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A 1-year-old boy presents with marked weakness. Parents report a weak cry and cough since birth, and the child cannot sit independently. Exam findings include a bell-shaped thorax, hypotonia, some movement of the hands and feet but minimal movement at the hips and shoulders, and there are tongue fasciculations. The diagnosis is best confirmed by
(a) genetic analysis.
(b) muscle biopsy.
(c) electromyography.
(d) repetitive nerve stimulation.
What is a.
This child’s presentation is typical for spinal muscular atrophy (SMA) type I, also known as Werdnig-Hoffmann disease. SMA is the second most common neuromuscular disease of childhood, occurring with an incidence of 1:6,000. Deletion of the survival motor neuron gene leads to degeneration of anterior horn cells and can be detected in over 90% of children with SMA. Prior to availability of genetic diagnosis, EMG and muscle biopsies were utilized. Repetitive nerve stimulation has been used in the investigation of botulism and congenital MG but is not helpful in SMA.