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Rare, congenital, slowly progressive inherited neuromuscular disease that usually is apparent at birth; characterized by extreme hypotonia.

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Nemaline Myopathy.

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Nemaline myopathy has been classified into four major subtypes:

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Severe neonatal form: Characterized by severe muscle hypotonia with little spontaneous movements, severe dysphagia and absence of sucking ability, and respiratory problems that are considered life-threatening during the first weeks or months of life. A fetal form (“fetal akinesia sequence") has been suggested but is still considered part of this entity. It is associated with large quantity of amniotic fluid, abnormal muscle growth, and underdevelopment of the lung.

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Mild congenital or “classic" form: Age at onset between birth and the first years of life. It is characterized by extreme muscle weakness, feeding difficulties, delayed motor development (e.g., walking difficulties, speech abnormalities), and respiratory complications that are not considered as severe at the neonatal form. The evolution of the disease is often static or very slowly progressive. Most affected individuals can have an active life; others may experience deterioration during the period of rapid growth before puberty.

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Childhood-onset form: Age at onset between 10 and 20 years. The apparition of muscle weakness is slowly progressive and allows motor development to be considered normal. The clinical symptoms typically progress during childhood, but exercises that increase muscle development and strength may offset the progression of the disease.

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Late-onset or adult-onset form: Characterized by the absence of family history or symptomatology before the apparition of the muscle weakness, which is mostly apparent in the extremities and the trunk.

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Affects females more than males; estimated incidence is 2:10,000 live births.

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Two clinically indistinguishable types are described. Type I is autosomal dominant with incomplete penetrance and is caused by a defect in the gene TPM3 (or NEM1) located on chromosome 1q21-q23 and encoding for tropomyosin-3. Type II is autosomal recessive and is caused by a mutation of the gene NEM2 located on chromosome 2q21.2-q22 and encoding for nebulin.

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All muscles, including the diaphragm, may be affected. The disease is nonprogressive or slowly progressive. Cardiac involvement is rare, but cases of cardiomyopathy in adults have been described.

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Serum creatine kinase levels are usually, but not always, normal. Muscle biopsy shows disproportion of type I muscles fibers and subsarcolemmal rod-like structures made of excessive Z-band material.

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In the neonatal period, hypotonia and muscle weakness produce swallowing problems and weak cry; severe cases progressing rapidly to respiratory failure are described. Motor development (walking) is delayed. Muscle mass is thin; truncal and proximal muscles are most affected, but distal limb, pharyngeal, and facial muscles can be involved. The face is usually long with a high-arched palate, malocclusion, prognathism, or micrognathia. Abnormal gait. Normal intelligence. Scoliosis and intercostal muscle weakness produce severe restrictive lung disease. Congenital heart disease or cardiomyopathy may be rarely present.

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