7.12: Peripherally and Centrally Acting Muscle Relaxants: A Comparison
Skeletal muscle relaxants can target the central nervous system [CNS] to reduce muscle tension or act directly at the neuromuscular junction to induce temporary paralysis. These two classes of muscle relaxants are called centrally acting muscle relaxants and peripherally acting muscle relaxants. They differ in their action, mechanism, administration route, and clinical uses.
Centrally acting muscle relaxants can be further divided into spasmolytic and antispasmodic drugs. Spasmolytic drugs, such as baclofen, diazepam, and tizanidine, primarily act on the CNS. Most of these drugs are administered orally, although some may be given intramuscularly or intravenously. The duration of action of these drugs typically ranges from 5 to 24 hours. Spasmolytic drugs are commonly used to manage severe and chronic spasms caused by conditions such as multiple sclerosis, cerebral palsy, stroke, spinal cord injuries, and muscle injuries. However, they may have side effects such as sedation, weakness, hypotension, and the potential for rebound spasticity upon discontinuing the medication. Antispasmodics have a shorter duration of action, typically lasting from 4 to 6 hours. They can produce strong antimuscarinic effects such as constipation, bradycardia, and urinary retention. An example of an antispasmodic drug is cyclobenzaprine, which is primarily given orally to treat acute spasms resulting from muscle injuries.
Peripherally acting drugs are also classified into neuromuscular blockers and directly acting agents. Succinylcholine, d-tubocurarine, cisatracurium, and rocuronium are some examples of neuromuscular blockers. These drugs are often administered intravenously. Their duration of action ranges from 5 to 60 minutes. These are given along with anesthetics to prolong muscle relaxation during surgeries. They can cause side effects like hypotension, prolonged apnea, intraocular pressure, and postoperative muscle pain. Dantrolene and botulinum toxin are examples of directly acting relaxants that are administered intramuscularly or orally. Duration of action ranges from 4 hours to 2 months. These medications weaken muscle contraction, leading to flaccid paralysis, and effectively treat spasms caused by conditions such as cerebral palsy, multiple sclerosis, and overactive bladder
