It is the most severe type of spinal muscular atrophy group. The disease shows symptoms in the period before 6 months. In some patients, symptoms may begin in the prenatal period. In the last stages of pregnancy, baby movements may be reduced. 
The majority of babies with type 1 do not have head control, arm and leg movements. They cannot sit without support. Babies do not have basic skills such as swallowing and sucking. They need respiratory support due to the effects of respiratory muscles, difficulty in coughing and respiratory tract infections. The majority of patients die before the age of two. Facial movements of these babies are normal and their gaze is lively. They make eye contact. Their communication skills are very strong, they are interested in the environment.




Symptoms in type 2 patients are seen in the period after the 6th month. Before, the baby's development is normal. These patients have head control, they can usually sit down. Most patients are unable to stand and walk without assistance. They cannot move from a lying position to a sitting position without assistance. Patients are susceptible to respiratory infections. Spinal curvatures (scoliosis), abnormalities in the hand, foot and chest wall can be seen.




In type 3 patients, onset is after the 18th month. Babies are normal at birth. The first symptoms may be seen as mild, and the disease may take years to be diagnosed. Sick individuals cannot demonstrate similar physical activity with their peers of the same age. For example, the child lags behind his peers while crawling and walking. Patients can walk, however, there is a muscle weakness. As the disease progresses, weakness in the hip and leg muscles prevents running, falls are frequent, walking becomes difficult. They have trouble climbing stairs and getting up from where they sit. They may need a wheelchair at the age of 20-30. In this period, skeletal disorders such as spinal curvatures (scoliosis) are seen to become prominent. Weakness of swallowing and chewing muscles is rare. Respiratory impairment is not as severe as in type 1 and type 2.




This type of SMA occurs in adulthood. The onset and progression of the disease is slow. There may also be pauses in the progression of the disease. Adult SMA patients can usually walk; few patients need a wheelchair. Weakness, scoliosis, tremors and twitching are seen in the arms and legs. Muscles used for swallowing and respiratory functions are rarely affected in Type 4 patients. Weakness and muscle loss are more on the parts of arms and legs close to the body. This weakness spreads slowly.