Neuromuscular Junction and Muscle Disorders

In Lambert-Eaton myasthenic syndrome (LEMS), autoantibodies disrupt the function of neuronal voltage-gated calcium channels, leading to a reduction in the release of acetylcholine at the neuromuscular junction and at synapses of autonomic nerves. Changes in membrane voltage and depolarization of the neuronal membrane by influx of potassium ions leads to the opening of calcium channels. Calcium ions flow into the nerve terminal, and neurotransmitter vesicles fuse with the neuronal membrane, releasing acetylcholine in the neuromuscular junction. When muscle biopsy specimens from LEMS patients receive a single supramaximal stimulus, there emerge miniature end-plate potentials that are normal in amplitude but reduced in number, suggesting a reduction in the number of quanta of acetylcholine being released into the neuromuscular junction. y

LEMS is either associated with malignancy, most commonly small cell lung cancer, or with other autoimmune disorders, including thyroid disease, pernicious anemia, vitiligo, and type I diabetes mellitus. General fatigue commonly precedes weakness. Gait dysfunction usually follows weakness on standing. Autonomic dysfunction, commonly cholinergic and involving nicotinic and muscarinic synapses, produces xerostomia and erectile failure. Orthostatic hypotension is not usually a feature. Although sluggish pupillary responses may occur, ocular symptoms are rare. Repetitive or sustained contraction can improve the lower extremity proximal muscle weakness. The combination of proximal muscle weakness and hyporeflexia are hallmarks of this disease. Neurophysiological studies help confirm the diagnosis. The weakened muscles exhibit reduced amplitude compound motor action potential (CMAP) with facilitation characterized by a twofold increase in CMAP after rapid stimulation at 20 to 50 Hz. Repetitive stimulation at rates of 2 Hz is associated with a decremental response.

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