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 . 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 . 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 .
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 . 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 . 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 . 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 . 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.
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