Type I SMA typically begins to affect infants within the first six months of life, with most showing initial signs of muscle weakness by three months. Types II and III begin to affect children between 7 and 18 months. Weakness is seen in muscles closer to the body, and children may be able to sit independently but are unable to walk.


Common symptoms of SMA include:

* Muscle weakness, hypotonia, and difficulty moving
* Delays in gross motor skills such as sitting, crawling, and walking
* Muscle twitching
* Poor head control
* Difficulty sucking and swallowing
* Bone and joint problems such as scoliosis
* Respiratory problems
* Difficulty eating and swallowing
* Difficulty breathing
* Weak voice and tongue fasciculations
* Frequent falls and decreased walking ability
* Difficulty rising from a seated position on the floor
* Decreased reflexes
* Cramps
* Tremors in the hands
* Developmental delays in infants