7.10: Skeletal Muscle Relaxants: Therapeutic Uses

Skeletal muscle relaxants are used to relax muscle tone and alleviate painful muscle contractions. However, the choice of skeletal muscle relaxants depends on the duration of the surgical procedure in order to minimize potential side effects. Skeletal muscle relaxants like neuromuscular blocking agents [NMBAs] are commonly employed as adjuvants alongside general anesthetics in clinical settings. NMBAs are also used to maintain controlled ventilation during surgery of the larynx or pharynx as well as emergency endotracheal intubation. By reducing reflex muscle contractions of the organs, they enhance the safety of surgical procedures. NMBAs additionally offer the advantage of reducing epileptic seizures and convulsions, which are often associated with the administration of local anesthetics.

In contrast, spasmolytics are skeletal muscle relaxants that primarily relieve spastic conditions such as cerebral palsy or multiple sclerosis. Centrally acting spasmolytics, including baclofen, diazepam, and tizanidine, act on the central nervous system [CNS] to provide relief from spasms. Others, like dantrolene and botulinum toxin, directly target skeletal muscle cells to reduce muscle spasms. Dantrolene is primarily useful in treating malignant hyperthermia, a potentially fatal genetic condition often triggered in susceptible patients after general anesthesia or following treatment with NMBAs such as succinylcholine. Conversely, botulinum toxin injections are routinely employed in cosmetic therapy to diminish wrinkles

Skeletal muscle relaxants are selected based on the nature and duration of the medical procedure.

For lengthy procedures like abdominal and thoracic surgery, nondepolarizing neuromuscular blocking agents or NMBAs, such as rocuronium, are administered alongside general anesthetics. NMBAs induce muscle paralysis to prevent target organs from contracting under reflex.

In contrast, succinylcholine—a depolarizing NMBA, is used for shorter procedures like laryngoscopy or endotracheal intubation.

Succinylcholine relaxes the laryngeal and pharyngeal muscles and opens airways. This facilitates mechanical ventilation while preventing the patients from choking on fluid or other materials entering the windpipe.

Additionally, both NMBA types reduce chest wall resistance to ensure good ventilation. This benefits critically ill patients suffering from pneumonia, bronchospasm, or atelectasis, where the lungs collapse, allowing only limited gaseous exchange.

Direct acting relaxants like Dantrolene and botulinum toxins treat spastic disorders like cerebral palsy and multiple sclerosis.

Dantrolene is also helpful in treating malignant hyperthermia, while Botulinum toxin injections are routinely used to reduce facial wrinkles.