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Clinical Concept of Visual Acuity Dr. Arpan B Patel, DNB Resident, Aravind Eye Hospital, Madurai Vision is the most sophisticated and highly developed special sense. The faculty of vision is a very complex phenomenon based on different physiological processes which are still poorly understood. Broadly, visual function comprises light sense, form sense, colour sense and the sense of contrast. The contrast sense is that faculty of vision by virtue of which the eye perceives slight changes in the luminance between regions which are not separated by definite borders. In its simplest terms, contrast sensitivity refers to the ability of the visual system to distinguish between an object and its background. For example, imagine a black cat on a white snowy background (high contrast ) against a white cat on a white snowy background (low contrast). The most common cause of decreased contrast sensitivity is ageing and the development of cataract. Although, the improvement of visual acuity is the major goal of any cataract surgery, with the present state of art, and the technology like phacoemulsification with foldable intraocular lens implantation, it is possible to provide a Snellens visual acuity 6/6 with almost normal field of vision. However, some patients are not entirely satisfied by the quality of visual acuity achieved. They complain of glare and “fogginess” or “haziness” of vision. These complaints are attributed to the reduction of contrast sensitivity. Contrast sensitivity is considered to be the best available measure of visual function in daily life. Incision of the cataract surgery getting smaller with phacoemulsification together with the better design and material of the intraocular lens implanted provide the best possible visual rehabilitation to the patients with cataract in the form of improvement in correct sensitivity. Excellent visual acuity measured with Snellens chart after intraocular lens implantation reflects the visual acuity at 100% contrast levels. However the real life situation are associated with various levels of illumination and objects with different shades and contrast. Contrast sensitivity, glare sensitivity and visual acuity evaluation are the best method of evaluating the problems of patient in daily circumstances. This study is undertaken to study and evaluate the optical performance of pseudophakic subjects with rigid PMMA, foldable spherical acrylic and foldable aspheric acrylic intraocular lens implantation after phacoemulsification cataract surgery using contrast sensitivity and visual acuity. Visual Acuity and Functional Vision Visual environment in every-day life consists of much more than just the simplified way in which vision is assessed using the letters or numbers on a standard eye chart. Visual acuity (VA) estimation is the central part of the ophthalmic examination and is the measure of the spatial resolution of the eye. Visual acuity testing is the “gold standard” for primary outcomes of clinical trials. Although Visual acuity testing is not the only measurement of visual function, it remains the most commonly used test. Despite being primarily a measure of the resolving power of the eye, it can be diminished in many different ocular conditions. Visual acuity is related to the angle subtended at the eye by the smallest recognizable optotype and is measured by the patient’s ability progressively smaller optotypes. The Snellens chart (Figure 2.1) introduced in 1862 by Dutch ophthalmologist Dr.Herman 2 Snellen, has been ubiquitously adopted as the standard for measuring visual acuity in clinical practice, because it is readily available as well as quick and easy to perform and is the universally accepted tool for testing visual acuity despite its poor reliability and reproducibility. The chart has letters of different sizes arranged from largest at the top to smallest at the bottom which are read,one eye at a time, at a distance of 6 meters (20 feet). Each letter on the chart subtends an angle of 5 minutes (min) of arc at the appropriate testing distance and each letter part (eg. limbs of the letter E ) subtends an angle of 1 min of arc. (Figure 2.2 ) Thus, it is designed to measure acuity in angular terms. In a healthy adult, the resolution limit is between 30 seconds and AECS Illumination 1 min of arc. The scoring method used is the line assignment method, where a patient gets credit for lines, not letters, read, Accepted convention does not specify Snellen acuity in angular terms; instead, Snellen acuities are usually expressed as a fraction with the numerator equal to the distance from the chart and the denominator being the size of the smallest line that can be read. The reciprocal of the Snellen Visual acuity equals the angle (in minutes of arc) that the strokes of the letter subtend at the person’s eye and is called the Minimum Angle of Resolution (MAR), which can also be given in log10 form,abbreviated as logMAR. The simplest method for computing the proper average visual acuity from any notation is to covert the value to the logMAR equivalent and then take the average of the logMAR values. The formulas for going from decimal to logMAR and then back are as follows: logMAR = -log (decimal acuity ) decimal acuity = antilog (- logMAR ) = 10-logMAR Figure 2.1 Figure 2.2 Standardization of the visual acuity task requires the following : 1. A logarithmic size progression (Constant ratio form one size to the next) 2. The same number of letters at each size level 3. Spacing between letters and between rows that is proportional to letter size 4. Equal (or similar) average legibility for the optotypes at each size level Newer logMAR charts (Figure 2.3) are now available that have negated the disadvantages of the Snellen chart. However, these charts are not being used regularly in daily practice. In 1976, Bailey and Lovie published a more standardized chart that was adapted and used for the Early Treatment of Diabetic Retinopathy Study (ETDRS). The ETDRS charts are now considered as the most accurate and standard method of VA recording, when precise data on VA measurement is required. The logMAR chart is based on Bailey and Lovie’s work and incorporates recommendations of the Committee on Vision of the American Vol. XVI, No.3, July - September 2016 Figure : 2.3 National Academy of Sciences-National Research Council. The technique for Visual Acuity measurement using a logMAR chart is very similar to the Snellen chart in that it requires patients to read to the lowest line possible. However, there can be practical difficulties with incorporating logMAR into clinical practice – many examination rooms are designed for Snellen charts at 6 m. The test distance for the logMAR chart is 4 m, but this problem is avoided by a very simple conversion factor. The regular geometric design of the optotypes allows easy conversion for other nonstandard distances. The change in size of the optotypes in the Snellen chart is arbitray and there is no regular progression from the easiest to the most difficult lines. Therefore, at the lower end of acuity level both over - and underestimation of the Visual Acuity can occur. A two-line reduction is Visual Acuity from 6/24 to 6/60 is more than a doubling of the visual angle, but the loss of two lines from 6/6 to 6/9 (on a chart with a 6/7.5 line ) is less than a doubling of the visual angle, that is, loss or gain of one line does not have the same meaning in different parts 3 of the chart. The geometric progression is size of the optotypes that occurs with the logMAR chart avoids this problem. The Snellen chart becomes progressively harder further down the chart the patient reads: only one letter is present at the 6/60 stage whereas upto eight letters are shown at the better acuity levels. This difficulty is compounded by the contour interaction or ‘visual crowding' phenomenon – the legibility of an optotype is less clear when presented with other optotypes in close proximity. Contour interaction varies throughout the Snellen chart and the 6/60 and 6/6 visual acuity are not entirely comparable.With the logMAR chart, the only parameter that determines visual acuity (VA) is the angular size of the letters. The smallest line a patient can read is used as a measure of his or her visual acuity. One or mistakes pre line are often compensated for, but this can have a different meaning at different levels of acuity with the Snellen chart, a patient must correctly identity seven out of eight letters (89%) on the 6/6 line, whereas on the 6/36 line, identification of only one letters (50%) may occur. The logMAR chart has an equal number of letters per line and each individual letter is assigned an individual score resulting in a more accurate and consistent measurement of visual acuity. Different Snellen charts may have different numbers of letters,that is,not all charts have a 6/7.5 line and there is no general standardization. Moreover, scoring Snellen visual acuity as the smallest line at which a majority of letters is correctly identified has been shown to restrict the sensitivity of the test to detecting changes over time. The Snellen chart is recongnizable and familiar to the layman and medical professional alike -‘6/6 vision’ is a common place term. This however, in the current climate of evidence-based medicine, does not justify a delay in adopting a better chart. Although the logMAR chart is regularly used as a research tool, it has to become more common in clinical settings. Indeed, for newer treatments such as Photodynamic Therapy, logMAR visual acuity 4 measurements are mandatory in order to be able to monitor treatment outcomes. LogMAR charts allow standardization of vision assessment and are, therefore, very useful in the research environment. As the 10 letters used in logMAR charts are chosen for their equality of readability and difficulty level, a three-line worsening of VA is equal to a doubling of the visual angle regardless of the intial acuity. The logMAR chart is said to be more timeconsuming and less easy to understand than the Snellen chart. This however, is not entirely accurate. The potential difficulty of use would be easily overcome by frequent use. The Snellen chart with its long precedent is so well entrenched in common use that this may be the single biggest factor preventing logMAR from superseding it. The logMAR chart also includes a conversion measure into Snellen equivalent if this is required for communication to those in other medical specialities who may not be as familiar with logMAR. In the context of assessment of Visual Acuity, the extra time involved is minimal and should be negated by increased accuracy and sensitivity. However, refraction on a logMAR chart is timeconsuming. Patients continually become ‘lost’ because of the crowding phenomenon, must read, and re-read the chart to locate the correct letters for fixation. Newer condensed forms of the chart have been shown to be as reliable as and quicker to perform than the full logMAR chart, which would allay this problem. Further refinement of the chart is required to overcome the impracticalities of the chart for refraction.The logMAR is, therefore, superior in its scientific principles, clinical accuracy and reproducibility. Acceptability of the LogMAR chart is less than Snellen chart because snellens chart is very well established in current clinical practice and ease in method of use. On the contrary few researchers still believe Snellens visual acuity is the best form of visual assessment in day to day clinical settings and practice. AECS Illumination LogMAR chart This chart has been designed for 4 meters,using high contrast lettering. It is based on the Bailey and Lovie’s work incorporating recommendations of the committee on vision of the American National Academy of sciences - National Research Council. As compared to the Snellen chart which does not have standard size progression e.g 6/60, the next line is 6/36 and then 6/24, as can be seen that the size progression is not linear, this problem is taken care of in the logMAR chart. In the LogMAR chart the progression of each line size is approximately 1.25 times greater than the line below. A change of 10 increments on this scale represents a change of exactly 10 times, and a change of three steps represents a change of approximately two times. Secondarily, the number of the letters is constant in each line. Therefore, it makes visual acuity measurement more accurate as the patient is able to read all letters of a particular line to be marked with visual acuity, whereas in snellens chart the number of letters change in each and this may be reason for the patient to guess where there is a single letter i.e at 6/60 and thus resulting in the wrong measurement of the visual acuity. Thirdly, letters are equally spaced on each line and the distance between each line also has a linear progression.Ten Sloan letters or pictures are selected for easy readability. The letters are C D H K N O R S V Z. Concept of Contrast Visual acuity measured using a Snellen chart has high-contrast letters; however, the world is almost never seen in such contrast. However, vision is not only about visual acuity, visual function comprises light sense, form sense,color sense and sense of contrast. The standard letter charts used by most ophthalmologists are not effective in the early detection of disease such as cataracts and glaucoma. However, other commonly performed tests (such as the slit lamp, intraocular pressure testing and Vol. XVI, No.3, July - September 2016 visual fiels ) can detect certain vision abnormalities caused by such diseases. Years of research and clinical trials has provided a more comprehensive, non-invasive technique for disease screening the contrast sensitivity testing. The standard high-contrast visual acuity chart measure the ability to see black letters (about 1 or 2 percent reflectance) on a white background (close to 100 percent reflectance) giving close to 100 percent contrast.The real world, however, is far from this ideal. It consists of objects with an average reflectance of only 18 percent, and the contrast between objects of interest and their background is usually much less than 100 percent. The jet black letters on the white background have great deal of reverse contrast so that even a patient with severely reduced contrast sensitivity can still read the chart. Patient perceives the letters as gray on white rather than black on white, but still be able to recognize them.The examiner has no idea how letters look to the patient, so proving the standard visual acuity charts insensitive to test visual function. For example, the contrast between the pavement and the sidewalk, which is the main clue that defines the edge of a curb, may typically be just a few percent. Contrast provides critical information about edges, borders, and variations in brightness. People with poor contrast sensitivity fail to see large, lowcontrast objects under conditions of poor visibility (such as fog) despite normal or near normal visual acuity. The importance of measuring contrast sensitivtity is that it can provide information, which cannot be obtained from visual acuity measures, and it is often a better predictor of performance than visual acuity. A meaningful measure of contrast sensitivity can provide a more complete picture of the visual function when used in conjunction with Snellens acuity. 5 Contrast sensitivity testing can help in the early detection of a wide range of visual problems including, cataracts, glaucoma, amblyopia, Alzheimer’s, AIDS, Macular degeneration, Diabetes, and exposure to toxic agents. Incorporating contrast sensitivity testing during routine eye examinations would arm clinicians with valuable information about their patient’s visual condition. This information can further aid in proper patient care, referrals and treatment. Contrast Sensitivity [CS] It is defined as the ability to detect the presence of minimal luminance difference between objects or areas. Example for this would represent the ability to read a road sign board at night in the fog. The minimal luminance difference between the letters on the sign and the background is minimal, rendering the letters more difficult to see.Viewing the same letters in the day light without the fog is much easier by comparison. There is some correspondence between the expected snellens acuity may be normal in the face of decreased CS. It has been noted that aging process is correlated with decreased CS. Contrast Sensitivity = (L max - L min) (L max + L min) L is luminance. In a patient with loss of low frequency contrast sensitivity may be able to read 6/6 but unable to see in the fog. While blur due to refractive error alone affects the lower frequencies, scatter of light due to cataract causes loss of all frequencies. Interestingly the patients evaluated after intraocular lens implanation were found to have decreased CS compared to normal eyes. A possible explanation to this is, crystalline lens when accommodates, will focus at its peak wave length of the spectrum and small amout of light is defocused which travel to retina. The implanted 6 IOL cannot accommodate thus causing the chromatic aberration to be greater. Measurement of contrast sensitivity : A test for the hidden loss of vision On routine ophthalmic examination, it is rare that patient is tested for the other most important functions of vision like Fields and Contrast Sensitivity. It has to be remembered that a 6/6 result, will not always uncover a possible hidden loss in contrast sensitivity. This will indicate a serious loss of visual function and this loss is usually a sign of eye disease. The detection of the low contrast is an important part of vision. It follows little consensus regarding the best method to use, to measure and to test Contrast Sensitivity. Unlike test for other elements of vision, there is no universal standard testing method to measure contrast. The tests currently available are either of grating type or optotypes as targets. Grating Types 1. The Functional Visual Acuity Contrast Chart Test [FACT] [Stereo Optical Company, Inc, Chicago IL] Originally developed by Arthur Ginsburg is available as handheld and wall mounted and illuminated with external source. It is forced choice test. 2. The VCTS6500 [Distich Consultans,Inc Dayton] is a grating wall chart with five spatial frequencies. Tested at 3 mts and it is also a forced choice test. 3. CSV-1000 (vector vision, Arcanum, OH) The test distance is 8 feet for CS testing. Consists of round sine-wave grating. Letter Optotypes 1. The small letter contrast sensitivity test [SLCT] is printed for use at a distance of 4 meters with normal over head illumination. 2. The Bailey-Lovie Letter Chart consists of optotypes also called as Contrast Acuity Charts. Five per size, with logarithmical increment in AECS Illumination declining optotypes size. The increment in size are usually 0.1 log units. This is essentially a refined snellens chart, which has logrithmical increment istead of the uneven increments to allow for the better comparison between patients. Typically, a high contrast setting usually greater than 90%, is held constant and acuity measured. A low contrast setting [13%] can also be held constant, and the acuity is measured. The result would be two points on contrast sensitivity function curve. 3. The Pelli-Robson Letter – Sensitivity Chart uses letters of only one size in the groups of three, each group then decreasing in contrast. The correct level of contrast is obtained by the line of the letters corresponding to two correct responses of the letters. Though there is strict controversy as which method of CS testing is the best, Grating chart are mostly research oritented and Optotype charts are also equally effective in contrast sensitivity testing. Pelli-Robson CS test is a reliable and easy to apply method and Contrast sensitivity the pelli-Robson chart can be useful in developing countries. Pelli-Robson Contrast Sensitivity Chart Method of examination of CS • This chart utilizes letters of the same size but with reducing contrast to provide a quick means of assessing patient contrast sensitivity. • There are two charts and two scoring pads. The charts have different letter sequence but otherwise they are similar. Each chart has six letters in a row organized into two triples of varying contrast, thus there are two contrast levels in each row. The illumination of the chart is 85cd/Mm2 and glare should be avoided. • The test is carried out at a distance of 1 meter (42 inches) with patient wearing the best correction and before dilating the pupils and with near addition if required. Vol. XVI, No.3, July - September 2016 • Patient is asked to read the alphabet starting from left hand upper corner, when he fails to respond. Several seconds are given to him to retry and guess the alphabet. The score of the test is recorded by the faintest triplet, out of which at least two letters are correctly identified. • The Log Contrast sensitivity value for this triplet is given by the number on the scoring pad nearest to the triplet, either on the left or on the right side. • Each eye is tested separately and then both the eyes. All three measurements should not take more than 8 minutes. • Usually the binocular CS is higher than the monocular vision by 0.15 units. However development of cataract may result in poorer binocular CS when compared to the eye been tested separately. Interpretation of Readings The score a single number is a measure of the subject log contrast sensitivity. Thus a score of 2 means the patient was able to read at least two of the letters of the three letters with contrast of 1% (contrast sensitivity = 100% or Log 2). A PelliRobson score of 2.0 indicates normal contrast sensitivity of 100%. Scores less than 2.0 means significantly poorer contrast sensitivity. A score of less than 1.5 is consistent with visual impairment and a score of less than 1.0 represents visual disability. This score represents an approximately 10 folds loss of contrast sensitivity that is the person with contrast sensitivity of 1.0 requires 10 times as much contrast to see as compared to normal vision. Mean value of the log CS 1.72 in a group of patients between 6070 years have been found in a study. A loss of this magnitude would be quite disabling and will have huge impact on one’s ability to drive or read. Poor CS adversely affects the ability to read the text, for example reading a newspaper. A CS of 1.0 or better is required to read high contrast print at a normal speed. Most people with a CS of 1.0 or less will read text slowly. This level of 7 contrast sensitivity is disabling with regard to walking speed. If CS is less than 1.3 the patient will have an increased likelihood of automobile accident. While performing typical manual tasks the contrast between different crucial parts of the task materials can be very low. It is likely that if one’s ability to see under such reduced contrast is impaired, then their ability to perform these tasks will also be affected and Pelli-Robson test, especially at 3m, could be a quick and reliable clinical test for contrast sensitivity at the region of peak sensitivity. The Pelli-Robson test with optotypes (letters) only measure one CPD (cycles per degree) region at a recommended distance and the examination must be done at different distances if more cycles per degree are needed. Limitation of the Human Eye The media can scatter light as it is being transmitted from an object. The optics of the eye may degrade the quality of the image. Retina has anatomical and physiological limits, and neural image processing can later the perception of a target and reduce or enhance contrast sensitivity. The highest level of the acuity attainable by the human eye is under photopic or bright light conditions. Clinical Significance of the Contrast Sensitivity While the achievement of uncorrected vision remains a laudable target for any cataract surgery, the goal of high quality vision increasingly reflects understanding of the visual system as a whole. Snellens visual acuity represent only a small portion of the functional vision. Contrast sensitivity has the ability to detect the difference in functional vision when snellens vision acuity measurement cannot. Blur images of the refractive error affect only the high spatial frequencies, the scatter of light due to the cataract and corneal opacities affect at all frequencies. Contrast testing thus offers critical information which helps to explain the patients’ complaints. 8 Unfortunately the CS decreases with age even with or without ocular pathology. This decline in the CS is due to the changes in the spherical aberration of the crystalline lens. In addition to understanding the visual complaints of the patients having good snellens acuity, it also helps in early diagnosis of ocular disease, to determine when a patient needs treatment, and to judge the efficiency of the given treatment. Contrast sensitivity can be used to determine exactly when does diminished functional vision from a cataract warrants surgery. It helps to explain the patients having 6/6 visual acuity and complaining about the quality of vision and then having cataract surgery and being happy with results. The sine wave grating contrast sensitivity of a pseudophakic patient with a spherical intraocular lens (IOL) implanted is no better than that of a phakic patient of a similar age who has no cataract. This may be the reason of non-specific visual complaints (‘washed-out images’), despite normal Snellen acuity, after cataract surgery and monofocal IOL implantation. Aberrations of the Eye The wave-aberrations is defined as the difference between the perfect (spherical) and the real wave fronts for every point over the eye’s pupil. These aberrations are classified as monochromatic and chromatic. Monochromatic aberrations are specific to a particular wave-length of visible light as it travels through optical system. Monochromatic aberrations can be further classified into; 1. Low-order aberrations, such as defocus (spherical refractive error) & astigmatism 2. Higher-order aberrations, such as coma, spherical aberration, trefoil, quadrafoil, or terafoil, secondary astigmatism, pentafoil. In phakic eyes, the decrease in visual acuity and in contrast sensitivity that occurs with age is usually attributed to lens changes. These changes AECS Illumination develop with presbyopia and are primarily related to increasing wavefront aberration.In young subjects, the crystalline lens compensates with its negative spherical aberration for the positive spherical aberration of the cornea, resulting in a low level of spherical aberration of the entire eye. Unfortunately after of 40 years, the spherical aberration of the lens progressively turns positive, adding to the spherical aberration of the cornea to increase the total aberrations of the eye. Thus aging lens loss its balance with the cornea, as both magnitude and the sign of its spherical aberration change significantly. Thus, a loses of balance between corneal and lenticular spherical aberration causes the degradation of the optical quality in the aging eye. Current IOLs also have positive spherical aberration, thus producing a pseudophakic eye that in this regard is no better than an aged eye with a transparent lens. Conventional pseudophakic IOLs fail to restore the potential maximal optical quality, because of imperfect centration, tilt, and increased positive spherical aberration due to the addition of the positive corneal spherical aberration of the IOL to that of the cornea. Thus, lens with optimized aspheric design could improve the optical performance in balancing the positive corneal spherical aberration, provided tilt and decentration of the optic of the IOL are controlled. Aspheric Optics The understanding of sophisticated optics and its relevance to clinical ophthalmology is credited to corneal laser refractive surgery. Vision symptoms that were not consistent with excellent high-contrast visual acuity rapidly led to an understanding of the impact of large-aperture optics and aberrations on the quality of vision. This understanding rapidly found its way into cataract and IOL surgery. The natural corneal curvature has an asphericity that averages +10.27μ RMS. In youth, this is balanced by a lenticular asphericity averaging Vol. XVI, No.3, July - September 2016 -0.27μ RMS, leading to minimal total spherical aberration. It has been suggested that the best optical is obtained if the entire amount of spherical aberration is corrected (i.e there is zero total spherical aberration after surgery). The original conventaional IOL designs all had optics designed on elementary optics, which resulted in positive spherical aberration in the IOL. This increased the total spherical aberration of the eye after cataract and IOL surgery, with resultant loss of image quality, loss of contrast sensitivity. IOL designs increasingly address the issue of the spherical aberration of the eye. Aspheric optics 9 clinically improves quality of vision and contrast sensitivity compared to spherical optics. Aspheric lens designs have targaetd different ranges of spherical aberration. The Alcon aspheric IOLs (e.g. SN60WF series) are designed for -0.17μ RMS of asphericity, with prolate posterior surface, strking a middle ground. The prolate surface of the aspherical IOL compensates for the spherical aberration of the cornea. Aspheric IOLs must be centered within 0.4mm of the pupil center to results in optical benefit. The lower the negative asphericity, the less benefit to a well-centered IOL but, correspondingly, the less degradation form decentration or tilt.