New method for quantifying visual midline shifts may aid understanding of the condition
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Key takeaways:
- Unlike the current clinical method for measuring visual midline shifts, the visual midline gauge provides quantitative results.
- Visual midline gauge measurements were repeatable.
Using a visual midline gauge to measure midline shifts in patients following a stroke may be more effective compared with the current manual clinical method, according to a study published in Optometry and Vision Science.
“The current clinical method is subjective on the part of the clinician, likely suffers from intra/interobserver variability and does not give quantitative results, which are needed for monitoring visual midline shifts and may be important for determining a prism to be used for treatment,” Amritha Stalin, PhD, a postdoctoral fellow at the University of Waterloo School of Optometry & Vision Science in Ontario, Canada, and colleagues wrote.
“To study visual midlines, their effects and the effectiveness of prism management, it is necessary to have a standardized method to quantify the visual midline shifts,” they continued. “In addition, to determine whether a person from a clinical population has a midline shift, it is necessary to compare against a normal range in people without disease.”
Thus, researchers conducted a cross-sectional study to standardize the test distance, speed and repositioning method of the new visual midline gauge, compare this method with the current clinical method and gather normative data across age groups.
The analysis included 93 adults with unimpaired vision and no history of stroke or other neurological disorders from the University of Waterloo and Hong Kong Polytechnic University who researchers divided into three age groups: 18 years to 44 years, 45 years to 64 years, and 65 years and older.
Participants attended two visits. During the first, the researchers measured horizontal and vertical visual midlines using the visual midline gauge at 50 cm with speeds of 1.15° or 2.3° per second and the adjustment and repeated method of repositioning the target for accuracy.
During the second visit about a month later, they measured vertical and horizontal visual midlines using the target speed of 2.3° per second and the adjustment method of repositioning, based on results from the first visit.
Both visits also included the current clinical method to measure visual midlines.
Between the first and second visits, the researchers assessed repeatability of both methods using Bland-Altman plots.
In the first visit, Stalin and colleagues observed no significant effects of repositioning method, speed, study location or age group on visual midline positions. However, in the second visit, they reported a significant effect of distance for the horizontal direction (P = .001), with significant differences between 25 cm and 50 cm (P = .03) and between 25 cm and 100 cm (P = .001).
In terms of repeatability, the Bland-Altman plots showed that bias was –0.12° in the horizontal direction and –0.42° in the vertical direction for the visual midline gauge, with corresponding figures of –0.06° and –0.01° for the clinical method.
The researchers also observed significant correlations between measurements using the visual midline gauge and the clinical method for the vertical direction (P < .0001) in both visits, but not for the horizontal direction.
“The study has shown that the measurement of the perceived midline is relatively insensitive to the target speed and testing distances within the range that was tested,” Stalin and colleagues wrote.
“Future studies are indicated to measure repeatability of the visual midline gauge and the agreement between the clinical measure and the gauge in post-stroke populations and others with neurological disorders,” they added.
Perspective
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In post-stroke patients, it is undetermined whether visual midline shift varies across personal space to a similar degree as that of spatial neglect. The effects of midline shift raise safety concerns regarding postural and balance disturbances that pose a significant fall risk for patients.
This study sets out to propose a new method for standardizing quantification of visual midline shift by testing a small sample size of 93 adults without neurological or visual impairment. Compared with the current clinical method, the mechanical gauge decreases subjectivity but otherwise determined that measurement of perceived midline was insensitive to the target speed and distances that were tested.
The study design and results are more academic and do not appear to have relevant clinical application. While it is suggested that there is some relationship between the degree of midline shift and the amount of yoked prism needed to correct abnormal posture, there is no discussion as to how results from post-stroke patients would be used to determine management.
It is questionable whether a new quantification method is necessary at all, or if trialing varying amounts of prism in a clinical setting would provide a more efficient and effective treatment regimen based on each patient’s individual deficits.
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