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The neuromuscular junction (NMJ) is an exquisitely organized and highly specialized synaptic interface between the motor nerve terminal and muscle fiber (Fig. 562-1). The NMJ transmits action potentials from the motor neuron to the muscle fiber (Fig. 562-2). The motor nerve terminal stores numerous presynaptic vesicles containing acetylcholine (ACh), and when it is activated, ACh is released by exocytosis across the NMJ synapse. It subsequently binds and activates the acetylcholine receptors on the postsynaptic muscle membrane, and this generates a postsynaptic action potential that translates into depolarization of the muscle fiber, which in turn leads to a contractile response. Diseases that lead to disruption of any of the structural components or function of the NMJ can impair transmission of the action potential and manifest as fatigue and muscle weakness.
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The most common disease of the NMJ is myasthenia gravis (MG). In children, there are 3 principal myasthenic disorders including autoimmune juvenile MG, neonatal MG, and congenital MG. All 3 have distinct pathophysiology with important consequences for diagnosis and treatment. Other diseases of the NMJ include paraneoplastic autoimmune myasthenia (Lambert-Eaton syndrome), toxins, and botulism. The NMJ diseases can be organized based on whether they affect presynaptic or postsynaptic function. This is a neurophysiologic characteristic that can aid in diagnosis.
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AUTOIMMUNE JUVENILE MYASTHENIA GRAVIS
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Pediatric patients with acquired autoimmune MG from birth to 19 years of age are typically defined as having juvenile MG (JMG), which is a postsynaptic NMJ disorder.
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Recent estimates suggest an overall ...