Peripherally and Centrally Acting Muscle Relaxants: A Comparison

Centrally acting muscle relaxants inhibit postsynaptic transmission in the central nervous system to decrease muscle tone, while peripherally acting relaxants block neuromuscular transmission directly at the neuromuscular junction, reducing muscle contraction and causing paralysis. Centrally acting drugs have notable CNS effects, whereas peripherally acting drugs do not affect the CNS.

Centrally acting muscle relaxants are generally administered orally, with some given intravenously or intramuscularly. They treat chronic back pain, fibromyalgia, and severe spasms caused by multiple sclerosis, cerebral palsy, stroke, and spinal cord injuries. Their duration of action typically ranges from 5 to 24 hours depending on the specific drug.

Centrally acting muscle relaxants are divided into spasmolytic and antispasmodic drugs. Spasmolytic drugs like baclofen, diazepam, and tizanidine have a duration of 5 to 24 hours. Antispasmodic drugs like cyclobenzaprine have shorter durations of 4 to 6 hours and produce strong antimuscarinic effects such as constipation and urinary retention.

Peripherally acting relaxants block neuromuscular transmission at the neuromuscular junction, reducing muscle contraction and inducing temporary paralysis without affecting the CNS. They are administered intravenously to relax muscles during surgeries or short medical procedures. These drugs are given alongside anesthetics to prolong muscle relaxation during operations.

Peripherally acting drugs are classified into neuromuscular blockers and directly acting agents. Neuromuscular blockers like succinylcholine and rocuronium are administered intravenously with durations of 5 to 60 minutes. Directly acting relaxants like dantrolene and botulinum toxin are given intramuscularly or orally with durations ranging from 4 hours to 2 months.

Centrally acting muscle relaxants may cause sedation, weakness, and hypotension. Upon discontinuing the medication, patients may experience rebound spasticity. Antispasmodic drugs specifically produce antimuscarinic effects including constipation, bradycardia, and urinary retention, which can limit their use in certain patient populations.

Peripherally acting muscle relaxants can cause hypotension, prolonged apnea, increased intraocular pressure, and postoperative muscle pain. Directly acting agents like dantrolene and botulinum toxin weaken muscle contraction, leading to flaccid paralysis. These side effects require careful monitoring during and after administration.