Spinal Muscular Atrophy (SMA) is an inherited neuromuscular condition that affects the nerve cells (motor neurons) in an area of the spinal cord called the anterior horn. Because the nerves are damaged, the muscles don’t receive signals from the brain correctly and so become wasted, or atrophied.
This muscle wasting can lead to problems with breathing as well as with movement (motor) activities such as crawling, sitting, walking, feeding and head control. Intelligence is not affected – indeed, many children with SMA appear to be particularly bright.
People with SMA have a missing or mutated gene (known as SMN1, or survival motor neuron 1) that produces a protein in the body called Survival Motor Neuron (SMN) protein. A deficiency of this protein affects the health of the motor neurons, causing them to shrink and eventually die.
SMA is inherited in an autosomal recessive pattern, which means that in order for a child to be affected, both parents must be carriers of the abnormal gene and both must pass this gene on to their child. Carrier parents have a 1 in 4 chance of having an affected child, a 2 in 4 chance of having a carrier child, and a 1 in 4 chance of having a child who is completely free of the affected gene.
SMA varies a great deal in severity – sometimes it is clear almost immediately after birth that something is wrong, while in other cases symptoms don’t become obvious until the teenage years. Because of this, it is traditionally separated into different “types” corresponding to different levels of severity and classified according to motor milestones reached. Broadly speaking, individuals with Type I never sit, those with Type II never walk, and those with Type III walk but may lose the ability later on in life.
Type I SMA is also called Werdnig-Hoffmann Disease. The diagnosis of this type is usually made before 6 months of age, often a lot earlier. Some mothers even notice their baby is moving less in the womb in the final months of pregnancy. Usually a baby with Type I is very “floppy” and is never able to lift his/her head or accomplish the normal motor skills expected in the early months of life. They are never able to sit up unsupported.
Most cases of SMA Type II are diagnosed by 15 months. Children with this type can sit unsupported when they are placed in a sitting position, although they are often unable to reach that position without help. Some children with this type may be able to stand with the aid of assistance or bracing and/or a standing frame.
Type III SMA is also known as Kugelberg-Welander Disease. It is much more variable in age of onset: diagnosis before the age of 3 is not uncommon, but some children may not show significant symptoms until adolescence. People with Type III SMA often achieve early motor milestones normally and can stand up on their own and walk, although it is often noticeable that they fall more frequently and have difficulty running or getting up from the floor. Most will gradually lose the ability to walk as they get older.
Children of carriers
The image below shows the inheritance pattern for autosomal recessive disorders where both parents are carriers but neither is affected. Further information about genetics can be found in the links section to the left.
Children of affected individuals
When an individual affected by an autosomal recessive condition has a child, that child will of necessity be at least a carrier of the condition, since the parent has no “normal” copy of the gene to pass on. But if the other parent is neither affected nor a carrier of the condition, the child will receive one normal copy of the gene from that parent and so will not be affected themselves. This means that the majority children born to people with a recessive condition are not affected. If the second parent happens to be a carrier, there is a 50% chance that the couple will have an affected child. If both parents are affected, all their children will be affected too.