Patients with different muscular-coordination conditions, or ataxia, exhibit different eye reflexes after head impulses, a finding that could help doctors determine the severity of a patient’s disease.
The video head impulse tests (vHIT) that researchers used showed differences in eye movements between patients with spinocerebellar ataxias and patients with Friedreich’s ataxia. The study included three kinds of spinocerebellar ataxia — types 1, 2 and 3, or SCA1, SCA2 and SCA3.
The research, “Vestibulo-ocular reflex dynamics with head-impulses discriminates spinocerebellar ataxias types 1, 2 and 3 and Friedreich ataxia,” was published in the Journal of Vestibular Research.
Researchers decided to compare patients’ vestibulo-ocular (VOR) ability, or ability to maintain visual stability during head movements.
The vHIT test allowed them to count saccades, rapid eye movements with changes of focus triggered by a head impulse.
Previous research has shown that rapid eye movements are a normal response to head impulse in patients with central nervous system disorders. Other research has indicated that patients with inherited ataxias also respond to head impulses with rapid eye movements.
Those findings prompted researchers to use vHIT tests to try to determine which type of ataxia a patient had.
They tested 23 patients with spinocerebellar ataxia, nine with Friedreich’s ataxia, and 40 healthy controls. The spinocerebellar patients included four with SCA1, four with SCA2, and 15 with SCA3.
One measurement they used was VOR latency, or the time required for the eyes to adjust to a head impulse. Another was instant VOR gain and a third was VOR loss. The gains and losses were measured at 40-, 60- and 80-millisecond intervals.
The VOR latency that researchers found in Friedreich’s ataxia patients was considerably higher than in spinocerebellar patients, enough that they concluded they could use the results to differentiate the two. They also found higher latency in SCA3 patients, but not to the degree in Friedreich’s patients.
In addition, the team found lower VOR-gain and VOR-loss levels in Friedreich’s and SCA3 patients, but only SCA3 patients had levels low enough to distinguish them from other patients.
The team looked at two kinds of rapid eye movements — those that occurred during a head impulse and those that occurred afterward.
They discovered that only SCA3 patients showed rapid eye movements during a head impulse, while all patients showed movements after an impulse.
Researchers also found that the higher the VOR regression score, the lower the patient’s score on the Scale for the Assessment and Rating of Ataxia (SARA). That suggested that increases in VOR gain may be used as a biomarker for determining disease severity.