Landmark-driven fundus perimetry using the scanning laser ophthalmoscope

Abstract

Purpose: To present a new method of performing scanning laser ophthalmoscope perimetry that compensates for eye movements so that the correct retinal location is tested even if fixation changes. This allows for accurate testing of patients with central scotomas and for repeating testing longitudinally at the same retinal locations even if central fixation is lost.

Methods: The operator views the retina and selects a retinal landmark, such as a vessel bifurcation, that can be identified easily. A testing strategy is preselected, and the computer saves the landmark and stimulus coordinates. To present each stimulus, the operator positions a cursor over the retinal landmark, and the computer adjusts the site of presentation of the stimulus for any change in landmark position caused by an eye movement. At the conclusion of the testing, the results are displayed in the proper retinal location on a fundus image.

Results: Sixty-seven eyes with macular disease were tested with the landmark-driven method, using the same preplanned strategy for each eye for both a bright and a dim stimulus. There was a low rate of inconsistent points (seen with dim but not bright stimuli), and virtually all of these bordered a dense scotoma. Those eyes with more inconsistent points had a significantly greater percentage of dense scotoma points and significantly lower visual acuity. The technique significantly corrected error in retinal localization resulting from large eye movement. There is no significant rotation or magnification change during the procedure, so specifying the change in location of one landmark is sufficient to describe movement of the retina. The technique is rapid and easy to administer to elderly patients and to children.

Conclusions: This technique allows for accurate and repeatable measures of retinal sensitivity in specific locations. It is useful in following change over time. It can be developed further to allow for fully automated, retinally correct testing.

FIGURE 1

Landmark-driven fundus perimetry. (left) The patient fixates the fixation cross, with a surrounding c-shaped area of scotoma (dotted curve). The large rectangle is the area seen on the monitor (corresponding to the small field size on commercial SLOs). The small solid square indicates the point to be tested. Two retinal landmarks (A and B) are selected by the operator, (center) At the time of testing, the retina has shifted downward. The retinal landmarks have changed, relative to their original positions on the monitor, from A and B to A′ and B′, respectively. B′ is now outside the field seen in the monitor, (right) The operator places the cursor on the new landmark location (A′). The computer corrects the location of the stimulus presented to its correct retinal location (black square). If uncorrected for eye movement, as in conventional perimetry, the stimulus would fall on the incorrect retinal position (shaded square).

FIGURE 2

The scanning laser ophthalmoscope landmark-driven fundus perimetry results of testing an eye with 20/500 visual acuity and an eccentric fixation locus at the nasal aspect of the area of geographic atrophy. (A) A fluorescein angiographic image. The central geographic atrophy appears as the hyperfluorescent (white) area. The optic nerve is seen to the right of the atrophy. The site of fixation near the border of the atrophy (corresponding to the white cross in B and C) is shown by the white arrow. (B) The display of the results of testing with the bright stimulus. Solid symbols indicate points that were seen, whereas open symbols indicate points that were not seen. (The color and shape of the symbol are not significant and are only used to make the symbol more visible.) The large white cross is the fixation cross seen by the patient. (The black cross is not seen by the patient; it is used in generating the stimulus by the computer.) The thin white cross (arrowhead) is the retinal landmark used for driving the perimetry, in this case the intersection of the atrophy with a vessel. The points not seen correspond well to the area of geographic atrophy. The optic disc is seen to the right. (C) The display of the results of testing with the dim stimulus. Same symbols as in B. (D) A composite map of the results of testing with the bright and dim stimuli. The solid squares are seeing points, the open squares are areas of dense scotoma, the shaded triangles are areas of relative scotoma, and x represents inconsistent points.

FIGURE 3

The scanning laser ophthalmoscope (SLO) computer display showing the results of testing using a rectangular grid for an eye with 20/167 visual acuity and central geographic atrophy, with unstable fixation near the nasal aspect of the atrophy. (A) The SLO computer display of the results of testing with a bright stimulus using the landmark-driven fundus perimetry technique. Symbols as in Figure 2. Here, the retinal landmark is the intersection of a retinal vessel with the disc, marked by the thin white cross. The fixation cross (thick white cross) is placed just nasal to the geographic atrophy. (The black cross is unseen by the patient.) The optic disc is mapped out as scotomatous, as is the area of geographic atrophy (with some points that were seen also plotted within the atrophy). (B) A drawing of the results of A. The area of geographic atrophy is shaded. Black circles show points that the patient indicated were detected, and white circles show points that were not seen. The retinal landmark (thin cross) and fixation cross are noted. (C) The black squares indicate the retinal locations that would have been tested if correction for eye movements was not made. These would be the points actually tested by a conventional perimetry technique that was attempting to map out a regular grid of stimuli. The patient often lost fixation during the testing, with the cross placed in the area of atrophy. As a result, no points would have tested the area of the optic disc. (D) The results that would be obtained if no correction for eye movement was made. Black circles show points that were detected, and white circles show points that were not seen. The operator would be unaware that the grid points did not fall on the desired retinal locations. The results would be mapped as though the stimuli fell on the retina in a regular grid, when in actuality the stimuli fell as in (C). No points actually would have tested the disc in C so that the disc grid points are registered as seeing. The shift in eye position caused many more points to fall in the area of atrophy, and these appear here as nonseeing points in clear retina.

FIGURE 4

The distribution of percentages of inconsistent points for each eye tested.

FIGURE 5

The amount of eye position change during testing as a function of LogMAR and Snellen visual acuity. The black circles indicate the average position change per stimulus presented for each eye. The error bars indicate the standard deviation of position change for each eye.