Muscle Relaxants
Muscle relaxants are comprised of a group of heterogeneous medications that work through various mechanisms to act as antispasmodics.
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Muscle Relaxants Robert M. Chow and Mohammed Issa
Muscle Relaxants Muscle relaxants are comprised of a group of heterogeneous medications that work through various mechanisms to act as antispasmodics.
Mechanism of Action As previously stated, muscle relaxants have varied mechanisms of action. Cyclobenzaprine (C), though heavily studied in terms of its use for muscle spasms, does not have a clear mechanism of action. In addition, metaxalone (M) also has not had its mechanism of action elucidated. Tizanidine (T) is an α2 adrenergic agonist, and baclofen (B) is a GABAB receptor agonist.
Examples Generic Cyclobenzaprine Metaxalone Tizanidine Baclofen
Brand name Flexeril Skelaxin Zanaflex Gablofen
Dose 5 mg TID 800 mg TID 2 mg daily 5 mg TID
Side effects: Common Dizziness (C,M,T) Drowsiness (B,C,M,T) Tachycardia (C) Weakness (B,T) Hypotension (T) Xerostomia (T)
Rare Hepatotoxicity (T) NMS (C) Arrhythmia (C) Seizures (C,M) Impotence (B) Angioneurotic edema (M)
Drug Interactions R.M. Chow, MD (*) Department of Anesthesiology, University of Maryland Medical Center, 22 South Greene ST, S11C13, Baltimore, MD 21201, USA e-mail: [email protected] M. Issa, MD Department of Anesthesiology and Psychiatry, Brigham and Women’s Hospital, 75 Francis Street, Boston, MA 02115, USA e-mail: [email protected]
Cyclobenzaprine has TCA-like qualities and antagonizes serotonin, histamine and muscarinic receptors. Therefore, cyclobenzaprine should be used with caution with other CNS depressants as well as with TCAs. In addition cyclobenzaprine should not be taken concomitantly with MAOIs or serotonergic drugs. Metaxalone is metabolized by the CYP 450 system, and caution is advised
© Springer International Publishing Switzerland 2017 R.J. Yong et al. (eds.), Pain Medicine, DOI 10.1007/978-3-319-43133-8_48
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when co-administering medications for liver metabolism.
Clinical Clinical Indications
Special Considerations • Baclofen should be avoided or reduced in renal failure patients, and with ESRD, baclofeninduced encephalopathies have been reported. • Tizanidine is potentially hepatotoxic and should be used cautiously in patients with impaired hepatic function.
Spasticity, musculoskeletal conditions
Clinical Pearls Initiation: Using Cyclobenzaprine Typical starting dose is 5 mg TID. Muscle relaxants are recommended only for short-term therapy when treating musculoskeletal conditions.
Weaning When using cyclobenzaprine 5 mg TID, no weaning is needed.
• Cyclobenzaprine 5 mg TID is as effective as cyclobenzaprine 10 mg TID, but with fewer side effects. • Baclofen should not be abruptly discontinued as this can lead to baclofen withdrawal. • Tizanidine can be used to decrease withdrawal symptoms from patients weaning off opioids.
Suggested Reading Chou R, et al. Drug class review on skeletal muscle relaxants. Oregon Health & Sciences University Review, Portland: Oregon Health & Sciences University; 2005.
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