SMA eye movements suggest brain changes, not weakness: Study
People with spinal muscular atrophy (SMA) may show subtle abnormalities in eye movements, but these abnormalities are unlikely to be caused by weak muscles; instead, they likely arise due to changes in the brain, a study found.
The study, “Central eye movement abnormalities in patients with SMA types II and III,” was published in Neurophysiologie Clinique.
SMA is a genetic disorder marked by muscle weakness and wasting. Some studies have indicated that people with SMA may experience issues with extraocular muscles (those that move the eyeballs and eyelids), but evidence is contradictory and has come from case reports published before genetic testing was available.
Scientists in Turkey used a technique called videonystagmography to track eye movements in 18 SMA patients and 21 people without the disease. Videonystagmography uses videos to capture details of eye movement that may not be discernible to human observers. According to the scientists, it has been used in people with vertigo and in several degenerative, hereditary, and metabolic neurological disorders.
Most of the patients in the study had SMA type 3, while the rest had type 2. The group included participants who could walk independently, those who could walk with assistance, and patients who needed wheelchairs.
Eye movement speed similar in patients, those without SMA
The researchers found that overall speed of eye movement in SMA patients was similar to that of people without the disease. This suggests that the SMA patients didn’t have weaker extraocular muscles, since weaker muscles wouldn’t be able to move as quickly.
But people with the disease did show some abnormalities. In measures of saccades — quick eye movements where the gaze quickly snaps to a new point — the SMA patients had less accuracy and shorter latencies. And in measures of smooth pursuit movement — the ability to track a moving object — SMA patients didn’t perform as well as those without the disease.
According to the scientists, these findings are more characteristic of changes in the brain than weakness in extraocular muscles. People with SMA may have subtle brain changes that lead to eye movement abnormalities, they said.
“These findings suggest that oculomotor abnormalities in SMA are more likely to reflect central dysfunction of [certain brain] pathways, rather than peripheral involvement of [extraocular muscles],” the researchers concluded. They stressed that their study was limited to a few patients at one center, so additional studies will be needed to validate the findings.
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