Traditionally SMA symptoms are divided into four types depending on the age at which symptoms appear and the related severity of muscle weakness, based on the motor milestones achieved. Even within these types there is a wide spectrum of severity as SMA affects everyone differently.

Babies diagnosed with SMA type 1 show symptoms within the first weeks or months of life (before six months of age) and they do not achieve the ability to sit independently. Babies with SMA type 1 have generalised muscle weakness and limited movement. They typically have a weak cry and difficulty coughing due to weak breathing muscles. These babies experience problems with swallowing and suffer from frequent respiratory infections. Life expectancy for babies with SMA type 1 is very limited but can vary. In many cases, babies may live for less than two years; especially without ongoing respiratory intervention. However, with round-the-clock care and respiratory support, some children with SMA type 1 may survive into their teens or longer. Type 1 is the most common type of SMA. Around 60% of cases of SMA are classified as type 1.

Symptoms of SMA type 2 appear between seven months and 18 months of age. Children are diagnosed with SMA type 2 if they were strong enough to maintain sitting position at some point, and some may have achieved the ability to stand, but they never gain the ability to walk unaided. Just like in SMA type 1, children and adults with SMA type 2 often have difficulties swallowing and are prone to respiratory infections due to weakness of the muscles used for breathing. They all also require mobility aids (for example, a wheelchair) due to progressive muscle weakness. However, with good care, they live to become teenagers and adults. Stronger people with SMA 2 may work, some start families.

When the first symptoms appear after the person learned to walk independently, the disease is classified as SMA type 3. People with this type still experience significant muscle weakeness over time and the vast majority soon lose the ability to walk or stand, but they tend to live full lives and oftentimes achieve a lot in personal and professional life.

Very rare cases when SMA symptoms develop in an adult person are termed as SMA type 4.

All the four SMA types have the same genetic cause and all drugs that act on SMA-causing genes should be of help in all types of the disease, although perhaps to a varying degree. However, we believe, along with many healthcare professionals, that all people with SMA will be helped greatly if treated with any of the upcoming powerful drugs for SMA.

For more information see NHS SMA Symptoms.