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A Review of Low Vision Rehabilitation

Mark E. Wilkinson, OD

Contents

The Examination Sequence

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.

 

 

 
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