Objective Evaluation of Inferior Oblique Overaction Using an Interocular Level Difference Gauge

The conventional method for evaluating inferior oblique overaction (IOOA) relies on subjective, qualitative grading. This study aimed to introduce and perform a pilot evaluation of a novel, objective, and quantitative measurement method for IOOA using a newly designed device, the interocular level difference gauge (IOLDG).

Sixty-two consecutive patients (124 eyes) with intermittent exotropia, IOOA or superior oblique palsy were recruited. The degree of IOOA was measured using two methods: (1) the conventional subjective grading scale (+0 to +4) by two examiners (A and B); and (2) the objective IOLDG method by two examiners (C and D). The IOLDG utilizes a spectacle-like frame with a 1 mm-spaced horizontal grid to measure the interocular height difference of the inferior corneal margin in millimeters. Interobserver repeatability was assessed using the paired t-test and the intraclass correlation coefficient (ICC).

The conventional method showed a statistically significant difference between examiners (paired t-test, p = 0.002) and moderate repeatability (ICC, 0.749). In contrast, the IOLDG method showed no significant difference between examiners (paired t-test, p = 0.064) and demonstrated excellent repeatability (ICC, 0.903; p < 0.001). The correlation analysis established quantitative equivalents for the conventional grades: grade +1 corresponded to 1.04 ± 0.75 mm, grade +2 to 1.75 ± 0.60 mm, and grade +3 to 2.81 ± 0.75 mm.

The IOLDG provides a highly repeatable, objective, and quantitative measurement for IOOA, largely independent of the examiner's subjective judgment. This simple, chair-side method offers superior reliability compared to the conventional grading scale.