Spinal muscular atrophy, also known as SMA, is an inherited neuromuscular condition that affects the nerve cells (motor neurons) of the spinal cord. These nerve cells transmit signals from the brain to the body’s muscles. Because these connecting nerve cells are damaged, the brain’s signals are not received properly by the muscles, and so the muscles gradually become weaker (atrophy).
SMA is caused by a defect in the gene called SMN1, which stands for survival motor neuron 1. This gene encodes a protein with a similar name, called Survival Motor Neuron (SMN). This is an important protein for maintaining the health of motor neurons; when it is faulty or missing, motor neurons start to shrink, and eventually will die. This is what leads to the broken signaling connection between the brain and muscles.
People with SMA can have troubles with breathing, crawling, sitting, walking, head control, and feeding. Often, babies with SMA are described as “floppy” because of weak muscles. SMA does not affect intelligence or learning.
The severity of SMA can vary greatly – the severity usually corresponds with the age at which the symptoms start to appear, and so the disease is often categorised into different “types”. The most severe type is usually diagnosed within the first 6 months of a child’s life, while less severe types may be diagnosed in the teenage years or adult life.
Type I SMA, also known as Werdnig-Hoffmann disease, is the most severe type of SMA. It is usually diagnosed within the first 6 months of age, though sometimes the symptoms are noticeable almost immediately after birth. Babies with Type I are described as very “floppy”. They are not able to lift their head or accomplish early motor skill milestones, and they cannot sit up without assistance or support.
Type II SMA, also known as Dubowitz disease, is a slightly less severe type of SMA. Children are usually diagnosed by 6-18 months of age. They are able to sit up with some assistance, but cannot walk by themselves. They may be able to stand with assistance items such as braces and standing frames.
Type III SMA, also known as Kugelberg-Welander disease, is more variable in severity and age of onset. Some children are diagnosed by three years of age, whilst others may be diagnosed well into adolescence. People with Type III SMA can often achieve normal early motor milestones, and can stand and walk on their own, but they are likely to lose the ability to walk as they grow older.
Type IV SMA, sometimes referred to as late-onset Type III SMA, is generally diagnosed in adults after symptoms of gradual muscle weakening start to manifest. People with type IV may require wheelchairs for mobility.