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.

Types of SMA

There are five types of spinal muscular atrophy (SMA): Types 0, 1, 2, 3, and 4. The type of SMA is based on the age that symptoms begin, and the highest physical milestone achieved. Even within each type, abilities can vary from person-to-person. In addition, individuals with SMA can lose function over time if muscles continue to weaken.

With treatment, individuals may gain more physical milestones than they would have otherwise. And, as newborn screening for SMA becomes more common, infants can receive treatment even before symptoms begin. Because of these factors, doctors believe that we may soon stop describing SMA as specific “types” and instead focus on the highest motor milestone achieved: non-sitter, sitter, and walker. But even then, there will still be a wide range of severity associated with SMA.

Type 0

SMA Type 0 is very rare and very severe. Symptoms begin prior to birth and is seen as decreased fetal movement in the weeks prior to delivery. At birth, the infant has severe weakness and often difficulty breathing, feeding, and may have joint contractures and cardiac defects. These infants typically require respiratory and feeding support prior to confirming the diagnosis. These infants may survive a few months.

Type 1

Also known as Werdnig-Hoffmann disease, SMA Type 1 is the most common (60%) and severe form, usually diagnosed during an infant’s first 6 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.

Type 2

Type 2 is usually diagnosed after 6 months of age, but before 2 years of age. The first sign is often a delay in meeting motor milestones, or failing to meet milestones entirely. Individuals with 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.

Type 3

Also called Kugelberg-Welander disease or juvenile SMA, Type 3 is usually diagnosed after 18 months of age, but before 3 years of age. However, SMA Type 3 can be diagnosed as late as the teenage years. Individuals with 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 4

SMA Type 4 is very rare, less than 1% of all diagnosed cases. It usually surfaces in adulthood, and it leads to mild motor impairment. While symptoms can begin as early as age 18 years, they usually begin after age 35 years.

Other Forms of SMA

Unlike the primary types of SMA described above, these other forms of SMA are caused by mutations in genes other than the SMN1 gene.

SMA 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


Kennedy’s Disease

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 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.