The types of SMA are based on the age at which symptoms first appeared and the effect of these symptoms on growth, development and quality of life. The different SMA types have varied impacts and different treatment options, and include:

• Type 1*: This is the most common type. Children with SMA Type 1 may begin showing symptoms between 0 and 6 months of age. Symptoms may include muscle weakness, difficulty holding their head up, a weak cry or cough, and trouble breathing, swallowing, or sucking. These children may not be able to roll or sit up on their own.
• Type 2*: Children with SMA Type 2 may begin showing symptoms between 7 and 18 months of age. Symptoms may include muscle weakness in their arms, legs, and torso (trunk), a weak cough and scoliosis (curvature of the spine). These children may not be able to walk independently.
• Type 3*: Children with SMA Type 3 may begin showing symptoms after 18 months of age. Symptoms may include muscle weakness, especially in the legs, hips, shoulders, arms and breathing muscles. These children may lose the ability to walk as they get older.
• Type 4: Also known as ‘adult-onset SMA’, symptoms may appear after 18 years of age. These can include mild muscle weakness, tremors and twitching. This type of SMA does not impact life expectancy and mobility may be mildly affected.

* However, in Australia, recent innovations facilitating timely diagnosis and treatment of SMA have started to change the way these types of SMA can be defined.

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There are also some rarer forms of SMA:

• Type 0: This type is the rarest and most severe form of SMA. This is also referred to as prenatal SMA as babies are diagnosed before birth based on symptoms such as reduced or absent movement in the womb in the later stages of pregnancy. Joint deformities, muscle weakness and issues with breathing and feeding are apparent at birth. Sadly, children with SMA Type 0 typically only survive for a few months. 
• X-linked SMA: This form of SMA is caused by a change in a gene located on the X     sex chromosome (UBA1). Because of this, X-linked SMA only affects boys. Babies born with X-linked SMA have severe muscle weakness, breathing difficulties, misshapen joints and are often born with broken bones.
• SMA-LED (SMA with lower extremity dominance): This is a rare form of SMA caused by a specific gene mutation. This mutation stops motor neurons from working properly, leading to muscle wasting which is most severe in the legs and thigh muscles (quadriceps). People with SMA-LED often have an unsteady ‘waddle’ when walking and struggle climbing stairs and standing up from sitting.
• SMARD1 (SMA with respiratory distress): SMARD1 is a type of SMA that specifically affects the breathing muscles, with symptoms usually starting between 6 weeks and 6 months of age. This is not the same as SMA Type 1. Sadly, there are no treatments available for people with SMARD1.