Skeletal Muscle Relaxants: Nursing Pharmacology Study Guide

Skeletal muscle relaxants are commonly tested on the NCLEX because they cross several body systems and carry important safety risks. This guide compares centrally and peripherally acting agents, focusing on mechanism, indications, side effects, and the nursing priorities most often tested.

Drug Classes at a Glance

Mnemonic: "Dantrolene is Direct, Others are Central."

Mechanism of Action

• Centrally acting muscle relaxants depress neuron activity in the brain or spinal cord to reduce muscle spasms.
• Baclofen is a GABA derivative that inhibits reflexes at the spinal level.
• Cyclobenzaprine is structurally related to tricyclic antidepressants and acts in the brainstem.
• Dantrolene is the only peripherally acting muscle relaxant. It works directly on skeletal muscle by blocking calcium release from the sarcoplasmic reticulum, preventing muscle contraction without affecting the CNS.

Clinical Uses

• Cyclobenzaprine — acute muscle spasms (short-term only).
• Baclofen — spasticity in Multiple Sclerosis or spinal cord injury.
• Dantrolene IV — malignant hyperthermia emergency.
• Methocarbamol — muscle spasms (may cause harmless urine color change).

Side Effects (Most Tested)

Centrally Acting Agents

• Drowsiness and sedation
• Dizziness
• Weakness and fatigue
• Cyclobenzaprine causes anticholinergic effects: dry mouth, blurred vision, constipation, urinary retention.
  • Mnemonic: "Can't see, can't pee, can't spit, can't sht."*

Dantrolene

• Drowsiness and dizziness
• Diarrhea
• Hepatotoxicity — carries a black box warning

Urine Color Changes (Harmless)

• Methocarbamol — urine turns brown, black, or green
• Chlorzoxazone — urine turns orange or red

Labs and Monitoring

• Monitor liver function tests (LFTs) before and during dantrolene therapy; stop immediately if liver impairment occurs.
• Monitor LFTs in patients on tizanidine or chlorzoxazone.
• For intrathecal baclofen pumps, ensure the patient keeps refill appointments to prevent withdrawal.
• During IV methocarbamol administration, monitor blood pressure and watch for extravasation.

Contraindications

• Cyclobenzaprine — contraindicated with MAOIs (within 14 days) and in patients with heart problems.
• Dantrolene — contraindicated in active liver disease.
• Intrathecal baclofen — must not be given IV or IM.
• IV methocarbamol — contraindicated in renal impairment.
• Use all skeletal muscle relaxants cautiously in the elderly due to fall risk.

Pregnancy Safety

• Cyclobenzaprine — considered safer in pregnancy.
• Baclofen — Category C; withdrawal can occur in the newborn if used near term.
• Dantrolene — Category C; use only for malignant hyperthermia in pregnancy.
• Methocarbamol — Category C; avoid if possible.
• Diazepam — Category D; avoid in pregnancy.

Nursing Safety Rules & High-Yield NCLEX Notes

• All centrally acting relaxants cause drowsiness and dizziness — implement fall precautions.
• Never combine these drugs with alcohol or other CNS depressants.
• Cyclobenzaprine cannot be given within 14 days of MAOIs due to serotonin syndrome risk.
• Dantrolene pearls:
  • Only direct-acting muscle relaxant.
  • Black box warning for hepatotoxicity.
  • For malignant hyperthermia: give 2.5 mg/kg IV push immediately.
  • Must be stocked wherever triggering agents are used.
• Baclofen pearls:
  • Intrathecal pump requires regular refills.
  • Never stop abruptly — withdrawal causes fever, confusion, and seizures.
• Methocarbamol pearls:
  • Urine may turn brown, black, or green — teach patients this is harmless.
  • IV form given too fast causes hypotension — give slowly.
• Carisoprodol is metabolized to meprobamate, a controlled substance with abuse potential.

Key Takeaways

• Skeletal muscle relaxants are either centrally acting (cyclobenzaprine, baclofen, methocarbamol) or peripherally acting (dantrolene only).
• Dantrolene blocks calcium release from the sarcoplasmic reticulum and is the antidote for malignant hyperthermia; it carries a black box warning for hepatotoxicity.
• Most muscle relaxants cause drowsiness and dizziness — fall precautions and no alcohol/CNS depressants.
• Never stop baclofen abruptly — withdrawal can cause fever, confusion, and seizures.
• Cyclobenzaprine has anticholinergic effects and must not be used within 14 days of MAOIs.
• Methocarbamol can harmlessly turn urine brown, black, or green; give IV slowly to prevent hypotension.