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.
More information and support for those affected by SMA can be obtained from the SMA patient organizations and advocacy groups in your country. Click here to find a group close to you.