5 Main Spinal Muscle Atrophy (SMA) Types

Spinal muscular atrophy (or SMA) is a progressive genetic disorder. It affects the muscular and nervous system of our body. Spinal muscular atrophy is a rare genetic disease and is found in an estimated one in every 5000 people. It is caused due to the loss or degeneration of certain nerve cells called lower motor neurons. These types of neurons are responsible muscle coordination and their degeneration can lead to muscle cell death and muscle weakness.

Lower motor neurons run from spinal cord to muscle cells. They enable muscle-controlled movements such as crawling, grasping, reaching, swallowing, and even breathing. Here are the five different types of SMA that are classified according to the patient’s age during the onset and the clinical course of the disease:

SMA type 0
Several SMA disease specialists do not consider type 0 as a separate type of atrophy because it takes place before birth. SMA type 0 only affects babies resulting in decreasing muscle tone and muscle weakness. The affected babies are unable to develop age-appropriate motor skills and most of them live longer than six months after their birth.

SMA type 1   
Also known as Werdnig-Hoffman disease, this type of Spinal Muscle Atrophy is the most common form of disease as it accounts an estimated 50% to 60% of all cases of childhood-onset of SMA. It is also an extremely severe form of SMA which has an onset within six months. Almost 60% percent of babies suffering from SMA type 1 die within two years and 80% succumb to the disease within the next four years. However, one can increase their survival rates through proper nutritional care.

SMA type 2
Dubowitz disease or SMA type 2 is a type of spinal muscle atrophy that usually occurs between the age six to 18 months. SMA type 2 can be characterized by poor muscle tone which can be evident at birth or within the first few months of the baby’s life. But, they may slowly gain motor control milestones such as sitting independently. However, they will not be able to stand and walk unaided. Flaccid muscle and trembling fingers are most commonly observed symptoms in SMA type 2. Severe symptoms might include respiratory problems, swallowing difficulties and scoliosis (abnormal curvature in the spine). The lifespan of people suffering from SMA type 2 is not fixed. However, majority of them live into early adulthood with proper nutritional care.

SMA type 3
This type of SMA is also called Kugelberg-Welander disease and it appears in the first 18 months of life. In some cases, they appear only in early adulthood. Individuals affected with SMA type 3 generally lack coordination while running or walking downstairs and have greater arm strength when compared with leg strength. The lifespan of people with SMA type 3 is same as rest of the population.

SMA type 4
It is an adult-onset SMA and is seen full-grown adults. It has an onset typically in the second or third decade of life. It consists of symptoms such as muscle weakness, scoliosis, and muscle tremors, and twitching. Individuals with SMA type 4 have a normal life expectancy. The quality of life can be improved with proper nutritional care.