Novel method objectively measures oblique muscle dysfunction

The authors developed and tested a method to objectively measure ocular oblique muscle dysfunction based on a three-dimensional eye model using simple photographic analysis of the corneal contour.

Subjects in this prospective observational study included 12 healthy volunteers and 27 patients with oblique muscle that was over- or underactive in at least one eye. The authors used nine-gaze photographs. A newly developed simulating program depicted corneal contour as the eyes moved and graded oblique muscle dysfunction by analyzing the angular difference between the two eyes. An angular difference of 5 degrees was defined as one unit of “overaction” or “underaction.” A masked observer compared the photographic analysis with goniometric readings in the model eye and magnetic search coil recordings  in healthy volunteers. The diagnostic accuracy of the photographic analysis was then compared with clinical grading of patients with oblique muscle overaction or underaction.

Goniometric readings of the model eye and measurements of the photographic analysis were nearly identical, and photographic analysis showed reliable agreement with the scleral search coil, supporting the validity and reliability of the software in evaluating ocular movement. The diagnostic accuracy of photographic analysis was approximately 90% for detecting ocular oblique muscle overaction (+1) and 80% for detecting underaction (−1). Comparing photographic analysis with the scleral search coil, the mean difference was mostly from the discrepancy of the horizontal angle.

The authors note that nine-gaze photographs may not fully show the amount of true dysfunction. In particular, subtle ocular oblique muscle underaction may not be fully revealed until the patient tries hard to look at an extreme gaze. This partly explains why the diagnostic accuracy of underaction (80%) was lower than that of overaction (90%) compared with clinical grading. Repeated photographs obtained at different time points may improve this error.

They conclude that photographic analysis could provide an objective and accurate means of evaluating ocular oblique muscle dysfunction. Expensive equipment and invasive techniques are not required, and the technique allows measurement of retrospective data in an objective and reproducible manner when acceptable photographs are available.