In each of the 31 eyes included in the study, both SW-AF and NIR-AF imaging revealed an autofluorescent ring (or arc) in the 30
° field. Examples of SW-AF (upper row) and NIR-AF (lower row) images obtained from seven patients and a healthy control are shown in
Figures 2 and
3. They have been chosen to illustrate the range of shapes and extent of rings/arcs in this group of patients. In the normal retina (
Fig. 2A) the SW-AF signal is attenuated centrally by the absorption of the excitation light by macular pigment and melanin. Outside the macula, the AF signal is relatively homogeneous and reflects the distribution of RPE lipofuscin. The optic disc and blood vessels are dark due to absent AF emission and blockage of the AF signal, respectively. The corresponding NIR-AF image (
Fig. 2E) is almost an inversion of SW-AF. Centrally, the signal from RPE melanin is greater, resulting in an increased NIR-AF signal. The extent and shape of the rings in the cohort of patients were similar for both SW-AF and NIR-AF, as can be seen in the examples presented in
Figures 2 and
3. It was usually possible to identify an inner and outer border of the ring. On SW-AF, for most of the patients (
n = 26) the intensity of the AF signal appeared to be higher across the ring than in the area outside the outer border. The intensity of the AF signal appeared to be at its lowest in the interior of the ring. On NIR-AF, the signal outside the ring was lower than that inside the ring, resulting in a high contrast between the two areas. In addition, in NIR-AF images, the signal across the ring was generally slightly higher than the area interior to the ring. On NIR-AF the choroidal vasculature was often visible outside the ring.
To assess the relationship between visual sensitivity and AF, the 10-2 MP-1 results were superimposed on the 30° SW-AF and NIR-AF images.
Figure 4 shows MP-1 results, in dB, superimposed on the 30° AF images for four patients (P2, P12, P21, P27). A higher dB value corresponds to better sensitivity. The dynamic range of the MP-1 is 2 log units, therefore the highest value is 20 dB and the lowest is 0 dB. Sensitivity values along the horizontal midline were calculated for each tested eye. As the 10-2 pattern has no test locations along the midline, 10 midline locations were assigned interpolated sensitivity values derived from actual values of the 10 test locations lying 1° above and 1° below each midline location. Sensitivities were relatively preserved interior to the ring (mean 14.1 ± 3.8 dB and 15.1 ± 4.1 dB for SW-AF and NIR-AF, respectively) but they were decreased across the ring (mean 9.2 ± 3.8 dB and 11.4 ± 4.1 dB for SW-AF and NIR-AF, respectively). Visual sensitivities outside the ring were markedly decreased or nonrecordable (mean 3.3 ± 4.6 dB, median 0.7 dB for SW-AF; mean 3.4 ± 4.6 dB, median 0.7 dB for NIR-AF).
A paired
t-test analysis was used to compare the measurements that were obtained from SW-AF and NIR-AF images and the SD-OCT scans (see
Table 2). There was no significant difference between the total ring-area measured on SW-AF and NIR-AF (
P = 0.197). There was also no significant difference between the measurements for the outer temporal ring border (
P = 0.088); however the distance from the fovea to the outer nasal border was significantly smaller on NIR-AF than SW-AF (
P = 0.043), as were the distances from the fovea to the inner border of the ring for both the nasal and the temporal side (
P = 0.004 and
P = 0.000, respectively). The Bland-Altman plots in
Figure 5 illustrate the differences in SW-AF and NIR-AF measurements from the fovea to the outer (
Fig. 5A) and inner temporal (
Fig. 5B) borders. For the outer temporal border measurements, the majority of data points are located around the zero axis, indicating a similarity between the SW-AF and NIR-AF measurements. For the inner temporal border of the hyperfluorescent ring, the majority of data points are located above the zero axis, indicating a decreased distance between the fovea and inner temporal border of the ring on NIR-AF.
We were also interested in comparing the location of ISe loss seen on SD-OCT with the location of the inner border of the ring measured on SW-AF and NIR-AF. There were no significant differences between measurements of the inner border of the ring on SW-AF and the location of ISe loss on SD-OCT for the temporal and nasal side (P = 0.928 and P = 0.062, respectively). However, for NIR-AF, the inner border of the ring was significantly closer to the fovea than the location of the ISe loss on SD-OCT for both the temporal and nasal side (P = 0.000 and P = 0.000, respectively).
ICCs revealed excellent agreement between observers for both SW-AF and NIR-AF (
Table 2).