What are the symptoms of spinal muscular atrophy (SMA) in children?

The symptoms of spinal muscular atrophy (SMA) in children include the following:

SMA Type I, also known as Werdnig-Hoffman disease, or infantile-onset SMA, is evident at birth or within the first few months. Symptoms include floppy limbs and trunk, feeble movements of the arms and legs, swallowing difficulties, a weak sucking reflex, and impaired breathing. A baby with SMA Type I will not be able to sit without support. The majority of babies with SMA Type I die of respiratory failure within the first two years. SMA Type II, also known as juvenile SMA, intermediate SMA, or chronic SMA, begins in babies between 6 and 18 months. Their legs tend to be more impaired than their arms. The first indication that a baby may have the disease is when he or she fails to crawl or walk. Children with SMA Type II are usually able to sit without support if placed in position. Some may be able to stand or walk with help. Although children with SMA II may not need artificial assistance to breathe, they are still at an increased risk for respiratory infections. These children often survive into adulthood but with a significant motor disability. SMA Type III, also called Wolhlfart-Kugelberg-Welander disease, or mild SMA, can begin as early as in the toddler years or as late as in adolescence. Children can stand alone and walk but may have difficulty getting up from a sitting position. Their fingers may tremble. Children with SMA Type III usually remain mobile well into adulthood. Like children with Type II, they are at an increased risk for respiratory infections.

This answer is based on source information from the National Institute of Neurological Disorders and Stroke.