Evaluating an alternative way to measure of visual acuity v1.0
Research type
Research Study
Full title
Evaluating the Stepwise sweep VEP as an alternative test of visual acuity compared to ETDRS logMAR visual acuity charts.
IRAS ID
316253
Contact name
Caitlin Gallagher
Contact email
Sponsor organisation
Aston University
Duration of Study in the UK
0 years, 7 months, 29 days
Research summary
Research Summary
This study aims to evaluate an alternative measure of visual acuity (a measure which is used to describe how well someone can see). The ‘gold-standard’ test to measure visual acuity requires patients to be able to recognise and name a target (e.g., letters) on a chart, known as an ETDRS chart. The ETDRS chart is a subjective test meaning its accuracy can depend on age, ability and cooperation. An alternative test of visual acuity can record the response generated in the brain and use this to estimate visual acuity objectively. Visual evoked potentials are used to assess visual pathway function and are a common ophthalmological test. Using the VEP to estimate visual acuity (a technique known as the stepwise sweep VEP) is not routine at present but may be easier for some patients to perform as it only requires them to watch a screen for 10 minutes with wires attached to the surface of their heads with easily removable paste.
This study will be conducted in the visual electrodiagnostic department (VED) in Newcastle Royal Victoria Infirmary. This is a regional service where patients regularly attend to have VEP tests performed. Healthy volunteers and patients referred to VED with subnormal vision will be recruited. The two tests of visual acuity (chart and stepwise sweep VEP) will be performed and how close the two measurements compare will be assessed. Secondly, in healthy volunteers, stepwise sweep VEP will be repeated to measure test-retest variability in two separate sessions. With this we hope to assess the real world application of the stepwise sweep VEP and determine if this is a reliable and suitable alternative to measure visual acuity clinically for patients unable to perform standard ETDRS chart.
Summary of Results
Visual acuity (units of logMAR) is a threshold of the finest image which someone can resolve. Routinely we measure this with calibrated letter charts. However, can perform this test accurately or reliably (e.g., non-verbal or pre-verbal populations). Visual Evoked Potentials (VEP) are cortical responses generated in the primary visual cortex in response to visual stimulation (e.g., checkerboard pattern, flashing light). The VEP can be configured to produce an estimation of visual acuity. This measure is objective and may be more reliable in people unable to perform standard letter charts accurately.This study aimed to evaluate and compare VEP measured visual acuity to a more widely used gold-standard letter chart in healthy participants and patients with different pathology of the eye/visual pathway, who would be able to perform both tests well. We measured how closely the two measurements compared, with the assumption that the letter chart was the "true" visual acuity. Furthermore, we measured how reliable the VEP estimation was with repeated measurements in healthy participants, to assess its use of this technique to monitor visual function.
We recruited 16 healthy participants and 2 patients. What we found is that in healthy participants when you compare the agreement (i.e., closeness/precision) between the two tests, there is a strong tendency for the VEP-based approach to underestimate visual acuity. For 95% of participants, the difference was within approximately two lines of a standard letter chart (i.e., +/- 0.2 logMAR). In patients, the agreement the between the two patients very different. One patient had very close agreement of the two measures of visual acuity, while the other patients visual acuity was markedly underestimated by the VEP. While this could suggest that presence of disease may affect the relationship between VEP measure of visual acuity and the letter chart, with such a small sample (N=2) we cannot confidently say which value is a more true or accurate representation.
The repeatability of VEP measurements of visual acuity, in healthy volunteers, was 0.19 logMAR. For context, when using a letter chart, a difference of 0.16 logMAR is considered clinically significant.
The VEP estimation of visual acuity may provide a potential alternative to quantify visual function, in an individual who is unable to provide an accurate or reliable acuity using standard methods. However care should be taken when we use it as a proxy measure, due to some fundamental differences in the two measurements.
REC name
North West - Haydock Research Ethics Committee
REC reference
22/NW/0338
Date of REC Opinion
27 Oct 2022
REC opinion
Further Information Favourable Opinion