Spinal muscular atrophy (SMA) is a group of genetic disorders that lead to a loss of control over the movement of muscles, following the loss of nerve cells in the spinal cord and the brain stem. It predominantly occurs in infants and children, and rarely occurs in adults. The neurological condition causes the weakening and wasting of muscles, with symptoms occurring within the first six months of life.

In nearly all cases, SMA is the result of inadequate production of the protein called survival motor neuron. These neurons transmit impulses from the spinal cord to the skeletal and smooth muscles for controlling all muscle movements.

Types of spinal muscle atrophy
There are generally four types of spinal muscular atrophy. Each stage is primarily marked by the age at which the symptoms start to occur. Here are the four key types of spinal muscular atrophy:

1. Type 1 SMA
This type is popularly known as infantile onset SMA, and is the most common and severe of all SMA types. It’s a serious condition, wherein an infant experiences breathing problems that can be fatal within a year, if left untreated. Muscle weakness and twitching are the common symptoms, including a lack of ability to move the limbs. The symptoms start showing within six months after birth, and these infants cannot hold their head up without support, and they may have floppy arms and legs. Eating and swallowing food becomes difficult, and the disease can lead to respiratory infections. Most kinds with Type 1 SMA don’t live beyond two years.

2. Type 2 SMA
Also known as intermediate SMA, type 2 SMA typically occurs between 6 to 18 months of birth, before the infant can stand or start walking by themselves. The rate of progression varies significantly, and the disease affects the legs more than the arms. Breathing problems may appear especially during sleep, and life expectancy depends on the severity of the symptoms. If timely treatment is not administered, then the child may never be able to sit for life without support. In other cases, infants may be able to sit, but never be able to stand or walk.

3. Type 3 SMA
Type 3, also known as juvenile SMA, sets in between 18 months to teenage. While children can stand and walk by themselves, they might fall frequently and have trouble getting up or climbing stairs. Muscle weakness in the arms and legs is common, and complications can cause respiratory infections. However, this is the mildest form of SMA, and most children can live a normal life with proper treatment.

4. Type 4 SMA
This type is also known as adult SMA, and is a rare condition that occurs after age 20. Sometimes, the symptoms don’t show until the individual reaches age 35 or above. The individual may experience weakness in their arms and legs, along with breathing issues. Type 4 is not as life-threatening as the earlier stages. Shaking and twitching muscles are common, and there may be difficulty in walking. Most people with this type of SMA can walk and move around throughout their lives without any difficulties. It is a progressive condition, and does not affect life expectancy.