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Congenital myopathy with a specific histologic pattern and high susceptibility to malignant hyperthermia.

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Muscle Core Disease; Muscular Central Core Disease; Central Fibrillary Myopathy; Shy-Magee Syndrome.

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Accounts for approximately 15% of all congenital myopathies. No sexual predilection.

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Autosomal dominant trait with the gene locus mapped to 19q13.1, which is in close proximity to one of the genes for malignant hyperthermia (ryanodine receptor gene). Expression is variable.

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Can be caused by at least 22 different mutations in the ryanodine receptor-1 gene (RyR1). According to one common hypothesis, these mutations could result in the formation of excessively leaky sarcoplasmic reticulum Ca2+ release channels. The ryanodine receptor is a protein involved in calcium release into the sarcoplasm from the sarcoplasmic reticulum. It has been postulated that after the exposition to trigger agents, the ryanodine receptor releases excessive amounts of calcium resulting in sustained muscle contraction and hypermetabolism.

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Requires muscle biopsy. The “central core” is a histopathologic description related to an area of low staining that extends the length of the muscle fibers and is devoid of mitochondria and other cell organelles. Type 1 muscle fibers are predominately affected. Nerve conduction studies are normal and electromyography is nonspecific. Normal plasma levels of creatinine phosphokinase do not rule out central core disease. Susceptibility to malignant hyperthermia is determined by in vitro contraction testing with halothane and caffeine.

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There is a wide variation in the clinical spectrum of this disease. The myopathy is asymptomatic in some patients, but in others (although rarely) it can be severe. Most commonly, it already presents in infancy with hypotonia and delayed motor milestones. A history of decreased fetal movement and/or breech presentation is often given. However, the first signs of this disease can be a slowly progressive limb-girdle syndrome with onset in adolescence or an attack of malignant hyperthermia. Central core disease may not be symptomatic until late in life. Proximal muscle weakness is common and manifests as difficulties in walking or climbing the stairs. Deep tendon reflexes may be significantly reduced or even absent. Muscle wasting occurs. Skeletal abnormalities may be part of the clinical picture and include kyphoscoliosis, congenital hip dislocation, patella dislocation, feet deformities (pes planus, pes cavus), and joint contractures. Mandibular hypoplasia has been reported in a few cases. These patients are mentally normal.

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As in all myopathies, evaluate cardiac (increased incidence of mitral valve prolapse) and respiratory function (e.g., clinically, chest radiographs, echocardiography, radionuclide scintigraphy, pulmonary function tests, arterial blood gases analysis), particularly in the presence of kyphoscoliosis. Blood work should include serum levels of electrolytes (including calcium) and creatinine phosphokinase.

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Patients are susceptible to malignant hyperthermia, and at least one death related to malignant hyperthermia has been reported. Until proven otherwise, these patients should be considered at high risk for malignant hyperthermia. Even asymptomatic family members should be considered at risk for malignant hyperthermia until susceptibility has ...

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