Spinal muscular atrophy (SMA) is a progressive genetic disorder that affects the nervous system and muscles, and is a very rare disease at that, found in an estimated 1 in every 6,000 to 1 in every 10,000 people. It is caused by a loss of specialized nerve cells, called lower motor neurons, leading to muscle weakness and muscle cell death.
Lower motor neurons run from the spinal cord to muscle cells, making possible such muscle-controlled movements as crawling and walking, grasping or reaching, breathing and swallowing.
There are 5 different types of SMA (type 0, 1, 2, 3, 4), classified accordingly to a person’s age at onset (before birth to young adulthood) and the disease’s clinical course. A number of disease specialists, however, do not consider type 0 a separate type onto itself, referring only to types 1-4, while other specialists do.
SMA types and life expectancy
Classifying SMA according to its type is not rigid, and overlap and a spectrum of severity exist between each type.
SMA type 0
Type 0 is the most severe form of the disease, and unusual in that onset that takes place before birth.
Affected babies show severe muscle weakness, decreased muscle tone, and are unable to develop age-appropriate motor skills.
Severe respiratory problems are common in infants with type 0 SMA, and few live longer than six months after their birth.
SMA Type 1Â (Werdnig-Hoffmann disease)
SMA type 1 is the most common form of the disease, accounting for an estimated 50% to 70% of all cases of childhood-onset SMA. It, too, is also an extremely severe form of the disease. (In general, the earlier SMA begins to manifest in a person, the more severe is the disease.)
Onset occurs within the first six months of life, when these babies first start showing evidence of notable muscle weakness, reduced muscle tone, and movement deficiencies. These children are unable to sit without support.
Overall, about 68% of children with SMA type 1 die before their second birthday and 82%  die before their fourth, but survival in these infants is improving with improvements in respiratory and nutritional care.
SMA Type 2Â (Dubowitz disease)
The onset of  SMA type 2 usually occurs between the age of 7 months and 18 months.
Poor muscle tone may be evident at birth or within the first few months of life. But children with SMA type 2 may slowly gain such motor milestones as sitting independently. But few are able to stand or walk unaided. Trembling in the fingers and a generally flaccid muscle tone are commonly observed, and respiratory problems and swallowing difficulties may be evident or develop.
Each child with this condition develops differently, often depending on their age at diagnosis. But many children with type 2 SMA will develop scoliosis as they age, an abnormal curvature of the spine resulting from weakness in muscles supporting the spinal column.
The lifespan of people who develop SMA type 2 is not known with certainty, although a majority live into early adulthood and, with proper care, many are living well into adulthood.
SMA Type 3Â (Kugelberg-Welander disease)
In patients with SMA type 3, symptoms usually appear after their first 18 months of life. In some cases, they may only appear in late childhood or early adulthood.
Individuals with type 3 can be affected by the disease in different ways. In many cases they are able to walk independently, but have a tendency to frequently fall and can have difficulties walking up and down stairs, or with running.
In general, children with SMA type 3 have greater arm strength than leg strength.
Around half of children with SMA Type 3 lose the ability to walk independently by age 14 years old, although some are still able to walk into adulthood. Special exercise programs are generally recommended for these children.
The lifespan of people with type 3Â is the same as the rest of the population, and breathing and swallowing problems are rare in these children. Still, breathing and cough strength should be checked regularly.
SMA Type 4Â (adult-onset SMA)
This type of SMA affects full-grown adults, with onset typically in the second or third decade of life. Type 4 symptoms consist of progressive muscle weakness, scoliosis (curvature of the spine), and muscle tremors and twitching. These symptoms progress very slowly throughout life, and are typically mild to moderate.
An exercise program aiming to improve muscle strength and balance, as recommended by a specialist, can be of help to people with adult-onset SMA.
Life expectancy is not affected, and those with this condition are usually able to walk unaided until age 60.
Follow the latest developments for all experimental Spinal Muscular Atrophy treatments on the SMA Therapy Tracker.
SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.