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Baclofen and Other Centrally Acting Muscle Relaxants Toxicosis 109

presynaptic neurons, resulting in excessive Advanced or Confirmatory Testing ○ Gastric lavage (p. 1117): consider for large
postsynaptic nerve firing. • Baclofen, carisoprodol, chlorzoxazone, ingestions (many tablets); protect airway
VetBooks.ir ally related to tricyclic antidepressants. bamol levels can be determined in urine and/ ○ Activated charcoal 1-2 g/kg PO or dose Diseases and Disorders
cyclobenzaprine, guaifenesin, and methocar-
• Cyclobenzaprine: structurally and function-
with cuffed endotracheal tube.
according to packaging label for commercial
Skeletal muscle relaxation may be related to
or serum; presence will confirm exposure.
sedative effects and inhibition of brainstem
dealing with an acute intoxication.
and spinal cord gamma and alpha motor ○ Turnaround time may limit usefulness in products. Repeated doses of activated
charcoal (q 8h with a cathartic for 24h)
neurons. In overdose situations, anticholin- ○ Cyclobenzaprine results in a positive serum are recommended for cyclobenzaprine and
ergic and antihistaminic effects become or urine assay result for amitriptyline. orphenadrine if signs persist; single doses
exaggerated. are recommended for the remaining SMRs.
• Tizanidine’s mechanism of action is like that TREATMENT • Anecdotally, intravenous lipid solutions have
of xylazine and clonidine. It stimulates hastened recovery of dogs with baclofen
alpha-adrenoreceptors in the brainstem, Treatment Overview toxicosis (p. 1127).
causing decreased vascular resistance, heart Treatment is aimed at early decontamination
rate, and blood pressure. At therapeutic of asymptomatic patients (emesis induction; Chronic Treatment
levels, skeletal muscle effects predominate, administration of activated charcoal) and Ventilatory support (p. 1185) may be required
but in overdose situations, pronounced supportive care to address overt signs when for several days (particularly with baclofen).
cardiovascular effects occur. they are present.
• Orphenadrine has pronounced anticholin- Nutrition/Diet
ergic and antihistaminic effects. Acute General Treatment For patients requiring long-term ventilatory
• Chlorphenesin carbamate, guaifenesin, and • Stabilize the patient. support, feeding tubes or parenteral nutrition
methocarbamol may act by blocking nerve ○ Maintain respiration: endotracheal intuba- may be considered (pp. 1107 and 1148).
impulse transmission within the brainstem, tion and positive-pressure mechanical
spinal cord, and subcortical levels of the ventilatory support may be required. Drug Interactions
brain. Some skeletal muscle relaxant effect ○ Control seizures: diazepam 0.5-1 mg/kg Use caution when administering drugs to
is due to sedation. slow IV to effect; propofol or isoflurane agitated animals.
• Carisoprodol causes skeletal muscle relaxation may be considered in cases refractory to
by depression of postsynaptic spinal reflexes. diazepam (induce, keep anesthetized 5-10 Recommended Monitoring
Some skeletal muscle relaxant effect is due minutes, then recover). Barbiturates Respiratory rate and rhythm, heart rate, blood
to sedation. should be considered a last resort because pressure, body temperature, blood gases, serum
• Chlorzoxazone and metaxalone have no their use may result in exacerbation of electrolytes, hydration status, fluid ins/outs
direct effect on skeletal muscles; muscle CNS depression; diazepam is also generally
relaxant properties are likely due to sedative useful for managing agitation. PROGNOSIS & OUTCOME
effects. • Manage cardiovascular abnormalities: most
arrhythmias and blood pressure irregularities • Signs can persist for hours to days, depending
DIAGNOSIS resolve during supportive care. Nitroprus- on the SMR involved and the dose ingested.
side constant-rate IV infusion (1-2 mcg/ • Most patients with mild to moderate signs
Diagnostic Overview kg/min [dogs] or 0.5 mcg/kg/min [cats], receiving prompt and appropriate veterinary
Diagnosis is based on history of exposure or increased incrementally q 3-5 minutes until attention have excellent prognoses.
presence of drugs in animal’s environment along blood pressure is reduced to < 180 mm • Patients exhibiting respiratory depression
with appropriate clinical signs (e.g., ataxia, Hg systolic) has been successfully used to requiring ventilatory support, seizures, or
depression, coma, vocalization). manage baclofen-induced hypertension in coma more guarded prognosis
dogs.
Differential Diagnosis • Atropine is contraindicated in the manage- PEARLS & CONSIDERATIONS
Toxicologic: ment of bradycardia due to orphenadrine
• Other CNS depressants: barbiturates, ben- or cyclobenzaprine toxicosis because it Comments
zodiazepines, opioids, avermectins, ethylene exacerbates anticholinergic effects. • Because the primary effect of these drugs is
glycol, tick paralysis, botulinum toxin, • Supportive care CNS depression, when treating animals
ionophore ingestion (dogs) ○ Fluid diuresis: for hypotension/hypovolemia showing paradoxical excitation, it is impor-
• Other causes of seizures: amphetamines, and to promote urine formation; may tant to use the lowest sedative dose necessary
ethylene glycol, metaldehyde, strychnine, enhance excretion of baclofen to relieve the stimulation to avoid overseda-
methylxanthines, zinc phosphide, tricyclic ○ Atipamezole (50 mcg/kg, give one-fourth tion after the agitation has resolved.
antidepressants, serotonergic drugs to one-third of dose IV, remainder IM) • Baclofen, cyclobenzaprine, and tizanidine
Non-toxicologic, spontaneous: or yohimbine (0.25 mg/kg slow IV) can have narrow margins of safety, and significant
• Spinal trauma, CNS trauma be helpful to reverse hypotension from (potentially life-threatening) signs can be
• Polyradiculoneuritis, tick paralysis tizanidine. seen at low doses.
• Organic brain disease (e.g., neoplasia, ○ Cyproheptadine (1.1 mg/kg PO or PR; ○ Baclofen: doses > 1 mg/kg can cause signs;
inflammation) can repeat once if needed) may be helpful doses ≥ 8 mg/kg can be fatal to dogs.
in reducing vocalization in dogs. ○ Cyclobenzaprine: doses of 0.07 mg/kg
Initial Database ○ Thermoregulation is essential, especially have resulted in clinical signs in dogs.
• CBC, serum biochemistry profile: minimal in comatose or recumbent animals. ○ Tizanidine: doses of 0.05 mg/kg have been
alterations expected from SMR (identify • Decontamination of patient (p. 1087) associated with clinical signs in dogs, with
pre-existing liver or kidney dysfunction that ○ Induction of emesis is contraindicated in hypotension occurring at doses as low as
may interfere with drug elimination) animals showing overt clinical signs. 0.08 mg/kg.
○ Orphenadrine overdose in humans ○ Induce emesis (p. 1188): because of the • Muscle relaxants methocarbamol, guaifenesin,
has been associated with hypokalemia, potential for rapid onset of clinical signs, and chlorphenesin carbamate have large
hypoglycemia, liver enzyme elevations, induction of emesis is best done under margins of safety, and overdoses rarely cause
and bleeding disorders. veterinary supervision. life-threatening problems.

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