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DESCRIPTION
Cyclobenzaprine hydrochloride (Flexeril) is an oral muscle relaxant.
FORMS AND USES
- Cyclobenzaprine is commonly prescribed as a relaxant for the skeletal muscles.
- The usual dose is 10 mg orally three times a day.
TOXIC DOSE
The toxic dose is poorly characterized, but several hundred milligrams may cause death.
PATHOPHYSIOLOGY
- Cyclobenzaprine has anticholinergic, antihistaminic, and sedative properties.
- Anticholinergic side effects are frequent at therapeutic doses.
- Deaths from this drug are primarily caused by CNS depression, not by cardiac toxicity.
DRUG AND DISEASE INTERACTIONS
Cyclobenzaprine potentiates CNS depression caused by other sedating drugs.
PREGNANCY AND LACTATION
US FDA Pregnancy Category B. Animal studies indicate no fetal risk and there are no controlled human studies, or animal studies show an adverse fetal effect, but well-controlled studies in pregnant women do not.
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DIFFERENTIAL DIAGNOSIS
Toxicologic causes of CNS depression include tricyclic antidepressants, ethanol, barbiturates, sedative-hypnotic drugs, and many others.
SIGNS AND SYMPTOMS
Typical overdose presentation. Tachycardia, tachypnea, confusion, and agitation, progressing to delirium, respiratory depression, and coma in severe cases.
HEENT
Mydriasis, dry mouth, blurred vision, hyperthermia.
Dermatologic
Warm, flushed, dry skin.
Cardiovascular
Sinus tachycardia; ventricular dysrhythmia occurs rarely.
Gastrointestinal
Decreased or absent bowel sounds.
Musculoskeletal
Rhabdomyolysis, rarely.
Renal
Urinary retention, renal failure secondary to dehydration, and rhabdomyolysis.
Neurologic
Drowsiness, dizziness, fatigue, confusion, agitation, somnolence, hallucinations, seizures
PROCEDURES AND LABORATORY TESTS
Essential Tests
No tests are usually needed for asymptomatic patients.
Recommended Tests
- Pulse oximetry, serum electrolytes, BUN, creatinine, and blood glucose should be measured to assess altered mental status.
- ECG, acetaminophen and aspirin level should be assayed to evaluate occult ingestion in an overdose setting.
- Serum creatine phosphokinase should be performed for patients with prolonged periods of unconsciousness.
- Despite the lack of clinical evidence of serious cardiac toxicity, continuous cardiac monitoring has been advocated in cyclobenzaprine overdose situations because of its structural similarity to tricyclic antidepressants.
- Judicious use of physostigmine 1 to 2 mg intravenously over 2 to 3 minutes may be used to diagnose anticholinergic syndrome; clinical response should be evident within a few minutes.
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- Therapy should focus on empiric therapy of altered mental status (e.g., oxygen, naloxone, glucose, and thiamine) and maintenance of airway.
- Dose and time of exposure should be determined for all substances involved.
- Other causes of altered mental status (infection, trauma, etc.) should be considered.
DIRECTING PATIENT COURSE
The health-care professional should call the poison control center when:
- CNS depression or other toxic effects develop.
- Toxic effects are not consistent with cyclo-benzaprine.
- Coingestant, drug interaction, or underlying disease presents an unusual problem.
The patient should be referred to a health-care facility when:
- Attempted suicide or homicide is possible.
- Patient or caregiver seems unreliable.
- Coingestant, drug interaction, or underlying disease presents an unusual problem.
Admission Considerations
Inpatient management is warranted for patients with an altered level of consciousness or hemodynamic instability following 6 hours of observation.
DECONTAMINATION
Out of Hospital
Do not induce emesis.
In Hospital
- Gastric lavage should be performed in pediatric (tube size 24-32 French) or adult (tube size 36-42 French) patients presenting within 1 hour of a large ingestion or if serious effects are present.
- One dose of activated charcoal (1-2 g/kg) should be administered without a cathartic if a substantial ingestion has occurred within the previous few hours.
ANTIDOTES
There is no specific antidote for cyclobenzaprine poisoning.
ADJUNCTIVE TREATMENT
Hypotension
- Saline (10-20 ml/kg 0.9%) should be administered and the patient placed in the Trendelenburg position.
- Further fluid therapy should be guided by central pressure monitoring to avoid volume overload.
- A vasopressor may be added if needed. Dopamine is preferred and norepinephrine is added for refractory hypotension.
Anticholinergic Syndrome
Reversal of anticholinergic toxicity by physostigmine is used to determine whether altered mental status is due to anticholinergic effects. (See SECTION III, Physostigmine.)
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PATIENT MONITORING
Respiratory and cardiac function should be monitored continuously in symptomatic patients.
EXPECTED COURSE AND PROGNOSIS
- Toxicity usually develops within hours, but may be delayed due to anticholinergic effects.
- Complete recovery over 1 to 2 days is expected unless sequelae of hypoxia or hypotension intercede.
DISCHARGE CRITERIA/INSTRUCTIONS
Asymptomatic patients may be discharged after 6 hours of observation and a psychiatric evaluation, if needed.
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DIAGNOSISFailure to consider other causes of altered mental status may result in missing life-threatening problems.
ICD-9-CM 968Poisoning by other central nervous system depressants and anesthetics.
See Also: SECTION II, Anticholinergic Syndrome and Hypotension chapters; and SECTION III, Physostigmine chapter.
RECOMMENDED READING
Ellenhorn MJ. Muscle relaxants. In: Medical toxicology: diagnosis and treatment of human poisoning. Baltimore: Williams & Wilkins, 1997:937-953.
Author: Luke Yip
Reviewer: Kennon Heard