McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 2 5 Muscle relaxants

TABLE 25.2

DRUGS IN FOCUS Direct-acting skeletal muscle relaxants

Drug name

Dosage/route

Usual indications

botulinum toxin type A (Botox, Dysport)

Adult: 20 units (0.5 mL solution) injected as divided doses of 0.1 mL (four units) into each of five sites (two in each corrugator muscle and one in the procerus muscle) repeated every 3–4 months; local injection associated with particular disorder—see manufacturer’s guidelines

Improvement of appearance in glabellar (frown) lines associated with corrugator or procerus muscle activity in adults; treatment of cervical dystonia; approved in 2004 for treatment of strabismus and blepharospasm associated with dystonia in people ≥12 years of age; treatment of severe primary axillary hyperhidrosis (sweating) when injected into the axillary area Management of upper motor neuron– associated muscle spasticity such as spinal cord injury, myasthenia gravis, cerebral palsy, multiple sclerosis, muscular dystrophy, polio, tetanus, quadriplegia and amyotrophic lateral sclerosis (ALS); prevention or treatment of malignant hyperthermia—a state of intense muscle contraction prophylaxis in susceptible people who must undergo anaesthesia and after acute episodes to prevent recurrence

Adult: initially 25 mg PO; increase based on spinal cord injuries; prevention and management of response to a maximum 400 mg/day for spasticity Paediatric: initially 0.5 mg/kg per day PO b.d., titrate to a maximum 100 mg PO q.i.d. for spasticity

dantrolene (Dantrium)

not used for the treatment of muscle spasms associated with musculoskeletal injury or rheumatic disorders. Table 25.2 presents additional information about these agents, including usual indications. Nutritional: Calcium in muscles Pharmacokinetics Dantrolene is used in oral or parenteral forms. Dantro- lene is slowly absorbed from the GI tract and metabolised in the liver with a half-life of 4 to 8 hours. Excretion is through the urine. Dantrolene crosses the placenta and was found to be embryotoxic in animal studies. Use should be reserved for those situations in which the benefit to the mother clearly outweighs the risk to the fetus. Dantrolene enters breast milk and is contra­ indicated for use during breastfeeding. Safety for use in children younger than 5 years of age has not been estab- lished; because the long-term effects are not known, careful consideration should be given to use of the drug in children. The botulinum toxins are not generally absorbed systemically, and there is no pharmacokinetic informa- tion available. Contraindications and cautions Dantrolene is contraindicated in the presence of any known allergy to the drug. It is also contraindicated in the following conditions: spasticity that contributes to locomotion, upright position or increased function, which would be lost if that spasticity were blocked ;

active hepatic disease, which might interfere with metab- olism of the drug and because of known liver toxicity ; and breastfeeding because the drug may cross into breast milk and cause adverse effects in the infant. The botulinum toxins are contraindicated in the presence of allergy to any component of the drug or with active infection at the site of the injection because injecting the drug could aggravate the infection. Caution should be used with dantrolene in the fol- lowing circumstances: in women and in all people older than 35 years because of increased risk of poten- tially fatal hepatocellular disease (Box 25.2); in people with a history of liver disease or previous dysfunction, which could make the liver more susceptible to cellular toxicity; in people with respiratory depression, which could be exacerbated by muscular weakness; in people with cardiac disease because cardiac muscle depres- sion may be a risk; and during pregnancy because of the potential for adverse effects on the fetus. Caution should be used with the botulinum toxins with any peripheral neuropathic disease; with neuromuscular dis- orders, which could be exacerbated by the effects of the drug ; with pregnancy and breastfeeding; and with any known cardiovascular disease. Adverse effects The most frequently seen adverse effects associated with dantrolene relate to drug-caused CNS depression: drowsiness, fatigue, weakness, confusion, headache and insomnia, and visual disturbances. GI disturbances may be linked to direct irritation or to alterations in smooth muscle function caused by the drug-induced calcium