muscle Relaxants)

·              Accidental overdose involving these drugs is not uncommon. Suicides have been reported with many of these drugs, and most of them have involved medical or paramedical personnel.

·              Homicides are increasingly being reported, especially with reference to neuromuscular blocking agents. There is even a report of a serial murder involving multiple victims with succinylcholine.

·              Some of these drugs are subject to being abused, e.g. orphenadrine, baclofen.


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