Comparison of Harms Tangent Screen and Search Coil Method in Patients with Trochlear Nerve Palsy

Abstract

Abstract: : Purpose: To compare a subjective with an objective method of measuring torsional, vertical, and horizontal gaze deviation in patients with different forms of trochlear nerve palsy (TNP). Methods: 23 patients were recorded in nine different gaze directions with the harms tangent screen that is a subjective test to evaluate three–dimensional binocular alignment. Also search coil recording was used that provides objective data as the patient is not able to influence vertical and torsional deviation between the fixating and the non–fixating eye. Results: Average horizontal gaze deviation measured with search coils ranged from 8,1° esophoria to 7,3° exophoria and correlated very well with Harms tangent screen (R²=0.81, p<0.01), as did the average vertical gaze deviation (range coils: 3.6°–15.0°; R²=0.58, p<0.01). Also the degree of vertical incomitancy (standard deviation of the 9 gaze directions) did correlate using both methods (R²=0.29, p<0.01). Congenital TNP was more comitant than acquired TNP in torsional deviation (2.8° vs 5.2°; p=0.047, analysis of variance) but not in vertical deviation (3.8° vs. 3.5°, measured with scleral search coils). Torsional incomitancy was less pronounced with Harms tangent screen compared to the search coils. Younger patients were better able to subjectively show the larger cyclovergence with the Harms tangent screen than older patients. Conclusions: Both tests are comparable in horizontal and vertical eye alignment. Subjective measurement of torsional deviation underestimates the torsional incomitancy measured at the oculomotor level. Therefore, central torsional fusional mechanisms can be assumed.

Keywords: strabismus • eye movements: recording techniques • eye movements