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epica

04/08/07 - 10:38 PM

A 45-year-old woman comes to the physician complaining of weakness in her legs for the past 3 months. She has difficulty climbing stairs and rising from a chair. She also states that she has been seeing double on vertical gaze and that her eyes appear droopy. Repetitive nerve stimulation of the femoral nerves at 1 Hertz shows action potential amplitudes in the rectus femoris muscles that are 10 percent of normal. Repetitive stimulation at 30 Hertz shows a doubling in the action potential amplitudes. Creatine kinase levels are normal. What is the most likely diagnosis?
A. Myasthenia gravis
B. Lambert-Eaton myasthenia syndrome
C. Polymyositis
D. Dermatomyositis
E. Amyotrophic lateral sclerosis

new_n_lost
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04/08/07 - 10:46 PM

B. Lambert-Eaton myasthenia syndrome

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epica

04/08/07 - 10:49 PM

Choice (B) is the correct answer. The patient is presenting with Lambert-Eaton myasthenia syndrome which is an autoimmune disease associated with auto-antibodies to voltage-gated calcium channels (VGCC). Weakness is characteristic, particularly in the proximal lower extremities. Approximately 25 percent of patients with LEMS have ocular manifestations, and many patients report dry mouth. Repetitive nerve stimulation shows that with low frequency stimulation, action potential amplitude is significantly abnormal but increases with increased stimulation frequency. Clinically, this is represented by increasing strength with sustained muscle contraction and is a distinguishing feature between LEMS and myasthenia gravis

(Choice A). In myasthenia gravis, patients get weaker with sustained muscle contraction, whereas patients with LEMS become stronger. In LEMS, the voltage-gated calcium channel autoantibodies prevent acetylcholine release into the neuromuscular junction. With sustained muscle contraction, more neurotransmitter is released which results in increased muscle contraction. Myasthenia gravis is characterized by autoantibodies to the acetylcholine receptor at the neuromuscular junction.

Polymyositis (Choice C) is characterized by symmetrical weakness in the proximal upper and lower extremities. Neck flexors may also be weak. The ocular muscles are spared. Creatine kinase levels are elevated, electromyography studies are abnormal, and muscle biopsy shows characteristic inflammation.

Dermatomyositis (Choice D) is similar to polymyositis but with dermatological manifestations.

Amyotrophic lateral sclerosis (Choice E) presents with a combination of upper and lower motor neuron symptoms, including muscle weakness, fasciculations, hyperreflexia, and a positive Babinski. Examination of the spinal cord in ALS reveals demyelination and sclerosis of the lateral corticospinal tracts as well as loss of anterior horn cells.

Edited by new_n_lost on 06/16/07 - 10:46 PM

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