A degeneration of the motor neurons in the spinal cord and brainstem characterize the spinal muscular atrophies (SMAs). These diseases, accounting for 10% of MNDs, are inherited and chiefly affect children of various ages. Based on age of onset, several types are distinguished (Table 5.9). Neonatal and early infantile, infantile, and juvenile types are transmitted as an autosomal recessive trait. They are associated with mutations in the survival motor neuron gene (SMN1) on chromosome 5. Adult-onset SMA, has an autosomal recessive or dominant mode of inheritance and a slowly progressive course. Clinical findings are weakness, muscle wasting, fasciculations, hypotonia, absent tendon reflexes and, late in the course, dysphagia and dysar-thria. The clinical course is the shortest in neonates, ranging from a few weeks to 2 months; it is longer in infants—2 to 3 years—and juveniles may reach adulthood. Early death is from respiratory failure and bulbar involvement.
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