Centrally acting skeletal muscle relaxants are commonly used drugs in the management of acute musculoskeletal pain and muscle spasm. Rather than acting directly on skeletal muscle fibers, these drugs exert their effects at the central nervous system (CNS) level, particularly the brainstem and spinal cord, to reduce excessive muscle tone.
The most important drugs in this group include Cyclobenzaprine, Baclofen, and Metaxalone. These agents are frequently tested in pharmacology, nursing, MBBS, and allied health examinations, and they are widely used in clinical practice for short-term symptom relief.
Therapeutic and Pharmacologic Class
These drugs are classified as:
1. Skeletal muscle relaxantsThey are not neuromuscular blockers and do not cause paralysis. Instead, they reduce muscle spasm by depressing CNS activity.
Indications for Use
Centrally acting muscle relaxants are primarily indicated for:
- Acute painful musculoskeletal conditions
- Muscle spasm due to strain, sprain, or injury
- Adjunct therapy with rest and physical therapy
Additional and Special Uses
Some drugs in this class are also used for:
- Fibromyalgia (off-label)
- Spasticity due to multiple sclerosis or spinal cord lesions
- Trigeminal neuralgia (certain agents)
- Intrathecal use for severe spasticity (baclofen)
Mechanism of Action (Core Concept)
These drugs act within the central nervous system rather than at the neuromuscular junction.
Their key actions include:
- Reduction of tonic somatic motor activity
- Inhibition of reflexes at the spinal cord level (especially baclofen)
- Sedative effects that contribute to muscle relaxation
Many centrally acting muscle relaxants also produce CNS depression, which explains both their therapeutic effects and adverse reactions.
Cyclobenzaprine
Cyclobenzaprine is one of the most commonly prescribed muscle relaxants for acute muscle spasm.
Mechanism of Action
Cyclobenzaprine:
- Acts at the brainstem level
- Reduces tonic motor activity
- Is structurally similar to tricyclic antidepressants (TCAs)
Therapeutic Uses
Cyclobenzaprine is used in:
- Acute muscle spasm
- Painful musculoskeletal conditions
- Short-term therapy (usually ≤ 2–3 weeks)
Adverse Effects
Common side effects include:
- Drowsiness and dizziness
- Dry mouth and blurred vision (anticholinergic effects)
- Confusion, especially in elderly patients
- Cardiac arrhythmias (rare but important)
Baclofen
Baclofen is particularly important for the treatment of spasticity.
Mechanism of Action
Baclofen:
- Is a GABA-B receptor agonist
- Inhibits reflex transmission at the spinal cord
- Reduces muscle spasticity more than simple muscle spasm
Therapeutic Uses
Baclofen is indicated for:
- Spasticity due to multiple sclerosis
- Spinal cord injury or lesions
- Severe cases via intrathecal administration
Adverse Effects
Baclofen may cause:
- Drowsiness and weakness
- Dizziness
- Hypotonia
- Seizures (especially with intrathecal use or abrupt withdrawal)
Important: Baclofen should never be stopped abruptly, as sudden withdrawal can cause severe CNS symptoms.
Metaxalone
Metaxalone is a centrally acting muscle relaxant known for being less sedating than many others in this class.
Mechanism of Action
The exact mechanism is not fully understood, but it:
- Acts centrally to depress CNS activity
- Leads to muscle relaxation and pain relief
Therapeutic Uses
Metaxalone is used for:
- Acute musculoskeletal pain
- Muscle spasm when sedation must be minimized
Adverse Effects
Side effects include:
- Drowsiness
- Dizziness
- Nausea
- Less anticholinergic effect compared to cyclobenzaprine
Adverse Reactions and Side Effects (Class Effects)
Central Nervous System
- Drowsiness
- Dizziness
- Confusion
- Fatigue
- Headache
Eye, Ear, Nose, Throat (EENT)
- Dry mouth
- Blurred vision
Cardiovascular
Arrhythmias (especially with cyclobenzaprine)Gastrointestinal
- Constipation
- Dyspepsia
- Nausea
- Unpleasant taste
Genitourinary
Urinary retentionNeurological and Metabolic
- Ataxia (intrathecal baclofen)
- Hyperglycemia
- Weight gain
Nursing and Patient-Care Considerations
Patients should be taught to:
- Take medication exactly as prescribed
- Avoid alcohol and other CNS depressants
- Avoid driving or operating machinery
- Take medication with food to reduce gastric irritation
- Increase fluid and fiber intake to prevent constipation
- Maintain good oral hygiene for dry mouth
Patients must be instructed not to discontinue therapy abruptly, especially with baclofen.
Monitoring and Safety (“Make the Connection”)
Healthcare providers should:
- Monitor level of sedation and CNS depression
- Assess improvement in pain, spasm, or spasticity
- Monitor for signs of urinary retention
- Be cautious of gastric irritation with long-term use
Contraindications and Precautions
Use caution in patients with:
- Elderly age (high fall risk)
- Cardiac disease
- History of seizures
- Liver impairment (metaxalone)
Nursing Diagnosis (Exam-Relevant)
Common nursing diagnoses include:
- Risk for activity intolerance
- Disturbed physical mobility
- Readiness for enhanced self-care activities
High-Yield Exam Points
1. Centrally acting muscle relaxants work at the CNS level, not at the neuromuscular junction.
2. Cyclobenzaprine is structurally similar to TCAs.
3. Baclofen is a GABA-B agonist and is most effective for spasticity.
4. Abrupt withdrawal of baclofen can cause seizures.
5. Sedation is a class effect of most muscle relaxants.
Simple Mnemonic for Recall
“CBM Relaxes Muscles Centrally”
C – Cyclobenzaprine (TCA-like)
B – Baclofen (GABA-B, spasticity)
M – Metaxalone (less sedating)
FAQs on Centrally Acting Muscle Relaxants
Are these drugs used for long-term muscle disease?
No, most are used short-term for acute conditions, except baclofen in chronic spasticity.
Do muscle relaxants cause paralysis?
No, they reduce muscle tone via CNS depression but do not cause paralysis.
Why is baclofen special among muscle relaxants?
It directly targets spinal reflexes and is effective for neurological spasticity.
Can these drugs cause drowsiness?
Yes, CNS depression is a common class effect.
Why should alcohol be avoided?
Alcohol increases CNS depression, raising the risk of falls and respiratory suppression.
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