Walking Ability in Type 3 SMA Declines Significantly in Adolescence, Study Finds
Walking ability in type 3 SMA patients is significantly impaired during adolescence, according to recent research. Type 3 SMA, also known as Kugelberg-Welander disease, is a milder form of SMA. Muscle weakness in type 3 SMA mainly affects patients’ limb movements, and children are generally able to stand and walk. These abilities decrease over time, however. Symptoms usually appear between 18 months and early teens. Symptom onset before age 3 is classified as type 3A SMA, and onset at later ages as type 3B — considered a milder SMA subtype. However, while SMA is thought to be a largely stable disorder, recent studies have highlighted different rates of progression based on age and function. In patients who cannot walk, gross motor function improves until about age 5, then worsens until about age 15, followed by a relatively stable phase in late adolescence and adulthood. While ambulatory patients have a similar disease progression to that of non-ambulatory patients, a more specific assessment may be necessary to identify changes in walking function. Researchers used data from three prospective natural history studies to evaluate changes in walking ability in 73 individuals with type 3 SMA who were able to walk without support for at least 10 meters. The team used the the six-minute walk test, a valid and reliable functional assessment that measures the distance a person can walk in six minutes. While the 6MWT was initially developed to evaluate functional exercise capacity in heart- and lung-associated diseases, it has become a valuable and reliable test to measure functional walking ability in SMA patients. The study showed that the ability to walk strongly deteriorated during adolescence with a loss of 20.8 meters per year, and slowly declined again through adulthood with a loss of 9.7 meters per year. The age around puberty appeared to be the most vulnerable period in ambulant patients, with researchers hypothesizing that weight gain and growth associated with puberty could contribute toward a greater decline in walking capacity. The study authors emphasized that when designing future clinical trials with SMA patients, it is important to understand the natural history of the disease and identify disease trajectories as measured by the 6MWT to better interpret patients' response to treatment.