Using OCT angiography, we have shown that ONH blood flow can be determined in both normal and glaucoma groups. We targeted two regions, the whole disc and a temporal ellipse area within the disc, for comparison between two groups. By excluding the major superior and inferior branches of the retinal vessels on the temporal side, we focused our quantifications mainly on ONH microvascular beds. Our preliminary results suggest that in early glaucoma the reduction of ONH microvascular flow is much more dramatic than that of whole ONH circulation. This suggests that quantification performed on microvascular perfusion may be more sensitive for detecting ONH circulatory changes in early glaucoma patients.
Our results confirm the finding of several other techniques that reported a significantly reduced ONH perfusion in glaucoma. Using fluorescein angiography, Wolf demonstrated that glaucoma is associated with an increased arteriovenous passage time and a decreased fluorescein velocity [33
]. Using single-point laser Doppler flowmetry, a few authors similarly reported decreased blood flow in the ONH of glaucoma and glaucoma suspects when compared with normal subjects [14
]. Using scanning laser Doppler flowmetry, Michelson reported that both neuroretinal rim blood flow and peripapillary retinal blood flow were significantly decreased in glaucoma patients compared with controls [15
]. Hafez also found open-angle glaucoma patients have lower blood flow in the ONH and suggested perfusion might be reduced before the manifestation of visual field defects [16
]. Conversely, Hollo failed to detect a significant difference in neuroretinal rim blood flow in their population of glaucoma patients compared with control subjects [35
]. By use of laser speckle flowgraphy, Sugiyama’s group reported less blood flow was observed at the superior and inferior sectors of the ONH rim in patients with PPG compared to normal control subjects [17
]. Overall, previous results agree with our finding that ONH perfusion is reduced in glaucomatous eyes.
In previous reports, the temporal sector has been studied separately for evaluating glaucoma patients’ disc perfusion. Kerr reported reduced blood flow in the lamina cribrosa and the temporal neuroretinal rim of the ONH of glaucoma patients in comparison with ocular hypertensives [36
]. Sugiyama reported the blood flow became reduced at the temporal sectors as open angle glaucoma progressed compared to patients with PPG [17
]. Our finding via OCT angiography also confirmed that the temporal sector is a critical study region for monitoring progression of glaucoma patients. Although the repeatability of the temporal disc measurements is not quite as good as whole disc measurements, it is still better than that of laser Doppler flowmetry [37
]. Using Doppler OCT with dual-circular scans, our group previously found TRBF was reduced by ~30% in patients with perimetric glaucoma in which eyes have significant glaucomatous visual field damage [38
]. In this pilot study, we found that the angiography-based flow index was reduced by 35% for the whole disc and by 57% for the temporal ellipse area of the disc in a group of PPG subjects. Our results indicate that the reduction in ONH flow in PPG patients may be more dramatic than reductions in TRBF. Thus, reductions in ONH blood flow detectable by OCT angiography precede detectable visual field damage, suggesting it could be useful in the early detection of glaucoma and the evaluation of glaucoma progression risk.
It should be noted that the amplitude-based method that we have developed is sensitive to motion that is perpendicular (transverse flow) or parallel (axial flow) to the OCT beam. We believe this characteristic of SSADA makes quantitation of microvascular flow independent on beam incidence angle. In addition, we chose to compute amplitude decorrelation rather than amplitude variance (also called speckle variance) [39
] because decorrelation is less affected by signal strength (i.e., variation due to media scattering, pupil blocking, focusing, polarization mismatch, etc.). This is important because previous techniques such as the laser Doppler flowmetry could not reliably compare flow values between individuals due to the effect of signal strength on the measurements [3
Quantitative SSADA has several limitations. First, flow projection artifact from superficial blood vessels to deeper tissue levels prevents us from separately measuring superficial and deep ONH flow. The artifact is caused by the moving shadow cast by flowing blood cells. Decorrelation is caused by both moving reflectors (blood cells) and moving shadows (projected on distal high reflectance tissue). The two type of decorrelation is not distinguished by SSADA – both appear as flow in the 3D angiogram. The artifact is not problematic if our analysis is confined to the 2D maximum projection angiogram. Therefore the study was limited to the use of 2D angiograms that measured superficial and deep vascular beds together. Second, the ONH flow index includes measurements on both local disc circulation and large retinal blood vessels. Thus it is a mixture of both disc and retinal circulation and not a pure measurement of a single vascular bed. However, since glaucoma and optic nerve diseases reduce both circulations, the mixed measurement is clinically useful. Third, OCT angiography cannot distinguish between perfusion reduction caused by tissue loss (a result of glaucoma) and ischemia (a cause of glaucoma). However, the OCT structural images can measure tissue loss. Thus, structural imagery and perfusion measurements could provide complementary information for both clinical assessment and pathophysiological investigation.