Development other than gross motor is normal in these children, which may give the family a false sense of improvement. Death may occur by the age of 6 months secondary to loss of gag reflex with resultant aspiration and pneumonia. Chronic Infantile SMA is an intermediate form of the disease. Age of onset is after three months of age. As the name suggests, it is slow to progress during infancy and childhood.
Weakness is usually symmetric and begins in the pelvic girdle.
Again, reflexes are depresses or not obtainable in affected muscles. Extraocular muscles are spared. Contractures tend to develop during childhood, and pseudohypertrophy of the calves may develop as in Duchenne's muscular dystrophy. Affected individuals live to a mean age of 30 years. Juvenile SMA, or Kugelberg-Welander disease, begins after age 7 months in its more severe form and after 18 months in mild forms.