Skeletal muscle relaxants are pharmacologic agents that induce relaxation or paralysis of skeletal muscle by interrupting neuromuscular transmission. These drugs are mainly used:
- As adjuncts in general anesthesia to facilitate endotracheal intubation and optimize surgical conditions by preventing muscle contractions.
- In ICU settings for mechanical ventilation.
- To manage spasticity from neurological disorders (central acting agents).
Mechanism of Action
Most skeletal muscle relaxants exert their effect by interfering with the transmission of acetylcholine (ACh) at nicotinic receptors on the motor end plate of skeletal muscles.
They can either:
- Block ACh receptor activation (non-depolarizing agents),
- Or mimic ACh, causing sustained depolarization (depolarizing agents),
- Or act centrally to reduce muscle tone via CNS pathways (centrally acting agents).
Classification
A. Centrally Acting Skeletal Muscle Relaxants
Used to relieve muscle spasms and spasticity due to neurological conditions (e.g., MS, cerebral palsy).
| Drug | Key Characteristics |
|---|---|
| Baclofen | GABAB agonist; reduces spinal reflexes. Used in spasticity (e.g., MS). |
| Tizanidine | α2-adrenergic agonist; reduces excitatory input to motor neurons. |
| Carisoprodol | Sedative muscle relaxant; abuse potential. |
| Methocarbamol, Chlorzoxazone, Metaxalone | General CNS depressants. |
| Diazepam | A benzodiazepine; enhances GABAA inhibition at spinal cord level. |
Baclofen is preferred in spasticity due to multiple sclerosis, while tizanidine is useful in muscle spasm-related pain.
B. Direct-Acting Skeletal Muscle Relaxants
| Drug | Mechanism | Use |
|---|---|---|
| Dantrolene Sodium | Blocks ryanodine receptor (RyR1) in sarcoplasmic reticulum, inhibiting Ca2+ release. | Treats malignant hyperthermia and neuroleptic malignant syndrome. |
Dantrolene is life-saving in cases of malignant hyperthermia (a rare but fatal reaction to succinylcholine or volatile anesthetics).
C. Neuromuscular Blocking Agents (NMBAs)
Used in surgery, ICU sedation, and rapid-sequence intubation.
1. Non-depolarizing Agents (Competitive antagonists)
These agents block ACh from binding nicotinic receptors without depolarizing the membrane.
| Agent | Duration | Notes |
|---|---|---|
| d-Tubocurarine | ~30 min | Obsolete; histamine release, ganglion blockade. |
| Pancuronium | Long | May cause tachycardia (M2 receptor blockade). |
| Vecuronium, Rocuronium | Intermediate | Fewer cardiac effects; rocuronium used for rapid intubation. |
| Atracurium | Intermediate | Organ-independent metabolism (Hofmann elimination); useful in renal/hepatic failure. |
| Mivacurium | Short | Rapidly hydrolyzed by plasma cholinesterases. |
The effects of non-depolarizing agents can be reversed using acetylcholinesterase inhibitors like neostigmine or pyridostigmine, often given with atropine or glycopyrrolate to block muscarinic side effects.
2. Depolarizing Agent
| Agent | Mechanism | Duration | Notes |
|---|---|---|---|
| Succinylcholine (Suxamethonium) | Persistent depolarization of the motor end plate → inactivation of Na⁺ channels → flaccid paralysis | Rapid onset (~60 sec), short duration (~5–10 min) | Used in rapid-sequence intubation |
- Not degraded by acetylcholinesterase, but by pseudocholinesterase in plasma.
- Initial fasciculations followed by flaccid paralysis.
Succinylcholine is contraindicated in:
- Patients with pseudocholinesterase deficiency (prolonged apnea),
- Risk of hyperkalemia (burns, crush injuries, neuromuscular diseases),
- Increased intraocular/intracranial pressure.
Adverse Effects of Neuromuscular Blockers
| Drug | Side Effects |
|---|---|
| d-Tubocurarine | Histamine release → bronchospasm, hypotension, urticaria; ganglionic blockade |
| Pancuronium | Tachycardia, hypertension due to M2 blockade |
| Succinylcholine | Hyperkalemia, malignant hyperthermia, bradycardia, increased IOP/ICP, fasciculations, myalgia |
| Atracurium | Can cause seizures (laudanosine metabolite) |
Reversal of Neuromuscular Blockade
- Non-depolarizing agents: Reversible with anticholinesterases (e.g., neostigmine + atropine/glycopyrrolate).
- Depolarizing agents (succinylcholine): Not reversed pharmacologically; effects wear off spontaneously. Prolonged effects in pseudocholinesterase deficiency.
