Spinal muscular atrophy

Spinal Muscular Atrophy (SMA) is a group of rare, genetically inherited neuromuscular conditions. They cause progressive (deteriorates over time) muscle weakness and loss of movement due to muscle atrophy (wasting) in all of the muscles in the body. The more severe forms of SMA can affect the breathing and swallowing muscles.

SMA affects the central nervous system and the peripheral nervous system. Classified as a motor neuron disease, it occurs when motor neurons (nerve cells in the spinal cord controlling muscle movement) progressively lose function.

 SMA can affect:

●        Walking and crawling

●        Arm and hand movement

●        Head and neck movement

●        Breathing and swallowing

The brain is not affected in SMA and affected individuals have normal cognitive development and do not show learning difficulties or behavioural problems.

A mutation or deletion in the SMN1 gene on chromosome 5 causes the most common form of SMA, known as 5q SMA. The faulty genes do not produce enough SMN protein to keep motor neurons healthy. This causes loss of the motor neurons connecting the brain and the spinal cord to the leg, arm and trunk muscles, resulting in muscle atrophy. SMA is often classified as one of the ‘Types’ described below.

Disease types

5q SMA is the most common form of SMA, named due to its genetic cause. It is often further classified as one of four main “types”. The different SMA ‘types’ are diagnosed based on the age symptoms appear and the motor development milestones reached. Greater severity is generally associated with earlier onset of symptoms, but the severity can vary widely from person to person, and within and between types. 

There are also rarer forms of SMA that have different genetic (non-5q SMA) causes.

Link to page 9 of the Guide to the 2017 International Standards of Care for SMA