July 13, 2010

MRCP 1 Question 11

A 22 year old woman complains of increasing fatigue and double vision. She has recently has a barium swallow examination because of dysphagia, but the test was normal.

On examination her reflexes, sensation and coordination are normal. She is noted to have difficulty speaking clearly. She has breathing problems which appear to improve with edrophonium chloride injections.

Which of the following tests would be most useful?
a) HLA B27
b) anti-cholinesterase antibody
c) rheumatoid factor
d) Antinuclear antibody
e) Anti-DNA

The correct answer is B


The typical features of Myasthenia Gravis are fatigability and weakness of skeletal muscles without reflex, sensory, or coordination abnormalities.

Many cases begin with involvement of ocular muscles such as diplopia and ptosis. Weakness may remain confined to ocular muscles for long periods or involve bulbar muscles, which control chewing, swallowing, or articulation.

The disorder remains confined to ocular muscles in up to 15 percent of patients, but becomes generalized in the majority.

Examination of neck extensors and flexors is often more sensitive in demonstrating generalized disease than the motor exam of other muscle groups.

Improvement of weakness after edrophonium chloride (Tensilon) injection is a useful sign, although false-negative and false-positive tests do occur. In recent years, however, the detection of anti-cholinesterase antibody in the serum, using a commercially available immunoprecipitation test, has significantly changed the evaluation of MG patients with a sensitivity of over 70%.

The usual AChR antibody test measures the binding of antibody to AChR labeled with radioactive alpha-bungarotoxin. The toxin itself is attached irreversibly to the ACh binding site of AChR. The antibody binding test result is positive in nearly all adults with moderately severe or severe MG, in 80% with mild generalized MG, and in 50% with ocular MG, but in only 25% of those in remission. The test is less reliable in juvenile than in adult MG.

In a few patients only antibodies that block the binding of ACh to AChR can be detected.

The antibody titer correlates only loosely with disease severity, but in individual patients a >50% decrease in titer for more than 12 months is nearly always associated with sustained clinical improvement.

Striated muscle antibodies also occur in MG patients. Their role remains unknown, but they often are associated with thymoma.

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