Type 1 SMA
Type 1 SMA, also known as Werdnig-Hoffman Disease, is considered to be the most severe type of SMA and the onset is usually within the first six months of life. Babies affected by type 1 SMA typically become floppy as their voluntary muscles become affected by the SMA. Whilst the problems associated with this can, to some degree, be overcome, it is usually respiratory problems that are fatal to babies living with type 1 SMA, and 80% of babies diagnosed with the condition do not live to their first birthday (JTSMA, 2012). The brain is unaffected by SMA, so babies with Type 1 SMA, in spite of their physical difficulties, are often described as bright, alert and responsive (JTSMA, 2012).

Type 2 SMA
Type 2 SMA is considered to be an 'intermediate' form of SMA. Symptoms usually appear in early childhood or infancy and usually involve generalised muscle weakness and wasting of the muscles. Most people affected by type 2 SMA are able to sit and some may be able to walk and stand in childhood, but these abilities are generally lost over time. People with type 2 SMA are susceptible to chest infections which can have serious consequences if SMA affects the respiratory muscles. Scoliosis (curvature of the spine) and joint contractures may cause problems for people with type 2 SMA and may require active medical management (e.g. through spinal fusion surgery). Many adults and children with type 2 SMA use powered wheelchairs for their mobility and lead very full and satisfying lives (Boardman, 2010).

Type 3 SMA
Type 3 SMA, also known as Kugelberg-Welander disease, is the mildest form of childhood onset SMA and onsets after 18 months of age. The generalised muscle weakness associated with SMA may cause some difficulties with walking, balance and other physical activities, such as bending. Many people with type 3 SMA experience a worsening of their muscle weakness over time. This is not thought to be because of a deterioration of the SMA itself, but rather the cumulative strain on the muscles overtime. Stress, growth spurts and illness may also be associated with a deterioration in muscle weakness. Like all types of SMA, there is a wide variety of presentations within type 3 SMA and everyone experiences it differently.

Type 4 SMA
Type 4 SMA is also sometimes referred to as adult-onset SMA. It typically begins in adult life with asymmetrical muscle weakness (e.g. in one leg but not the other), but then spreads to become symmetrical muscle weakness. There is wide variation in how type 4 SMA affects people, depending on the degree of muscle weakness as well as the muscles that it affects. Unlike types 1-3 which are usually associated with an autosomal recessive inheritance pattern, type 4 SMA can be both recessively or dominantly inherited. Sometimes type 4 is non-hereditary and is caused by a spontaneous gene mutation.

Variant Forms of SMA
SMA has many varient forms, these include:
Spinal Muscular Atrophy with Respiratory Distress (SMARD). SMARD is a rare variant form of SMA, with symptoms usually beginning in the first months of life, and sometimes even before birth. Breathing difficulties is the first symptom of SMARD as the respiratory muscles are severely affected. Generalised muscle weakness may follow, affecting the distal muscles (those furthest away from the centre of the body- e.g. the muscles in the hands and feet) more severely than proximal muscles (those closest to the centre of the body). The prognosis for babies affected by SMARD is highly variable, some die before their first birthday, having experienced rapid deterioration in their symptoms, whilst some live, with ventilatory support, to the age of two and beyond. SMARD is inherited in a similar way to SMA types 1-3 (in an autosomal recessive pattern), but with the involvement of different genes.
Spinal and Bulbar Muscular Atrophy (SBMA). SBMA, also known as Kennedy's Disease, only affects males as it is associated with a mutation on the X-chromosome. It onsets at some point between adolescence and mid-life and symptoms include generalised muscle weakness, cramping and spasms, as well as difficulties with speech and swallowing. Unlike SMA types 1-3, SBMA can also involve the endocrine system sometimes leading to diabetes, and the androgen receptor gene involvement may also cause breast enlargement and low sperm count in affected men. As SBMA is X-linked, only males are affected, and females are carriers. The son of a carrier female has a 50% chance of developing SBMA.