Impact of Viewing Distance on the Occurrence Rate of Intermittent Exotropia Revealed by Ambulatory Eye Tracking

ABSTRACT Purpose Eye tracking in ambulatory patients permits measurement of the occurrence rate of intermittent exotropia. This information is useful for deciding which patients need surgery. However, viewing distance can influence occurrence rate, creating a potential confound. To address this issue, an approach was devised to break down the overall occurrence rate into separate components, compiled during either near viewing or far viewing. Design Prospective cross-sectional study. Subjects 35 patients with intermittent exotropia, ranging in age from 3 to 79 years, median 10 years. Methods Eye tracking glasses were used to record each eye's horizontal position while subjects engaged in their customary daily activities. Scene video was reviewed to parse the data into epochs of near viewing versus far viewing. Main Outcome Measures Four exotropia occurrence rates were calculated: 1) overall rate before segmentation by viewing distance, 2) overall rate after segmentation by viewing distance, 3) rate during near viewing only, 4) rate during far viewing only. In addition, eye positions during near viewing and far viewing were compared to assess convergence effort during exotropia. Results In participants with a low occurrence rate, exotropia occurred mostly during distance viewing. As the occurrence rate increased, exotropia became more common during near viewing. For 28/35 patients, segmentation of data into near versus far viewing had a negligible impact on the overall occurrence rate. However, in 7 patients this process revealed a near exotropia peak partially hidden in the orthotropia far peak, which resulted in a higher overall exotropia occurrence rate after segmentation by viewing distance. Comparison of exotropia amplitude during near versus far viewing showed that convergence effort varied widely among deviated subjects, and surprisingly, bore no relationship to the overall rate of exotropia occurrence. Conclusions Partitioning of data recorded with eye tracking glasses into near viewing versus far viewing allows a more accurate assessment of the overall rate of exotropia occurrence. A major advantage is that the impact of viewing distance associated with various activities is neutralized, because separate exotropia occurrence rates are provided for near versus far. Incorporation of range finding capability into currently available mobile eye trackers would enhance their utility for evaluation of patients with exotropia.