Skeletal Muscle Relaxants CHAPTER 9 Skeletal Muscle Relaxants

Clinical Indication A. Prevent or interrupt muscle spasm associated with Spastic diseases Spinal cord damage Overexertion of muscles B. Adjunct medication for anesthesia intubation, surgical and orthopedic procedures

Types of Muscle Relaxants Muscle Relaxants differ by the location and mechanism that muscle contraction is inhibited Centrally- within the spinal cord Peripherally- within the muscle

Paralysis Spastic – upper motor neuron lesion Can use central or peripheral relaxer Stroke Brain trauma Multiple sclerosis Spinal cord transection Flaccid – lower motor neuron lesion  atrophy

Peripherally Acting Muscle Relaxants Mechanism of Action Peripherally acting muscle relaxants interact with nicotinic-muscle (Nm) receptors to block acetylcholine (ACH) attachment on the receptors nondepolarizing blockers curare, gallamine, pancuronium OR • stimulate the receptor followed by inability of the receptor to respond to ACH depolarizing blockers succinylcholine

Neuromuscular blockers - uses Any situation where impaired relaxation interferes with function Before surgery Allows intubation Relax abdominal muscles Orthopedic procedures fibromyalgia

NMJ blockers Tubocurarin (Curare) – arrow poison Nondepolorizing-lasts 20-30 min – releases histamine – not in asthma Pancuronium (Pavulon) – nondepolorizing does not release histamine Nondepolorizing blockers – competitive – reversed by prostigmine Succinylcholine (Anectine) depolarizing – rare pseudocholinesterase deficiency

Peripherally Acting Muscle Relaxants Mechanism of Action (continued) OR The muscle relaxant may interact with the biochemical pathway within contractile proteins that make up the muscle fibers direct acting skeletal muscle relaxant dantrolene Used in MS, cerebral palsy and cord injury

Other Effects Cardiovascular blockade of ACH vagolytic increase heart rate (tachycardia) histamine release or decreased sympathetic tone vasodilation Respiratory histamine release bronchospasm increase in bronchial secretions

Cautions, Toxicity, and Drug Interactions All peripheral muscle relaxants Paralysis of respiratory muscles Succinylcholine Enhanced relaxation in Myasthenia Gravis, spinal cord lesions, ACHesterase inhibitors Malignant hyperthermia Dantrolene Hepatotoxicity elevated serum liver enzymes Peripheral and Central muscle relaxants Potential muscle relaxation- CNS depressants- alcohol, sedatives, tranquilizers

Central Skeletal Muscle Relaxants Mechanism of Action Centrally acting muscle relaxants inhibit reflexes within the spinal cord chlordiazepoxide, diazepam inhibit gamma-aminobutyric acid (GABA) receptors baclofen inhibit alpha-2-adrenergic receptors in the CNS tizanidine

Central muscle relaxents Used – chronic spasticity, overexertion, muscle trauma – sprains and strains, nervous tension Cyclobenzaprine (Flexeril) Diazepam (Valium) Methocarbamol (Robaxin) Often used with aspirin or anti-inflammatory drug

Other Effects CNS blurred vision, dizziness, lethargy, decreased mental alertness Vascular-decreased sympathetic tone vasodilation, orthostatic hypotension Dependence with prolonged use

Drug Interactions Neuromuscular blocking drugs Potentiate muscle relaxation- antibiotics, antiarrhythmic drugs, general anesthetics Potentiate potassium loss- diuretics, digitalis