Figure 1. This is an example of an optotype from the Feinbloom number chart.
By Mark E. Wilkinson, OD
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Blindness and vision impairment represent a significant burden, not only to those affected by sight loss, but also to our national economy. It is estimated that $468 billion is spent annually on care and services for the blind and visually impaired in the US. (Source: Testimony, National Alliance for Eye and Vision Research before the Labor, Health and Human Services, Education and related Agency Sub-Committee of the House of Representatives Appropriation Committee, March 2005)
Based on this testimony, it should be clear that helping individuals who are visually impaired to function at their highest potential will allow many to remain independent, which will directly impact the personal and economic/social burden vision loss causes.
Low vision rehabilitation is the management of individuals who have a congenital or acquired impairment of visual acuity, visual field, and/or other functional vision factors. This loss of vision can interfere with the process of learning, vocational or avocational pursuits, social interaction, and activities of daily living. Most importantly, the impairment of vision cannot be adequately improved by conventional refractive measures.
Low vision rehabilitation involves a continuum of care, which begins with medical and surgical intervention and proceeds through to the prescription of low vision devices and vision rehabilitation services.
Vision rehabilitation maximizes the use of residual vision and provides the individual with practical adaptations for their normal activities of daily living and any other desired tasks. As the result of vision rehabilitation, the individual will attain the maximum function of their remaining vision, a sense of wellbeing, and a personally satisfying level of independence.
Maximum functioning is achieved through the use of optical, non-optical, and/or video magnification devices, or by teaching compensatory non-visual techniques. Vision rehabilitation may be necessitated by any condition, disease or injury that causes a visual impairment serious enough to result in functional limitations or disability.
There is no required amount of visual acuity or visual field loss necessary before an individual can be referred for low vision rehabilitation, however, the process of vision rehabilitation is felt to be more effective if it is started as soon as functional visual difficulties are identified and any medical conditions are cured or stabilized. This will allow the low vision rehabilitation team an opportunity to minimize the resultant visual disability and subsequent visual handicap.
Before we get started on our discussion of how to care for individuals who are visually impaired, it is important to be sure we are talking the same language. Here are a few definitions:
Traditional Classifications/Definitions of Vision Loss include the following:
An individual cannot be legally blind in one eye, and/or legally blind without the use of their glasses or contact lenses. Misuse of this definition is the cause of much public confusion about vision loss.
Based on this information, the status of legal blindness using the US definition is much easier to attain (along with its resultant benefits), than is the status of legal blindness using the WHO definition.
The conditions requiring low vision rehabilitation services are specified using the usual ICD-9 codes. In addition, for compensation for low vision rehabilitation services, another standard set of diagnostic and procedure codes is used.
The International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) is based on the World Health Organization's Ninth Revision, International Classification of Diseases (ICD-9). ICD-9-CM is the official system for assigning codes to diagnoses and procedures associated with hospital utilization in the United States. (http://www.cdc.gov/nchs/about/otheract/icd9/abticd9.htm)
The set of codes used for specifying the degree of a patient's visual acuity impairment is based on the ICD-9 Classification of Visual Acuity Impairment designating medical necessity for rehabilitation:
Impairment can also take the form of visual field loss. Field loss codes and definitions from the ICD-9 Classifications of Visual Field Loss designating medical necessity for rehabilitation are as follows:
Along with disease codes, the ICD-9 classification codes for visual acuity and visual field loss should be used when billing Medicare, Medicaid, or other insurance providers so as to better demonstrate the need for your services.
What follows are classifications for describing visual acuity or visual field loss that avoid the use of the term “legally blind” and are designed to give be a better description of the vision loss.
The WHO Classification of Visual Acuity Loss provides the following definitions and acuity ranges:
American Medical Association's Classification of Visual Field Loss (Based on the American Medical Association's Guide to the Evaluation of Permanent Impairment, 5th Edition, Chapter 12 - The Visual System American Medical Association's Guide to the Evaluation of Permanent Impairment, 5th Edition, Chapter 12 - The Visual System) provides the following definitions and usable remaining field ranges:
Vision loss is a common problem as we age. The estimated number of individuals who are visually impaired in the US varies from 3.5 to 14 million, depending on which definition of visual impairment is used.
In 1995, the Lighthouse National Survey on Vision Loss estimated the following:
Risk factors for developing visual impairment include:
Low vision care is no longer just about prescribing optical or electronic devices. Maximizing the potential of individuals with vision loss requires a much larger scale and integrated rehabilitative approach. For this reason, it is important to understand the following definitions of terms commonly used in rehabilitation:
Disease/Disorder:
Impairment:
Abnormality in visual system functioning includes difficulties with:
Medical/Surgical Services:
Disability:
Visual Disability results in reduced ability for:
Handicap:
Rehabilitation:
Low Vision Rehabilitation Services:
Habilitation Services:
Human Services:
This table illustrates the use of rehabilitative terms for two specific conditions. It is important to note that an individual with 20/50 (6/15) vision may feel terribly disabled or handicapped whereas an individual with 20/200 (6/60) vision may not feel at all disabled or handicapped at all.
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Disorder
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Impairment
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Disability
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Handicap
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Age-Related Macular Degeneration
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Decreased visual acuity
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Cannot read road signs
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Loss of driver’s license
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Glaucoma
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Decreased visual field
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Orientation problems
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Loss of ability to travel independently
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Now that a set of commonly agreed upon definitions have been established, the low vision examination can be presented.
The low vision exam is a structured process to be followed in a sequential manner when evaluating individuals who are visually impaired. It is important to note that the visual needs and functioning of the individual cannot be predicted based on the diagnosis or distance visual acuity alone. Individual variations make it impossible to generalize a rehabilitation/treatment plan for a given diagnosis or visual acuity levels. It must also be emphasized that there is no single device that works best for a given diagnosis or acuity level.
The components of the structured low vision examination are as follows:
These components will be discussed individually.
This is the most important aspect of the examination. It includes the following components:
During history taking, it is important to determine what types of visual difficulties your patient is experiencing because of their vision loss. There are many difficulties above and beyond reading or driving that may need to be explored. This is why a comprehensive history is so important.
Patients should also be asked about the occurrence of Charles Bonnet Syndrome in which formed, non-psychotic hallucinations of people, animals, etc. are seen by about 10 to 20% of patients with vision loss. Management of this syndrome includes physician recognition, empathy, reassurance, and patient education, which form the cornerstone of treatment. For this reason, consider initiating a discussion on Charles Bonnet by saying the following: “I often find that patients with a loss of vision experience phantom visions — perhaps streaks, flashes, or even faces or scenery—that seem unusual or hard to understand. Have you noticed anything like that?”
It should also be noted that depression is common among the elderly in general and is even more common among those who have experienced a significant loss in vision. This depression can be severe enough so as to require medical intervention to reduce the probability of self-destructive acts.
Ocular history should include a classification of vision loss. (Source: E. E. Faye, MD) Knowing the cause of your patient’s vision loss is important because it will help direct your examination and assist in the selection of devices that will be demonstrated to the patient. This is because each eye disease has a predictable effect on function, and the type and severity of the disease influences the ultimate effectiveness of any intervention.
The causes of visual impairment can be defined by the location of the pathology affecting the visual system: ocular media, retina, and/or brain.
Vision loss can be classified as: overall blur with no field defect, central field defect, and peripheral field constriction. The causes and management strategies for each classification will be discussed separately.
Overall Blur with No Field Defect. This condition typically occurs when the refractive media (cornea, pupil, lens or vitreous) become cloudy. In many cases of cloudy media, the individual’s complaint often seems out of proportion to their measured distance visual acuity. This is usually due to a significant change in their contrast sensitivity.
There are a variety of medical conditions that can cause this type of vision loss. They include:
When there is overall blur with no field loss, the patient will typically report these symptoms:
However, there will be little effect on the following:
Central Field Defect This condition can occur from a variety of medical conditions including:
Patients with central field defects will often report the following symptoms:
However, the following will not typically be affected:
Peripheral Field Constriction Medical conditions that can cause peripheral field loss include the following:
Patients with peripheral field constriction will often report the following:
This function will probably not be affected in peripheral field loss:
Health history should include a general health review and a discussion of all medications currently and recently taken by the patient. Information on hearing/other impairments/conditions, orthopedic limitations, and self-care needs (e.g., ileostomy, diabetes) should also be obtained.
Specific questions about any co-morbidities such as the following should be asked:
An extensive discussion regarding enhancement needs should focus on what may be very different requirements for distance, intermediate, and near tasks. Mobility, occupational, recreational, and daily living concerns should be explored in depth.
The low vision examination should include the following steps:
Selected aspects of the examination sequence will be discussed in detail.
Visual Acuity Assessment After history taking, the next most important task in the care of visually impaired individuals is accurate assessment of visual acuity.
Visual acuity (VA) testing should accomplish the following:
Factors to consider when measuring VA include:
Types of Visual Acuity Testing Acuity can be assessed by the ability of the patient to localize, detect, resolve, and/or recognize test stimuli.
Using the correct chart will provide the most accurate assessment of visual acuity. Commonly used low vision charts include Feinbloom and Bailey-Lovie type charts.
The Feinbloom number chart (available from Designs for Vision, Inc.) has the following characteristics:
Figure 1. This is an example of an optotype from the Feinbloom number chart.
The ETDRS/Bailey-Lovie chart (available from the Lighthouse International or the UC Berkley Optometry School) has the following characteristics:
Figure 2. The ETDRS/Bailey-Lovie chart.
The Standard Snellen chart has the following characteristics:
Acuities measured by specialized low vision techniques may not correlate by simple ratio to a standard 20–foot or 6-meter acuity measured with a projected chart. For example, 10/40 (3/12) does not necessarily equal 20/80 (6/24) and 10/100 (3/30) does not necessarily equal 20/200 (6/60). In addition, there is not always a one-to-one relationship between different chart types.
Shorter test distances allow for greater accuracy when measuring lower levels of acuity. Typical starting test distances are 5 to 10 feet or 2 to 4 meters, depending on the chart used. Remember to account for accommodative demands at closer distances.
Record the visual acuity as actual test distance over size of character read. For the Feinbloom chart, the test distance in feet becomes the numerator, and the size of the number read (noted in foot size) is the denominator. For example a 400 size optotype at 10 feet (10/400) (3/120) is the equivalent of an 80 size optotype at 5 feet (5/80) (1.5/24).
When the ETDRS chart is viewed 1, 2, or 4 meters, use the testing distance as the numerator and the M size of the letters read as the denominator. M size is given in the far left column of the chart. The next column on the chart gives the conversion to Snellen equivalent (not the letter size). For example, when testing at 2 meters and the patient reads the 32M line (160 Snellen equivalent), the acuity is recorded as 2/32, which is the numeric equivalent of 20/320.
Count fingers, hand motion, and light detection. It is important to accurately measure visual acuity to determine if the patient's rehabilitation plan is helping. For this reason, do not use “counts fingers” if at all possible. If the patient can see fingers, he or she can read the larger figures on a chart if it is brought close enough.
If “counts fingers” must be used, note the distance at which the patient can count fingers. Most fingers are equivalent in size to a 20/200 (6/60) size letter. Therefore, counting fingers at 3 feet (1 meter) is equivalent to about 3/200 (1/60), which is equal to 20/1300 (6/360).
If the patient’s visual acuity is reduced to the point at which he or she can only see hand motions, note in which quadrant(s) and at what distance the motions can be seen. If the patient can only see light, determine if he or she has “light perception with projection” versus just “light perception.” If direction can be determined, note in which quadrant(s) and at what distance the light can be seen.
Pinhole Visual Acuity For individuals who do not have any type of ocular disease, a pinhole aperture can be a useful tool for determining if a refractive error is present or if a refractive correction change is needed. The most useful pinhole diameter for clinical purposes is 1.2 millimeters. This size pinhole will be effective for refractive errors of plus to minus 5.0D.
A pinhole improves visual acuity by decreasing the size of the blur circle on the retina resulting in an improvement of the individual's visual acuity. However, if the pinhole aperture is smaller than 1.2 millimeters, the blurring effects of diffraction around the edges of the aperture will actually increase the blur circle, causing the vision to be worse.
Individuals with macular disease, as well as those with other ocular diseases that affect central vision, may have similar or even reduced acuity when looking through a pinhole. This is because the reduced amount of light entering through the pinhole makes the chart less easy to read. Additionally, it can be difficult to use an eccentric fixation point through a pinhole. For this reason, individuals with ocular disease should not be told that a spectacle correction change will not improve their vision, based solely on their looking through a pinhole. Careful retinoscopy along with a trial frame refraction (in most cases) is needed to determine whether an individual with pathology induced vision loss will benefit from a spectacle correction change.
During measurement of visual acuity, the clinician should evaluate any eccentric viewing techniques used by the patient.
Near Acuity Measurement For measuring visual acuity at near, charts designed for individuals who are visually impaired (i.e., charts with single letters, isolated words, or short sentences) should be utilized and testing distances must be measured and recorded.
Use of the M system is preferred for specifying near acuities because it yields a Snellen fraction that is more easily compared to distance visual acuities. The designation of letters signs (e.g., 1M, 2M) indicates the distance in which the print is equivalent in angular size to a 20/20 optotype. For example, 1M print subtends 5 minutes of arc at 1 meter.
There are a variety of cards that can be used for assessing near visual acuity.
The M unit chart was developed by Bailey in 1978. The International Council of Ophthalmology as well as the International Society for Low Vision Research and Rehabilitation recommends metric acuity testing, because it is the most accurate and reproducible test available.
The Lighthouse near chart uses Sloan optotypes that range in size from 8M to 0.3M.
Figure 3. The Lighthouse Near Acuity chart.
The ETDRS near chart, like distance version, has a logarithmic progression in optotype sizes, with proportional spacing of letters and rows. This allows the task to remain constant at different distances.
The Lighthouse “Game” and “Number” cards present words and triple digit numbers. This allows assessment of crowding factors as well as cognitive influences on reading ability. These are the cards most commonly used by the author for evaluating near vision.
Figure 4. Lighthouse Game card.
Figure 5. Lighthouse Number card.
The Bailey-Lovie word reading chart presents logarithmic progression of unrelated words.
The MN Read charts present sentences.
Figure 6. The MN Read chart.
Sloan Continuous Text reading cards provide a more accurate measure of reading ability than do single optotype acuity cards.
Figure 7. Sloan Continuous Text reading cards.
Jaeger Acuity. This is the least desirable letter-size designation (Source: International Council of Ophthalmology and the American Academy of Ophthalmology). Jaeger numbers are a printer’s designation that refers to the boxes in the print shop in Vienna where Jaeger selected his print samples in 1854. The print box numbers were are not proportional to the letter sizes, and the system has never been standardized. In addition, print size is not the same from one Jaeger test card or chart to another
Recording Near Acuity Near visual acuity is typically recorded as testing distance in meters over M-size letter read, thus yielding a true Snellen fraction. For example, if a 4M letter is read at 40 cm, the acuity is recorded as 0.40/4M, which is equivalent to 20/200 (6/60) distance acuity. As a second example, if a 1.6M letter is read at 20 cm, the acuity is recorded at 0.20/1.6M, which is equivalent to 20/160 (6/48) distance acuity
Use of the M system also facilitates calculation of addition power (i.e., the dioptric power required to focus at a specific metric distance). For example, if a patient reads 0.40/4M and wants to read 1M print, he or she must hold the material at distance X where X is determined by the equation 0.40/4M = X/1M. Solving the equation for X yields X = 0.10M or 10cm. The lens that focuses at this distance is +10D. This will be discussed more fully later in the course.
The goal of a subjective refraction is to achieve the best possible clear and comfortable binocular vision. The ability of the clinician to maintain patient control during the refraction is directly related to his or her ability to communicate clearly and directly.
There are many disadvantages to using a phoropter for refractive error determination. These include:
Conversely, there are many advantages to using a trial frame for refractive error determination. These include:
Just Noticeable Difference (JND) Refraction Techniques
The JND is the amount of lens power needed to elicit an appreciable change in clarity or blur; the poorer the visual acuity, the larger the JND will be. Numerically, the JND in diopters equals the denominator of the 20 foot Snellen acuity divided by 10. For example, for a 20/150 Snellen letter, 150 divided by 10 equals 1.50D. The patient would have an initial range of clarity of plus and minus 0.75D around their best correction. (This calculation also works for metric notation.)
JND techniques allow accurate refraction at any acuity level, the techniques apply to both sphere and cylinder corrections, and JND techniques elicit reliable answers.
As an example of JND refraction to determine sphere power, consider the following patient:
Figure 8. Determining sphere power using the JND method.
Finding the best cylinder axis and power requires using a Jackson Cross Cylinder (JCC) with the appropriate value the same JND technique described above:
Figure 9. Determining cylinder power using the JND method.
Here is an example of the JND technique used to determine cylinder power and axis:
Final Comments on JND Refraction
These tests are designed to provide a better understanding of an individual’s quality of vision, not just the quantity number clinicians get by testing distance acuity alone. Distance acuity tells us the patient's quantity of vision, not how well they are able to use their vision, i.e., their quality of vision. Near acuity testing, along with the following tests, helps the clinician to better understand how vision loss has effected the patient's visual functioning.
Visual function tests include the following:
Contrast Sensitivity Contrast indicates the variation in brightness of an object. When a vision chart uses perfectly black ink on perfectly white paper, 100% contrast is achieved. Printed acuity charts that approximate 100% contrast are helpful for characterizing central visual acuity. However, they are less helpful for examining visual function away from fixation.
Figure 10. Contrast sensitivity test chart.
Contrast sensitivity is typically tested using alternating light and dark bars with varying contrasts. The number of light bands per-unit width or per-unit angle is called the spatial frequency. During clinical testing, patients are presented with targets of various spatial frequencies and contrasts. The minimum detectable contrast is the contrast threshold. The reciprocal of the contrast threshold is defined as the contrast sensitivity, and the manner in which contrast sensitivity changes as a function of target spatial frequency is called the contrast sensitivity function.
Figure 11. Contrast sensitivity recording forms showing the range of normal contrast sensitivity functions (shaded areas).
Contrast sensitivity can be tested with sine wave gratings presented using either charts or video gratings. Because standard Snellen acuity assesses only high spatial frequency (e.g., 20/20 (6/6), which is equivalent to a grating frequency of 30 cycles per degree), they do not provide an accurate picture of the entire range of an individual's visual functioning, particularly when the individual has an ocular disease. Snellen acuity does not assess mid- or low-spatial frequency contrast sensitivity.
Acuity charts provide a quantitative assessment of visual functioning whereas contrast sensitivity charts provide a qualitative assessment of visual functioning. Contrast sensitivity testing is similar to audiological testing, which assesses an individual's ability to hear the entire range of sound frequencies.
Contrast sensitivity testing can detect changes in visual function even if Snellen visual acuity is normal. This can occur when corneal pathology, cataracts, glaucoma, and various other ocular diseases are present. Contrast sensitivity testing helps to predict illumination, contrast and magnification needs as well as predict success with optical magnification.
Amsler Grid Amsler grid testing is useful in low vision rehabilitation to locate and characterize scotomas, to determine if the patient is using eccentric viewing, and/or train the individual to use eccentric fixation. Amsler grid testing can also be useful for predicting an individual's success with the use of optical/electronic devices.
A scotoma’s location relative to fixation, size, shape and density can all be estimated using the Amsler grid. The location of a scotoma may have prognostic value - scotomas above the midline may have a better prognosis for reading than scotomas to the right or directly on the midline.
Figure 12. Amsler grid showing significant field distortion.
When using the Amsler grid, if a dense central scotoma exists but the center of grid is visible, eccentric fixation is likely. In this case, the scotoma is mapped relative to fixation, not relative to the center of the fovea. It is sometimes possible to train a patient to move his or her eyes around until the center of the Amsler grid appears. For some patients, this is easier to do this when using a video monitor.
While doing Amsler grid testing, if the grid has more distortion when viewed binocularly than it does with either eye individually, occlusion or fogging of the worse eye may be necessary for best test results. If the grid has less distortion when viewed binocularly than it has when viewed with either eyes alone, then binocular devices may be more helpful.
It is important during Amsler grid testing to explain and demonstrate to patients the location of their scotomas, the concept of eccentric viewing, which eye is their dominant eye, and why they may function better with their poorer eye occluded or fogged.
There are a number of problems with Amsler grid testing. These include the fact that the sensitivity of both standard and threshold Amsler grid testing is very low. Almost 50% of scotomas are missed during Amsler grid testing. Larger scotomas are more likely to be detected. However, when larger scotomas are detected, the full extent of the scotoma is frequently underestimated. These problems can occur because of unsteady/eccentric viewing and perceptual filling (visual completion).
Despite these problems, Amsler grid testing is still very useful. When Amsler grid testing shows the location and the size of the scotoma, this information helps to predict problems the patient might have during vision rehabilitation. Additionally, it helps predict which patients will do better when viewing monocularly versus binocularly. Finally, Amsler grid testing can provide an indication as to which patients are likely to benefit from eccentric viewing training. Those individuals who are able to see their scotomas and maintain a preferred retinal locus are more likely to benefit from this training.
Preferred Retinal Locus Determination Typically the visual systems of individuals with central scotomas naturally, consistently, and unconsciously choose an eccentric retinal area to perform the visual task that the fovea previously performed. One study found that about 85% of patients with a central field loss were found to have established such a Preferred Retinal Locus (PRL). For individuals with central scotomas, visual tasks are performed by aiming the eye so that the image of a visual target is placed within the PRL.
The PRL, in essence, becomes a pseudo-fovea and assumes many of the tasks of the nonfunctioning fovea. For example, object recognition and detail discrimination. Therefore, the ability of the PRL to perform fixation, as well as pursuit and saccadic movements determines the performance ability in many activities of daily living. Some individuals use more than one PRL, depending on the visual task.
Under higher illumination conditions, the PRL tends to be closer to the fovea, sometimes in an area of a relative scotoma, whereas under lower illumination conditions, the PRL is switched to an area further from the fovea.
The relative location of one or more PRLs to a macular scotoma also indicates the degree of difficulty the individual will have in adapting to the scotoma. Individuals with macular scotomas that encircle the PRL, (called a ring scotoma), often experience greater difficulty in activities of daily living despite having fairly good visual acuity.
PRL testing is easily done with a scanning laser ophthalmoscope. Unfortunately, the cost of the instrument prohibits its widespread use.
Visual Field Testing Accurately detecting peripheral and central field loss is important because visual field changes can affect visual functioning. Visual field integrity is important for reading as well as for independent travel. Visual field testing is also important for determining eligibility for services and for driving.
Confrontation visual fields provide a quick screening which can be helpful for detecting unrecognized peripheral defects. Confrontation visual fields can also be useful as an educational tool for patients with central loss by demonstrating that their peripheral vision is intact.
As opposed to automated perimetry, traditional Goldmann perimetry is easier for individuals who are visually impaired, particularly for those with poor fixation, fatigability, and reduced visual thresholds. The problem with this type of testing is that it requires a trained technician to perform the test.
Automated perimetry has the advantage of standardized protocols, longitudinal databases, and macular assessment capability. Additionally, this type of perimetry can be performed without a specially trained technician. The problem with threshold related automated perimetry is that it tends to overestimate visual field loss for individuals who are visually impaired. With this in mind, it is important to not rely on the perimetry gray scale when interpreting visual field loss in individuals with inherited eye diseases or those with reduced central acuity. The gray scale will indicate that the visual field loss is much worse than it really is as compared to super-threshold visual field testing.
Color Vision Testing It is important to ask if discriminating colors is difficult for low vision patients. Not only the patient, but also the parent/family/spouse should be asked about color discrimination difficulties.
Most acquired color vision defects are blue-yellow confusions as opposed to the typical red-green inherited confusions. However, many pseudoisochromatic plate tests do not detect blue-yellow problems. Also, acquired color vision loss may be monocular so eyes should be tested individually. For individuals who are visually impaired, the Farnsworth D-15 may be the best test to use. If motor skills do not allow manipulation of the color caps, assistance should be provided, but care must be taken to not provide clues to the proper arrangement sequence when moving caps for the patient.
It is also important to remember that inherited color vision deficiencies may also be present in individuals that are visually impaired.
Brightness Acuity Testing (BAT)/Glare Testing It is important to ask about glare problems during history taking. This is because routine test conditions may miss glare symptoms. Knowing about glare problems will help to direct lighting recommendations. Glare and poor contrast sensitivity can make management of vision loss using optical magnification difficult.
It is obviously important to review the patient's ocular and systemic health with respect to preexisting and new diseases, and to conduct a complete ocular health examination.
When cataracts are adversely affecting visual functioning (they would normally be removed if no other ocular problems were present), and cataract surgery is not contraindicated by some other ocular disease (e.g. active diabetic retinopathy or choroidal neovascular membrane, etc.), surgery should be considered to enhance visual functioning and maximize visual potential.
When considering postoperative refractive error correction for individuals with additional ocular diseases, Lighthouse International suggests the following: individuals undergoing cataract surgery who will require magnification because of coexisting conditions such as macular degeneration or diabetic retinopathy, should almost never be made emmetropic postoperatively.
Lighthouse suggests that a postoperative refractive error of -2.00 D to -3.00 D will leave the individual happier because he or she can remove his or her spectacles to read more comfortably (with or without an optical device) than would be possible with bifocals or separate reading glasses. Additionally, if the patient needs reading glasses, they will be of a lower power. If the patient needs higher amounts of magnification, the reading glasses will be lower powered with less weight and distortion. Additionally, patients will be able to use hand-held magnifiers without their glasses on. Obviously, these individuals will still need a lens correction to have their best distance vision, but the reading advantages more than make up for this.
Many individuals that are visually impaired will benefit from the prescription of optical and electronic devices, and from changes in their visual environments.
Assuming it has been concluded that an optical aid is appropriate for a patient, the magnification to be provided by the aid must be determined.
There are 4 types of magnification that individuals with visual impairments can employ to enhance their visual abilities. They are: relative size, relative distance, angular, and electronic.
Relative Size Magnification (RSM) enlarges an object while maintaining the same working distance. Numerically, RSM is equal to size after magnification divided by size before magnification.
Consider an example for print:
Because it is difficult to enlarge reading materials much beyond the 2M (18 point) level, this option is of relatively little value for individuals who have experienced a significant loss of vision. Additionally, there are many things individuals who are visually impaired want to read (e.g., newspapers, mail, bank statements, etc.) that are not readily available in large print formats.
Relative Distance Magnification (RDM) The easiest way to magnify an object is to bring it closer to the eye. By moving the object, the image size on the retina is enlarged. Children with visual impairments do this naturally. Adults with less accommodative ability will require reading glasses to keep the object in focus as it is moved closer.
RDM is defined as r/d where r equals the reference or original working distance and d equals the new working distance. For example:
With reading glasses, as the lens power increases, the working distance decreases. For example, a +5D lens focuses at 100/5 = 20cm (40/5 = 8 inches) and a +10D lens focuses at 40/10 = 4 inches (100/10 = 10cm). Reading glasses do not magnify by their power alone when worn in the spectacle plane. Magnification occurs simply because the lens strength requires the individual using the glasses to hold things closer to have them in focus.
Angular Magnification Angular magnification occurs when the object is not changed in position or size but has an optical system interposed between it and the eye to make it appear larger. Examples of devices that produce angular magnification are telescopes and hand magnifiers.
Optical Magnification Ratings
Some companies use lens focal length/4 (defined as Rated Magnification) while others use the quantity (lens focal length/4) + 1 (defined as Conventional Magnification) to specify magnification strength for their devices. This is why dioptric power, which is an absolute value and is the same under all conditions, is a better way to specify the magnification needs of an individual.
Rated Magnification (Mr)
Rated magnification assumes that the individual can accommodate up to 4.00 diopters when doing close work, which gives a working distance of 25cm (25cm is the standard reference distance always used when talking about magnification).
Conventional Magnification (Mc)
The underlying assumption in the equation ((lens focal length/4) + 1) is that the patient is “supplying” one unit (1X) of magnification
Effective Magnification (Me)
Effective magnification is based on the reference distance in meters to the object (image is formed at infinity). (d is reference distance and F is dioptric power of the lens)
Determining Needed Magnification Magnification needs are based on an initial reference value and the desired final value. Clinically, needed magnification is defined as the entrance distance (or near) acuity divided by the goal acuity (VA entrance/VA goal).
Electronic Magnification is magnification that can be provided by a closed circuit television system or computer software. These systems can make an image appear larger and with greater.
Magnification Estimation Techniques Accurate near visual acuity testing is essential for determining magnification needs required for reading and other near point activities. There are several ways to determine a starting point for near magnification. We will discuss the two most common ones that are used today.
Kestenbaum’s Rule To determine the power necessary to read 1M size print (newsprint), take the reciprocal of the patient's distance acuity to establish a starting add power.
As examples:
However, because distance acuity is a poor predictor of near visual functioning, this is not a very accurate method for determining a reading add power. The more accurate and more frequently used approach is the Lighthouse method.
Lighthouse Add Determination To determine the power needed to read 1M print, measure the patient's near acuity at a 16 inch/40 cm working distance (WD). Multiply the M acuity by 2.50D to arrive at the theoretical add power needed to read 1M print
As examples:
The Lighthouse method establishes a good starting power. However, patients may need additional power if their contrast sensitivity is reduced, if they have multiple scotomas, if they need to read smaller than 1M print, or if they have less than ideal illumination when reading. The clinician should adjust the add power using normal reading materials under task lighting to determine how much add power is actually needed.
Devices should be considered and presented to the patient in a sequence that roughly follows increasing cost and complexity:
Regular spectacles Always start by determining whether a change in the spectacle correction will enhance distance and/or near acuity as determined by a trial frame refraction. Regular spectacles can provide:
Stronger bifocal corrections will be required to use relative distance magnification early in the vision loss process. As higher amounts of reading addition are needed (greater than 6D), a +4.00D add may prove beneficial as an intermediate distance add. Individuals benefiting from this type of intermediate correction are those who need lower amounts of magnification for less detailed tasks such as signing their name, cutting their finger nails, seeing the food on their plate as well as cooking and reading larger print, such as the headlines, etc.
Spectacle Magnifiers can take several forms. These can include:
Figure 13. Reading spectacle designs.
When prescribing reading spectacles, is important to consider whether the patient functions better monocularly or binocularly. If patient is monocular, he or she will not need prism incorporated into the reading spectacles but it may be necessary to occlude/fog the fellow eye if it interferes with the better eye.
The need for occlusion or fogging the poorer seeing eye is often found in situations where the sighting/dominant eye has the greatest vision loss. In this situation, the now poorer seeing/dominant eye confuses the better seeing non-dominant eye resulting in poorer visual performance. Occlusion or fogging of the poorer seeing eye will ameliorate this problem and allow the individual to function at the highest potential.
For those individuals who have similar near acuities between their two eyes and whose binocular acuity is better or the same as their monocular acuity, base-in prism will provide more comfortable, sustained reading ability. Base-in prism is used for adds of +4.00 to +12.00D. The prism power equals the add strength plus 2 prism diopters base-in for each eye. As an example, a +6.00D add would have 8 prism diopters base-in added to each eye's lens.
Advantages of spectacle magnifiers:
Disadvantages of spectacle magnifiers:
Working distance for these lenses is determined by taking the reciprocal of the equivalent add power (+20D lens will have a working distance of 100/20 = 5cm).
Figure 14. Close working distance resulting from use of a high dioptric power lens.
Training considerations for spectacle magnifiers:
Absorptive Lenses These lenses can absorb uniformly across the spectrum (e.g., gray sunglasses) or selectively in certain wavelength bands (e.g., blues, so the lenses appear yellow). Absorption of blue may be advantageous because chromatic aberration and glare can be reduced.
Advantages of absorptive lenses include:
Figure 15. Absorptive lenses with broad and selective absorbance spectra.
Disadvantages of absorptive lenses include:
Hand/Stand Magnifiers
Hand magnifiers are typically positioned so that the material being viewed is at the focal point of the lens. Patients need to be made aware that the larger the lens diameter, the weaker the lens power will be.
Hand magnifiers are used with the individual's distance spectacle correction in place. They come in both illuminated (standard or LED bulbs) and non-illuminated versions.
Hand magnifier considerations include the optical design, which may be:
Figure 16. Hand magnifiers.
For aspheric hand magnifiers, the front surface gradually flattens toward the edge of the lens. This design reduces or eliminates distortions induced when looking away from the optical center of the lens. Aspheric lenses have directionality. This means that the more curved surface should face toward the individual using the magnifier.
Aplanatic magnifier systems are created by using two plano-convex lenses with convex surfaces facing each other. This results in a distortion-free image right up to the edge of the lens. Patients vary in their appreciation of aspheric and aplanatic systems when compared to conventional spherical lenses.
Advantages of hand magnifiers include:
Disadvantages of hand magnifiers include:
Training considerations for use of hand magnifiers include:
Stand Magnifiers are available in illuminated and non-illuminated designs. The larger the lens diameter, the weaker the lens power. Stand magnifiers need to be used with a reading correction.
Figure 17. Stand magnifiers.
Advantages of stand magnifiers include the following:
Disadvantages of stand magnifiers include the following:
Training considerations for stand magnifiers include:
Telescopes can be hand-held or spectacle-mounted, and they can be monocular or binocular. Fixed focus, manual focus and auto focus systems are available with Galilean or Keplerian designs. They can be used for distance, intermediate, or near vision enhancement.
Figure 18. Telescopes.
Figure 19. Spectacle mounted bioptics.
Telescopes are afocal optical systems consisting of two lenses, separated in space by the sum of their focal lengths. Galilean telescopes have a plus power objective lens and a minus power ocular lens. They form an erect/upright image. Keplerian telescopes have a plus power objective lens and a plus power ocular lens. Keplerian (astronomical) telescopes form an inverted image and require an erecting lens or prisms to make them into terrestrial telescopes.
Galilean telescopes have several practical advantages for low vision work. The image is upright without the need for erecting prisms, and the device is shorter than a Keplerian telescope. Galilean telescopes typically are 2, 3, or 4x in strength, inexpensive, lightweight, and have a large exit pupil, which makes centering less difficult.
Four power (4X) telescopes and stronger are usually Keplerian in design, which gives an optically superior image, but they are more expensive with a smaller exit pupil requiring better centering and aiming. Keplerian binoculars, contain prisms to erect the otherwise inverted image.
Advantages of telescopes for vision remediation:
Disadvantages of telescopes:
Training considerations for telescopes:
Telemicroscopes (a.k.a. reading telescopes or surgical loupes) can be hand-held or spectacle mounted. Spectacle-mounted reading telescopes can be in full diameter (center-mounting) or bioptic configurations. They are available in Galilean or Keplerian designs.
Galilean telescopes used as surgical loupes require an add to be combined with the objective lens. The field size is far smaller than that obtained with bifocal spectacles.
Telescopic loupes can produce asthenopia when the patient has any type of refractive error. If binocular loupes are not aligned properly, vertical or horizontal phorias can be induced. Adopting a working distance too far inside the focal distance of the add can require excessive accommodation, even for a myope.
When viewing a near object through an afocal telescope, the telescope acts as a vergence multiplier. The approximate accommodation required is given by Aoc = M2U, where Aoc equals vergence at the eyepiece which also equals accommodation, U equals object vergence at the objective which equals 1/u (u is the distance between the objective lens and the object being observed), and M equals the magnification of the telescope.
Advantages of telescopes include the following:
Disadvantages of telescopes include the following:
Training considerations for telescopes include:
Video Magnification Devices Closed circuit video magnification systems are available in a variety of different styles ranging from full sized systems with their own or separate monitors, to hand-held camera systems that plug into the user’s own television, to portable battery powered systems.
Figure 20. Video system used for magnification.
Figure 21. Maximum magnification obtained with a video system.
Figure 22. Portable video display system used for reading.
Advantages of video magnification systems include:
Disadvantages of video magnification systems include:
Training considerations for video magnification systems include:
Head-Borne Video Magnification Devices As the name implies, these devices are worn on the patient's head and consequently move as the head turns.
Advantages of head-borne devices include:
Disadvantages of head-borne devices include:
Training considerations for head-borne devices:
Non-Optical Devices Objects used in daily living can be modified to facilitate use by low vision patients. Some modifications and special aids include:
Figure 23. Typoscope reading and signature guides and other special aids for low vision patients.
Reading stands and clipboards can be helpful for maintaining proper placement of reading material. Use of these devices can reduce postural fatigue and facilitate placement of adequate light on reading materials.
Figure 24. Use of a lapboard and magnifier for reading.
Figure 25. Use of a reading stand.
Use of typoscope signature and reading guides can reduce glare from glossy paper and minimize figure-ground confusion.
Some aids make use of relative size magnification, which can be used in conjunction with other forms of magnification (i.e. use of low powered reading lenses with large print). There are a variety of options available.
Illumination is probably the single most important factor in enhancing visual functioning. The median illumination found to give optimum performance in a low vision clinic was 1188 lux, whereas normal home conditions have a median value of only 177 lux. More than 90% of low vision patients showed some improvement in near or distance visual acuity when illumination was improved. (Silver JH, Gould ES, Irvine D, Cullinan TR, Visual Acuity at Home and in Eye Clinics, Trans. Ophthalmol. Soc. UK (1978) 98: 262-266)
Types of illumination that can be used include:
Light fixtures are as important as the bulbs used in them. They must be flexible to allow proximity to the paper and placement at a non-glaring angle. The position of the source must be adjustable to allow maximum comfort/contrast enhancement. Normally, light angled in from the side of the better seeing eye is best.
Figure 26. Light fixture providing proper illumination.
Adaptive Technology A comprehensive review of adaptive technology is beyond the scope of this course, however a brief review of computer systems that can be beneficial for low vision patients is included as an Appendix to this course.
Rehabilitation instruction involves the teaching of visual skills to improve overall visual functioning, both with and without the use of devices. Skills can include:
These skills can be taught by the clinician, a trained member of the staff, or a member of the low vision rehabilitation team. Some practitioners prefer to provide instruction/training on the initial visit, whereas others prefer to schedule a separate visit.
Teaching patients about non-visual approaches to some tasks may also be helpful:
For many individuals who are visually impaired, referral to additional resources such as state blind rehabilitation agencies or Veterans Administration programs will be of value for financial assistance, vocational training, etc.
It is also important to return patients to their primary (eye) care practitioner when rehabilitation is completed.
Formal communication to the referring doctor and other health care providers concerning the low vision rehabilitative care being provided is important for individuals of all ages. Communication is consistent with the objectives of the Government’s Healthy People 2010 initiative, it is a good practice builder, and documentation is required by most insurance plans when a consultation code is used. Consider sending a report not only to the referring doctor, but also to the patient’s other eye care practitioner(s), the primary care physician, and other specialists who are involved in care of the patient.
The first thing you should know is that you already have much of the equipment you need to provide low vision rehabilitation care in your office.
Equipment typically needed for low vision care includes:
Staff training for low vision patient care can include:
Scheduling
Currently, there are not enough optometrists providing low vision rehabilitative care to take care of those who are already visually impaired. As the population ages, the need for low vision rehabilitation services will increase significantly. Optometry is the profession best suited to provide this care. We understand optics, refraction, and ocular diseases. By providing low vision care, you will strengthen your connection with your patients and offer a unique service that will be greatly appreciated.
Vision Problems in the US-2002, Prevalence of Adult Vision Impairment and Age Related Eye Disease, Demographics of Visual Impairment, 4th edition, National Eye Institute, Prevent Blindness America (Source: www.usvisionproblems.org)
The Lighthouse Ophthalmology Resident Training Manual – A New Look at Low Vision Care, Faye, Albert, Freed, Seidman, Fischer (2000) Lighthouse International
Low Vision Rehabilitation: Caring for the Whole Person – Ophthalmology Monographs - 12, (1999), American Academy of Ophthalmology
American Academy of Ophthalmology Preferred Practice Pattern, Rehabilitation: The Management of Adult Patients with Low Vision (1998), American Academy of Ophthalmology
Optics, Refraction, and Contacts Lenses, Basic and Clinical Science Course, Section 3 (2004-2005), American Academy of Ophthalmology
Foundations of Low Vision, Clinical and Functional Perspectives, Corn and Koenig, (1996) AFB Press
Foundations of Rehabilitation Counseling with Persons who are Blind or Visually Impaired, Moore, Graves & Patterson, (1997) AFB Press
Foundations of Orientation and Mobility, Second Edition, Blasch, Wiener & Welsh, (1997) AFB Press
Visual Impairments: Determining Eligibility for Social Security Benefits (2002) Board on Behavioral, Cognitive, and Sensory Sciences and Education, National Research Council, National Academies Press
(Source: http://www.nap.edu/books/0309083486/html/)
Clinical Low Vision, Second Edition, Faye, (1984) Little, Brown and Company
Vision and Aging, General and Clinical Perspectives, Second Edition, Rosenbloom and Morgan, (1993) Butterworth-Heinemann
The Lighthouse Handbook on Vision Impairment in Vision Rehabilitation, Silverstone, Lang, Rosenthal, Faye, (2000) Oxford University Press
Remediation and Management of Low Vision, Cole and Rosenthal, (1996) Mosby
The Art and Practice of Low Vision, Second Edition, Freeman and Jose, (1997) Butterworth-Heinemann
Screen Enlarging Software
Windows-based systems:
Apple Macintosh-based systems:
Screen Reading Software
Windows-based software:
Apple Macintosh-based software:
Operating System Software
Windows-based:
Apple Macintosh-based:
Contact this author:
Mark E Wilkinson, OD Associate Professor, Clinical Director, Vision Rehabilitation Service Carver College of Medicine Department of Ophthalmology and Visual Sciences 200 Hawkins Drive, 11190A PFP Iowa City, IA 52242-1091Dr. Wilkinson has no proprietary interest in any of the products mentioned in this course.
Pacific University College of Optometry provides On-Line CE as a service to optometrists. The college does not endorse or recommend any products, equipment, or services that might be discussed in the courses. Courses are prepared by individuals believed to be experts in their areas of specialization who are compensated for their efforts. The College relies on their expertise to produce accurate and timely courses. Questions or concerns about courses should be directed to the individual authors and/or the Continuing Education Department at the College of Optometry at kundart@pacificu.edu.
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