Spinal muscular atrophy or SMA, as it is commonly known, is a form of genetic disorder that restricts the voluntary movement of the muscles, as the nervous system is affected. There are two major spinal muscle atrophy (SMA) types and the subcategories can be classified broadly under these two. One SMA is directly linked to chromosome 5, while the other is not linked to chromosome 5. There are four spinal muscle atrophy (SMA) types which are directly linked to chromosome 5:

1. SMN1 gene

In general, all of the 1-4 types of SMAs occur due to a deficiency in the SMN protein that is known as survival of motor neuron. This SMN deficiency occurs as a result of a flawed mutation within the SMN1 gene, which is present in chromosome 5. A normal SMN1 gene produced protein would be both full length and functional in nature, without any glitches.

2. SMN2 genes

However, when there is a flawed mutation, the SMN1 protein production would function but the lack of length would cause problems. This happens when the SMN2 genes, which are present nearby, end up offsetting the SMN1 gene. The SMN2 genes do not have to be full length or functional in nature as a mandate, but there could still be ones that are. More importantly, the number of SMN2 genes differ in each individual.

3. SMA and age of onset

In the case of spinal muscle atrophy (SMA) due to chromosome 5, it occurs when a person with more number of SMN2 genes would have more functional and full-length proteins. As a result, the disease set in only later in their life. Therefore, the impending course also tends to be milder. The major classification primarily depends on the age at which this condition occurs.

When the age of SMA occurrence is between 7-18 months, it is classified as spinal muscle atrophy (SMA) type 2. In this case, the occurrence of SMA causes the child to not stand or even walk independently.

The spinal muscle atrophy (SMA) types 3 and 4 are usually late-onset SMAs, and they occur after 18 months or in a person’s adulthood. The degrees of the SMN2 genes vary, and the impact on each individual can vary as well. The modality of treatment also varies based on the degree and impact it has on a person.

4. SMA symptoms

If the symptoms of SMA are evident from the time of birth to the age of about 6 months, it is known as spinal muscle atrophy (SMA) type 1. In this case, the infant would experience muscle weakness and also have breathing distress along with a weakened crying. This hinders the essential milestones they are expected to reach within the first six months.