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A Review of Low Vision Rehabilitation

Mark E. Wilkinson, OD

Contents

Determination of Refractive Errors

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:

The image cannot be displayed, because it contains errors.

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

Visual Function Tests

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.

 

 

 

 

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