Skeletal muscle relaxants

This topic falls into one of the most confusing parts in pharmacology, the names,duration of action etc can be confusing.

Some of the names we come across very often are:

  • Succinyl choline(Analogue of acetyl choline,acts on nicotinic Nm receptors)
  • Pancuronium
  • Rocuronium
  • Mivacurium
  • Atracurium

These names are similar so learning and memorizing their properties can be a bit difficult.But finding some peculiarities in these names will help you.

Succinyl choline

Pancuronium(Steroidal)

Atracurium(d-Tubocurarine group)

Classification

Skeletal muscle relaxants are classified into:

  1. Centrally acting
  2. Peripherally acting

Peripherally acting again subdivided into:

1.Directly acting

2.Acting on neuromuscular junction

Drugs acting on NMJ are again classified into:

1.Depolarizing

2.NonĀ  depolarizing

Depolarizing:Succinyl choline

  1. Also called suxamethonium
  2. Only depolarizing skeletal muscle relaxant
  3. Fastest onset of action
  4. Shortest duration of action
  5. Trigger malignant hyperthermia when used with halothane
  6. Sch can stimulate autonomic ganglia
  7. Cause hyperkalemia in patients with nerve and muscular disorders so CI in nerve disorders(Paraplegia,hemiplegia,GBS) and muscular disorders(muscular dystrophy,Myasthenia,crush injury,burns,rhabdomyolysis)
  8. Increases all pressures so CI in glaucoma,head injury,increase BP, nausea and vomiting due to intragastric pressure.

Non depolarizing

1. Steroidal

  1. Pancuronium(Longest acting among the curonium group, have the longest SPANcuronium)
  2. Rapacuronium(RAPIDcuronium,fastest acting but withdrawn from market)
  3. Rocuronium(Replaced rapacuronium and came to the front row as ROWcuronium,fastest acting non depolarizing in the market)
  4. Vecuronium(For the Weakhearted, WEAKuronium better cardiovascular stability but CI in heptic disease and biliary obstruction.
  5. Pipercuronium

Benzylquinoline derivatives

  • Release histamine largest with d-TC and minimum with cis-atracurium
  • Block autonomic ganglia max with d-TC and metocurine.

d-TC cause hypotension and broncho constriction.

Doxacurium( DEXAcurium is longest acting and most potent SMR.)

Mivacurium(Shortest acting non depolarizing SMR due to metabolism by esterase)

Atracurium and cis atracurium metabolized by hoffman elimination and are DOC’s in hepatic and renal insufficiency.

Atracurium metabolized to laudanosine—>seizures

3.Gallamine- Least potent SMR,nephrotoxic and teratogenic

Directly acting SMR

Dantrolene and quinine

Dantrolene inhibits release of Ca2+ from sarcoplasmic reticulum by inhibition of ryanodine receptors.

DOC in malignant hyperthermia also useful in neuroleptic malignant syndrome.

Adverse effects are hepatitis and muscle weakness

Quinine can be used in nocturnal leg cramps.

Centrally acting SMR

They act on CNS and can cause sedation.

Mephenesin group includes carisprodol,chlorzoxazone,chlormezanone,methocarbamol selectively inhibit polysynaptic reflexes.

Benzodiazepines like diazepam and clonazepam inhibit both poly and monosynaptic reflexes.Useful in muscle spasms

Baclofen is a GABAb agonist which increase K+ conductance thus inhibiting mono and polysynaptic reflexes.MS and spinal injuries

Tizanidine is centrally acting alpha2 agonist with no effect on BP, used in MS,ALS and spinal injuries

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