Over ninety-five percent of spinal muscular atrophy (or SMA) cases affect children and infants. This genetic disease is caused by improper production of the survival motor neuron protein. This protein is important for the motor neuron that lies in the anterior horn of the spinal cord, and controls the skeletal muscles of the body directly. Improper production of the protein causes the motor neurons of the spinal cord to shrink and eventually die, resulting in the brain being unable to control the voluntary muscles of the body (i.e., neck, head, legs, and arms).

Subsequently, the muscles will start weakening and wasting, making movements like breathing, swallowing, neck and head control, crawling and walking more difficult. There are 4 various types of SMA. The classification for each type is ascertained by the developments seen in a child being affected over time:

1. Type 1 SMA
The first type of spinal muscular atrophy (SMA) is known as Werdnig Hoffman disease. This type of spinal muscular atrophy is common as well as severe. It is found to be very obvious at the time of birth or at least in the initial six months since birth. The most common symptom found in the child is the weak movements of the trunk and limbs. The ability to move in children who suffer from this type of spinal muscular atrophy is limited. The child will find it difficult to swallow and feed themselves. Keeping their head up is also a challenge and at times, breathing becomes difficult. The progression seen in this type of spinal muscular atrophy is rapid wherein the muscles get weakened often causing infections in the respiratory tract of the child followed by the death of the child by the time they turn two. The children who suffer from this type of spinal muscular atrophy will never be able to sit.

2. Type 2 SMA
In this type of spinal muscular atrophy (SMA), the symptoms are mostly found to develop when the child is between seven to eighteen months. As compared to the arms of the kids the legs get affected in this type of spinal muscular atrophy, therefore, such children will never be able to stand. The infections in the respiratory tracts are also commonly found among children who suffer from this type of spinal muscular atrophy. The life span of the child gets affected and can range from childhood to adulthood based on the severity of the condition of the patient.

3. Type 3 SMA
The third type of spinal muscular atrophy (SMA) is known as the Juvenile Spinal Muscular Atrophy or the Kugelberg-Welander. Under this type of spinal muscular atrophy, the appearance of the symptoms can be at varying ages starting from eighteen months to the initial adulthood of a person. In such cases, the patients can walk and stand but may face difficulty in getting themselves up while they are in a sitting position. There is also mild muscle weakness that such patients under this type of spinal muscular atrophy may experience along with respiratory infections. The life expectancy of such patients is usually found to be normal.

4. Type 4 SMA
The type 4 spinal muscular atrophy (SMA) is a rare type and will only develop post the person’s life’s second or third decade. Such patients will be able to walk in their adulthood but will slowly face weakening of the muscles as well as other disease-related symptoms.