Types of SMA

There are four primary types of SMA: type I, II, III, and IV. The type of SMA is based on age of onset and the highest physical milestones achieved.

No two people with SMA have identical experiences. Even among those with the same type, the progression of the disease can differ. Decisions about care and treatment should be made according to each individual’s needs.

SMA Type I

Type I—the most severe and the most common—is usually diagnosed during an infant’s first six months. Babies with SMA type I face many physical challenges, including muscle weakness and trouble breathing, coughing, and swallowing. They may need breathing assistance or a feeding tube. Type I is often fatal early on in life. 60% of all SMA cases are type I. 

SMA type I is also know as Werdnig-Hoffmann disease. 


Type II is usually diagnosed after six month 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 II 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 III is usually diagnosed after 18 months of age, but before three years of age. However, SMA type III can be diagnosed as late as the teenage years. Individuals affected by SMA type III are initially able to walk, but have increasingly limited mobility as they grow and eventually, many need to use a wheelchair. 

Type III is also called Kugelberg-Welander disease or juvenile SMA.


SMA type IV 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 I 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

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.

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 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, major muscle cramps, as well as other symptoms.

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