A new research study has determined that the muscle weakness found in children who have spinal muscular atrophy (SMA) type 1 may be due to decreases in muscle thickness over time.

Researchers at Washington University School of Medicine in St. Louis used ultrasound technology to measure the muscle thicknesses in the arms and legs of three young children with SMA type 1. They discovered that the children were losing muscle thickness as time progressed.

The study, called “Quantitative Muscle Ultrasound Measures Rapid Declines Over Time in Children with SMA Type 1,” was published in October in Journal of the Neurological Sciences.

Researchers noted that muscle changes do not occur in children who have SMA type 2 or 3, but wanted to determine if muscles changed in children who have the most severe form of the condition. The research team, led by Dr. Kay W. Ng, tested infants who were 1 month, 6 months and 11 months old, then repeated the tests two or four months later.

According to a news announcement about the study, “Although at baseline the children showed normal muscle thickness, except for the quadriceps (thigh) muscle in the oldest child, at the later time point, muscle thickness decreased. All three children showed lower than normal quadriceps muscle thickness.

Negative changes were also noted in the biceps of two children and the anterior forearm of one child, but the tibialis anterior (shin) muscles were unchanged in all three. This indicates that not all muscles are affected equally throughout time by SMA type 1 — muscles closer to the body were more affected than those further from the body.”

Researchers chose to use ultrasound technology, they added, because it “is a relatively simple and less painful technique to measure muscle thickness and function in children.” Other studies have used magnetic resonance imaging (MRI) technology, which the researchers noted can require higher levels of training for personnel and can be painful for the children involved.