Signs of muscular dystrophy possible in SMA: Case report
The case of a man with clinical signs of muscular dystrophy who was ultimately diagnosed with adult-onset spinal muscular atrophy (SMA) was described in a recent case report.
Researchers hope the findings highlight that when signs of muscular dystrophy, such as enlarged muscles, are observed it does not necessarily rule out SMA. Care must be taken to look holistically at all clinical signs and symptoms.
The findings were detailed in a brief report, “Limb-girdle muscle weakness and muscle hypertrophy: Do not dismiss spinal muscular atrophy,” in Revue Neurologique.
In SMA, the motor nerve cells that control voluntary muscle movements (motor neurons) degenerate, leading to progressive muscle weakness and low muscle tone. The main symptom of SMA is increasing weakness in the proximal muscles, or the ones closest to the center of the body including the core (trunk), upper arms, and upper legs.
This can look similar to a group of conditions called myopathies, where damage directly to muscle fibers causes muscle weakness and wasting. That includes muscular dystrophy, a group of inherited diseases caused by mutations in genes important for muscle health.
Patient’s symptoms included leg weakness, swelling calf muscles
The recent report described the case of a 24-year-old man who was eventually diagnosed with adult-onset SMA type 4 and who presented to the clinic with symptoms that mimicked muscular dystrophy.
He came to the researchers’ hospital in Spain reporting progressive weakness in both of his lower limbs over the last six years, manifesting as difficulty standing up or walking up stairs. In the last two years, the man said his calf muscles had been swelling, but that it was painless. He had no family history of these symptoms.
The researchers found signs of muscle wasting, or atrophy, near the shoulders and armpits, with increased size, or hypertrophy, of muscles in the calves and shoulders.
A limb-girdle pattern of muscle weakness was observed, where weakness mainly affects the muscles in the shoulder and pelvic areas. Some loss of muscle stretch reflexes and involuntary muscle twitches (fasciculations) were observed.
Blood biomarkers of muscle damage were elevated, and MRI imaging showed swelling and fatty deposits in the calf muscles.
Electromyography was then used to look at the health of motor neurons and the muscles they control. Essentially, the test looks at how muscles respond to stimulation from motor neurons and can distinguish whether any problems are coming from the nerve cells (neurogenic) or the muscles (myopathic). Results from this test showed signs of neurogenic changes in several muscles.
Certain symptoms the man experienced, such as a limb-girdle pattern of muscle weakness and hypertrophy, are signs of limb-girdle muscular dystrophy and are rarely seen in SMA. Still, there were a few factors that led the doctors to suspect adult-onset SMA.
That included the reduced reflexes and fasciculations, as well as the neurogenic changes seen on the electromyography exam. Those types of changes are consistent with the motor neuron damage observed in SMA, whereas in diseases such as muscular dystrophy, the problem would be myopathic.
As such, the man underwent genetic testing and SMA was confirmed when a mutation in the SMN1 gene, the known cause of SMA, was identified.
The researchers believe the findings highlight that neurological disorders should not be ruled out when muscle hypertrophy is observed, because while “encountering muscle hypertrophy on examination in SMA is rare,” it is still possible.
Still, the mechanisms underlying how this might arise secondarily to nerve damage is not established.
“We hope further research will elucidate possible mechanisms of muscle hypertrophy in neurogenic conditions,” the researchers concluded.
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