Using the system described above, B-scans (collection of adjacent A-scans along a line) of the cornea at the apex and retina across the optic disc were acquired for 20 consecutive seconds. The B-scan length on the retinal surface was estimated to be approximately 4.7 mm. Out of these B-scans, the longitudinal movement of the cornea at apex, the base of the optic disc cup, and the nasal and temporal peripapillary retina were measured. The locations of these measurement points are shown in
Figure 2A. A typical recording of the measured movement is shown in
Figure 3. From this figure, it can be seen that different regions of the fundus and cornea move with slightly different amplitudes and phases. Moreover, the ocular tissues pulsate not only with the heart beat but also at its multiple frequencies. Pulsations of the ocular tissues at multiple frequencies of the cardiac cycle have been reported previously by our group,
9 as well as by other groups.
12,15 In order to justify that the observed multiple pulsations are not due to the data processing, a recording of the raw movement of the cornea in a healthy subject is given as Supplementary Figure S1, with the corresponding FFT power spectrum in Supplementary Figure S2 (see Supplementary Material and
Supplementary Figs. S1,
S2), from which the multiple pulsations in the cornea movement can be clearly seen. The RMS value of the movements of different ocular regions in normal subjects and glaucoma patients is shown in
Figure 4 as a solid box plot. In normal subjects, the means ± standard deviations of the RMS value of the movement were found to be 22.7 ± 6.5 μm, 25.4 ± 9.7 μm, 24.8 ± 6.3 μm, and 23.7 ± 7.7 μm for the cornea, base of the optic disc, nasal peripapillary retina, and temporal peripapillary retina, respectively. In glaucoma subjects, the corresponding values were 25.1 ± 8.9 μm, 30.4 ± 11.0 μm, 27.5 ± 10.0 μm, and 27.8 ± 13.4 μm (
P = 0.29 to 0.52). In the glaucoma group, the data distribution was skewed toward larger movements. It is interesting to examine the pulsatile expansion of the globe, the fundus pulsation amplitude (FPA), by subtracting the corneal movement from that of the retina or ONH. This process also removes any contribution to these movements from the orbital pulse and was done from the original scans, taking into account the phase differences in these movements. The results are shown in
Figure 5. In normal subjects, the mean RMS FPA of the base of the cup was 13.0 ± 2.5 μm, while the same value was 9.0 ± 2.1 μm and 8.7 ± 2.9 μm in the nasal and temporal peripapillary retina, respectively. The corresponding FPA values in the glaucoma group were 16.7 ± 6.8 μm, 17.3 ± 10.9 μm, and 12.7 ± 6.2 μm (
P = 0.26,
P = 0.008, and
P = 0.12, respectively).