Other skeletal muscle relaxants
Two other drugs used as skeletal muscle relaxants are baclofen and diazepam. Diazepam, however, is primarily an antianxiety drug. (See Diazepam as a skeletal muscle relaxant)
Baclofen and diazepam are absorbed rapidly from the GI tract. Ba-clofen is distributed widely (with only small amounts crossing the blood-brain barrier), undergoes minimal liver metabolism, and is excreted primarily unchanged in urine.
Diazepam is metabolized in the liver and mostly excreted in the urine, with a small amount excreted in the feces.
It can take from hours to weeks before the patient notices the beneficial effects of baclofen. The elimination half-life of baclofen is 21⁄2 to 4 hours. Abrupt withdrawal of the drug can cause halluci-nations, seizures, and worsening of spasticity.
It isn’t known exactly how baclofen or diazepam works. Diazepam probably depresses the CNS at the limbic and subcortical levels of the brain. It suppresses the spread of seizure activity in the cortex, thalamus, and limbic areas.
Baclofen is chemically similar to the neurotransmitter gamma-aminobutyric acid (GABA) and probably acts in the spinal cord. It reduces nerve impulses from the spinal cord to skeletal muscle, decreasing the number and severity of muscle spasms and reduc-ing associated pain.
Because baclofen produces less sedation than diazepam and less peripheral muscle weakness than dantrolene, it’s the drug of choice to treat spasticity.
Baclofen’s primary clinical use is for paraplegic or quadriplegic patients with spinal cord lesions, most commonly caused by MS or trauma. For these patients, baclofen significantly reduces the number and severity of painful flexor spasms. However, it doesn’t improve stiff gait, manual dexterity, or residual muscle function. Baclofen may be administered intrathecally for patients who
are unresponsive to oral baclofen or who experience intolerable adverse reactions. After a positive response to a bolus dose, an implantable port for chronic therapy is inserted.
Diazepam relieves anxiety, muscle spasms, and seizures, and it induces calmness and sleep.
Baclofen has few drug interactions:
§ The most significant drug interaction is an increase in CNS de-pression when baclofen is administered with other CNS depres-sants, including alcohol.
§ Analgesia can be prolonged when fentanyl and baclofen are ad-ministered together.
§ Lithium carbonate and baclofen taken together can aggravate hyperkinesia (an abnormal increase in motor function or activity).
§ Tricyclic antidepressants and baclofen taken together can in-crease muscle relaxation.
§ Baclofen may increase blood glucose levels, resulting in the need for increased doses of oral antidiabetic agents and insulin.
§ Intrathecal baclofen shouldn’t be discontinued abruptly be-cause doing so has resulted in high fever, altered mental status, exaggerated rebound spasticity, and muscle rigidity that, in rare cases, has progressed to rhabdomyolysis, multiple organ system failure, and death. (See Adverse reactions to baclofen.)