There are four primary types of SMA: types 1, 2, 3, and 4. The type of SMA is based on the age that symptoms begin, and the highest physical milestone achieved.
No two people with SMA have identical experiences. Even among those with the same type, the experience of the disease can be different. Decisions about care and treatment should be made according to each individual’s needs.
Four Primary Types of SMA
SMA Type 1
Type 1—the most severe and the most common—is usually diagnosed during an infant’s first six months. Babies with SMA type 1 face many physical challenges, including muscle weakness and trouble breathing, coughing, and swallowing. They may need breathing assistance or a feeding tube. If not treated, type 1 can be fatal early on in life. 60% of all SMA cases are type 1.
SMA type 1 is also known as Werdnig-Hoffmann disease.
SMA Type 2
Type 2 is usually diagnosed after six months of age, but before two years of age. The first sign is often a delay in meeting motor milestones, or failing to meet milestones entirely.
Individuals affected by SMA type 2 can typically sit up without help, though they may need assistance getting into a seated position, but they are unable to walk and will require a wheelchair.
SMA Type 3
Type 3 is usually diagnosed after 18 months of age, but before three years of age. However, SMA type 3 can be diagnosed as late as the teenage years. Individuals affected by SMA type 3 are initially able to walk, but have increasingly limited mobility as they grow and eventually, many need to use a wheelchair.
Type 3 is also called Kugelberg-Welander disease or juvenile SMA.
SMA Type 4
SMA type 4 is very rare. It usually surfaces in adulthood, and it leads to mild motor impairment. While symptoms can begin as early as age 18, they usually begin after age 35.
Other Forms of SMA
Unlike the four primary types of SMA, these other forms of SMA are caused by mutations in genes other than the SMN1 gene.
Spinal Muscular Atrophy Respiratory Distress (SMARD)
SMARD is a very rare form of SMA type 1 that affects the upper spinal cord more than the lower spinal cord. Babies with SMARD experience severe respiratory distress, and weakness in the arms and nearby muscles. SMARD is caused by a specific mutation and can be diagnosed through genetic testing.
Distal SMA is another rare form of SMA. Unlike other forms of SMA, distal SMA can be inherited from just one parent. Weakness from distal SMA affects the hands and feet.
Another rare form of SMA, Kennedy’s disease is an X-linked genetic disease, meaning it only affects males. It usually appears between the ages of 30 and 50. It causes muscle weakness and wasting (atrophy) throughout the body, which is most noticeable in the legs and arms. It is also especially noticeable in the face and throat, and causes speech and swallowing difficulties and major muscle cramps.