Three-dimensional Hess screen test with binocular dual search coils in a three-field magnetic system

PURPOSE. To establish an objective Hess screen test that allows a simultane ous and binocular analysis of all three axes of eye rotation. METHODS. in orthotropic and strabismic human subjects, both eyes were recor ded with dual scleral search coils in a three-field magnetic system. Before mounting the search coil annuli on the eyes, the voltage offsets of each c hannel and the relative magnitudes of the three magnetic fields were determ ined. For calibration, subjects were only required to hx monocularly on a s ingle reference target. During fixation of targets on the Hess screen by th e uncovered eye, the three-dimensional eye position of both the occluded an d the viewing eye was simultaneously measured. RESULTS. For clinical interpretation, an easy to understand graphical descr iption of the three-dimensional Hess screen test was developed. Positions o f orthotropic and strabismic eyes tended to follow Listing's law, which in both eyes allowed the determination of the primary position, that is, the p osition of gaze from which pure horizontal and pure vertical movements do n ot lead to an ocular rotation about the line-of-sight. To a first approxima tion, the location of primary position is a result of the summation of the individual rotation axes of the six extraocular muscles and thus can be use d to infer which muscle is paretic. CONCLUSIONS. The three-dimensional Hess screen test with binocular dual sea rch coils in a three-field magnetic system is an objective method to assess the ocular alignment in three dimensions with high precision. From these r ecordings, the clinician can relate deviations of primary position to speci fic eye muscle palsies.