protein that is critical to the function of the nerves that controlour muscles without it those nerve cells cannot function properly and eventually die leading to debilitating and sometimes fatal muscle weakness there are four primary types of sma 1 2 3 and 4 based on the age that symptoms begin and highest physical milestone achieved the severity of symptoms and age of onset varies by the type some types are apparent at or before birth while others are not apparent until adulthood all the four conditions generally result in worsening of muscle weakness associated with muscle twitching arm leg and respiratory muscles are generally affected first associated problems may include problems with swallowing scoliosis and joint contractures individuals with sma have difficulty performing basic functions of life like breathing and swallowing