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HomemieyecareThe Sense of Vision

The Sense of Vision

Out of the five senses, people generally consider vision to be the most important. Eye care professionals (ECPs) are responsible for maintaining this valuable sense but as technology advances, what will ECPs need clinically to advance vision care into the 21st century? [/vc_column_text][/vc_column]

There’s no doubt about the importance of vision to patients. The 2007 American Eye-Q survey showed that almost half of respondents indicated that eyesight was the sense they worry most about losing.1 The Needs, Symptoms, Incidence, Global Eye Health Trends (NSIGHT) study indicated that among a spectacle or contact lens corrected population, vision ranked at the top of the hierarchy according to patient needs.2 Eye care professionals’ (ECPs) responsibilities in relation to maintaining a patient’s sense of vision can include a number of clinical duties; correcting ametropia and/or presbyopia to improve a patient’s range of vision, diagnosing and treating ocular disease to preserve vision, and prescribing visual aids to rehabilitate vision.

Technology continues to advance with regards to testing, corrective devices, and procedures. With this in mind, this article will explore how visual performance testing, contrast sensitivity, higher order aberrations, and neural adaptation all play a role for the future of vision.

Visual Acuity Testing and Subjective Performance

The vast majority of testing for visual acuity takes place in the confines of an exam room. The universal method involves having the patient read a standardised high contrast eye chart to determine best-corrected acuity. A patient’s real word performance is then gauged from this measurement. Examples of this include determining visual eligibility to qualify for a driver’s licence or candidacy to be a police officer.

Participants were sent outside the office to carry out real world tasks, which included driving, using a computer, and reading…

A limitation of standardised visual acuity testing is that the testing does not take into account a number of environmental factors that patients encounter the moment they leave the exam room. Some of these factors may include variances in illumination and contrast, glare, and movement. Of course, contrast sensitivity and glare testing are done for select patients, but they are not routinely done for everyone. In addition, ECPs routinely make recommendations of spectacle or contact lens prescriptions based upon objective in-office visual acuity measurements. Too often a patient indicates they are having a vision problem, we measure good high contrast VA and don’t consider and don’t seek deeper understanding that they are talking about a problem of low contrast visual acuity, glare, or halo issues.

Some interesting research involving multifocal contact lens wear addresses the differences and relevance of standardised testing versus subjective real-world performance. A 2009 study by Papas et al. concluded that static acuity based measurements were insensitive indicators of performance compared to a subjective alternative when evaluating multifocal contact lens performance.3 Generally, the objective testing data did not reflect a patient’s change in visual experience after a four day trial period of multifocal soft contact lenses. The authors point out that if the subjective responses are perceived as meaningful, then the in-office visual acuity measurements lacked sensitivity for this particular application.

Another study by Woods et al. evaluated objective and subjective visual performance for patient’s trialling multifocal contact lenses versus other soft lens correction modalities.4 Office testing included objective standardised acuity testing and simulated real world tasks. As well, the participants were sent outside the office to carry out real world tasks, which included driving, using a computer, and reading.

A novel aspect of the actual real world tasks was that participants issued real-time subjective ratings through the use of a smart phone device. The measured performance in the exam room generally showed monovision to objectively perform better, while real-time assessment of real world tasks showed multifocal lenses to have better overall subjective performance. The authors reason that the real-time subjective assessment is probably a better indicator of performance than objective acuity taken in an exam room.

The conclusions of these studies point out the weaknesses of standard objective acuity testing that ECPs rely on for indicators of successful visual performance. Although practitioners may query patients at follow-up exams when assessing new vision correction prescriptions by taking a history, Papas et al. point out that generalised questioning can be too vague to be accurately analysed.3 More sophisticated subjective questionnaires that the patients can take with them or the use of electronic devices may improve accuracy.

A recent quality of vision (QoV) questionnaire has been used to measure quality of vision in patients with all types of refractive correction, eye surgery, and ocular disease.5 As well, perhaps novel computer based in-office visual simulation testing could be developed to improve success in determining actual real-world performance. Crucial aspects to this will be the ability to standardise these tests and make them efficient so that they don’t dramatically increase chair time.

Contrast Sensitivity

Contrast sensitivity is an important indicator of the quality of vision that a patient has and significantly affects visual performance.5,6 Contrast can be defined as the difference between a test stimulus’ maximum (light) and minimum (dark) luminances divided by their sum.8 Contrast threshold is determined for a given spatial frequency by lowering the contrast until detection is impossible.5 The inverse of contrast threshold is contrast sensitivity and this can be plotted versus spatial frequency to give the contrast sensitivity function. Although optics plays a prominent role in determination of contrast sensitivity function, retinal and brain processing are also factors.7,8 In general, most ECPs do not include contrast sensitivity testing during a comprehensive eye examination.

Typical projected charts that are used to measure visual acuity use optotypes that are near 90 per cent contrast.9 Thus, the practitioner is not measuring an important component of a patient’s visual perception.5 A patient may read the 20/20 row of letters, but complain that the letters are dim because they have reduced contrast sensitivity. Without additional information, this patient would appear to have equal quality of vision as a patient who has 20/20 vision and near normal contrast sensitivity. However, their visual performance in real-world situations will be vastly different, as they have to recognise and detect a variety of targets
of varying illumination, size, and shape.

Age-related changes of the eye, such as cataracts, result in significant declines in contrast sensitivity.10 However, ocular disease such as glaucoma, macular degeneration, and diabetic retinopathy also can significantly lower contrast sensitivity function.9,11-13 Conventional refractive laser procedures that induce higher order aberrations and diffractive multifocal intraocular lenses may also negatively impact a patient’s contrast sensitivity.14-16

Measurement of a patient’s low contrast acuity or contrast sensitivity can be done in a standard exam room with relatively inexpensive wall charts or computerised monitors. However, is contrast sensitivity testing really necessary or useful for clinicians as they examine and follow particular patients?

There are published studies that show contrast sensitivity testing can be used not only to detect subclinical ocular diabetic eye disease, but also assess treatment outcomes in macular degeneration.17,18 Also, Richman et al. reported in a 2010 study that visual acuity and contrast sensitivity best predicted the ability of a patient with glaucoma to perform activities of daily living.6 It has also been suggested that because contrast sensitivity provides significant information regarding visual performance, that these measurements be included in presbyopic contact lens fitting.19

Contrast sensitivity testing may also be a more accurate indicator of increased higher order aberrations after refractive surgery than visual acuity testing.8 Low contrast acuity is a fast way of gathering substantial information on a patient’s overall performance at the resolution threshold. We know that most keratoconus patients have worse low contrast visual acuity even in the presence of good gas permeable contact lens correction compared to normals.

Another important and relative issue with regards to contrast sensitivity testing is whether or not the ECP can provide significant help to improve visual performance for those patients with decreased contrast sensitivity function. There is some suggestion that short wavelength filters would improve visual function for patients with measurable decreases in contrast sensitivity function.20 Interestingly, enhancing contrast sensitivity through neural plasticity by way of action video game training may help some individuals.7 Low vision rehabilitation is probably the best course for patients with severe declines in contrast sensitivity.

Higher-Order Aberrations

Ocular aberrations that degrade retinal image quality are divided into two main categories, which include monochromatic and chromatic aberrations. Monochromatic aberrations can be further subdivided into lower and higher order aberrations. Lower-order aberrations include defocus and cylinder. These are the aberrations that ECPs typically correct with conventional spectacles, contact lenses, or refractive surgery. The advent of the aberrometer has enabled researchers and ECPs to measure higher-order aberrations, which has led to investigations of how they affect the performance of the visual system. The higher-order aberrations can be represented mathematically by Zernike polynomials, which can be modelled three-dimensionally to show how a perfect wavefront is distorted.21 The common Zernike modes have been given names such as coma and trefoil. A point spread function, which shows how aberrations affect a point source of light, can be another way to represent how these aberrations cause optical disturbances.

One interest in measuring and correcting for higher-order aberrations centres around whether or not ECPs can improve upon the visual performance of patients with normal visual acuity, or in other words, give the patient “super vision”.

The higher order aberrations are affected by pupil-size and become more relevant with increasing pupil size. The benefits of correcting for HOA on a healthy eye with a pupil less than 3mm are negligible.22 The predominant HOA for normal individuals include coma, and spherical aberrations.23,24 Coma will cause the patient to see points of light as doubled or with a comet-like tail, while spherical aberrations will cause halos around point sources of light.21 Although aberration levels increase with age, these degradative changes may be offset by senile miosis.23,25,28

Although in theory, correcting for all higher-order aberrations is complicated because not all aberrations affect the visual system equally and some Zernike modes may interact when combined to improve acuity despite increasing the total wavefront error,29, 30 correction of individual aberrations such as spherical aberration may improve visual quality.

A commercially available way to correct for aberrations in spectacle lenses has been available for several years, however, spectacle lenses are currently somewhat limited because the optimal effect is diminished when the visual axis is off optical centre. The increased cost of the spectacle lenses for a small benefit is a further deterrent for some patients. Contact lenses may be a better alternative because they stay relatively central to the visual axis with eye movement for the correction of spherical aberration; however, limitations may result from contact lens rotation and/or translation when correcting rotationally asymmetric aberrations like coma.31,32

There are commercially available GP and soft contact lenses that incorporate asphericity to improve visual performance in patients based on correcting the population average of spherical aberration. These lenses may allow for better vision in low light situations. Custom wave-front guided soft contact lenses are also being developed that may help patients with corneal irregularity and secondary higher-order aberrations who cannot tolerate GP lenses or where GP lenses have insufficiently corrected for their aberrations.33-38 Wave-front guided refractive surgery has improved results for some patients over conventional treatments by reducing the amount of aberrations induced during the treatment.31 Finally, cataract patients
who have significant amounts of positive corneal spherical aberration may have visual improvement with commercially available aspheric intra-ocular lenses.31,39

Neural Adaptation

Eye care professionals emphasise optics when assessing and correcting patients for their visual needs. However, retina and cortical processing are contributing factors. This neural component is an important part in determining a patient’s visual perception. This subjective interpretation is why most glasses scripts are prescribed from a manifest refraction and not a retinoscopy or auto-refractor measurement. There isn’t a computer system or test that can objectively determine what a patient perceives, so we rely on their subjective responses as we change sphere and cylinder.

Evidence is beginning to accumulate that the neural visual component has the ability to adapt to optics that are less than ideal in order to give the perception of visual clarity. George et al. reported a study where patients looked through a +2.50 D spherical lens over their best correction for a two hour time period.40

The study participants demonstrated a perceptual adaptation to the sustained blur and improved acuity through the blur that was equivalent to a one diopter reduction of myopia. This adaptation has been hypothesised to take place in a central binocular site within the visual cortex.40 This is supported by another study that showed a 35 per cent improvement in an occluded eye after the fellow eye was allowed to adapt to a blur-inducing lens.41 Additional studies suggest that the improved resolution may last up to 10 days and is not affected by brief periods of corrected clear vision.40,42,43 This type of adaptive process may help patients with low amounts of myopia and cataract formation.41,44 There are data that suggest patients who have had LASIK surgery undergo an adaptive phase after about 10 weeks that improves their unaided visual acuity.45

The phenomenon is most likely similar to what happens for patients whose vision improves over time after new multifocal soft lens fitting or implantation of multifocal IOLs for cataract surgery. Artel et al. reported results from a study that shows the neural visual system adapts to the eye’s particular aberrations so that images subjectively appear sharp even though some retinal blur exists.46 A follow-up paper indicates that the adaptation is incomplete and the best image results when 88 per cent of a subject’s original aberrations are removed.47

Clinically, all of this brings up a number of corrective situations that the ECP should consider in light of a patient’s neural adaptive ability. Some patients who have undergone corrective eye surgery or are newly fit into multifocal contact lenses may need longer adjustment periods before they are able to make the necessary neural adjustments to fully benefit from the correction. Additionally, patients with irregular corneas who are fit with gas permeable lenses may slowly experience better visual acuity over time as they adapt to the improved optics.

Conclusion

It’s apparent that going into the 21st century, ECPs are going to have to step beyond more traditional ways of testing and correcting patient’s vision to improve subjective visual satisfaction. Utilising novel ways to assess visual performance will play an important role in this endeavour. Additionally, continued research into higher-order aberrations and neural adaptation will be necessary to determine how to utilise new corrective devices or procedures in order to maximise vision.

Enhancing Your Contact Lens Practice:

Aspheric Designs

Writers: Dr. Nidhi Satiani and Dr. Kathryn Richdale

As optometrists, we took an oath to “advise [our] patients fully and honestly of all which may serve to restore, maintain or enhance their vision.”

Unfortunately, correction of lower order aberrations (myopia, hyperopia and astigmatism) alone does not always provide for full restoration, maintenance, and/or enhancement of our patients’ vision. We have all experienced the patient who sits in our chair, reads 20/20 at distance and near, yet still complains of blur or halos. Previous studies have indicated that once lower order aberrations are corrected, higher order aberrations become significant factors in visual quality.1,2 Perhaps, instead of having a conversation about “realistic expectations,” we should consider refitting such patients into an aspheric contact lens design.

Measuring and Correcting Higher Order Aberrations

Shack-Hartmann style aberrometers are the most common clinical aberrometers and have many uses including the objective measurement of refractive error and pupil size, but are most often used to quantify higher order aberrations.3-7 Clinically meaningful higher order aberrations are generally those in the third to sixth order (Figure 1). In a normal, healthy population, spherical aberration is one of the largest of the higher order aberrations and occurs because light rays from the periphery of the pupil are bent more or less than those through the center (Figure 2).8 Ultimately, aberrometers give us a better understanding of our patients’ visual system.

Spherical aberration can cause symptoms of blur, glare, halos and starburst effects.9 The visual effects of spherical aberration are pupil size dependent with symptoms being less or absent in those with small pupil sizes. These symptoms are often exacerbated under low light or low contrast conditions. Fortunately, spherical aberration is relatively easy to reduce as it is rotationally symmetric about the visual axis, unlike other higher order aberrations which vary by orientation.10,11 The popularity of “high definition” in our society has extended from television sets to refractive surgery and intraocular lens implants. One way optical devices deliver on the high definition promise is by reducing higher order aberrations; this has brought the importance of aberration correction to the fore. As primary eye care providers, we can also deliver on this promise with the use of contact lenses that correct for spherical aberration.

Aspheric Contact Lenses

Traditionally, soft contact lenses are designed with spherical surfaces which can introduce additional spherical aberration to the eye. Aspheric contact lenses are created by altering the shape of the lens from the center to the edge. There are a number of disposable aspheric contact lenses available on the market today, including both hydrogel and silicone hydrogel materials, with replacement schedules from daily to monthly. It is important to note that not all aspheric designs are equal. Some aspheric contact lenses are designed to counteract the spherical aberrations introduced by the contact lens alone, while others minimise the combined aberrations of the contact lens and the population average. The latest advances in aspheric technology include more precise correction for each 0.25 D step along the entire power range, and designs that account for the change in contact lens shape, and therefore optics, when the lens is placed on the eye. Practitioners can educate themselves on the specific attributes of various lens designs by contacting their local representatives or visiting the company websites.

Building your Contact Lens Practice

Fitting aspheric contact lenses is just one more way practitioners can increase patient satisfaction and retention. Often, patients complain of problems with night driving or reading the overhead in conference rooms, yet doctors dismiss these issues as being “normal.” Such complaints may be due to uncorrected spherical aberration. Doctors do not often measure patients’ aberration profiles or test their full visual function using low-illumination or low-contrast charts. Fortunately, practitioners need not change their testing routines or purchase expensive equipment to be successful fitting aspheric contact lenses. It’s more important to listen to patients’ subjective complaints and be proactive in educating ourselves and our patients on the latest technology in contact lens design.

Aspheric contact lens designs may be beneficial for patients with subjective complaints of blur, glare, halos or who have large pupils or a high refractive error. As refractive error increases, so does the magnitude of spherical aberration induced by spherically surfaced contact lenses. Aberrations also increase with increasing pupil size, so patients with pupils over 5 mm, or who work or study in dimly lit environments may be more affected by spherical aberrations. However, it is important to remember that a decrease in higher order aberrations is most relevant when lower order aberrations are minimised.

There are a few caveats to keep in mind when fitting aspheric contact lenses. Decentered aspheric optics may actually degrade vision more than a spherical design, so centration should be evaluated. Also, some aspheric contact lenses are correct for different amounts of spherical aberration due to limitations in design or manufacturing capability. Finally, not every patient’s higher order aberrations will fall within the normal population range. While one design may decrease aberrations and improve visual quality for a certain patient, that design may not improve vision for the next patient. Without access to an aberrometer, it’s best to educate the patient on the benefits of aspheric contact lenses and encourage them to compare the visual quality in their own environment. Patients may need to try one or two brands to find the lens that provides their best vision quality. But, in heeding your oath to enhance patient’s vision and taking the time to address your patient’s subjective complaints of visual quality, you may gain a patient for life.

Nidhi Satiani earned her BS, OD, and MS degrees from The Ohio State University where she also completed an Advanced Practice Fellowship in Cornea and Contact Lenses. In addition to having served as a clinical instructor in various clinics at OSU’s College of Optometry, she has been a researcher and lecturer for private- and industry- funded projects. Dr. Satiani has received financial support from the NIH for her work and currently practices in Columbus, Ohio.

Dr. Kathryn Richdale OD, completed a Cornea and Contact Lens Advanced Practice Fellowship while at the Ohio State University and is currently an Assistant Clinical Professor at the SUNY College of Optometry. She has been an investigator for federal-private-and industry-funded studies and holds a Mentored Research Career Development Award (K23) from the National Eye Institute.

Dr. Gregory W. DeNaeyer, OD, FAAO is the Clinical Director for Arena Eye Surgeons in Ohio in the United States, and President of the Scleral Lens Education Society.

References
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20. Nowakowski R.W. Primary Low Vision Care; 1994.
21. Shaw-McMinn P.G. How to provide high-resolution vision. Review of Optometry 2006 (Dec).
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29. Applegate R.A., et al. Interaction between aberrations to improve or reduce visual performance J Cataract Refract Surg. 2003; 29:1,487-1,495.
30. Applegate R.A., Sarver E.J., Khemsara V. Are all aberrations equal? J Refract Surg. Sep-Oct 2002; 18(5):S556-562.
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33. Katsoulos C., et al. Customized hydrogel contact lenses for keratoconus incorporating correction for vertical coma aberration. Ophthalmic Physiol Opt. May 2009; 29(3):321-329.
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36. Marsack J.D., et al. On-eye performance of custom wavefront-guided soft contact lenses in a habitual soft lens-wearing keratoconic p

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