Methocarbamol
**Methocarbamol** is a centrally acting skeletal muscle relaxant structurally related to guaifenesin. It is widely used in veterinary medicine to treat acute inflammatory and traumatic conditions of skeletal muscle, as well as to manage severe muscle spasms and tremors. **Clinical Pearls:** * Highly effective and frequently used as a life-saving intervention for **permethrin toxicity in cats** and **tremorgenic mycotoxin** (e.g., moldy food) ingestions in dogs. * Available in both oral and injectable formulations. * The injectable formulation contains polyethylene glycol (PEG) 300, which can exacerbate pre-existing renal pathology; therefore, the IV form should be used with caution or avoided in patients with significant kidney disease. * Does not directly relax contracted skeletal muscles but works via central nervous system depression.
Mecanismo: The exact mechanism of action of methocarbamol is not fully understood, but it is known to act centrally rather than peripherally. * **Central CNS Depression:** It is believed to cause skeletal muscle relaxation by general central nervous system depression, likely depressing polysynaptic reflexes in the **internuncial neurons** of the spinal cord. * **No Peripheral Action:** It has **no direct relaxant effects** on striated muscle, nerve fibers, or the motor endplate. * **Secondary Effects:** Produces a secondary sedative effect due to its generalized CNS depressant properties.
Dosificación por especie
- Relief of moderate conditions · 44 mg/kg IV · IV · As directed · Administer half the estimated dose rapidly, then wait until animal starts to relax and continue administration to effect.
- Controlling severe effects of strychnine and tetanus · 55-220 mg/kg IV · IV · As directed · Do not exceed 330 mg/kg/day. Administer half the estimated dose rapidly, then wait until animal starts to relax and continue administration to effect.
- General muscle relaxation (Tablets) · Initially, 132 mg/kg/day PO divided q8h-q12h, then 61-132 mg/kg divided q8-12h · PO · q8h-q12h · Discontinue if no response in 5 days
- Adjunctive treatment (control of seizures/muscle tremors) of permethrin toxicity · initially administered at 50-150 mg/kg IV · IV · As directed · First half is given IV slowly over approximately 5-10 minutes and the second half of the dose being given as needed to effect. The dose can be repeated up to a maximum of 330 mg/kg/day; in severely affected cases, the total daily dose can be calculated and given over 24 hours as a constant rate infusion.
- General muscle spasms, tetanus, toxicities · 20-45 mg/kg · PO · q8h · As needed · Very high doses may be required for tetanus. Recommended not to exceed 330 mg/kg. Limited literature in cats.
- Moderate conditions · 4.4-22 mg/kg IV to effect · IV · As directed · ARCI UCGFS Class 4 Drug
- Severe conditions · 22-55 mg/kg IV · IV · As directed
- General muscle relaxation · 15-25 mg/kg IV by slow infusion · IV · As directed
Vías de administración
Contraindicaciones
- Food-producing animals
- Renal disease (injectable formulation only, due to PEG 300 vehicle)
- Known hypersensitivity to methocarbamol
- Subcutaneous (SC) administration
- No specific contraindications available, but use with caution in renal or hepatic impairment.
Efectos adversos
- Sedation
- Salivation
- Emesis
- Lethargy
- Weakness
- Ataxia
Interacciones farmacológicas
- CNS Depressants (e.g., opioids, benzodiazepines, barbiturates) · Additive CNS depression may occur when given concurrently.
- Pyridostigmine · May cause severe weakness (reported in a human patient with myasthenia gravis).
- Other CNS depressants · Additive CNS depression · moderate
Monitorización
- Level of muscle relaxation
- Degree of sedation
- Respiratory rate and effort (especially with high IV doses or concurrent CNS depressants)
- Degree of muscle relaxation
- CNS depression/sedation levels
- Renal and hepatic function (if prolonged use)
Sobredosis
**Signs of Toxicity:** Overdosage is generally characterized by profound CNS depressant effects, including loss of righting reflex and prostration. Excessive doses in dogs and cats may manifest as emesis, salivation, severe weakness, and ataxia. **Management:** * **Decontamination:** If the overdose occurred recently via oral administration, emptying the gut (e.g., emesis induction or gastric lavage) may be indicated. **Do not induce emesis** if the patient's continued consciousness and airway protection are not assured. * **Supportive Care:** Treat other clinical signs supportively if severe. Monitor respiratory and cardiovascular function closely.
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