[/vc_column_text][/vc_column]An aberration can be described as anything that prevents light rays entering an optical system from coming to a perfect focus. Many factors can induce aberrations. For example, something as simple as an irregular corneal shape or a tilt of the crystalline lens can lead to aberrations, as can more complicated issues such as keratoconus. Ultimately, aberrations work to reduce the clarity of the retinal image by introducing blur, which is experienced by the individual. When aberrations are minimised, the best visual image and best visual perception occurs.1
Defocus due to refractive error and astigmatism are examples of low order aberrations. In most cases these can be corrected with spectacle lenses or contact lenses. Higher order aberrations are typically those not corrected with spectacle lenses. Imagine a patient with keratoconus; their vision with spectacle lenses or soft contact lenses may be adequate, but they experience poor image quality. However, when wearing a gas permeable contact lens, with its induced tear lens, they have better vision since the combined affect of the rigid lens surface and tear lens help neutralise some of the corneal surface irregularities. These corneal surface irregularities can be thought of as a combination of various higher order aberrations.
Spherical aberration, coma, trefoil and secondary astigmatism are a few examples of higher order aberrations. Tip and tilt are classed as aberrations from radial order one, and defocus and astigmatism from radial order two. Anything above that would be classed as higher order. Coma and trefoil are third order aberrations and spherical aberration, tetrafoil and secondary astigmatism are fourth order aberrations. In fact, depending on how many radial orders to which you continue, you can continue naming higher order aberrations. Typically, instruments that measure higher order aberrations will measure to the region of fifth, sixth or seventh order.
Ultimately, aberrations work to reduce the clarity of the retinal image by introducing blur, which is experienced by the individual…
Measurement
Aberrometers are devices used to measure monochromatic aberrations. They can represent this information in either graphical form or as departure from an ideal wavefront. The root mean squared value (RMS) represents the difference between the ideal wavefront and the aberrated1 wavefront. In general, if the value of the RMS error is low then the aberrations in the optical system are also low.3The RMS error is a good indicator of the general level of aberration in the eye and allows a quick
comparison of aberrations between individual eyes with similar aberration profiles. However, RMS is not a good metric of overall image quality because not all aberrations degrade image quality the same. There are other common methods of displaying higher order aberrations, for example Tscherning ellipses or Zernike polynomials. Here the complex information is broken down into bite size pieces of information.
To discuss the methodologies of these here would be beyond the realm of this article. However, it is important to know what pupil diameter or entrance pupil the aberrometer is taking its measurements over. Usually the measurement is given for a 6mm pupil. If the pupil size is larger, more off axis light will enter the system, giving an increase in higher order aberrations (such as oblique astigmatism, for example). Conversely a smaller pupil size will result in lower levels of higher order aberrations. This is particularly important for some patients who experience visual symptoms such as glare or halos under low light conditions like night driving. Under these mesopic or scotopic conditions, the pupil diameter will be larger and the induced higher order aberrations greater.
The eye is essentially an optical system made up of different powered or refracting surfaces – the front and back of the cornea and the front and back of the crystalline lens. The image formed by these surfaces is created on the retina.
Altogether the eye has a refracting power of around 60 diopters. It seems to have built in mechanisms to reduce its own aberrations from this high power. For example the prolate shape of the anterior cornea and the steeper central curvature of the posterior cornea help to minimise spherical aberration. Similarly the unique shape and structure of the crystalline lens can also help to reduce spherical aberration; the lens has prolate surfaces on the front and back, the back being steeper and curved in the opposite direction to the front, plus the centre of the lens is denser and with a higher refractive index than the cortical regions.
If an optical system had perfect refracting surfaces then the image formed would still be subject to chromatic aberrations. This is caused by the fact that the refractive index depends on the wavelength of the light and since visible light is polychromatic the formed image will suffer this type of aberration. An example of chromatic aberration in spectacle lenses would be the coloured fringes on a high contrast border.4 In an optical system such as a telescope, the chromatic aberration can be corrected with a lens system like an achromatic doublet – where materials of differing refractive indices are cemented together to complement each other.
When observing side-by-side images simulating uncorrected aberrations and partially corrected variant images, a significant difference of subjective preference was found with either a partial (50 per cent) or full correction of both spherical aberration and coma. This gain was comparable to 1/8 D of defocus blur.5 Piers et al. explored the impact of spherical aberration on contrast sensitivity and found that peak performance was achieved when spherical aberration was completely corrected.6
Correction and Accommodation
Spectacle correction of some higher order aberrations is possible, in fact the shape factor of the correcting spectacle lens is considered during manufacture when deciding on the base curve of a spectacle lens. Aspheric spectacle lenses are also used for higher prescriptions to minimise spherical aberration. However, to try to correct some other higher order aberrations is more problematic. The spectacles would need to be adjusted and fitted to the ‘exact vertex distance and pantoscopic tilt’. Fixation through spectacle lenses would need to be steady and any eye movements behind the lenses would degrade the retinal image.7
Laser refractive surgery to correct higher order aberrations was first performed more than a decade ago and nowadays it is used as the treatment of choice for many patients undergoing laser vision correction. The initial excitement of giving patients hyper acuity – hawk eye vision or super vision was soon realised to be not possible.8 Although what did emerge was the need for caution in altering an individual’s higher order aberrations since the patients were often adapted to their own aberrations.
Similarly, a patient with symptoms that could be related to higher order aberration may benefit if their aberrations were brought into a ‘normal’ range. Another good use for wavefront laser vision correction is to help patients with unusual corneal shapes since the unusual corneal shape would induce higher order aberrations too.
Similar changes to spherical aberration can occur during accommodation. When the crystalline lens steepens to accommodate for near objects, the resultant increase in the overall refractive power of the eye would show an increase in spherical aberration.
But this is not necessarily so since during accommodation we also see a decrease in pupil size, miosis, and, as mentioned above, we would see a reduction in the higher order aberrations with a smaller pupil. The net result is that during accommodation the higher order aberrations do not change significantly.9
Impact of Contact Lenses
Contact lenses can also affect our higher order aberrations for better or for worse. A keratoconic patient for example may have poor vision with spectacles and soft lenses but better visual acuity with a gas permeable lens as the tear lens may neutralise the irregularities from the corneal surface hence reducing higher order aberrations. Research carried out by de Brabander and colleagues, demonstrates that the deformed surface of the cornea is neutralised by the spherical anterior surface of the rigid lens.10
A high-powered positive contact lens may increase a patient’s spherical aberration and a high-powered myopic lens could decrease the spherical aberration in the eye (in the same way as laser vision correction). However, lenses designed with aspheric front and/or back surface asphericity may induce less spherical aberration. Aberrations from soft contact lenses were shown to be predictable and the main limitations for precise correction of aberrations were the rotations and translations of the lens with respect to correct position on the eye.11
Dietze and Cox showed that negative lenses induced less spherical aberration than their equivalent positive powered lenses.12 However their results showed that lenses that still induced small amounts of negative spherical aberration gave better visual performance than the lenses that induced no spherical aberration at all. The majority of the subjects that took part in Dietze and Cox’s study had positive spherical aberration. The main finding was that the spherical aberration of soft contact lenses placed on the eye is similar to the spherical aberration of the lenses produced in air. A conclusion was reached that soft contact lenses with aspherical front surfaces would be free from aberration on eye if corrected for spherical aberration in air with negligible back vertex power changes.
Due to the advances made in measuring wavefront aberrations, some lens manufacturers have introduced contact lenses that aim to compensate for the reduction in image quality caused by higher order aberrations. Spherical aberration is the aberration targeted by many manufacturers to be corrected and counterbalanced with soft contact lenses. Since spherical aberration is rotationally symmetrical, the contact lens required for correction does not require any stabilisation criteria to prevent rotation.
If the soft contact lens is decentred, coma aberration is induced.13 Contact lenses need to have rotational and translation stability. Lopez-Gil and colleagues found that the correction of aberrations was limited by rotation and translation despite the correct fitting of the lens.11 However, all lenses need to move to a degree to enable tear exchange.7 Soft lenses, although inhibited to an extent by lens flexure, are favoured over gas permeable lenses as they move and rotate less on the cornea than gas permeable lenses.12 However, the tear lens effect in gas permeable lenses may compensate for some aberrations. Furthermore, it has been shown that gas permeable lenses can bring about a reduction in aberrations.14 The amount of reduction in aberrations that can be achieved by a contact lens is dependent upon the baseline aberration of the eye.
Research has shown that image quality is compromised if the lens decentration is more than 0.5mm and rotation is more than 10 degrees. These tolerances are dependent on the type and magnitude of the aberrations and size of the pupil diameter in each individual.10,13 Higher order aberrations induced are variable among different lens types. These variations can be attributed to the different manufacturing techniques of each lens type.
The material and method of manufacture, therefore, also influence higher order aberrations in terms of the regularity of lens surfaces and refractive index. Hong and colleagues found similar results; their study concluding that visual performance was found to be slightly better when viewing through an aberrated contact lens compared to an aberration free contact lens.15 Custom wavefront designed contact lenses which are fabricated to neutralise each individuals’ measured higher order aberrations may be beneficial. It is possible to produce such lenses with the use of rotationally asymmetric lathe techniques or excimer laser ablation.7
The Role of Tear Film
The tear film plays an important role in influencing wavefront aberrations due to differences in tear layer thickness and refractive index, which result in optical path differences presented as variations in wavefront aberration. Aberrations reduce after a blink as the normal tear film becomes stable. When the tear film becomes irregular and breaks up, aberrations increase especially in dry eye patients who present with higher levels of aberrations than normal. With contact lenses the tear break up time is shorter and irregularities have a more profound effect on visual performance.13
Clinical Manifestations
The visual needs of our patients are ever changing. The need for reading spectacles alone, for example, may no longer be enough to help a presbyopic patient. They may now require additional corrections for reading/computer distances or under certain lighting conditions. For example, between 50 and 75 per cent of people surveyed state that they watch television, use a computer or read before they sleep,16 and about 20 per cent of respondents reported that they commute to work between midnight and sunrise.17 More than half of frequent fliers on American Airlines stated that they read while travelling on an aeroplane.18
Patients in need of vision correction due to refractive errors may experience a wide range of eye-related symptoms that they expect vision care products to address. Despite differences between men and women and individuals of different ages, some common symptoms stand out. Recognition of these symptoms, who suffers and how often, and their impact on quality of life affords eye care professionals (ECPs) the opportunity to tailor management strategies and better ensure patient satisfaction with prescribed treatments.
As part of its comprehensive survey of 3,800 vision-corrected patients worldwide, the Needs, Symptoms, Incidence, Global Eye Health Trends (NSIGHT) study sought to identify the incidence, frequency, and perceived severity of 14 eye-related symptoms, as well as how those symptoms are currently addressed and to what degree of success.19 Among the symptoms most frequently reported by both spectacle and contact lens wearers were halos (the appearance of rings around sources of light, especially at night) and glare (difficulty seeing in the presence of bright light). One possible source of these symptoms is spherical aberration.
Globally, one third (34 per cent) of patients surveyed reported experiencing halos, while more than half (53 per cent) reported experiencing glare (see Figure 1). These incidences were generally similar among patients irrespective of the type of vision correction: spectacles (34 and 54 per cent for halo and glare, respectively) and contact lenses (39 and 46 per cent, respectively) (see Figure 2).
Halos and glare were commonly reported by both men and women, and young and old patients. While the incidence of halos was fairly consistent between men and women (32 and 37 per cent) and among patients of all ages (33-36 per cent for the age categories of 15-19 years, 20-30 years, 31-45 years, 46-65 years), glare was reported by a greater proportion of women and patients in older age groups (see Figure 3). Among female patients, 61 per cent reported experiencing glare, compared with 46 per cent of male patients. Similarly, the oldest group of vision-corrected patients in the survey (age 46-65 years) were more likely to experience glare than were members of the youngest cohort (age 15-19 years), 59 per cent versus 41 per cent.
More than half of the patients experiencing symptoms of halos and glare reported encountering them in the evening or late at night, and several common circumstances were identified as leading to their cause. Halos and glare shared an association with bright lights, sunlight, driving, and being in the dark or night time viewing. Overall, more than half of the patients reporting glare (52 per cent) experience the symptom in the presence of bright light, while 39 per cent reported the problem when driving and 22 per cent at night. Although less common, halos occurred most frequently for patients while in the dark or in bright light (31 per cent and 30 per cent, respectively), and at a slightly lower rate (25 per cent) when patients were driving.
NSIGHT provided valuable detail on how often patients experience halos and glare and the degree to which they found them bothersome. About half of the spectacle and contact lens wearers reported suffering from the symptoms of halos (52-56 per cent) and glare (47-50 per cent) more than three times a week, and more than four of five patients found each symptom bothersome (84 per cent and 89 per cent for halo and glare, respectively). Clearly, the severity of these symptoms is sufficient enough to impact patient satisfaction with prescribed vision correction.
Coupled with how bothersome patients tended to find halos and glare, about 90 per cent of patients (90 per cent for halos, 91 per cent for glare) reported having either no solution or one that was unsatisfactory or less than complete. Moreover, about the same rates of patients (89 per cent for halos, 87 per cent for glare) expressed interest in an intervention that more adequately addressed these symptoms.
Summary
There are numerous causes and solutions to halo and glare complaints including ergonomic factors and optical approaches. Because spherical aberration often contributes to these two symptoms, particularly halos, ECPs should give close consideration to options such as aspheric lenses. With nearly 90 per cent of spectacles and contact lens wearers lacking a solution and looking for better ways to relieve or prevent symptoms of halos and glare, ECPs have the opportunity to provide optimal visual correction for their patients by addressing these common and bothersome symptoms, especially in low-light situations.
One thing ECPs must take on board is the relevance of a more detailed visual task analysis for each of our patients. Thinking about the tasks that the patients perform and under what conditions as well as thinking beyond the high contrast, highly illuminated consulting room is important. Likewise, thinking about those situations where higher order aberrations start to cause effects such as halos and glare will aid ECPs in formulating a treatment plan for each patient.
Dr. Shehzad Naroo is a senior lecturer at Aston University, the Editor-in-Chief of the BCLA Journal ‘Contact Lens and Anterior Eye’ and the Global Vice-President of the International Association of Contact Lens Educators. He has authored and co-authored over 60 scientific publications, has been invited to present keynote addresses over 40 times and has presented more than 50 further free paper or poster presentations at international scientific meetings. He has given over 200 other invited seminars/ guest lectures and continued education and training presentations.
