Parapapillary Deep-Layer Microvasculature Dropout in Glaucoma: Topographic Association With Glaucomatous Damage

Abstract

Purpose: The purpose of this article was to compare the frequencies of the parapapillary deep-layer microvasculature dropout (MvD) detected by optical coherence tomography angiography in eyes with primary open-angle glaucoma (POAG) and healthy eyes and to determine the topographic correlation between the MvD and the glaucomatous retinal nerve fiber layer (RNFL) defect in POAG eyes.

Methods: Microvasculature in the peripapillary deep-layer was evaluated in 150 POAG patients and 45 healthy controls using swept-source optical coherence tomography angiography to identify an MvD. Frequencies of MvDs were compared between the POAG and control groups. In POAG eyes with MvD, topographic correlation was assessed between the MvD and the RNFL defect defined based on the optical coherence tomography circumpapillary RNFL thickness measurement.

Results: MvD was observed as a sectoral filling defect in the parapapillary deep-layer in 53.9% (n = 62) of the POAG eyes, whereas none of the control eyes exhibited an MvD. POAG eyes with MvD had a thinner global RNFL (P < 0.001) and worse visual field mean deviation (P = 0.042) and were more myopic (P = 0.029), with axial length being longer (P = 0.046) than those without MvD. There was a good agreement between the circumferential extent and location of MvD and those of RNFL defect (95% limits of agreement of the difference ranged from −23.4 to 21.9° and −16.2 to 17.0°, respectively).

Conclusions: MvD was identified in the parapapillary deep layer exclusively in POAG eyes at the location of glaucomatous damage using optical coherence tomography angiography. The finding suggests that peripapillary deep-layer circulation is directly related to the glaucomatous optic neuropathy.

Glaucoma is a multifactorial disease whose precise mechanism remains elusive.¹ Although increased IOP is considered the greatest risk factor for the development² and progression of glaucoma,^3–5 vascular factors are also thought to play an important role in the pathogenesis of glaucoma.⁶ 

Optical coherence tomography (OCT) angiography (OCTA) is a new imaging technique that enables visualization of the individual retinal and choroidal microvasculatures. Recent studies using OCTA have demonstrated decreased vascularity in the optic nerve head (ONH)^7,8 and superficial peripapillary retina^9–13 in glaucoma patients. A regional microvasculature dropout (MvD) has also been demonstrated in glaucomatous eyes using OCTA images of the parapapillary deep layer.¹³ Recently, our group demonstrated that such MvD accurately corresponded to the perfusion defect shown by indocyanine green angiography.¹⁴ The finding indicates that the parapapillary deep-layer MvD shown in OCTA represents a true perfusion defect in the choroid or inner sclera. The deep-layer microvasculature within the peripapillary area is of particular clinical interest¹³ because it is downstream from the short posterior ciliary artery,^15–19 which also perfuses the prelaminar tissue and lamina cribrosa.^17–19 However, whether the MvD is a clinically meaningful finding or a nonspecific feature that can be found in general population remains to be determined. 

We hypothesized that an MvD would be found more frequently in glaucomatous eyes and a significant topographic correlation would be observed between MvD and glaucomatous damage if the MvD has pathogenic relevance with glaucomatous optic neuropathy. Therefore, we conducted this study to compare the frequencies of the parapapillary deep-layer MvD between eyes with primary open-angle glaucoma (POAG) and healthy eyes and to investigate whether the MvD is topographically correlated with the location of glaucomatous optic nerve damage represented by defects in the retinal nerve fiber layer (RNFL). 

Methods

This study investigated the peripapillary circulation using OCTA in consecutive POAG patients who were enrolled in the Investigating Glaucoma Progression Study, which is an ongoing prospective study of glaucoma patients at the Glaucoma Clinic of Seoul National University Bundang Hospital Glaucoma Clinic. Healthy controls were recruited from patients who visited the clinic because of an incipient cataract. Written informed consent to participate was obtained from all participants. The study protocol was approved by the Institutional Review Board of Seoul National University Bundang Hospital, and it followed the tenets of the Declaration of Helsinki. 

All participants underwent comprehensive ophthalmic examinations that included best-corrected visual acuity, Goldmann applanation tonometry, a refraction test, slit-lamp biomicroscopy, gonioscopy, stereo disc photography, and red-free fundus photography (EOS D60 digital camera; Canon, Utsunomiyashi, Tochigiken, Japan), central corneal thickness measurement (Orbscan II; Bausch & Lomb Surgical, Rochester, NY, USA), axial length measurement (IOLMaster version 5; Carl Zeiss Meditec, Dublin, CA, USA), spectral-domain OCT (SD-OCT) scanning of the circumpapillary RNFL and ONH using enhanced-depth imaging mode (Spectralis; Heidelberg Engineering, Heidelberg, Germany), standard automated perimetry (Humphrey Field Analyzer II 750, 24-2 Swedish interactive threshold algorithm; Carl Zeiss Meditec), and OCTA (DRI OCT Triton; Topcon, Tokyo, Japan). 

POAG was defined as the presence of an open iridocorneal angle, signs of glaucomatous optic nerve damage (i.e., neuroretinal rim thinning, notching, or an RNFL defect), and a glaucomatous visual field defect. A glaucomatous visual field defect was defined as a defect conforming with one or more of the following criteria: (1) outside normal limits on a glaucoma hemifield test, (2) three abnormal points with a P < 5% probability of being normal and one abnormal point with P < 1% by pattern deviation, or (3) a pattern standard deviation of P < 5% confirmed on two consecutive reliable tests (fixation loss rate of ≤20% and false-positive and false-negative error rates of ≤25%). The healthy controls had an IOP of ≤21 mm Hg, no history of increased IOP, an optic disc with a normal appearance, and a normal visual field. 

The eyes included in the POAG group were required to have a record of untreated IOP, which was measured prior to the initiation of ocular hypotensive treatment or identified in the referral notes. In patients with an untreated IOP of ≤21 mm Hg, the diurnal variation was measured during office hours (9 AM to 5 PM). In patients who were undergoing treatment with ocular hypotensive medication at the time of the initial visit, the diurnal variation was measured after a 4-week washout period. The exclusion criteria were eyes with a best-corrected visual acuity worse than 20/40, a spherical equivalent of <−6.0 D or >+3.0 D, a cylinder correction of <−3.0 D or >+3.0 D, a history of intraocular surgery with the exception of uneventful cataract surgery or trabeculectomy, or retinal or neurologic diseases. When both eyes were eligible, one eye was randomly selected for inclusion in the study. 

Optical Coherence Tomography Angiography

The optic nerve and peripapillary area were imaged using a commercially available swept-source OCTA device (DRI OCT Triton; Topcon), with a central wavelength of 1050 nm, an acquisition speed of 100,000 A-scans per second, and an axial and transversal resolution of 7 and 20 μm in tissue, respectively. Scans were taken from 4.5 × 4.5 mm cubes with each cube consisting of 320 clusters of four repeated B-scans centered on the optic disc. 

The deep-layer microvasculature in the peripapillary area was evaluated on the en face images of the peripapillary deep-layer generated based on the automated layer segmentation performed by the OCT instrument software. The en face images of the deep-layer were derived from an en face slab, extending from the retinal pigment epithelium to the outer border of the sclera. 

MvD was defined as a focal sectoral capillary dropout without any visible microvascular network identified in the deep-layer en face images.¹³ When circumferential width of the area with capillary dropout looked more than two times greater than the width of visible juxtapapillary microvessels, it was considered a disruption of the microvascular network, and an MvD was defined. Two independent observers (E.J.L. and S.H.L.) identified MvDs while blinded to the clinical information of the participants. Disagreements between these two observers were resolved by a third adjudicator (T.W.K.). All OCT B-scan images had to have an image quality score ≥30, according to the manufacturer's recommendation. When the quality of the OCTA images was poor (i.e., blurred images that hampered the delineation of the MvD), the eye was excluded from the analysis. 

Determination of the Circumferential Extent and Location of the MvD and RNFL Defects

To determine the topographic correlations between MvD and glaucomatous RNFL damage, the circumferential extent and location of the MvD and RNFL defects were determined in the POAG group. The circumferential extent and location of the MvD were measured as the angular extent of the MvD (α), and as the angular deviation of the midpoint of the MvDs relative to the foveal-disc axis (β), respectively (Fig. 1B). These measurements were made by first identifying the two points at which the borders of an MvD area met the optic disc margin. Lines connecting the disc center and the two points were then drawn, and these lines were used to measure the angular extent and location. The foveal-disc axis in OCTA images was determined by superimposing and manually aligning the OCTA images on the infrared fundus image provided in the SD-OCT circumpapillary scanning using commercial software (Photoshop CC; Adobe Systems, Mountain View, CA, USA). 

Figure 1

 

Determination of the circumferential extent and location of the parapapillary MvD and RNFL defect. Color disc photograph (A), OCTA image obtained in the deep-layer (B), magnified image of the image in the left lower column (C). (D) OCT circumpapillary scanning of the RNFL. Dashed ellipses indicate optic disc margin. Area demarcated by red dotted line indicates the area with MvD. Blue line on the infrared image indicates the foveal-disc axis (x: fovea). α and γ are angular extents of the MvD and RNFL defect, respectively, and β and δ are locations of the MvD and RNFL defect relative to the foveal-disc axis, respectively.

The circumferential location and extent of the RNFL defect were assessed using SD-OCT circumpapillary RNFL scans (Fig. 1D). The RNFL defect was defined on the temporal-superior-nasal-inferior-temporal profile as the area with a thickness of <1% limit of the normative database (red color). The extent of the RNFL defect was determined on the x axis of the temporal-superior-nasal-inferior-temporal graph, as the angular width between the start and end points of the RNFL defect (γ). The location of the RNFL defect was defined as the angular location of the midpoint of the RNFL defect from the foveal-disc axis (δ). The extent and location of the MvD and RNFL defect were recorded by two observers (E.J.L. and K.M.L.), who were blinded to the participants' clinical information. Each observer measured the values twice, and the mean of the four values was used for the analysis. 

Data Analysis

The interobserver agreement regarding confirmation of the presence of MvD was assessed using κ statistics (κ value). The interobserver reproducibility in the determination of the circumferential extent and location of the MvD and RNFL defect was assessed by calculating intraclass correlation coefficients. The topographic correlation between MvD and the RNFL defect was assessed using Bland-Altman analysis. All statistical analyses were performed using SPSS software (version 19.0; SPSS, Chicago, IL, USA). The data are presented as mean ± SD values except where stated otherwise, and the cutoff for statistical significance was set at P < 0.05. 

Results

A total of 120 POAG eyes and 47 healthy control eyes were initially included. Of these eyes, five POAG eyes and two control eyes were excluded due to poor-quality angiography images. The baseline characteristics of the remaining 115 POAG eyes and 45 control eyes are provided in Table 1. 

Table 1

 

Baseline Characteristics of the Participants

 

Parapapillary MvD

There was excellent interobserver agreement regarding the detection of MvD (κ = 0.955). MvD was observed in 62 of the 115 POAG eyes (53.9%). Of the 62 eyes, 8 and 48 had single MvDs in the superior and inferior hemispheres, respectively, and 6 eyes had double MvDs in both the superior and inferior hemispheres, resulting in a total of 68 MvDs. In the remaining 53 POAG eyes, MvD was not evident in the OCTA images. None of the eyes in the control group exhibited an MvD. 

Clinical characteristics of the eyes with MvD and those without are compared in Table 2. Eyes with MvD had a thinner global RNFL (P < 0.001) and worse visual field MD (P = 0.042) and were more myopic (P = 0.029), with an axial length longer (P = 0.046) than those without MvD. Parapapillary atrophy (PPA) was more prevalent in the eyes with MvD than those without (P = 0.001): PPA was accompanied in all eyes with MvD, and none of the eyes without PPA had MvD. 

Table 2

 

Comparison of Clinical Characteristics Between POAG Eyes With and Without MvD

 

Topographic Correlation Between the MvD and RNFL Defects

A total of 89 RNFL defects were identified using SD-OCT circumpapillary scans in 62 eyes having the MvDs. Of these, 68 RNFL defects were found in the same hemisphere as the MvDs. The topographic correlation was evaluated between these 68 MvDs and RNFL defects. The remaining 21 RNFL defects from 19 eyes that did not have MvDs in the same hemisphere were not included in the topographic correlation analysis. The intraclass correlation coefficients for the interobserver reproducibility in measuring the circumferential extent and location of the MvD were 0.963 and 0.991, respectively, and those for the RNFL defect were 0.997 and 0.996, respectively. 

There was good topographic agreement between MvD and the RNFL defects defined using SD-OCT circumpapillary scan in terms of both the circumferential extent (Bland-Altman 95% limit of agreement, −23.4° to 21.9°; Fig. 2A) and location (Bland-Altman 95% limit of agreement, −16.2° to 17.0°; Fig. 2B). The extent and location of the MvD did not differ with those of RNFL defect (P = 0.602 and 0.704, respectively, paired samples t-test). 

Figure 2

 

Bland-Altmann plots showing the angular extent (A) and location (B) of the MvD in the parapapillary deep-layer versus RNFL defect. The solid lines represent the mean difference, and the dashed lines represent the 95% limits of agreement. Positive and negative values of the locations of the RNFL defect and MvD indicate locations that are superior and inferior relative to the foveal-disc axis, respectively.

Figure 3 shows representative cases with MvD, where the location and extent of the MvD correlates with those of the OCT-defined RNFL defects. Figure 4 shows cases of a POAG eye without MvD and a healthy control eye. MvD was not identified in this POAG eye despite an extensive RNFL defect, as is in the control eye. 

Figure 3

 

Color disc photography (A, D), OCTA image of the deep-layer (B, E), and OCT circumpapillary RNFL scanning (C, F) of two glaucomatous eyes with parapapillary MvD, one of which having more localized damage (A–C) and the other having more diffuse damage (D–F). Dashed ellipses indicate optic disc margin. Areas demarcated by red dotted lines (B, E) indicate MvDs. Red colored areas (C, F) are OCT-defined RNFL defects, and the red lines (B, C, E, F) indicate the margins of the RNFL defects. Note that the location and extent of the MvDs correlate with those of the OCT-defined RNFL defects in both cases.

Figure 4

 

Color disc photography (A, D), OCTA image of the deep-layer (B, E), and OCT circumpapillary RNFL scanning (C, F) of a glaucomatous eye without parapapillary MvD (A–C) and a healthy control eye (D–F). Dashed ellipses indicate optic disc margin. Red-colored areas (C) are OCT-defined RNFL defects. MvD is not identified either within or outside of the locations of RNFL defects in this POAG case.

Discussion

This study investigated the parapapillary deep-layer MvD using OCTA in POAG eyes and healthy control eyes and topographically correlated the findings with glaucomatous RNFL damage represented by an OCT RNFL defect. Approximately half of the POAG patients (53.9%) showed MvD in OCTA in the parapapillary deep layer, which showed a strong topographic correlation with OCT-defined RNFL defect, whereas none of the healthy control eyes exhibited an MvD. To the best of our knowledge, this is the first study that examined the frequency of MvD in a relatively large number of healthy controls and the topographic correlation between the MvD and OCT-defined RNFL defects. 

In the present study, MvD was universally located at the juxtapapillary area and topographically corresponded to the RNFL defect. It remains elusive whether the MvD is a pathogenically meaningful primary event or a simple secondary effect of glaucomatous damage (i.e., compromise of proximal vessels secondary to decreased metabolic need because of the damaged ONH). In relation to this question, it is noteworthy that the MvD was seen as a sectoral nonperfusion that was devoid of signal from small vessels. This finding suggests that the nonperfusion involves choriocapillaris together with larger microvessels. Because the surface choriocapillaris is not directly related to the prelaminar tissue supply, the choriocapillaris cannot be obliterated as a secondary change to the loss of prelaminar tissue. Therefore, the MvD may well be a primary event. However, the possibility that the MvD developed secondary to glaucomatous RNFL thinning should also be noted. In the present study, MvD was identified in approximately half of the POAG eyes, and those eyes had more advanced visual field damage. This may suggest that the MvD was caused by regression of supplying vessel due to decreased metabolic need from damaged axons. This issue remains to be clarified by further studies. A longitudinal study to determine the temporal relationship between the development and MvD and glaucomatous optic nerve damage may be needed. 

If the MvD is a primary phenomenon, the finding may have important relevance with glaucoma pathogenesis. The MvD would induce ischemia within the prelaminar and/or laminar tissue that should have been supplied by the compromised vessels. Axonal transport is an energetically costly process that is performed by adenosine triphosphate (ATP)-dependent motor proteins such as dynein.²⁰ Thus, hypoperfusion of the prelaminar and/or laminar region and the resulting ATP depletion will hamper axonal transport at the prelaminar tissue level, which in turn will cause depletion of neurotrophic factors in the cell body and ultimately promote the apoptosis of retinal ganglion cells.²¹ This hypothesis explains the topographic correlation between the MvD and RNFL defects. 

Irrespective of whether the MvD is a cause or result of glaucomatous optic neuropathy, it may deserve interest as a potential factor influencing the disease prognosis. It is conceivable that the blood–brain barrier is damaged in the area of choroidal nonperfusion, thereby releasing vasoactive or toxic substances into the ONH, which may promote further axonal damage.^22,23 More severe glaucoma damage in eyes with MvD supports this possibility. Further longitudinal studies are required to confirm the influence of the MvD on glaucoma progression. 

A limitation of the current study was that retinal vessel signals were observed in the en face OCTA images segmented in the deep-layer, which hampered a precise evaluation of the boundary of MvD. These could have been flow projection artifacts caused by the moving shadows cast by flowing blood cells.^7,24 This possibility should be considered when evaluating OCTA images. 

In conclusion, the MvD was exclusively found in POAG eyes than in control eyes and had a strong topographic correlation with the RNFL defect. The finding suggests that the MvD is directly related to glaucomatous optic nerve damage as either a cause or result. Future studies are needed to investigate the pathogenesis of MvD and its relationship with the development and progression of glaucomatous optic neuropathy. 

Acknowledgments

Supported by Grant 02-2016-023 from the Seoul National University Bundang Hospital Research Fund. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. 

Disclosure: E.J. Lee, None; S.H. Lee, None; J.-A. Kim, None; T.-W. Kim, None