Motor neurons send signals to muscles from the CNS. These neurons need SMN protein in order to work. Without SMN protein, motor neurons die off. With no signals from the CNS, muscles get weaker and weaker.
The rate of disease progression varies from person to person, and there is no sure way to tell when someone with SMA will start to lose motor function.
It can be hard to notice motor function loss with annual checkups because it may be happening slowly. But that doesn’t mean it isn’t happening. Such loss becomes more obvious as it continues over time.
Because everyone experiences SMA differently, there is no way to predict when motor function loss will happen or even who will experience it.
Genetic testing is an important first step in accurately diagnosing rare diseases like SMA. In cases where a family history of SMA exists or symptoms suggestive of the disease present themselves, genetic testing can help to confirm the diagnosis by looking at genetic material to determine if the SMN1 gene is missing or damaged. Some states also have newborn screening for SMA to help identify and plan for treatment before symptoms even develop. Above and beyond this, genetic testing is often required to initiate SMA treatment.