Every clinician has seen patients who have 20/20 visual acuity (VA), yet they still complain about their vision. Conversely, we’ve seen patients who have the same acuity who sing our praises for giving them such wonderful, crisp vision. The reason that patients who have the same VA can have very different visual experiences is likely related to visual quality—something that traditional methods of measuring acuity do a poor job of assessing.
Consider traditional Snellen VA testing, the central element of the vision exam in clinical practice. It typically measures vision in a high-contrast, high-luminance, static environment that bears little resemblance to visual tasks in the real world, where patients encounter visual targets of many different spatial frequencies, levels of luminance, and contrast. In this more dynamic environment, visual quality becomes much more important than it is in the examination room.
Tear Film Any number of factors can degrade visual quality in well-corrected eyes, beginning with the tear film. The tear film constitutes the first refractive interface through which light passes. Any disturbance at this interface will lead to various degrees of vision degradation—and because it’s very dynamic, it can change with each and every blink. To support good vision, the tear film needs to be smooth and stable over the entire inter-blink interval. This, in turn, requires that the eye produces not just a sufficient quantity of tears, but high-quality tears with the right balance and configuration of lipids, mucins, and aqueous to maintain tear spreading and provide that clear refracting surface. If the tear film quality is poor, light is scattered before reaching the retina, reducing image quality.
Higher-Order Aberrations Beyond the tear film, we know that the eye is not a perfect optical system. Besides defocus and astigmatism, higher-order aberrations (HOAs) from the cornea and crystalline lens can also compromise visual quality. Monochromatic aberrations of the eye can be objectively measured through wavefront aberrometry. Zernike polynomials are a common way of characterizing aberrations in the field of optics.
In the human eye, second-order Zernike aberrations—more commonly known as defocus and astigmatism—are typically the largest in magnitude and, therefore, have the greatest impact on vision. HOAs (i.e., the third-order Zernike aberrations onward) include coma, spherical aberration, trefoil, and others. Wavefront aberrations may be expressed as root mean square (RMS) error. The average HOA RMS error in normal eyes is relatively low, about 0.33µm at average pupil sizes.1 At a pupil size of 7.5mm in a normal population, the average amount of HOA is equivalent to the wavefront error produced by less than 0.25D of defocus.2 For perspective, highly aberrated eyes, such as keratoconic or post-keratoplasty eyes, can have a very high HOA RMS error of > 2.00μm.3 It is important to note that the magnitude of HOAs is pupil-dependent. In low light, as the pupil expands, the eye is exposed to a greater amount of aberrations, which can have a greater effect on visual performance.
Neural Processing In addition to optical aberrations, light scattering from any optical element in an optical system can also affect the performance of an optical system, whether in cameras, telescopes, or the eye. Unique to the human optical system, though, is the role of the brain, which adds another layer of complexity to visual performance. Neural processing allows the brain to fill in gaps in an image based on prior experience, but it can also limit visual perception through the selective prioritization of visual stimuli and factors such as mood, attention, and fatigue.
INTRODUCING A CONTACT LENS
When an artificial component such as a contact lens is introduced into the complex human optical system, a host of additional factors related to lens material and design come into play.
For example, the way in which a soft contact lens drapes on the cornea can affect visual quality. Toric contact lens performance in real-world settings is heavily dependent on stability of the lens to keep the optics correctly aligned. In addition, design features such as prism ballast and other stabilization methods can introduce aberrations.
Aberrations may also be purposely introduced into a contact lens to achieve a particular goal, such as introducing asphericity to create an increased depth of focus, as seen in some presbyopic lens designs. More typically, though, manufacturers of mass-produced contact lenses strive to produce high-optical-quality lenses that have no unintended aberrations. However, the potential exists for aberrations to be introduced by extrinsic factors after manufacturing. If the lens becomes folded or stuck to the foil package, for example, there could be subtle deformations that affect visual quality.
Material properties of the contact lens (e.g., wettability and surface roughness) have been shown to impact comfort and measures of in-vitro or ex-vivo lens performance (e.g., coefficient of friction or frictional energy), but the polymer structure of the lens can also affect visual performance. In particular, the way in which the lens material interacts with the tear film can promote or disrupt visual quality. Homogeneity of the lens material may also affect visual quality due to the impact of different refractive indices, different levels of scatter as light moves through the different polymer compositions, and the potential for non-homogeneity of hydration across the polymer and over time.
A contact lens placed on the eye immediately splits the tear film and isolates the membrane-bound mucins (MBMs) from their free-floating counterparts in the pre-lens tear film.4 Unless an effort is made to mimic the form and function of the natural tear film in the lens polymer to try to re-establish the natural gradient of mucins that exists without a contact lens, an unstable tear film can result.
One approach to solve this problem is to incorporate a molecule such as polyvinylpyrrolidone (PVP), which has properties similar to the mucins in the tear film, throughout the lens polymer. When formulated as a long-chain (high molecular weight) pure PVP form, it has similar properties of hydration and lubricity that may help “bridge the gap” between the trapped MBMs behind the lens and the free-floating mucins in the pre-lens tear film.5 By helping to attract and maintain moisture throughout the lens and providing a more “familiar” environment to the significantly thinned pre-lens tear film, this approach is thought to contribute to the support of a stable tear film.
PUTTING IT INTO PRACTICE
We’ve discussed many of the factors that can maintain or degrade the quality of an optical system, with and without the presence of a contact lens. But how do we put all of this together to determine the quality of the entire optical system and, ultimately, the visual performance experienced by patients?
In optics, the modulation transfer function (MTF) is one of the classic metrics for assessing overall optical quality. Modulation transfer function curves can be obtained in the laboratory to determine the quality of a contact lens. To determine the visual quality of the eye, contrast sensitivity function (CSF; a physiological analogue of MTF) can be measured by presenting the eye with visual stimuli of various combinations of contrast and spatial frequencies to derive a detection threshold at each combination. The map of those thresholds becomes the contrast sensitivity curve or function. The area under the contrast sensitivity curve describes the total visual “space” that a patient can perceive. A larger area under the CSF curve indicates better spatial vision (Figure 1).
In contrast, the VA routinely measured in clinical practice, such as high-contrast Snellen VA, is essentially one data point on the contrast sensitivity curve and, therefore, is limited in representing a patient’s visual function compared to CSF. The difference between VA versus CSF metrics is apparent in many conditions, including cataract, dry eye, and age-related macular degeneration, in which the loss in VA is disproportionate to the subject-reported vision loss.6 The latter is better represented by the significant loss in CSF.
CSF can be accurately measured in research settings via sophisticated psychophysical testing paradigms. However, such testing is tedious, time-consuming, and requires experienced subjects. Many clinical contrast sensitivity tests, although suitable for detecting disease conditions, may lack the required resolution for detecting difference in visual quality among healthy eyes corrected with different contact lenses. Newer, computerized CSF tests are being developed to streamline the testing procedure to yield results with laboratory-grade precision in a much shorter time, limiting subject fatigue. However, it is still unlikely that busy clinicians will begin performing CSF testing during routine eye exams.
There are, however, a number of things that clinicians can do to maximize visual quality before introducing a contact lens and to assess whether the lenses prescribed are contributing to or degrading visual quality:
- Pay close attention to the tear film. Treat ocular surface disorders such as dry eye, meibomian gland dysfunction, and ocular allergy. Then, the tear film is in the best possible state to support contact lens wear.
- Ask patients more probing questions about how well they see in various visual environments such as at night and during low-light conditions, during active tasks such as when playing or watching sports, or in other challenging situations. The advantages of contact lenses that provide greater optical quality may be more noticeable for real-world tasks versus examination room testing; this will be particularly noticeable in situations in which contrast is already reduced, such as in the presence of smoke, rain, or glare from oncoming headlights (Figure 2).
- Listen to how quickly and confidently patients read an acuity chart, and ask them to describe their vision with contact lenses. Words like “sharp,” “bright,” and “crisp” are indicative of high visual quality, whereas “good” or “fine” probably fall short.
- Choose lens brands from companies that have a reputation for high-precision design and manufacturing.
- Pay close attention to the science behind contact lens materials to ensure that polymer networks integrate well with the tear film to support comfort and optical precision, particularly with regard to challenging environments such as during heavy digital device use.
- Keep in mind that visual quality may also affect the subjective perception of comfort. In a clinical study, emmetropic subjects rated their comfort significantly lower when viewing blurred targets versus clear targets (p < 0.001), demonstrating an association between clarity of vision and ocular comfort.7
- Ensure that the contact lens prescription is up to date, with full correction of sphere and cylinder. Remember, these lower-order aberrations have a much bigger impact on optical quality compared to the HOAs discussed previously. Fitting low astigmats in toric lenses can improve patient-perceived image quality compared to fitting them in contact lenses with the best spherical equivalent.8
Achieving the highest quality visual performance requires all of the components of the optical system to work well together. The introduction of a contact lens presents additional challenges but also many opportunities to enhance visual performance. And finally, contrast sensitivity measurements offer us a clinical way to better evaluate and verify patient subjective reports of either exceptional vision or subpar vision, especially when these reports are at odds with our standard Snellen VA testing. CLS
- Salmon TO, van de Pol C. Normal-eye Zernike coefficients and root-mean-square wavefront errors. J Cataract Refract Surg. 2006 Dec;32:2064-2074.
- Thibos LN, Hong X, Bradley A, Cheng X. Statistical variation of aberration structure and image quality in a normal population of healthy eyes. J Opt Soc Am A Opt Image Sci Vis. 2002 Dec;19:2329-2348.
- Pantanelli S, MacRae S, Jeong TM, Yoon G. Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor. Ophthalmology. 2007 Nov;114:2013-2021.
- Craig JP, Willcox MDP, Argüeso P, et al. The TFOS International Workshop on Contact Lens Discomfort: Report of the Contact Lens Interactions With the Tear Film Subcommittee. Invest Ophthalmol Vis Sci. 2013 Oct;54:TFOS123-TFOS156.
- Sterner O, Karageorgaki C, Zürcher M, et al. Reducing friction in the eye: a comparative study of lubrication by surface-anchored synthetic and natural ocular mucin analogues. ACS Applied Materials & Interfaces. 2017 Jun 14;9:20150-20160.
- Owsley C. Contrast sensitivity. Ophthalmol Clin North Am. 2003 Jun;16:171-177.
- Rao SB, Simpson TL. Influence of vision on ocular comfort ratings. Optom Vis Sci. 2016 Aug;93:793-800.
- Milton A, Murphy M, Rose B, et al. Demonstrating correction of low levels of astigmatism with realistic scenes. Cont Lens Anterior Eye. 2016 Feb;39:26-37.