McKenna's Pharmacology for Nursing, 2e

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P A R T 4  Drugs acting on the central and peripheral nervous systems

M any injuries and accidents result in local damage to muscles or the skeletal anchors of muscles. These injuries may lead to muscle spasm and pain, which may be of long duration and may interfere with normal function- ing. Damage to central nervous system (CNS) neurons may cause a permanent state of muscle spasticity —sus- tained muscle contractions—as a result of loss of nerves that help to maintain balance in controlling muscle activity. Neuron damage, whether temporary or perma- nent, may be treated with skeletal muscle relaxants. Most skeletal muscle relaxants work in the brain and spinal cord, where they interfere with the cycle of muscle spasm and pain. However, the botulinum toxins and dantrolene enter muscle fibres directly. See Box 25.1 for discussion of the use of these muscle relaxants in various age groups. NERVES AND MOVEMENT Posture, balance and movement are the result of a con- stantly fluctuating sequence of muscle contraction and relaxation. The nerves that regulate these actions are the spinal motor neurons. These neurons are influenced by higher-level brain activity in the lower areas of the brain, the cerebellum (associated with conscious muscle CHILDREN The safety and effectiveness of most of these drugs have not been established in children. Agents have been used, with adjustments to the adult dosage based on the child’s age and weight. Baclofen is often used to relieve the muscle spasticity associated with cerebral palsy. A caregiver needs intensive education in the use of the intrathecal infusion pump and how to monitor the child for therapeutic as well as adverse effects. Dantrolene is used to treat upper motor neuron spasticity in children.The dosage is based on body weight and increases over time.The child should be screened regularly for CNS and gastrointestinal (including hepatic) Adults being treated for acute musculoskeletal pain should be cautioned to avoid driving and to take safety precautions against injury because of the related CNS effects, including dizziness and drowsiness. Adults complaining of muscle spasm pain that may be related to anxiety often respond very effectively to diazepam, which is a muscle relaxant and anxiolytic. toxicity. ADULTS BOX 25.1 Skeletal muscle relaxants

movements) and basal ganglia (associated with uncon- scious muscle movements). This brain activity provides coordination of contractions, and the cerebral cortex allows conscious thought to regulate movement. Nutritional: Muscle contraction Spinal reflexes The spinal reflexes are the simplest nerve pathways that monitor movement and posture (see Figure 25.1). Spinal reflexes can be simple, involving an incoming sensory neuron and an outgoing motor neuron, or more complex, involving interneurons that communicate with the related centres in the brain. Simple reflex arcs involve sensory receptors in the periphery and spinal motor nerves. Such reflex arcs make up what is known as the spindle gamma loop system ; they respond to stretch receptors or spindles on muscle fibres to cause a muscle fibre contraction that relieves the stretch. In this system, nerves from stretch receptors form a synapse with gamma nerves in the spinal cord, which send an impulse to the stretched muscle fibres to stimulate their contraction. These reflexes are responsible for maintain- ing muscle tone and keeping an upright position against the pull of gravity and are important in helping venous return when the contracting muscle fibres massage veins PREGNANCY AND BREASTFEEDING Women of childbearing age should be advised to use contraception when they are taking these drugs. If a pregnancy does occur, or is desired, they need counselling about the potential for adverse effects. Women who are breastfeeding should be encouraged to find another method of feeding the baby because of the potential for adverse drug effects on the baby. Premenopausal women are also at increased risk for the hepatotoxicity associated with dantrolene and should be monitored very closely for any change in hepatic function and given written information about the prodrome syndrome that often occurs with hepatic toxicity. OLDER ADULTS Older people are more likely to experience the adverse effects associated with these drugs—CNS, gastrointestinal and cardiovascular. Because older people often also have renal or hepatic impairment, they are also more likely to have toxic levels of the drug related to changes in metabolism and excretion. If dantrolene is required for an older person, lower doses and more frequent monitoring are needed to assess for potential cardiac, respiratory and liver toxicity. Older women who are receiving hormone replacement therapy are at the same risk for development of hepatotoxicity as premenopausal women and should be monitored accordingly.

Drug therapy across the lifespan