Beyond BCVA: Thorough Vision Assessment For Retinal Physicians

Snellen acuity alone is insufficient for proper understanding of visual performance.

August Colenbrander, MD • Donald C. Fletcher, MD

The retina is an important transit point for visual information. Retina specialists want to make sure that the precious cargo that we call “vision” travels to its final destination, where it prompts visual perception and visually guided behaviors, with as little distortion as possible.

On this journey there are three stages that may present with very different problems.

First is the optical stage. This stage must present a sharp image of the outside world onto the retina. Most retina specialists would prefer to leave it to others to deal with problems in this area, but a basic understanding of its significance is essential. When optical problems cause a blurred image, magnification usually is an adequate solution.

Next is the receptor stage, in which the light energy of the ptical image is converted to neural energy. This conversion happens in the outer retina. For this stage, retinal sensitivity (contrast) and the topography of lesions are important.

Third is neural processing. This stage starts in the inner retina and proceeds through the optic nerve to the visual cortex and then to higher cerebral centers, where the visual information meets with input from other senses and with stored memories to provide the basis for visual perception and visually guided action.

What is often overlooked is the significance of the first steps in this last stage — the preprocessing in the inner retina. There are approximately 100 million receptors in each eye but only roughly one million fibers in each optic nerve. Therefore, the inner retina must achieve a 100:1 data compression. We know that there are more than 60 different cell types in the retina and at least 20 ganglion cell types,but we have only vague ideas about how they function.¹

One thing seems certain: The retinal ganglion cells and their optic nerve fibers do not convey a pixel-by-pixel image to the brain, like a camera sensor transmitting an image to the camera memory. Some cells probably convey information about edges, others about brightness and color, still others about movement, and so on. A better insight into these processes is needed for a complete understanding of retinal functioning and of the functional differences between subretinal, epiretinal and cortical implants.

VISION TESTS

Unfortunately, we cannot test each of these stages separately; we can only test the end result. Therefore, it is important to recognize the differences between problems that can arise at each stage.

The most common vision test, letter chart acuity, is so common that many consider it as a test for vision in general; statements such as “His ‘vision’ is 20/40” are not uncommon. Actually, letter chart tests assess only the small retinal area upon which each letter is projected; they tell us nothing about the function of the surrounding retina.

Visual acuity is a good test to assess the optical stage, because optical blur at one point predicts equal blur for the surrounding areas. For retinal disorders, however, knowledge about the surrounding area is essential. The condition of a patient with 20/40 letter chart acuity from a slight cataract with a full visual field is very different from the condition of a patient with end stage glaucoma or retinitis pigmentosa with 20/40 in a small remaining field. The condition of a patient with a central scotoma that pushes fixation to an eccentric location is different still.

Accurate plotting of scotomata requires special equipment, such as a scanning laser ophthalmoscope (SLO) or MP-1. Approximate plotting is possible with tangent screen devices²; in the latter case, the center of the plot will represent the new center of fixation (also called the preferred retinal locus, or PRL), and the scotoma will be displaced correspondingly.

Reading tests are the simplest tests to obtain a more complete impression of actual retinal function. Reading not only requires the recognition of letters but the recognition of words, which occupy a larger retinal area. Furthermore, fluent reading requires a series of saccades to move from one word to the next. To target these saccades properly requires an even larger area, especially to the right. Thus, reading tests may detect scotoma interference that is not obvious on letter chart tests.

SLO studies have shown us that the topography of scotomata in the central visual field can vary enormously. Of 1,339 eyes of low vision patients, only 12% did not demonstrate a scotoma within two degrees of fixation, while 88% had scotomata, with 41% having scotomata in only one quadrant, 19% in two quadrants, 9% in three quadrants, and 19% on all four sides, completely surrounding the PRL/fixation.³

Patients with a complete (Figure 1) or partial (Figure 2) ring scotoma may exhibit a typical slow-fast-slow phenomenon when plotting their reading speed vs print size.⁴ When confronted with large words, they will read slowly, because they must scan each word that does not fit in their small central island. When they read medium print, they may speed up, until they reach very small print, for which they will slow down again.

Figure 1. The word "wonderful" projected onto the macula on the SLO is equivalent to headline size letters held at 20 cm. A small island within a ring scotoma may catch a letter but not the whole word.

Figure 2. Even if foveal fixation is maintained, it may be surrounded by irregular scotomata.

VISION TRAINING

When visual acuity is reduced because of an optical problem,the patient can be helped with various magnification devices. There is no need to retrain fixation and saccade behavior, because the fovea still serves as the center of fixation.

However, when fixation has been forced to an eccentric retinal area, the situation is different. The original fixation reflexes will bring the object of regard to the fovea, which no longer functions. Thus, patients may experience a jack-in-the-box effect: When they try to look directly at an object, it may move in and out of the scotoma, making interpretation very difficult. They have to recalibrate their oculomotor reflexes.

Eventually, this will happen, but the process can be facilitated with appropriate training by a vision rehabilitation specialist or by an occupational therapist trained in vision rehabilitation. Retina specialists can assure themselves of grateful patients by recognizing the importance of such training and by making the appropriate referrals.

Although it would seem natural that patients would discover the presence of their scotomata, this is usually not the case. When 134 consecutive AMD patients with binocular scotomata were asked whether they had ever seen their scotoma, only two (1.5%) answered affirmatively.⁵ They reported seeing it only briefly when waking up and looking at the blank ceiling. Fifty-nine of 134 (44%) related experiences of things disappearing on them, and 75 of 134 (56%) were totally unaware of their binocular central field defects. The vast majority needed an explanation about the need for eccentric viewing.

Scotoma awareness can be enhanced by appropriate exercises. Once patients are aware of their scotoma and its consequences, they will be able to take better corrective action and develop better search strategies.

CONTRAST

Visual acuity and the plotting of the central field for scotomata are not the only important parameters of vision. Equally important is contrast sensitivity. Some patients notice that something is wrong with their vision but cannot describe the problem, because their visual acuity (tested with high-contrast letters) is still normal.

Unfortunately, contrast sensitivity is not often tested in routine ophthalmology. One of the reasons is that it is an extra test, which takes time in a busy office. The other reason is that a finding of reduced contrast sensitivity does not have immediate therapeutic consequences.

The traditional contrast sensitivity test is the Pelli-Robson chart.^6,7 This chart has to be viewed from a one-meter distance, which often means repositioning the patient. The newer Mars cards^8,9 are handheld, which makes testing easier; they have been shown to be equally sensitive. Even easier are tests in the Mixed Contrast format.^10,11 These are reading cards on which half of the sentences are printed with high contrast and half with low contrast.

The Mixed Contrast format (Figure 3) has several advantages: (1) Because the high- and low-contrast sentences are printed on the same card, they are automatically viewed at the same distance and under the same illumination; (2) The high-contrast section can replace the traditional reading test, so that one card serves two purposes; (3) Because it is a reading test, it assesses a larger retinal area than a letter test; and (4) Comparing the low-contrast performance of a patient to his/her own high-contrast performance as a baseline is more informative than only comparing the low-contrast performance to a population average.

Figure 3. A Mixed Contrast reading card.

For most patients, we have found a one- or two-line difference between high- and low-contrast performance (HC-LC). However, for many AMD patients, we have found differences up to five or six lines. Occasionally, we have found HC-LC differences of up to 10 lines. We have also found that the HC-LC difference is largely independent of visual acuity. There are patients with good acuity and poor contrast; there are others with poor acuity but a normal HC-LC difference.¹² When using separate HC and LC tests, this finding would not be obvious.

Because treatments for dry AMD may be available in the foreseeable future, early diagnosis of AMD will be increasingly important. When testing the presumably normal central field in the fellow eyes of AMD patients, we have documented islands of reduced contrast sensitivity outside the fovea.¹³ This finding should not be surprising,as AMD does not need to start in the fovea and does not need to start homogeneously in all locations. Others¹⁴ found that, in a longitudinal study, contrast losses were the strongest predictor of deterioration in the next five years.

We feel that the Mixed Contrast reading cards provide the easiest, fastest, and least expensive test currently available for the early detection of AMD. We hope that wider use of Mixed Contrast cards in retina practices and follow-up over time will provide data to validate this claim.

Even when no therapeutic decisions result from the documentation of a contrast deficit, making patients aware of their deficits is important. The Mixed Contrast format is very demonstrative in this regard, so lengthy explanations are not needed. For patients who have observed their own HC-LC differences, it will be obvious that serving milk in a Styrofoam cup and serving white rice on a white plate on a white tablecloth is asking for problems. Warning patients about poor visibility of steps and curbs may prevent a broken hip.

AVAILABLE TESTS

Many reading tests are available. In the United States, the most commonly used ones use Jaeger notation. This notation is meaningless. Jaeger's original reading samples were numbered according to the reference numbers in a print catalog in Vienna in 1854.¹⁵ Because this reference no longer exists, today's Jaeger cards vary widely in their print sizes. All current tests are easier than Jaeger's original.

Snellen¹⁶ defined a physical reference standard in 1862. In the United States, this standard is known as the M-unit (name introduced by Louise Sloan to emphasize its metric origin). One M-unit subtends five minutes of arc at one meter (=1.454 mm or almost 1/16 of an inch). It is the only notation that is applicable to letter charts, as well as to reading tests.

Conventional reading cards have short paragraphs of large print and longer paragraphs of smaller print. To be able to compare performance at various print sizes by measuring reading speed, it is desirable that all sentences have the same length. It is also desirable that the progression of print sizes is a regular one, which requires a logarithmic progression.

Several sentence sets are available that conform to these requirements. The MNread cards¹⁷ are the oldest, Colenbrander cards¹⁸ are newer and Radner cards¹⁹ are the latest. Of these, only the Colenbrander cards are available in the Mixed Contrast format. The SKread test²⁰ specifically looks for patterns of errors caused by scotomata adjacent to the PRL.

For accurate measurements, it is necessary to use a standardized reading distance. The Colenbrander cards come with a 40-cm (16-inch) cord attached; for the other cards, users must provide their own separate rulers. This is an important point, because changing from 40 cm (16 inches, 2.5 D add) to 32 cm (13 inches, 3 D add) causes a full-line difference in recorded reading acuity. A small change in reading acuity may go unnoticed if the patient uses a different reading distance at different visits.

CONCLUSION

Retinal physicians should not only consider the structural integrity of the retina but also its function. Letter chart acuity alone does not suffice. Reading and contrast tests are recommended in addition, especially as early detection of AMD gains in importance. RP

REFERENCES

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