Diana Castro, MD, founder of the Neurology Rare Disease Center in Dallas, Texas discusses newborn screening for spinal muscular atrophy (SMA) in the United States.
SMA is a genetic disease that affects motor neurons in the spinal cord and control of muscle movement. It is caused by a mutation in the survival motor neuron gene 1 (SMN1) which affects nerve cells and leads to loss of muscle function. Interestingly, there is more than one gene that encodes for the SMN protein. The predominate gene responsible for producing SMN protein is the SMN1 gene. However, there is a second gene, SMN2 gene, that can also produce a form of SMN protein. The number of copies of that second gene can play an important role in the severity of the SMA symptoms and age of onset.
There are several types of SMA based on age of onset and the manifestation of the disease. Type 0 SMA begins prenatally while those with type 1 start to show symptoms at birth or soon thereafter. Type II, diagnosed in children between the ages of 6 months and 2 years, results in a delay or failure to reach motor milestones. Children with Type III, usually diagnosed before age 3 and sometimes as late as their teenage years, learn to walk but have mobility problems as they grow. Other forms of SMA that are caused by mutations in genes other than SMN1 are spinal muscular atrophy respiratory distress (SMARD), distal SMA, and Kennedy’s disease.
SMA is a progressive disorder. Therefore, the sooner a person with SMA can be diagnosed and be given proper treatment, the better their chances of maintaining a certain level of activity and quality of life.
Due to the progressive nature of the disease, and the fact that treatment is available to attenuate that progression, SMA is part of newborn screening in most states.
As Dr. Castro explains, testing positive for SMA in a newborn screening still requires confirmation with genetic testing. Further testing can also determine the type of SMA the baby may have that can guide treatment.
Dr. Castro also noted that newborn screening can detect about 95% of SMA children but there is a small percentage of people with SMA that are not diagnosed by newborn screening. Early signs and symptoms for SMA in those patients should necessitate genetic testing
