Topographic mapping of residual vision by computer

Journal of Visual Impairment & Blindness, Oct, 2008 by Manfred MacKeben

Many persons with low vision have diseases that damage the retina only in selected areas, which can lead to scotomas (blind spots) in perception. The most frequent of these diseases is age-related macular degeneration (AMD), in which foveal vision is often impaired by a central scotoma that impairs vision of fine detail and causes problems with reading and recognizing faces. One would think that differences between damaged and relatively intact areas should be painfully obvious to those who have AMD. However, as clinical practice and scientific studies have shown, this is often not the case (Safran & Landis, 1996, 1999; Schuchard, 1995). Thus, patients and rehabilitation workers need to know where on the retina or in the visual field vision is still good enough to be usable. Having this information increases the chances for successful rehabilitation, the necessity and feasibility of which have been demonstrated (Backman & Inde, 1979; Nilsson, Frennesson, & Nilsson, 2003; Otto, 1969; Park, 1999). The fact that rehabilitation is possible emphasizes the need for a topographic assessment of remaining vision. In this article, I describe the Macular Mapping Test (MMT) and illustrate with case reports how it can be used in clinical practice to assist in the assessment and rehabilitation of patients with central vision loss.

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).

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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

Procedure

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.