Franka Risch

Franka Risch

Master's Thesis

Measurement of Ocular Deviation with an VR Hess Screen Test Using Eye Tracking


09/2020 – 03/2020

Strabismus is with a prevalence of approximately 4% one of the most common chronic eye diseases among children and juveniles in Germany [1]. As strabismus in young age can cause restriction in the visual development an early diagnose is crucial to prevent a permanent damage of
the visual system, such as cortical suppression of one eye, called amblyopia [2]. Multiple approaches measure ocular deviation, like the Hirschberg Test and the Cover/ Uncover Test. While the Prism Cover Test is still regarded as the gold standard in measuring strabismus, the Hess Screen
Test provides detailed information about the angle of strabismus in nine gaze directions with less effort. Thus, conclusions on impaired muscle function and muscle entrapment can be drawn [3]. The original Hess Screen Test was proposed in 1908 by Walter Rudolf Hess who intended to develop an easy method that allows to visualize the deviation of the eyes [4]. The Hess Chart consists of vertical and horizontal lines that subtend a visual angle of 5 where the junctions serve as targets and can individually be highlighted. The fundamental principle is based on presenting two images which are color dissociated by using red/green complementary filters. The task of the subject is to fixate a specific target only visible for one eye and simultaneously overlap it with a pointer only visible to the other eye. The very first version of the test was built of stitched red dots on black cloth and used a simple pointer with a green arrow at the end. Modern versions worked with LEDs and laser pointers but still with a manual recording of the results by the examiner [5]. Digital Hess Screen Tests performed on computers are now available through several providers such as VISUS GmbH 1 or Thomson Software Solutions 2. While influences by the level of experience and the subjectivity of the examiner can be overcome with these programs, some limiting factors, referring to reproducibility, especially when it comes to lighting conditions, proper dissociation, time and cost, still remain. Virtual Reality (VR) technology yields great potential to overcome these issues especially focusing on lighting conditions and proper dissociation of the eyes [6]. Furthermore, the applications can be implemented in a way that no additional support by medical staff is required. Several approaches for the measurement and treatment of strabismus and amblyopia with VR were already implemented [7, 8, 9]. However, most of the strabismus measurement methods rely on autonomous eye tracking systems [10, 11, 12]. The recent combination of eye tracking with head mounted displays (HMDs) presents the opportunity to detect, measure and treat strabismus in a single device. Miao et al. developed a VR-based Prism Cover Test in which the ocular deviation was tracked using infrared cameras inside of the HMD [13]. Even though the system showed good performance the great expanse by the elaborate image processing which had to be carried out has to be considered. Moreover, their approach only measures the squint in central gaze.

The main goal of this thesis is the measurement of ocular deviation with an VR-implemented Hess Screen Test using the inbuilt eye tracking system. The Unreal Engine 3 is used for the implementation. The program is supposed to run on a HTC Vive Pro Eye 4 system. The inbuilt eye tracking system has – according to HTC – an accuracy of 0;5–1;1. Similarly to the original Hess Screen Test, the subject will be presented the Hess Chart. As HMDs allow to deliver different images to each eye, perfect ocular dissociation is achieved without the need of additional complementary color filters. During the examination, a highlighted junction will be visible to the fixating eye and the remaining non-highlighted chart to the non-fixating eye. A subject with a healthy visual system will still be able to align both eyes on the target, whereas a patient with
strabismus will show a deviation with the eye where the target is not present. The intrinsic focus of the non-fixating eye will be evaluated on the basis of the data delivered by the inbuilt eye tracking system. The main expected advantage is an enhanced reproducibility within the ranges
of the accuracy of the eye tracking system as the HMD eliminates environmental influences, such as light, positioning or distance differences to the chart, that might affect the subjective perception or cause distraction. Within the scope of the squint angle measurement, the distance compression aspect in VR will also be regarded by varying the chart distance.

1] Schuster, A. K. et al.: Kindlicher Strabismus in Deutschland: Prävalenz und Risikogruppen: Ergebnisse der KiGGS-Studie.Bundesgesundheitsblatt – Gesundheitsforschung – Gesundheitsschutz 60.2, 2017.
[2] Wright, K. W. et al.: Handbook of Pediatric Strabismus and Amblyopia. 2006.
[3] Thorisdottir, R. L. et al.: Comparison of a new digital KM screen test with conventional Hess and Lees screen tests in the mapping of ocular deviations. American Association for Pediatric Ophthalmology and Strabismus 22.4, 277-280.e6, 2018.
[4] Roodhoft, J. M.: Screen Tests Used to Map out Ocular Deviations. Bull. Soc. belge Ophtalmol 305, 57-67, 2007.
[5] Roper-Hall, G.: The Hess Screen Test. American Orthoptic Journal 56.1, 166-174, 2006.
[6] Nesaratnam, N. et al.:Stepping into the virtual unknown: feasibility study of a virtual reality-based test of ocular misalignment Macmillan Publishers Limited, part of Springer Nature. 2017.
[7] Blaha, J. et al.: Diplopia: A virtual reality game designed to help amblyopics. IEEE Virtual Reality (VR). 2014.
[8] Cepeda-Zapata, L. K. et al.: Implementation of a Virtual Reality rendered in Portable Devices for Strabismus Treatment based on Conventional Visual Therapy. 41st Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC). 2019.
[9] Qiu, F. et al.: Interactive Binocular Amblyopia Treatment System with Full-Field Vision Based on Virtual Realty. 1st International Conference on Bioinformatics and Biomedical Engineering. 2007.
[10] Bakker, N. M. et al.: Accurate Gaze Direction Measurements With Free Head Movement for Strabismus Angle Estimation. IEEE Transactions on Biomedical Engineering. 2013.
[11] Chen, Z. et al.:Eye-tracking-aided digital system for strabismus diagnosis. Healthcare Technology Letters. 2018.
[12] Chen, Z. et al.:Strabismus Recognition Using Eye-Tracking Data and Convolutional Neural Networks. Journal of Healthcare Engineering. 2018.
[13] Miao, Y. et al.: Virtual reality-based measurement of ocular deviation in strabismus. Computer Methods and Programs in Biomedicine 185, 2020.