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Proximal spinal muscular atrophy (SMA) is an autosomal recessive disease caused by a genetic defect in the SMN1 gene, which encodes SMN, a protein widely expressed in all eukaryotic cells. SMN is apparently selectively necessary for survival of motor neurons, as diminished abundance of the protein results in loss of function of neuronal cells in the anterior horn of the spinal cord and subsequent system-wide muscle wasting (atrophy). Spinal muscular atrophy manifests in various degrees of severity, which all have in common progressive muscle wasting and mobility impairment. Proximal muscles and lung muscles are affected first. Other body systems may be affected as well, particularly in early-onset forms. SMA is the most common genetic cause of infant death. The term ''spinal muscular atrophy'' is used as both a specific term for the genetic disorder caused by deficient SMN, and a general label for a larger number of rare disorders having in common a genetic cause and slow progression of weakness without sensory impairment caused by disease of motor neurons in the spinal cord and brainstem – see spinal muscular atrophies for a comparison chart. ==Signs and symptoms== The symptoms vary greatly depending on the SMA type involved, the stage of the disease, and individual factors; they commonly include: * Areflexia, particularly in extremities * Overall muscle weakness, poor muscle tone, limpness or a tendency to flop * Difficulty achieving developmental milestones, difficulty sitting/standing/walking * In infants: adopting of a frog-leg position when sitting (hips abducted and knees flexed) * Loss of strength of the respiratory muscles: weak cough, weak cry (infants), accumulation of secretions in the lungs or throat, respiratory distress * Bell-shaped torso (caused by using only abdominal muscles for respiration) * Clenched fists with sweaty hands * Head often tilted to one side, even when lying down * Fasciculations (twitching) of the tongue * Difficulty sucking or swallowing, poor feeding * Weight loss 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Spinal muscular atrophy」の詳細全文を読む スポンサード リンク
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