Muscle Disorders and General Anaesthetics

Posted by e-Medical PPT Thursday, August 23, 2012
Muscle disease (Myopathy ) can be divided into
Myositis – inflammatory disease
Muscular dystrophy – inherited disorder with progressive weakness
Myotonia -  sustained contraction and slow relaxation
Channelopathies – disorders of ion channels within skeletal muscle cells

Myasthenia Gravis and LEMS
Myasthenia Gravis (MG) is an autoimmune condition in which IgG autoantibodies interact with the postsynaptic acetylcholine receptors (AChR) at the nicotinic neuromuscular junction (NMJ).
The AChR antibodies reduce the number of functional receptors by:
blocking attachment of ACh molecules
increasing the rate of degeneration of the receptors
complement-induced damage to the NMJ.
On average MG patients have 30% of the normal number of functional AChR

LEMS
Lambert-Eaton Myasthenic Syndrome - a rare syndrome occurring in association with small cell carcinoma of the bronchus.
LEMS results from an autoimmune attack directed against the Voltage Gated Calcium Channels (VGCC) on the presynaptic motor nerve terminal (in 98% of those with cancer)
Antibodies have been found in the majority of patients with LEMS.
3% of patients with SSLC.
Clinical features similar to MG but facial and bulbar muscles relatively spared
Distinguishing between the two diseases on EMG: Facilitation (strength improvement after exercise) is common in LEMS. Facilitation differentiates the 2 diseases only if it is noted after repeated testing of many separate muscle groups
They improve with the edrophonium test but not as markedly
Very sensitive to both depolarising and non depolarising drugs

There are other rare myasthenic syndromes including congenital

Duchenne’s Muscular Dystrophy
X linked recessive disorder, A third of cases are from a spontaneous mutation
1 in 3000 live male births
Gene located at Xp21 region of X chromosome
Gene product is the protein Dystrophni, which is a cytoskeletal muscle protein
Clinical features of proximal limb weakness occur by the age of 4 and the diseases usually causes death by 20 years.
Becker’s MD is similar but milder and presents later.

Clinical features
Difficulty running
Gower’s sign of proximal leg weakness
Pseudohypertrophy of the calves
Myocardium is affected and a cardiomyopathy occurs
Usually disabled by the disease by 10 years of age

Myotonic dystrophy
Autosomal dominant disease. A triplet repeat disorder.
1 in 20,000
Characterised by myotonia = incomplete muscle relaxation after exercise.
Defect in sodium-chloride channels in muscle membrane
Onset around 20 to 50 years
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