Muscle Relaxants Nursing Pharmacology Study Guide

Generic and Brand Names

Centrally acting skeletal muscle relaxants

• baclofen (Lioresal)
• carisoprodol (Soma)
• chlorzoxazone (Paraflex)
• cyclobenzaprine (Flexeril)
• methocarbamol (Robaxin)
• orphenadrine (Banflex, Flexoject)
• tizanidine (Zanaflex)

Direct acting skeletal muscle relaxants

• botulinum toxin type A (Botox)
• botulinum toxin type B (Myobloc)
• dantrolene (Dantrium)
• incobotulinumtoxin A (Xeomin)

Disease Spotlight: Neuromuscular Abnormalities

Muscle spasm. Disease, infection, toxins, and injury disrupt the normal flow of information in the CNS, producing anything from spasm to paralysis. A muscle spasm is a violent, painful involuntary contraction, usually from overstretching, a wrenched joint, or torn tendon or ligament. The injured area floods the spinal cord with sensory impulses, the cord answers with intense contraction, blood flow is cut off, lactic acid builds up, and the pain feeds another round of contraction. That vicious cycle is what these drugs interrupt.

Spasticity. Spasticity comes from damage to neurons inside the CNS rather than the periphery, which makes it permanent. The balance of excitatory and inhibitory influence is interrupted, leading to hypertonia (excessive muscle stimulation) with contractures and structural changes. Coordinated muscle activity is not lost.

Centrally Acting Muscle Relaxants

Centrally acting relaxants work in the CNS to interrupt the reflexes that drive muscle spasm, which is why they are called spasmolytics. Pair them with rest, heat, and physical therapy.

Therapeutic Action

The exact mechanism is not fully understood, but it involves upper and spinal interneurons. These drugs inhibit monosynaptic and polysynaptic spinal reflexes and act as CNS depressants.

Indications

The primary use is relief of discomfort from acute, painful musculoskeletal conditions, as an adjunct to rest, physical therapy, and other measures. They may also ease the signs and symptoms of spasticity in spinal cord injury or disease.

Children. Safety and effectiveness are not established in children. Metaxalone may be given to children older than age 12. Baclofen relieves spasticity associated with cerebral palsy. Methocarbamol is the drug of choice for children with tetanus; monitor closely for CNS and GI toxicity.

Adults. Caution patients against activities requiring alertness (such as driving), since these drugs cause confusion and drowsiness. Muscle spasm tied to anxiety can be treated with diazepam. Pregnant and lactating patients should use contraception and an alternative feeding method, respectively.

Older adults. They are more likely to have adverse effects. Note that carisoprodol is a Schedule IV controlled substance: it is metabolized to meprobamate and carries real sedation and abuse potential, so it is used cautiously, especially in older adults, who are more prone to falls and confusion (Carisoprodol: StatPearls). When a centrally acting relaxant is needed in older patients or those with hepatic or renal impairment, dosing is conservative and shorter agents are preferred.

Pharmacokinetics

Here are the characteristic interactions of centrally acting muscle relaxants and the body in terms of absorption, distribution, metabolism, and excretion:

Contraindications and Cautions

• Allergy to centrally acting skeletal muscle relaxants, to prevent hypersensitivity reactions.
• Spasm from rheumatic disorders, which does not benefit from these drugs.
• History of epilepsy. Drug-induced CNS depression and imbalance can worsen a seizure disorder.
• Cardiac dysfunction. Muscle function may be depressed.
• Conditions marked by muscle weakness, which the drugs can worsen.
• Hepatic or renal dysfunction, which impairs drug metabolism and excretion.
• Baclofen is not for spasticity that supports locomotion, upright posture, or function. Blocking it costs the patient those functions.

Adverse Effects

• CNS: depression, drowsiness, fatigue, weakness, confusion, headache, insomnia
• CV: hypotension, arrhythmias
• GI: nausea, dry mouth, anorexia, constipation
• GU: urinary frequency, enuresis, urinary urgency
• Chlorzoxazone may turn urine purple-red.
• Tizanidine is associated with liver toxicity and hypotension.
• Baclofen is tapered over 1-2 weeks to prevent psychoses and hallucinations.

Interactions

• Other CNS depressants and alcohol: increased CNS depression.

Direct-Acting Skeletal Muscle Relaxants

Direct-acting relaxants enter the muscle and block contraction at the fiber.

Therapeutic Action

Dantrolene acts inside skeletal muscle fibers, interfering with calcium ion release from the muscle tubules so the fibers cannot contract. It does not affect neuromuscular transmission or the skeletal muscle surface membrane. This same mechanism makes intravenous dantrolene the first-line drug for malignant hyperthermia, the rare but life-threatening rise in muscle metabolism and temperature triggered by general anesthesia or succinylcholine. It is given right away in a crisis and may be used before and after surgery in susceptible patients (Dantrolene: StatPearls).

Indications

Children. Safety and effectiveness are not established in children. Dantrolene treats upper motor neuron spasticity in children; dose it by body weight and titrate up over time. Children are at higher risk of CNS and GI toxicity.

Adults. Caution patients against activities requiring alertness because of confusion and drowsiness. Pregnant and lactating patients should use contraception and an alternative feeding method. Premenopausal women carry an increased risk of hepatotoxicity with dantrolene.

Older adults. They are more likely to have adverse effects. Older women on hormone replacement therapy carry the same dantrolene hepatotoxicity risk as premenopausal women.

Pharmacokinetics

Here are the characteristic interactions of direct-acting skeletal muscle relaxants and the body in terms of absorption, distribution, metabolism, and excretion:

Contraindications and Cautions

• Allergy to direct-acting skeletal muscle relaxants, to prevent hypersensitivity reactions.
• Spasticity that supports locomotion, upright posture, or function, which is lost if the spasticity is blocked.
• Active hepatic disease, which interferes with metabolism.
• Pregnancy, given potential fetal adverse effects.
• Lactation, since the drug may cross into breast milk.
• Women and patients older than age 35, because of increased risk of potentially fatal hepatocellular disease.
• History of liver disease or prior dysfunction, which raises susceptibility to cellular toxicity.
• Respiratory depression, worsened by muscle weakness.
• Cardiac disease, given the risk of cardiac muscle depression.

Adverse Effects

• CNS: drowsiness, fatigue, weakness, confusion, headache, insomnia, visual disturbances
• GI: irritation, diarrhea, constipation, abdominal cramps
• GU: urinary frequency, enuresis, urinary urgency, crystalline urine with pain or burning on urination
• Other: acne, abnormal hair growth, rashes, photosensitivity, abnormal sweating, chills, myalgia
• Dantrolene can cause direct hepatocellular damage and potentially fatal hepatitis.
• Botulinum toxins are associated with anaphylactic reactions: headache, dizziness, muscle pain, paralysis.
• Botulinum toxins carry an FDA boxed warning for distant spread of the toxin's effect away from the injection site. Hours to weeks after injection, this can cause swallowing and breathing trouble that may be life-threatening, with the greatest risk in children treated for spasticity. Report any new difficulty swallowing, speaking, or breathing right away (BOTOX FDA Prescribing Information).

Interactions

• Estrogens: increased hepatocellular toxicity with dantrolene.
• Neuromuscular junction blockers, lincosamides, quinidine, magnesium sulphate, anticholinesterase, succinylcholine, polymyxin, aminoglycosides: increased risk of additive effects.

Nursing Considerations

Assessment, diagnoses, and evaluation are the same for both classes. The implementation differs, so it is split out below.

Nursing Assessment

• Assess for the contraindications and cautions above (drug allergy, cardiac depression, rheumatic disorder, pregnancy, lactation) to prevent complications.
• Do a full physical: temperature, skin color and lesions, CNS orientation, affect, reflexes, bilateral grip strength, spasticity, bowel sounds, and urine output for baseline data.
• Monitor liver and renal function tests to catch adverse effects early.

Nursing Diagnoses

• Acute pain related to GI and CNS effects
• Disturbed thought processes related to CNS effects
• Risk for injury related to CNS effects

Implementation: Centrally Acting Relaxants

• Add rest periods, heat, ordered NSAIDs, and positioning to boost musculoskeletal pain relief.
• Discontinue at any sign of liver or renal dysfunction to prevent severe toxicity.
• Monitor respiratory status and arrange dose adjustment or discontinuation as needed.
• Provide comfort measures and safety measures (adequate lighting, raised side rails) to prevent injury.
• Educate the patient on the drug to support understanding and compliance.

Implementation: Direct-Acting Relaxants

• Assess the area before botulinum toxin injection; active infection there will be made worse by the injection.
• Monitor dantrolene IV sites for extravasation. The drug is alkaline and very irritating to tissue.
• Periodically discontinue dantrolene for 2-4 days as ordered to gauge therapeutic effect.
• Discontinue at any sign of liver dysfunction.
• Provide comfort measures and safety measures (adequate lighting, raised side rails) to prevent injury.
• Educate the patient on the drug to support understanding and compliance.

Evaluation

• Monitor response to therapy: less spasm and pain, improved spasticity, movement, and activity.
• Monitor for adverse effects (CNS changes, GI depression, diarrhea, liver toxicity, urinary urgency).
• Confirm understanding by having the patient name the drug, its indication, and the adverse effects to watch for.
• Monitor compliance.

Frequently Asked Questions

What is the difference between centrally acting and direct-acting muscle relaxants?

Centrally acting relaxants (such as cyclobenzaprine, baclofen, and tizanidine) work in the brain and spinal cord to dampen the reflexes that drive muscle spasm, which is why they are called spasmolytics. Direct-acting relaxants (dantrolene) work inside the muscle fiber itself, blocking calcium release so the fiber cannot contract.

Why is dantrolene the drug used for malignant hyperthermia?

Malignant hyperthermia is a runaway rise in muscle metabolism and temperature triggered by general anesthesia or succinylcholine, caused by uncontrolled calcium release in muscle. Dantrolene works exactly there, interfering with calcium release from the sarcoplasmic reticulum, so intravenous dantrolene is the first-line treatment given immediately in a crisis (Dantrolene: StatPearls).

Is carisoprodol (Soma) a controlled substance?

Yes. Carisoprodol is a Schedule IV controlled substance. It breaks down into meprobamate and carries sedation and abuse potential, so it is prescribed cautiously, used short term, and generally avoided in older adults (Carisoprodol: StatPearls).

What is the most important teaching point for any muscle relaxant?

These drugs are CNS depressants that cause drowsiness, dizziness, and confusion, so patients are warned not to drive or operate machinery until they know how the drug affects them, and to avoid alcohol and other CNS depressants, which intensify the sedation.

Why is baclofen tapered instead of stopped abruptly?

Stopping baclofen suddenly, especially intrathecal baclofen, can cause withdrawal with hallucinations, psychosis, high fever, rebound spasticity, and seizures. It is tapered over 1 to 2 weeks to avoid this.

What is the boxed warning on botulinum toxin?

Botulinum toxin products carry an FDA boxed warning for distant spread of the toxin's effect beyond the injection site, which can cause life-threatening swallowing and breathing difficulty hours to weeks later. The risk is greatest in children treated for spasticity, and any new trouble swallowing, speaking, or breathing is reported immediately (BOTOX FDA Prescribing Information).