What is Spinal Muscular Atrophy?
Spinal Muscular Atrophy (SMA) is an inherited neuromuscular condition that very specifically affects nerve cells in the spinal cord called lower motor neurons. Acting as a message delivery system allowing the conscious contraction of muscles, electrical signals originating in the brain are fired down the spinal cord, along upper motor neurons and on to skeletal muscles via the lower motor neurons.
In SMA, the lower motor neurons degenerate, impairing the link between the brain and muscles. These muscles can no longer be excited, which causes them to atrophy, or wither, due to inactivity. The motor neurons mainly affected in SMA are those which enable walking, crawling, arm and hand movement, head and neck movement, and swallowing. The muscles used in breathing can also be affected leading to breathing complications. The brain and sensory nerves, which allow us to feel sensation such as temperature and touch, remain relatively unaffected in SMA.
SMA is a relatively rare disease that affects approximately 1 in 6,000-10,000 babies born. SMA is a recessive genetic disorder, meaning that two faulty copies of the disease gene have to come together, one from each parent, for the disease to occur. When both parents carry a faulty copy of the disease gene, there is a 1 in 4 chance in each pregnancy of the baby being affected by SMA. About 1 in 40-60 of us carries one of these faulty gene copies that cause the condition. For more information on the population statistics of SMA, click here.
There are 4 main types of SMA - Types 1, 2 and 3 appear in childhood, while Type 4 affects adults, and is known as Adult Onset SMA. The disease gene behind Types 1-4 is called Survival Motor Neuron 1 (SMN1).
Other rarer forms of SMA include SMA with Respiratory Distress (SMARD), Spinal Bulbar Muscular Atrophy (SBMA), and Distal SMA (DSMA). These diseases are genetically distinct and are caused by faults in different genes.
The inheritance pattern of the adult and rarer forms of SMA can be different from the childhood forms. All types of SMA produce muscle weakness but in varying degrees of severity. Of the conditions caused by faults in the SMN1 gene, Type 1 SMA is the most severe and is usually fatal with two years of birth.
Please click on the relevant type of SMA to see the available information.
