Face Fields and Microperimetry for Estimating the Location of Fixation in Eyes with Macular Disease

Central scotomas from macular diseases, such as age-related macular degeneration (AMD), that necessitate the use of the eccentric retina have a major impact on reading, the recognition of faces, and the performance of other activities of daily living that require fine vision. Since the fovea is specialized for fine resolution and excellent visual acuity in a way that the peripheral retina is not, a person with a central scotoma must use magnification or large-print text. Likewise, with the loss of central vision, there is a drop in contrast sensitivity, and improved contrast and lighting are often required. However, the third important component in restoring optimal vision to a person with a central scotoma has not been as easy to quantify or address—the impact of having a blind spot in the central field of vision. If a person places what he or she wants to see at the location of the (now blind) fovea and within the scotomatous area, he or she will not be able to see it. If a person cannot establish a way to use the peripheral retina as a “pseudo-fovea” (Guez, Gargasson, Rigaudiere, & O’Regan, 1993; White & Bedell, 1990)—that is, to direct the object of interest to the eccentric preferred retinal locus (PRL) for fixation—the person will waste time and effort scanning, moving his or her head in all different directions, and having letters and words appear and disappear as they enter and emerge from the scotoma. Some persons with central scotomas are able to adopt a regular and reliable fixation pattern; they look to the side or above the object of interest in a consistent way and thereby move the central scotoma out of the way. They may or may not be consciously aware of what they do, but they do it consistently. Other persons cannot seem to place the object of interest on the seeing retina regularly and, according to anecdotal evidence, have their visual acuity measured at levels that are far worse than one would expect from the observed retinal lesions. Most persons seem to fall in the middle; they have a PRL, but it is not strongly established. They may, for example, keep missing the first letters on the line in the acuity chart, implying that they are moving the scotoma to the left to read, but they do not compensate for this situation and may not do it consistently. With the development of fundus perimeters (more commonly called microperimeters), such as the Rodenstock scanning laser ophthalmoscope (SLO) and the Nidek MP-1, the clinician can see the actual site of the PRL on the retina, evaluate how stable it is, and measure the size and location of the scotoma relative to the location of the PRL (Fletcher & Schuchard, 1997; Rohrschneider et al., 1995; Timberlake et al., 1986). Studies that have used microperimeters in geographic atrophy (GA) from AMD have shown that with the acquisition of a PRL over time, measured visual acuity can actually improve despite progressive enlargement of the atrophy (Sunness, Applegate, & Gonzalez-Baron, 2000). These microperimeters are the gold standard for characterizing the PRL and fixation pattern relative to the central scotoma and give the clinician or low vision provider an awareness of the PRL that can be strengthened through verbal coaching and specific exercises. However, microperimeters are not widely available; the Rodenstock SLO is no longer being manufactured, and the MP-1 costs about $48,000, so that most low vision providers do not have access to them. Therefore, simpler inexpensive ways to assess fixation are desirable, particularly ways that can be used both in a clinic and at a person’s home, where low vision instruction may be taking place. It has long been recognized that, presumably because of perceptual completion (or “filling in”), more than 40% of persons with definite scotomas in their central visual field cannot detect them on an Amsler grid (Fine et al., 1986; Schuchard, 1993). In addition, despite an effort to be clear in the instructions to persons who use the grid, clinicians often do not know whether these persons are centering their fixation (using the corners of the grid, for example) at the fovea or whether they are using an eccentric PRL to fixate the center spot (Schuchard & Raasch, 1992). Even if a person reports an abnormality, it is difficult to interpret its location clearly. Conventional visual field testing that does not correct for eye movements cannot accurately measure the location of the PRL or the size of the scotoma (Sunness, Schuchard et al., 1995). In examinations of persons with low vision, it is clear that they are aware of the blurring of facial features that is associated with their scotomas. It is easy and quick to do a “face field”—to map out where the blurred areas of the face are relative to fixation. As is described later, the face field is a much simpler and more familiar task than is using clock hours and numbers for looking for PRLs (Watson, 1999). To validate the face field technique, a retrospective study was performed of persons with central scotomas who, during a single visit, underwent first face fields and then SLO or MP-1 analysis.