3.1. The method
(left panel) shows a pseudo-color representation of the RGC + IPL thickness of a healthy control based upon the macular cube scan. As expected, the greatest RGC + IPL thickness is just outside the foveal center at about 4°. By around 10°, the thickness is reaching an asymptotic level.
In contrast, the RNFL thickness derived from the same cube scan (, left panel) shows a different pattern. As expected, the RNFL is relatively thin except in the arcuate regions. A better picture of the arcuate RNFL can be seen in (left panel), which depicts the thickness of the RNFL for the optic disc, cube scan. Because the macular and optic disc cube scans provide overlapping information about RNFL thickness, they were combined into one map as in
by co-registration based upon blood vessel alignment in an en face view of the fdOCT scan. The same maps are shown in with iso-thickness contours added, while shows the RGC + IPL map in the same format. Note that both maps were flipped across the horizontal axis so as to be presented in the same way as the visual field. Additionally, the results for left eyes were flipped across the vertical axis so that all eyes were presented as right eyes in all figures.
Fig. 2 RGC + IPL and RNFL thickness maps for the control in . Combined RNFL thickness maps from scans of the macula and optic disc. B. Same scans as in A shown with iso-thickness contours and after flipping across the horizontal axis to present in field (more ...)
presents the 24-2 and 10-2 visual fields and the 3 thickness maps illustrated in for the healthy control in . The red circles have a radius of 10°. show the results from 3 patients. (Note that in all figures, the OCT results are flipped along the horizontal axis to coincide with the field view.) Patients 1 and 2 have clear arcuate defects that can be seen on the 10-2 and all three OCT thickness maps. Patient 3’s visual field damage is more subtle, although again there appears to be an indication of an upper field arcuate defect that appears to be confirmed on the OCT maps (red arrows). However, we cannot tell if the cluster of abnormal points on the visual field (green arrow) corresponds to the region that appears thinned (red arrows) in the OCT scans. In particular, is the RGC + IPL thinning in the same place as the abnormal points on the 10-2 (green arrow); and, does the OCT thinning reach statistical significance?
Fig. 3 Sample visual fields and OCT thickness maps. A. 24-2 (left panel) and 10-2 (second column) visual fields are shown for a healthy control along with the thickness maps for the macula RGC + IPL (middle panel) and RNFL thickness (fourth panel), and the optic (more ...)
To address the first question, we need a way to relate the location of the visual field points to the region of the RGCs activated by these points. To make this comparison, we must take into consideration the displacement of the RGCs near the fovea. That is, light falling on receptors near the foveal center excite RGCs that are some distance from this location. As previously reported [5
], we can use the results of Drasdo et al. [10
] to locate the average location of the excited RGCs for each of the visual field points. Note, this does not take into consideration any variation in RGC displacement that may occur from one individual to another.
shows the thickness maps for patient 1 presented as in . In , the superimposed small circles show the retinal location of the RGCs associated with the 10-2 (small circles) test locations.
RGC + IPL and RNFL maps for patient 1. A. OCT thickness maps as in . B. Maps from panel A with 10-2 test locations displaced based upon displacement of RGCs. C. Probability maps for OCT thickness.
To address the second question, the OCT thickness maps need to be transformed into probability maps. To do this, the local OCT thickness for an individual is compared to the distribution of thicknesses of the 128 healthy control eyes. The maps in show the results as a continuous probability scale. This scale is shown in the middle of this panel, where dark green indicates a p value of greater than 10%, yellow 5%, red about 1% and dark red <1%.
The final step involves combining the probability information from the visual fields and the OCT images.
shows the combined OCT/10-2 probability maps for patient 1. The significantly abnormal points from this patient’s 10-2 visual field (, second column) are shown as the yellow (5%) and red (<1%) circles. It is easy to see the agreement between the probability maps for the 10-2 and OCT thicknesses.
A-C. Combined OCT/10-2 probability maps for the 3 patients from .
show the combined OCT/10-2 probability maps for the other 2 patients in . The combined probability maps () for patient 2 resemble those for patient 1. For patient 3, while the visual fields suggest that the damage is relatively confined and subtle, the RGC + IPL probability map suggests more extensive damage.
3.2. Patient data
The 10-2 visual fields of the 16 patient eyes were classified as arcuate or partial arcuate (8), diffuse (1), and normal (7). shows the field and OCT data for 3 of the 8 patients with arcuate or partial arcuate visual fields. The combined OCT/10-2 probability maps for these 3 patients are shown in . For all 8 arcuate visual fields, the RGC + IPL and RNFL probability maps showed arcuate damage in the same region in which the visual field contained abnormal points. In six cases, both the significantly abnormal portions of the OCT and fields coincided as seen for Patients 1 and 2 (). In two eyes (same patient), the OCT abnormal region included both hemifields, while the visual field showed abnormalities only in the superior field (). For the eye with the diffuse visual field, the OCT probability map showed damage in both hemispheres.
Five of the 7 eyes with 10-2 visual fields classified as normal had OCT probability maps within the normal range. The other 2 are of particular interest. In one case, Patient 4, there were abnormal points in the upper field of the 10-2 (
), although the visual field was normal based upon MD, PSD and cluster criteria. Of note is that the combined OCT/10-2 maps () showed evidence of an arcuate defect in both hemispheres.
shows the results for Patient 5 (OD), the 7th eye with a normal visual field. Note the arcuate damage on the OCT corresponding to a lower field arcuate defect. The 10-2 Total Deviation plot did not show a single abnormal point (). In fact, these points were nearly all positive (more sensitive than normal), as if this patient had a higher baseline sensitivity or lower criterion for saying “I see it”. In any case, the Pattern Deviation map (), although normal based upon MD and PSD criteria, showed significantly abnormal points in the region corresponding to the lower field arcuate seen on the OCT maps.
Results for patient 4. A.B. Combined OCT/10-2 probability maps. C. 10-2 Total Deviation map.
Results for patient 5. A, B. Combined OCT/10-2 probability maps. C. 10-2 Total Deviation map. D. 10-2 Pattern Deviation map.
3.3. Future directions
There are a number of possible improvements that can be made to this approach. First, if 10-2 visual field data were available on a large group of healthy controls, then the visual field data could be referenced to the same continuous probability scale used for the OCT data. Second, summary statistics can be generated for the OCT maps analogous to the mean deviation, pattern standard deviation and glaucoma hemifield test of the visual field tests. In addition, interocular comparisons could be obtained similar to those we have used in our multifocal visual evoked potential reports [11