Spinal muscular atrophy or SMA is a genetic disease that affects the nervous system. The disease affects that part of the nervous system that controls voluntary muscle movement. The muscles don’t receive signals from these nerve cells that are located near the spinal cord and as a result the muscles begin to waste away. The inactivity of the muscles causes them to become smaller and smaller and hence the word “atrophy”.
The age of the person decides the degree to which the motor functions are affected. The sooner the onset, the higher the effect on motor functions. Infants who display symptoms will typically have very low levels of functioning. This is Type 1. SMA in toddlers(intermediate), children(juvenile) or adults – type 2, 3, 4 respectively – will see higher levels of motor function.
SMA is generally caused by a mutation in the SMN1 gene – a deficiency of a motor neuron protein called survival of motor neuron (SMN). This protein is necessary for normal motor neuron function and its deficiency is the result of genetic flaws or mutations on chromosome 5. There are other forms of SMA (non-chromosome 5) that are rare and caused by mutations in genes other than SMN.
The main symptom of SMA is the weakness of proximal muscles – the ones closer to the center of the body. These are affected much more than the distal muscles. Proximal muscles include the shoulders, hips, thighs and upper back and the weakening of these can mean curving of the spine and even difficulty in swallowing and breathing.
Since SMA is a purely muscular disease, the person suffering from it has completely normal functioning of sensory, mental and emotional states.
Spinal muscular atrophy is inherited when both copies of the SMN1 gene in an individual have mutations. Generally, the parents of one who suffers from SMA each carry one copy of the mutated gene but they do not show signs or symptoms of the disease itself. Occasionally, a person who suffers from SMA will have inherited an SMN1 gene mutation from one parent and then acquires a new mutation in the other copy of the gene. This might occur during the formation of eggs or sperm or very early in embryonic development.