9 MUSCLE RELAXANTS.pdf

Skeletal Muscle Relaxants Dr. Anuroop Singhai Oral Surgery Division Skeletal Muscle Relaxants Introduction Skeletal muscle relaxants are drugs that act peripherally at neuromuscular junction/ muscle fibre itself or centrally in the cerebrospinal axis to reduce muscle tone and/or cause paralysis. Cla...

Skeletal Muscle Relaxants Dr. Anuroop Singhai Oral Surgery Division Skeletal Muscle Relaxants Introduction Skeletal muscle relaxants are drugs that act peripherally at neuromuscular junction/ muscle fibre itself or centrally in the cerebrospinal axis to reduce muscle tone and/or cause paralysis. Classification Skeletal Muscle Relaxants Peripherally acting Neuromuscular blocking agents Nondepolarizing blockers Long acting D-Tubocurarine Intermediate acting Vecuronium Atracurium Short acting Mivacurium Depolarizing blockers Succinyl choline Centrally acting Directly acting agents Mephenesin congeners Dantrolene sodium Chlorzoxazone Gaba mimetic Baclofen Thiocolchicoside Benzodiazepines Diazepam Central a2 agonist Tizanidine Neuromuscular blocking agents - Mechanism of Action The site of action of both competitive and depolarizing blockers is the endplate of skeletal muscle fibres. Competitive block (Nondepolarizing block): This is produced by curare and related drugs. The competitive blockers have affinity for the nicotinic (NM) cholinergic receptors at the muscle endplate but have no intrinsic activity. Depolarizing block: Decamethonium and SCh have affinity as well as submaximal intrinsic activity at the NM cholinoceptors. They depolarize muscle endplates by opening Na+ channels (just as ACh does) and initially produce twitching and fasciculations. Neuromuscular blocking agents - Uses 1. The most important use of neuromuscular blockers is as adjuvants to general anaesthesia; adequate muscle relaxation can be achieved at lighter planes. They are specially valuable in abdominal and thoracic surgery. In dentistry they may be needed for setting of mandibular fractures. 2. Assisted ventilation of critically ill patients in intensive care units can be facilitated by continuous infusion of a competitive neuromuscular blocker which reduces chest wall resistance to inflation; vecuronium is generally selected. 3. Convulsions and trauma from electroconvulsive therapy can be avoided by the use of muscle relaxants. 4. Severe cases of tetanus and status epilepticus, may be paralysed by a neuromuscular blocker (repeated doses of a competitive blocker) and maintained on intermittent positive pressure respiration. Neuromuscular blocking agents - Toxicity 1. 2. 3. 4. 5. 6. Respiratory paralysis and prolonged apnoea is the most important problem. Flushing can occasionally occur with atracurium and mivacurium. Fall in BP and cardiovascular collapse can occur, especially in hypovolemic patients. Cardiac arrhythmias and even arrest have occurred, especially with SCh. Precipitation of asthma with histamine releasing neuromuscular blockers. Postoperative muscle soreness after SCh. Centrally acting muscle relaxants These are drugs which reduce skeletal muscle tone by a selective action in the cerebrospinal axis, without altering consciousness. Comparative features of centrally acting and peripherally acting muscle relaxants S. No. Centrally acting Peripherally acting 1 Decrease muscle tone without reducing voluntary power Cause muscle paralysis, voluntary movements lost 2 Selectively inhibit polysynaptic reflexes in CNS Block neuromuscular transmission 3 Cause some CNS depression No effect on CNS 4 Given orally, sometimes parenterally Practically always given i.v. 5 Used in chronic spastic conditions, acute muscle Used for short-term purposes (surgical spasms, tetanus operations) Thiocolchicoside It is believed to be a GABA mimetic and glycinergic muscle relaxant. Uses: Combined with NSAIDs, it is being used for painful muscle spasms, Sprains, Torticollis, Backache Tizanidine This recently introduced clonidine congener is a central a2 adrenergic agonist—inhibits release of excitatory amino acids in spinal interneurones. Facilitation of inhibitory transmitter glycine has also been demonstrated. It inhibits polysynaptic reflexes; reduces muscle tone and frequency of muscle spasms without reducing muscle strength. Uses: 1. It is indicated in spasticity due to neurological disorders. 2. In painful muscle spasms of spinal origin. Tizanidine Side effects: 1. Dry mouth, 2. Drowsiness, 3. Night-time insomnia 4. Hallucinations 5. Dose-dependent elevation of liver test values has been noted. Centrally acting muscle relaxants - Uses 1. Acute muscle spasms: Overstretching of a muscle, sprain, tearing of ligaments and tendons, dislocation, fibrositis, bursitis, etc. cause painful spasm of muscles. The mephenesin like and BZD muscle relaxants are often combined with analgesics. They may help to relieve trismus occurring after a dental procedure. However, efficacy of these drugs is modest or uncertain. 2. Torticollis, lumbago, backache, neuralgias respond in the same way as acute muscle spasms. 3. Anxiety and tension is often associated with increased tone of muscles and bruxism (involuntary grinding of teeth during sleep; mostly stress related). Diazepam like drugs may afford some relief. Centrally acting muscle relaxants - Uses 4. Spastic neurological diseases like hemiplegia, paraplegia, spinal injuries, multiple sclerosis, and cerebral palsy are somewhat benefited by baclofen, diazepam, tizanidine and dantrolene. 5. Tetanus: Most commonly diazepam is infused i.v. and the dose is titrated by the response. Methocarbamol is an alternative. 6. Electroconvulsive therapy: Diazepam may be used to suppress convulsions. 7. Orthopaedic manipulations may be performed under the influence of diazepam or methocarbamol given i.v. Thank You