A Comparison of Prism Cover Testing and a Virtual Reality Headset-based Test in the Objective Assessment of Ocular Deviation

Strabismus, or squint, describes any misalignment of the eyes, and can affect both children and adults. As a result, the images of the outside world conveyed to the brain from the two eyes do not correspond, which can result in diplopia, or 'double vision'. As well as impacting on a patient's functionality, strabismus can have cosmetic and psychosocial ramifications. Furthermore, if left untreated in childhood, strabismus can lead to life-long visual impairment, despite best spectacle correction.

Current treatment is guided by the exact nature of the strabismus and the patient's age; options include spectacle prescription to correct underlying refractive error, use of prisms to control deviation, or in certain cases, surgery. In order to investigate the size and type of strabismus present, and determine the best management, tests of ocular deviation are performed in Eye Clinics.

Quantitative objective assessment of the angle of horizontal and vertical strabismus is currently performed using the prism cover test - the patient is asked to look at a fixation target, and prisms of increasing power are placed in front of one eye until the ocular deviation is neutralised (i.e. there is no movement of the eyes to take up fixation). The test can provide accurate numerical recordings to aid in monitoring and surgical planning.

However, the prism cover test requires a range of different prisms, the presence of a near and distance fixation target, a trained examiner, and a patient able to fixate at an appropriate target without moving their head for the duration of the test. In this study, it will be investigated whether a virtual reality headset-based test can produce comparable measurements of ocular deviation to the traditional prism cover test, without the need for prisms, or control of head posture.