Topographic mapping of residual vision by computer.
THE IDEAL SOLUTION
Topographic assessment is best done by a scanning laser ophthalmoscope (SLO) that shows live images of the retina and the stimulus that is scanned directly onto it. A useful vision assessment in AMD does not need to encompass the entire visual field. It can be restricted to the macula, a central area of 20 degrees diameter, where most preferred retinal loci (PRL) develop (Fletcher & Schuchard, 1997). If small spots of light are flashed onto the retina (microperimetry), the patient indicates which are detected and which are not. For precise placement of stimuli, modern software compensates for involuntary eye movements during testing, so that precise fixation is not necessary (MacKeben & Gofen, 2007). Since SLOs are expensive and rare, they are not available to most vision professionals.
WHAT OTHER MEANS ARE AVAILABLE?
Conventional perimetry can provide relevant data, but it is time-consuming and requires sophisticated equipment and special expertise. In addition, the resulting intensity thresholds are not needed to find usable remaining vision for rehabilitation. The standard tangent screen test has shortcomings in that it makes it hard to include a discrimination (recognition) paradigm for a better measurement of visual performance. Furthermore, it is difficult to time the appearance and disappearance of a target precisely at defined locations, and the results can vary with the examiner's level of skill and personal habits. The Amsler grid is a subjective test that is sensitive only to drastic changes, but not to those that develop over longer periods. At best, it indicates where damage has been done, but not visual performance. In addition, the normal "filling-in" mechanism often prevents the detection of scotomas of less than 5 degrees diameter (Ramachandran & Gregory, 1991; Schuchard, 1993).
[FIGURE 1 OMITTED]
Note that measuring visual acuity alone is insufficient, despite the importance of visual acuity for prescribing optical aids, because it has limited utility in predicting the performance of activities of daily living, like reading (Backman, 2000; Crossland, Culham, & Rubin, 2005) or visually guided mobility (Brown, Brabyn, Welch, Haegerstrom-Portnoy, & Colenbrander, 1986; Hassan, Lovie-Kitchen, & Woods, 2002; Marron & Bailey, 1982) after the loss of central vision. Also note that subsets of the data presented here were published previously (MacKeben & Colenbrander, 2000; MacKeben, Colenbrander, & Gofen, 1999).
DESCRIPTION OF THE TEST
Although MMT is not new (MacKeben & Colenbrander, 2000; MacKeben et al., 1999), its role in vision rehabilitation has not been sufficiently described. It noninvasively tests the detection and recognition of briefly shown single targets in the center and near periphery of the visual field. The duality of the task (detection plus recognition) distinguishes it from conventional perimetric procedures. Targets can be single letters, words, Landolt rings (Cs), or tumbling Es. The tested locations lie either in the center (4 trials) or on one of 4 concentric rings at 2, 4, 6, and 8 degrees eccentricity, each of which contains 8 locations (1 trial each). Target sizes are scaled according to eccentricity up to 8 degrees (see Figure 1), which removes eccentricity as a variable. For a demonstration of the MTT, visit the web site <www.ski.org/Rehab/ MacKeben/General/MMTest-webpage.html>.
Thus, 33 locations of the visual field are tested in 36 trials that take about 3 minutes, on average. Each target appears at an unpredictable location and typically remains visible for a quarter of a second. In the context of rehabilitation, the goal of MMT is to assess and graphically display the remaining macular vision, which aids in finding intact retinal areas that are usable for eccentric viewing of which the patient may be unaware. Moreover, MMT can also serve as a tool for assessing a patient's status after therapy or training.
The examples shown here were drawn from a group of tests that were run without dilating the pupils of 134 eyes of 103 patients with AMD of all stages or Stargardt's disease who were recruited from the Low Vision Service at the Smith-Kettlewell Eye Research Institute. All subjects gave their informed consent, and the experiments were approved by the institutional review boards at California Pacific Medical Center, San Francisco, and the Smith-Kettlewell Eye Research Institute. All procedures were in compliance with the tenets of the Declaration of Helsinki.
It is crucial for patients to stabilize their gaze as much as possible during the test. The "wagon wheel" background pattern helps this purpose by providing peripheral feedback when an inadvertent eye movement occurs. The following instructions are recommended: "Do you have a sense where the center of the circular display area is?" The patients invariably answer yes. And then, "Direct your gaze at that center and keep it there as still as possible--even if this makes the center disappear." Without such instructions, it is hard to know upon what part of the display the patients center their gaze. Those who have had a central scotoma for a long time are likely to use PRL automatically (White & Bedell, 1990). Note that such patients may benefit less from this test, since they already have established eccentric viewing habits. On the other hand, if good visual acuity indicates that foveal vision has been spared so far, one can use a central fixation mark. Stabilizing the head with a chin rest is helpful, but not essential. It is necessary to use best optical correction for the viewing distance--typically 30 inches (73 centimeters)--and to cover the nontested eye.
MMT can be run on any PC with Windows (3.1 and later) by anyone without special computer expertise. It requires little hard disk space and memory and no hardware additions.
The patient's task is to indicate verbally whether the target was detected and, if possible, recognized, which information the examiner enters via the keyboard. There are three possible responses: "recognized," "detected but not recognized," and "not detected." The program scores each response and enters it into a database, from which the responses can be recalled and graphically displayed. To simplify the assessment of overall performance, MMT provides a field score that combines the scores from all tested locations to a single number. Thus, a score close to 0 represents poor performance, and one close to 36 signifies good performance. Target and background brightness can be set at 16 gray levels each to reduce contrast, which makes the test more sensitive to subtle changes that may not be noticeable by the patient if they do not affect the fovea, as in a ring scotoma (Messias et al., 2007).
Case 1. An 82-year-old man, with AMD and a visual acuity of 20/80, was unaware of topographic vision variations. He had a central scotoma of at least 4 degrees in diameter; some damage also in the upper right, right, and below (see Figure 2A), a field score of 8 and read at 40 words per minute (WPM) with difficulty. Two locations showed the possibility for PRLs in the upper and lower left (marked by "+" signs) because of intact space to the right of them. After training, his reading speed had doubled, and his subjective sense of unease had diminished.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Case 2. A 72-year-old woman, with no diagnosis of AMD and a visual acuity of 20/40, had an absolute scotoma in the upper quadrant of the visual field (see Figure 2B). She was not aware of the scotoma, but said that her "vision is not as good as it used to be." She read fluently at 120 WPM. She had sustained a mild head injury from a collision with a shelf.
Case 3. A 74-year-old woman, with early AMD and a visual acuity of 20/30, performed well at 92% contrast (field score = 30) and poorly at 14% contrast (field score = 5.5). She required very bright light for reading, but then read 100 WPM.
These examples illustrate conspicuous topographic differences among individuals with AMD. Although many develop a dense central scotoma, this does not mean that early stages of AMD always affect the fovea first (see Case 3, Figure 3, left). Most of the patients considered here had visual acuities of 20/40 or better and thus did not necessarily have a reason to consult an eye care professional yet. However, they did so, possibly for a check-up because the fellow eye had already been diagnosed with AMD.
Rehabilitation workers are concerned with finding areas of usable remaining vision, that is finding relatively intact areas that may be candidates to become a PRL for eccentric viewing. This topographic knowledge enables them to give patients specific advice to redirect their gaze, such that the object of interest falls on the candidate PRL (see Figure 2A, locations marked "+") and then to note whether the adjustment of their gaze was beneficial.
Most patients in this study reported that the MMT procedure was helpful, either by confirming what they had tentatively noticed before or by teaching them something new. This is an important feature of MMT, since the results are immediately visible and intuitive and can easily be explained by the examiner.
MMT is not a high-precision procedure, but a simple tool to estimate the topography of remaining vision without the use of expensive and sophisticated equipment. A comparison of MMT data with kinetic manual perimetry showed good correspondence of the detection aspect of the task (Trauzettel-Klosinski, Biermann, Hahn, & Weismann, 2003). In addition, many locations yielded suboptimal scores owing to the added requirement of recognition.
The success and accuracy of MMT depend mostly on the steadiness of gaze, as is the case with conventional perimetry. In people with beginning AMD, a relatively steady gaze can be achieved in two ways: (1) by residual foveal vision, which allows for the use of the fixation mark, or (2) by instructions to "use the old center of the retina," which many can do by their volition. For rehabilitation, people with already established eccentric viewing habits are not the best candidates for MMT anyway because they have already adjusted to their absolute central scotoma.
Advice given to patients on the basis of the results of MMT does not need to be highly precise, since the patients receive immediate sensory feedback that makes the process self-correcting. In essence, if the recommendation goes in the right direction, the patients will take care of the rest because they will know instantaneously what looks best to them and intuitively correct the adjustment of their gaze.
The author thanks August Colenbrander for encouragement and for recruiting patients, and Alexander Gofen for transferring the original program to the PC platform and for years of software improvement. Financial support was obtained from Prevent Blindness of Northern California, the National Institute on Disability and Rehabilitation Research, and the Smith-Kettlewell Eye Research Institute. The Macular Mapping Test is a commercial product published by MMTest (San Francisco), a small business owned by the author.
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Manfred MacKeben, Ph.D., scientist, Smith-Kettlewell Eye Research Institute, 2318 Fillmore Street, San Francisco, CA 94115; e-mail: <firstname.lastname@example.org>.
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|Title Annotation:||Practice Report|
|Publication:||Journal of Visual Impairment & Blindness|
|Article Type:||Clinical report|
|Date:||Oct 1, 2008|
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