In two articles looking at blur sensitivity, Professor Edward Mallen defined blur sensitivity, blur adaptation and blur discrimination1 and then looked at how blur sensitivity can influence aspects of the refraction undertaken during the routine eye examination.2 These two articles were then used as the source material for an interactive CPD exercise.
Definitions
Blur sensitivity is a measure of the ability of the visual system to detect blur. In research experiments, blur sensitivity can be assessed subjectively using relatively simple equipment, and can be expressed as a dioptric threshold. Such experiments are generally conducted monocularly and under cycloplegia and with a standardised artificial pupil.
Blur discrimination is the ability to determine whether one target is blurred by a different amount in comparison to a reference target.
Blur adaptation describes the ability of the visual system to improve its character recognition abilities following exposure to blur, but without a change in refractive error.
Why is this relevant?
A significant part of any routine eye test is the measurement of the refractive error of the eye. While this may be undertaken objectively, by retinoscopy or autorefraction, for most patients the final refractive correction is established subjectively and may differ from the objective result as it better reflects the preferred viewing of the patient. However, the very subjective nature of the testing is a potential source of variation between patients.
Someone with poor blur discrimination may find it difficult to accurately report the difference between two sequentially presented images during the test and be undecided as to whether ‘it is better with lens 1 or lens 2.’ Also, the introduction of a deliberate blur is how the final sphere is verified. To ensure that there is no residual accommodation in place for a distance correction, a patient should report some blur on the introduction of a +0.25DS lens. However, someone with poor blur discrimination or, indeed, low blur sensitivity, may in fact be happy with the slightly fogged image and feel it is not sufficiently different to the actual final sphere.
This phenomenon is also a source of error when using the +1.00DS blur test to establish the final sphere (figure 1). Someone with poor blur sensitivity may be happy to report the smallest letters they see despite them being blurred, while a more blur sensitive individual will only accept a much larger letter line as these are seen crisply and clearly compared to those beneath, even if those beneath might still be discerned.
There is also an influence upon the prescribing of spectacles. A patient who is offered a refractive correction for the first time may find a significant improvement in the clarity of their vision. However, because of poor blur sensitivity and adaptation, they may complain that the spectacles are making their vision worse because, when they now take the spectacles off, their unaided vision ‘is a lot worse than it was before they had the specs.’
The point at which a patient decides they want new spectacles is influenced by their blur sensitivity. Someone with poor sensitivity may be able to tolerate a correction that is increasingly out of date for longer. Also, practitioners using automated phoropters will know that, when smaller changes in refractive error have been found, being able to instantly swap between the new refraction and the old will know that patients will be much better able to appreciate the change than if demonstrated using the much slower process of a multiple lens swap in a trial frame.
Case scenario for discussion
The case scenario you were asked to discuss was designed to show how a number of factors might influence the reported acuity of a patient independent of the refractive correction in place. Most of your responses highlighted the influence of subjective interpretation, but there were still some answers looking for a purely refractive reason for the results shown. For example, one respondent thought the poorer acuity with the +1.00DS blur for patient B was most likely due to a failure to relax accommodation that had remained after the initial sphere checks. Though this is a possibility, the scenario was written as to assume that the +1.00DS blur was being used after the accurate distance correction had first been established.
During one particularly busy morning clinic, you undertake a full eye examination of three successive patients. You remember them because of marked difference in their response to one part of the subjective refraction. The patients were as follows:
1 Patient A
A 48-year-old male who is attending your practice for his first ever eye examination. He has never had any concerns regarding his vision or eye health but, recently, has started to find his near vision becoming blurry and he finds small text increasingly difficult to read. This is especially so in the evenings when he is tired and the ambient light is not as helpful. Though tempted by cheap ready-made readers from a local shop, his wife persuaded him of the benefits of a full eye examination. His spectacle refraction was found to be as follows:
2 Patient B
A 48-year-old male who is attending your practice for his two-yearly check-up. He has no particular worries and does not feel as if his eyes or sight have changed in any way. He was first prescribed spectacles for distance viewing several years ago and advised to wear them when driving or when visiting the cinema. He likes the extra clarity offered by the spectacles and has always tended to wear them full time, though recently he is becoming increasingly aware of his reading vision being clearer without the spectacles. His spectacle refraction was found to be as follows:
3 Patient C
An 80-year-old female patient is attending your practice for her annual ‘eye health check.’ She has no concerns about her eyes or vision, but has always attended for her check-ups as ‘the eyes are so important.’ She is also very loyal to your practice as she always feels she receives a very thorough eye examination; for example, you always use drops to dilate her pupils because, as you have explained to her many times, it is the only way you can be completely confident that you have full examined the back of her eyes. Her spectacle refraction was found to be as follows:
Though the best corrected distance acuity for the three patients is the same (6/6), the acuity with the +1.00DS blur test differs considerably, as follows:
Why might this be?
Patient A
Patient A is a classic hypermetrope who is finally having to give in to the fact that progressive presbyopia is making near vision and, to some extent, distance clarity increasingly difficult to cope with uncorrected. Such patients are often ideal candidates for simultaneous vision multifocal contact lens correction. They tend to be quite indignant about the prospect of increasing spectacle wear after a life of no wear, while also tend to have established blur adaptation and poor sensitivity related to a longer time of uncorrected refractive error. Such poor sensitivity would mean they would tend to try to read much further down the chart with a fogging lens and report blurrier targets as seen.
As one respondent put it: ‘Because this patient has never worn a spectacle correction it is usually the case that they are not so blur sensitive so the acuity only drops to 6/9.’
Patient B
In many ways, patient B is the opposite to patient A. They have always worn a correction, state clearly how clarity of vision is important to them. Their insistence of regular and thorough eye testing might also betray a certain fastidiousness. Also, myopes with low cylinder refractive components tend to be aware of clear near targets, which also offer a clear reference point even as refractive error progresses. For these reasons, the patient reports a much greater impact of the +1.00DS blur test as they have high blur sensitivity and remain intolerant to anything other than a sharp and defined image.
Patient C
This example seemed a familiar one to most respondents as it seems to centre on the impact of small pupils. Indeed, some readers may remember the heyday of pilocarpine for glaucoma treatment and reports of elderly patients suddenly reported improved vision without any spectacles that had, prior to treatment, been relied upon. The smaller the pupil, the longer the depth of focus and so the easier to interpret images that are fogged by lenses introduced over the final error correction (figure 2).
Figure 2: Small pupils increase the depth of focus and so reduce blur sensitivity and discrimination
As one of you put succinctly, ‘Age related miosis in this case maintains the acuity at 6/7.5.’
- Edward Mallen is Professor of Physiological Optics at the University of Bradford, and Past President of the College of Optometrists.
References
- Mallen, E. Blur sensitivity 1. What is it? Optician, 09.09.2022, pp19-23
- Mallen E. Blur sensitivity 2. Impact on clinical practice. Optician, 14.10.2022, pp17-21