McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 2 5 Muscle relaxants

TABLE 25.1

DRUGS IN FOCUS Centrally-acting skeletal muscle relaxants

Drug name

Dosage/route

Usual indications

Adults 15 mg PO daily. Via intrathecal infusion pump, dosage individualised according to response Paediatric: intrathecal infusion pump, 24–1199 mcg/day—base dose on child’s response 2–10 mg deep IM or slow IV injection q 3 hours–q 4 hours, 10–60 mg PO daily 100 mg PO, a.m. and at bedtime, or 60 mg IV or IM q 12 hours

Treatment of muscle spasticity associated with neuromuscular diseases such as multiple sclerosis, muscle rigidity and spinal cord injuries

baclofen (Lioresal)

diazepam (Valium)

Treatment of muscle spasm or spasticity, including cerebral palsy Relief of discomfort of acute musculoskeletal conditions in adults; under investigation for relief of quinidine- induced leg cramps

orphenadrine (Norflex)

be anticipated with their use. See Table 25.1 for usual indications for each of these agents. Pharmacokinetics Baclofen is available in oral and intrathecal forms and can be administered via a delivery pump for the treat- ment of central spasticity. Baclofen is not metabolised but, like the other skeletal muscle relaxants, it is excreted in the urine. Contraindications and cautions Centrally-acting skeletal muscle relaxants are contra­ indicated in the presence of any known allergy to any of these drugs and with skeletal muscle spasms resulting from rheumatic disorders. In addition, baclofen should not be used to treat any spasticity that contributes to locomotion, upright position or increased function. Blocking this spasticity results in loss of these functions. All centrally-acting skeletal muscle relaxants should be used cautiously in the following circumstances: with a history of epilepsy because the CNS depression and imbalance caused by these drugs may exacerbate the seizure disorder ; with cardiac dysfunction because muscle function may be depressed ; with any condition marked by muscle weakness, which the drugs could make much worse ; and with hepatic or renal dysfunc- tion, which could interfere with the metabolism and excretion of the drugs, leading to toxic levels. No good studies exist regarding the effects of these agents during pregnancy and breastfeeding; therefore, use should be limited to those situations in which the benefit to the mother clearly outweighs any potential risk to the fetus or neonate. Adverse effects The most frequently seen adverse effects associated with these drugs relate to the associated CNS depression:

drowsiness, fatigue, weakness, confusion, headache and insomnia. Gastrointestinal (GI) disturbances, which may be linked to CNS depression of the parasympa- thetic reflexes, include nausea, dry mouth, anorexia and constipation. In addition, hypotension and arrhythmias may occur, again as a result of depression of normal reflex arcs. Urinary frequency, enuresis and feelings of urinary urgency reportedly may occur. Clinically important drug–drug interactions If any of the centrally-acting skeletal muscle relaxants are taken with other CNS depressants or alcohol, CNS depression may increase. People should be cautioned to avoid alcohol while taking these muscle relaxants; if this combination cannot be avoided, they should take extreme precautions. Prototype summary: Baclofen Indications: Alleviation of signs and symptoms of spasticity; may be of use in spinal cord injuries or spinal cord diseases. Actions: Gamma-aminobutyric acid (GABA)

analogue; exact mechanism of action is not understood; inhibits monosynaptic and polysynaptic spinal reflexes; CNS depressant. Pharmacokinetics: Route Onset Peak

Duration 4–8 hours 4–8 hours

Oral

1 hours

2 hours

Intrathecal 30–60 mins 4 hours

T 1/2 : 3 to 4 hours; not metabolised; excreted in the urine. Adverse effects: Transient drowsiness, dizziness, weakness, fatigue, constipation, headache, insomnia, hypotension, nausea, urinary frequency.