Spinal Muscular Atrophy (SMA) is a rare, genetic neuromuscular condition causing progressive muscle wasting (atrophy) and weakness leading to loss of movement. This may affect walking and upper body movement, breathing and swallowing. Importantly, SMA does not affect a person’s ability to think, learn or build relationships with others.
Patients with the disease lack a protein called 'survival motor neuron' (SMN), which is essential for the normal functioning and survival of motor neurons. Without this protein, the motor neurons deteriorate and eventually die. This causes the muscles to fall into disuse, leading to muscle wasting (atrophy) and weakness.
The SMN protein is made by two genes, the SMN1 and SMN2 genes. Most patients with spinal muscular atrophy lack the SMN1 gene but have the SMN2 gene, which mostly produces a 'short' SMN protein which cannot work properly on its own. There are different forms of SMA and a wide spectrum of how severely children and adults are affected. The most common form is known as ‘5q SMA’; referring to the chromosome containing the genetic cause.
When a child or adult is first diagnosed, the doctor gives a clinical classification - SMA Type I, II, III or IV - which reflects the age of onset of symptoms and the motor milestones that someone would be expected to achieve. The severity of the condition varies from person to person, both within and between ‘Types’ - each is affected differently.
The four primary types of SMA
SMA Type I
The most severe and the most common — is usually diagnosed during an infant’s first six 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 I can be fatal early on in life. 60% of all SMA cases are Type I. SMA Type I is also known as Werdnig-Hoffmann disease.
SMA Type II
Type II is usually diagnosed after six months 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.
SMA Type III
Type III SMA 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
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.
SMA in their own words
Our beautiful child started missing key development milestones and we knew something was wrong. A genetic test confirmed SMA. We were devastated, but in the years that followed we have been amazed and just so proud of everything our little girl has achieved. Sure, there are difficult days, but we wouldn't swap her for the world.
When I was diagnosed with Spinal Muscular Atrophy in the 1980s there was no medical treatment available. In more recent years we have seen amazing developments, such that a child born with SMA today has the possibility of reducing the worst effects of the condition. Who knows what new discoveries lie ahead?
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) 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 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.
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 and major muscle cramps.