What is SMA

SMA stands for Spinal Muscular Atrophy. It is a rare inherited motor neurone disease. Motor neurones are the cells that relay messages from the brain and spinal column to the muscles to make them contract and allow us to move. It is estimated as many as 1 in 40 people carry the mutated gene for SMA and when 1 person who carries the mutated gene has children with another person who also carries the mutated gene there is a 25% chance in each pregnancy of the baby inheriting both defective genes – resulting in a child with SMA. As was the case for us, often the carriers of SMA have no idea that they carry the gene until they have a child affected with the condition.

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The 2 defective genes that result in SMA are called SMN1 genes and they are located on the pairs of chromosome 5. SMN1 in healthy individuals makes a protein called Survivor Motor Neurone (SMN), however in SMA it doesn’t produce any SMN. SMN is the protein that maintains healthy motor neurone cells. However there is an almost identical gene copy in the body called SMN2, this gene only produces around 10% of the functional SMN protein that is made by the SMN1 gene and this is not enough to maintain healthy motor neurone cells. Usually people have 2 copies of SMN2, but some people can have as many as 4. Although around 10%, the amount of functional protein made by the smn2 gene can vary widely from one person to the next and this, along with SMN2 copy number, can lead to different levels of severity in SMA classified into different “Types”.

There are 4 types of SMA. Type 1 is the most severe and symptoms are displayed before 6 months and the child never achieves the ability to sit unsupported. Children with Type 1 are not expected to live past 2 years of age. Type 2 displays symptoms between 6 and 18 months and although these children are usually able to sit unsupported, they are usually unable to stand or walk. The majority of children with type 2 are expected to live into adulthood. Type 3 children display symptoms after 18 months and usually these are difficulty running or climbing stairs, problems with balance or a slight tremor of the fingers. Type 3 doesn’t usually effect life expectancy. Type 4 is adult onset and is the mildest form, which may present symptoms of muscle weakness in the hands and feet, tremors and difficulty walking.

Due to the damage to motor neurone cells in SMA, children with the most severe forms experience increasing muscle weakness over time. The muscles closest to the spinal cord are affected the most severely. This means that these children struggle to do the things we take for granted, such as walking, swallowing and breathing. Cognitive function is unimpaired and children with SMA are often considered to have above average intelligence. As problems with swallowing present themselves a feeding tube may have to be used in order to sustain nutrition. As breathing difficulties develop non-invasive ventilation may need to be considered in the first instance, but longer term problems maintaining the ability to breathe become life threatening. For children with SMA getting a cold has the potential to become life threatening event very quickly and must be treated aggressively by medical staff.

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