Sangeetha N, B.S.Optometry .

Research Optometrist, Medical Research Foundation, Sankara Nethralaya, Chennai

Introduction
In our day-to-day life, we either voluntarily or involuntarily align our eyes to the natural scenes at various ranges for perceiving the image with high quality. Eye movements are guided and controlled by different regions of the brain (see Figure 1, Table 1), which helps in finding, fixating, focusing and following the target of interest whose integrity can be clinically assessed to guide in the diagnosis of Ocular Motor Dysfunction (OMD) by delineating the affected region of the brain¹ ².OMD is a sensorimotor anomaly of the oculomotor system which is characterized by the inability to perform accurate and effective eye movements and fixations³.

Types of Eye Movements

1) Saccades and Pursuits
Saccades are rapid eye movements that shift the fovea from one static object to another while pursuits are tracking movements of the eyes to maintain clear fixation of a moving object^¹,². Clinically saccades and pursuits are subjectively assessed using North Eastern State University College Of Optometry’s Oculomotor Norms (NSUCO) by considering Ability, Accuracy, Head and Body movements⁴. Horizontal and vertical saccades are evaluated with the Developmental Eye Movement test (DEM) by calculating the ratio score with the time taken to complete the task⁴. Readalyzer is an objective method for assessing the reading skill of the patients⁴.

Figure 1: Schematic representation of neural circuits controlling saccades and smooth pursuits.
(Image source: https://www.sciencedirect.com/science/article/pii/S2590112519300155)

Table 1: Summary of structures involved in the control of saccades and smooth pursuits².

2) Vergence

Vergence are disconjugate eye movements that maintain binocular fusion when an object is moved towards or away from the subject (Convergence & Divergence)^¹,². Near Point of Convergence, Fusional Vergence (positive and negative) and Vergence Facility are the parameters considered for evaluating vergence⁵.

3) Vestibular and Optokinetic Movements

Vestibular movement is the fixation of eyes on an object when there is a head movement while Optokinetic movements are slow phase and fast phase eye movements elicited by tracking of a moving field^¹,². Vestibular and Optokinetic movements are clinically assessed by Head Impulse Test (HIT) and Optokinetic Nystagmus Drum respectively.

Recent Advancements

Conventional clinical measurements guide in the diagnosis of OMD with specific criteria and test score ratios. Recent advancements with eye trackers provide dynamic and quantitative parameters like Amplitude, Velocity, Peak Velocity, Gain, Latency or Reaction Time, and Duration, which are more precise and accurate compared to the parameters measured with conventional clinical methods (such as NSUCO grading, DEM test) due to their high inter-examiner and test-retest variability⁶. The integrated method for quantification of eye movement parameters allows the examiner to access video of the test for visual confirmation, gaze plot for assessing the accuracy of the measurement and graphs with numerical values⁶.

Programmed eye movement tasks along with eye trackers, provide quantitative values that reflect the magnitude of the condition and will be very useful in understanding and exploring the stages of disease progression and also for evaluating the post-treatment efficacy⁶. Even though there are wide ranges of clinical studies on quantitative measurement of eye movements, at present there are no defined protocols and stimulus parameters for testing different types of eye movements. Eye tracking technology which is currently being used as a research tool, if implemented with defined testing protocols will serve as a reliable method for quantitative evaluation of eye movements, thereby, becoming a potential screening and diagnostic tool for OMD.

References

1. Downey, D. L., & Leigh, R. J. (1998). Eye movements: pathophysiology, examination and clinical importance. The Journal of neuroscience nursing : journal of the American Association of Neuroscience Nurses, 30(1), 15–24.
2. Lynch, J.C.. (2010). Oculomotor Control: Anatomical Pathways. Encyclopedia of Neuroscience. (vol 1,17-23)
3. Iyer, J., Taub, M.B., & Wc, M. (2011). The VisionPrint System: A new tool in the diagnosis of ocular motor dysfunction.
4. Allison, C. (2005). Part 3 of 3, eyedentify your patient’s efficiency problems: by performing the appropriate tests, we can improve academic performance in many school-aged children. Review of Optometry, 142(6), 45-52.
5. Kapoula, Z., Morize, A., Daniel, F., Jonqua, F., Orssaud, C., & Brémond-Gignac, D. (2016). Objective Evaluation of Vergence Disorders and a Research-Based Novel Method for Vergence Rehabilitation. Translational vision science & technology, 5(2), 8.
6. Blignaut, P., van Rensburg, E. J., & Oberholzer, M. (2019). Visualization and quantification of eye tracking data for the evaluation of oculomotor function. Heliyon, 5(1), e01127.