Types of SMA

The severity of SMA varies greatly between individuals, so the disease is divided into four types. These are referred to as SMA type I, SMA type II, and SMA type III and SMA type IV. The type of SMA is determined based on the age at which symptoms appear, the related severity of muscle weakness, and the highest motor function a child achieves.
Even within these types there is a wide spectrum of severity as SMA affects everyone differently.

SMA Type I (Werdnig–Hoffmann disease)

It is the most severe form of SMA and defined as children who are not

able to sit independently. It is usually evident at birth, or in the

first few months afterwards (0-6 months). Symptoms include floppy limbs and

weak trunk movement. They will also have a hard time feeding and swallowing,

holding their head up, and breathing. They may need breathing assistance

or a feeding tube. Type 1 SMA progresses rapidly, with the weakening of muscles leading to

frequent respiratory infections and usually death by the age of 2.

SMA Type II(Dubowitz disease)

Symptoms of SMA type 2 appear between seven months and 18 months of age. Children are diagnosed with SMA type 2 if they were strong enough to maintain sitting position at some point, and some may have achieved the ability to stand, but they never gain the ability to walk unaided. The rate of progression can vary greatly. Just like in SMA type 1, children and adults with SMA type 2 often have difficulties swallowing and are prone to respiratory infections due to weakness of the muscles used for breathing. They also require mobility aids (for example, a wheelchair) due to progressive muscle weakness. However, with good care, they live to become teenagers and adults. Stronger people with SMA 2 may work, and some start families.

SMA Type III (Kugelberg–Welander disease)

Symptoms for this type can first appear from 18 months to early adulthood.

Patients with Type 3 SMA can stand and walk but may have trouble getting up

from sitting position, running and climbing stairs. They may also experience mild muscle

weakness and are at greater risk for respiratory infections. Later in life, they may need a wheelchair

to get around. Most patients with Type 3 SMA have a life expectancy close to normal.

SMA Type IV

SMA type 4 is rare. Symptoms for this rare type of SMA do not usually emerge until the second or third decade of life. Patients with Type 4 SMA can walk during adulthood but will usually experience slowly progressive muscle weakness and other typical SMA symptoms.

Overview of SMA clinical classification

SMA Type Usual age of symptoms Impact of muscle weakness on sitting / walking
Type 1 Younger than 6 months Unable to sit or roll independently
Type 2 6 – 18 months Able to sit but not walk independently
Type 3 18 months – 3 years Able to walk, though may lose this ability over time.
Type 4 Over 18 years Mild walking difficulties

Adapted from Tillmann et al. 2018