Patients with spinal muscular atrophy (SMA) are more susceptible to becoming tired during physical activity, which, in addition to muscle weakness, may contribute significantly to their decreased motor function, a study finds.
The study, “A continuous repetitive task to detect fatigability in spinal muscular atrophy,” found that a simple, repetitive manual exercise, such as the repeated nine-hole peg test (r9HPT), can help detect and quantify this kind of muscular fatigue in SMA patients, in particular those with SMA type 2. It was published in the Orphanet Journal of Rare Diseases.
SMA patients experience muscle wasting and weakness, and many also report a lack of stamina and a tendency to get tired, or fatigability, which can further limit their daily activities and social life.
Walking tests and muscle contraction exercises suggest that muscle fatigue also plays an important part in patients’ motor incapacity.
Motor scales that have been used in SMA research do not specifically address muscle fatigability. As a result, there is the need for “additional tools to determine the presence, extent and causes of fatigability,” the authors believe.
A team led by researchers at the Brain Center Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht in the Netherlands, conducted a study to evaluate if repeating a simple exercise called the nine-hole peg test (9HPT) would help quantify and determine the impact of fatigability on impaired motor function of SMA patients.
The study included 52 patients with SMA older than 5, of whom 23 had SMA type 2, 27 had type 3, and two had type 4, who were recruited from the Dutch SMA register. All of them had a deletion or a combination of a deletion and a mutation in the SMN1 gene.
Two control groups were included: a disease control group of 29 patients with other neuromuscular diseases (mostly Duchenne muscular dystrophy, hereditary motor sensory neuropathy, and limb-girdle muscular dystrophy), and a group of 17 healthy individuals.
All participants in the intervention group were asked to perform five rounds of the 9HPT. This is a test commonly used by occupational therapists as a simple and quick assessment for finger dexterity that has also been widely used to assess upper extremity function in people with neurological problems such as multiple sclerosis.
The 9HPT consists of taking out nine lightweight plastic pegs one by one from a container and placing them in nine holes on a board as fast as possible, then removing them one at a time and putting them back in the container.
Participants were asked to perform a repetitive version of the test, or r9HPT, in which they did five consecutive rounds without a break, using the same, preferred hand.
The time needed to complete each round was recorded and used to assess performance and how quickly participants would tire, as well as associations with disease characteristics.
The analysis revealed that the time required by patients with SMA type 2 increased with each round, indicating a reduced ability to maintain a simple activity that mimics hand function in daily life.
Patients with SMA type 2 performed the test significantly slower than all other groups, and disease controls were slower than healthy controls.
In contrast, no significant slowing down of 9HPT performance was observed between patients with SMA type 3 or type 4 and disease controls or healthy controls.
The time needed to complete each round increased in 15 patients with SMA type 2 (65%), eight with SMA type 3 (30%), nine disease controls, (31%) and one healthy control (6%).
“These observations imply that fatigability in SMA type 2 is, at least partially, a separate dimension next to muscle weakness in SMA,” the researchers wrote.
There was no effect, however, of age at disease onset and disease duration, both surrogate markers for disease severity, on the fatigability of SMA type 2 patients.
These findings indicate that “fatigability may represent an important dimension of reduced motor function, in addition to weakness” and that a simple, and reproducible test like the r9HPT test can be used to address this dimension even in SMA patients with minimal arm function.
Researchers propose that future studies may include heart rate monitoring, muscle electrical activity recording, and questionnaires to assess pain and motivation while the test is performed to create a more robust measure of muscle fatigability.
Other types of repetitive exercises may also be tailored for specific patient groups, namely in those with SMA types 3 and 4, as their performance in the r9-HPT was as good as healthy controls, despite often having muscle weakness and complaining of fatigability in their daily life.
“Developing tailored measures to quantify fatigability, implementable in clinical trials, could be an important step towards the development of (add-on) medication to treat fatigability in SMA, improving quality of life,” the team concluded.