Measuring inferior oblique muscle overaction

Purpose: We developed a reliable and reproducible method of quantifying inferior oblique muscle (IOM) overaction. Strabismus surgery is usually quantified based on measurements of the deviation. Unfortunately, there is no method to quantify IOM overaction to support surgical decisions. Methods: Patients with IOM overaction (cases) and normals (controls) underwent a complete ophthalmic exam and the IOM function was estimated "clinically" on a scale of 0 to +4. Binocular photographs were taken of the patient's eyes with the abducted eye fixating in right and left gaze and in upper right and left positions; five ophthalmologists estimated the "clinical" IOM function. Monocular photographs of the adducted eye were made in the same fields of gaze with the patient wearing a lens trial frame. The IOM action was quantified in degrees using the photos with the trial frame; the measurement in degrees was calculated by drawing a line from the light reflex through the limbus to the trial frame. The "clinical" values by the five ophthalmologists analyzing the binocular photographs were compared as were the values found in the office charts; "clinical" chart values were compared with those from the binocular photos. Both "clinical" groups were compared with the degrees (trial frame). All comparisons were statistically analyzed. Results: We analyzed data from 50 controls and 25 cases. There was no significant difference in the measurements of the IOM function with the 0 to +4 scale in the office and by binocular photography; there were considerable interobserver differences. The monocular assessment with the trial frame was reproducible. Conclusions: This study showed that measuring the IOM function in degrees using a trial frame is an easy, inexpensive and reproducible method and gives the surgeon a support in the decision regarding an appropriate surgery.