Faculty Bibliography

Glaucoma is a neurodegenerative disease of the visual system and is the leading cause of irreversible blindness worldwide. To date, its pathophysiological mechanisms remain unclear. This study evaluated the feasibility of advanced diffusion magnetic resonance imaging techniques for examining the microstructural environment of the visual pathway in glaucoma. While conventional diffusion tensor imaging (DTI) showed lower fractional anisotropy and higher directional diffusivities in the optic tracts of glaucoma patients than healthy controls, diffusion kurtosis imaging (DKI) and the extended white matter tract integrity (WMTI) model indicated lower radial kurtosis, higher axial and radial diffusivities in the extra-axonal space, lower axonal water fraction, and lower tortuosity in the same regions in glaucoma patients. These findings suggest glial involvements apart from compromised axonal integrity in glaucoma. In addition, DKI and WMTI but not DTI parameters significantly correlated with clinical ophthalmic measures via optical coherence tomography and visual field perimetry testing. Taken together, DKI and WMTI provided sensitive and comprehensive imaging biomarkers for quantifying glaucomatous damage in the white matter tract across clinical severity complementary to DTI.

Purpose: Rigorous clinical testing has established that Schlemm"™s canal cross-sec-tional area (SC-CSA) is reduced in glaucomatous eyes. However, to date, it is unclear whether trabecular bypass procedures impact the morphology of the proximal aqueous outflow tract, or if the introduction of a local region of low outflow resistance adversely affects SC-CSA elsewhere, specifically presenting as SC diminution. This study quantifies changes in the morphology of the distal outflow pathway after iStent Trabecular Micro-Bypass stent (Glaukos Corp, Laguna Hills, CA, USA) implantation in living eyes by anterior segment optical coherence tomography (OCT). Design: This was a prospective observational study. Subjects: This study included six patients (eight eyes) with primary-open angle glaucoma. Methods: Patients underwent iStent placement in the nasal anterior chamber angle quadrant. OCT imaging was obtained of both nasal and temporal eye quadrants before and after surgery. For each SC parameter, an average of ten consecutive, evenly spaced measurements were manually obtained over a 1 mm segment of SC on FIJI ImageJ. Linear mixed effects modeling quantified the effect of the iStent on these parameters. Main outcome measures: Main outcome measures were changes in SC-CSA, inner-to-outer wall distance (IOD), and trabecular meshwork (TM) thickness following iStent placement. Results: Following iStent placement, total SC-CSA increased an average of 1,039.12 µm² (P = 0.05). Individually, there were no significant changes in SC-CSA in the nasal or temporal quadrants. Total SC-IOD and nasal SC-IOD increased an average of 2.35 µm (P = 0.01) and 2.96 µm (P = 0.04), respectively. There were no significant changes in temporal quadrant SC-IOD. There were no significant changes in TM thickness in either quadrant. Conclusions: Implantation of the iStent Trabecular Micro-Bypass stent significantly increases SC-IOD in the nasal quadrant at the location of implant, with no evidence of SC diminution in the temporal quadrant. It remains unclear how these observa-tions relate to the surgical efficacy of trabecular bypass procedures.

Purpose/UNASSIGNED:The purpose of this study was to develop a machine learning model to forecast future circumpapillary retinal nerve fiber layer (cpRNFL) thickness in eyes of healthy, glaucoma suspect, and glaucoma participants from multimodal temporal data. Design/UNASSIGNED:Retrospective analysis of a longitudinal clinical cohort. Participants/UNASSIGNED:Longitudinal clinical cohort of healthy, glaucoma suspect, and glaucoma participants. Methods/UNASSIGNED:(LTBE). Main Outcome Measures/UNASSIGNED:The mean absolute difference and Pearson's correlation coefficient between the true and forecasted values of the cpRNFL in the healthy, glaucoma suspect, and glaucoma patients. Results/UNASSIGNED:< 0.01) for the 3 groups, respectively. Conclusions/UNASSIGNED:The performance of the proposed forecasting model for cpRNFL is consistent across glaucoma suspect and glaucoma patients, which implies the robustness of the developed model against the disease state. These forecasted values may be useful to personalize patient care by determining the most appropriate intervisit schedule for timely interventions.

Purpose : Glaucoma is thought to involve neurochemical changes not only in the eye but also the brain's visual system. While excitotoxicity may play a role in glaucoma pathogenesis, it remains controversial whether excess glutamate occurs in this process. In the current study, we investigated alterations in the excitatory-inhibitory balance (E/I balance) in the visual cortex of glaucoma patients. In addition, we examined whether the altered neurochemical balance in the visual cortex is associated with projections of basal nucleus of Meynert (BNM), a major source of cortical cholinergic innervation in the basal forebrain. Methods : 10 glaucoma patients with a wide range of disease severity and 4 age-matched healthy subjects underwent 3-Tesla anatomical MRI, resting-state functional MRI (fMRI), and magnetic resonance spectroscopy (MRS). We used MEGA-PRESS and PRESS sequences to measure the levels of gamma-aminobutyric acid (GABA) and combined glutamate and glutamine (GLX), respectively. Both GABA and GLX were obtained from the same single voxel (2.2x2.2x2.2 cm³) placed along the calcarine sulci and fitted by LCModel software. We normalized the amount of GABA and GLX to N-acetyl-aspartate (NAA) values obtained from MEGA-PRESS, following LCModel guidelines. E/I balance was calculated by dividing the amount of GLX by the amount of GABA. The resting-state fMRI data were analyzed by CONN software. Results : Glaucoma patients had 16.51% higher E/I balance in the visual cortex compared to the healthy control group (Figure 1a). This difference in E/I balance was apparently driven by a 16.85% reduction in GABA (Figure 1b) with no apparent difference in glutamate or glutamine levels between groups (Figure 1c). Furthermore, the E/I balance in the visual cortex was correlated with the functional connectivity between BNM and the visual cortex (Figure 2). Conclusions : The current study shows that the visual cortex of glaucoma patients adopts an excitatory-dominant state that is driven by reduced GABA. This imbalance was associated with the functional connectivity between BNM and the visual cortex, suggesting that weaker projection of BNM to the visual cortex may play a role in the neurochemical changes in the visual cortex of glaucoma patients. Taken together, these findings suggest that widespread functional and metabolic alterations are involved in the brain during glaucoma pathogenesis

Purpose : In mild glaucoma, RNFL thinning and visual field (VF) loss are often localized, but structure-function modeling is impeded by variability due to individual eye anatomy. We perform high-resolution spatial correlations of RNFLT maps for each VF location to identify relevant areas and study further improvements by geometrically aligning RNFLT maps based on artery trajectories. Methods : In 419 SITA Standard 24-2 Humphrey VFs with at most mild glaucoma (mean deviation >=-3dB) with accompanying circumpapillary Cirrus HD-OCT RNFLT maps, we computed pixel-wise correlations (52 VF locations x 40401 pixels). We then performed an alignment operation, ensuring that the two major retinal arteries follow the same lines in all scans. We piecewise linearly approximated the trajectories of the arteries on 4 concentric circles around ONH (Fig. 1a), determined the necessary rotation for each pixel, and morphed the images accordingly (Fig. 1b). Results : For the pre-alignment RNFLT (correlation maps Fig. 2 top) we observed: (1) relatively high correlations (max 0.29); (2) most of the high-correlation regions are highly localized around the median trajectories of the major arteries at most VF locations, possibly due to the stacked character of the fiber bundles close to ONH, which impedes precise spatial mapping to the VF. This observation suggests general retinal vulnerability zones rather than highly VF location-specific areas as assumed by many previous structure-function models. Accordingly, morphing the RNFLT maps by aligning the eyespecific artery locations increased the maximal correlations on 25 of the 52 VF locations (Fig. 2 bottom, marked in green), particularly in nasal and inferior VF, with improvements of up to 0.1 (inferior arcuate region of VF). At many locations, aligned vulnerability areas become substantially more conspicuous (e.g. the location enlarged on the top left) and might have been missed without aligning. Conclusions : High-resolution structure-function correlations reveal retinal vulnerability zones in mild glaucoma. At many VF locations, these zones become better correlated with VF regions when RNFLT maps are aligned along the arteries. Specific attention to RNFL thinning in these zones in glaucoma suspects may improve the detection of initial VF loss glaucoma

Purpose : The glymphatic system has been postulated to play a crucial role in the central nervous system via metabolic waste removal from brain tissues by the cerebrospinal fluid (CSF). However, it remains unclear whether there is a direct glymphatic pathway in the visual pathway, partly due to limited in vivo methods for assessing the physiology of CSF dynamics in the optic nerve (ON). Contrast-enhanced MRI has been shown to be capable of monitoring the dynamics of glymphatic system in the brain using paramagnetic contrast agents. Investigating the same in and around the ON might give insights into the mechanisms of vision-related diseases such as glaucoma. Methods : In the present study, we infused a small molecular weight gadolinium-DTPA contrast agent intrathecally into the lumbar region (L4-L5) of 3 healthy adult C57BL/6J mice and imaged its flow, accumulation and clearance in the brain and the optic nerve over time using a 7-Tesla MRI scanner under isoflurane anesthesia. Contrast dynamics was monitored using a 3D T1-weighted imaging sequence at an isotropic resolution of 78x78x78 mum . Each scan lasted 10 min and a total of 12 continuous scans were acquired. These scans included 3 baseline acquisitions followed by 30 min of gadolinium contrast infusion using an automated pump while the scanning continued until the 12th time point. The intensity-time curves of the ON parenchyma, ON subarachnoid space (SAS), olfactory bulb, lateral ventricles and muscle tissues were generated and compared quantitatively. Data are represented as mean+/-SEM. Results : The ON parenchyma, ON-SAS, olfactory bulb and lateral ventricles showed a gradual increase in contrast enhancement (Figures 1 and 2A) with peak intensities at 92.03+/-16.21% (p<0.05), 440.50+/-39.41% (p<0.01), 210.54+/-20.69% (p<0.01) and 196.63+/-38.63% (p<0.05) respectively relative to baseline (Figure 2B). Peak intensity 3 occurred first in the olfactory bulb followed by ON-SAS, ON parenchyma and finally the lateral ventricles (Figure 2B). No apparent contrast uptake was observed in the nearby muscle tissues. Conclusions : This study illustrates direct communications between CSF and ON parenchyma and supports the evidence of the glymphatic system in the ON. In vivo imaging of CSF dynamics in and around the ON may open up new avenues for understanding ON function in health and disease with the possibility of devising novel drug delivery routes and therapeutic targets

Purpose : Gradient class activation maps (grad-CAM) generated by convolutional neural networks (CNN) have qualitatively indicated that these networks are able to identify important regions in OCT scans. Here, we quantitatively analyse these regions to improve our understanding of the CNN decision making process when detecting glaucoma in OCT volumes. Methods : A total of 1110 OCT (Cirrus HD-OCT, Zeiss, Dublin, Ca) scans from both eyes of 624 subjects (139 healthy and 485 glaucomatous patients (POAG)). An end-to-end 3D-CNN network was trained directly on 3D-volumes for glaucoma detection. Grad-CAM was implemented to highlight structures in the volumes that the network relied on. Grad-CAM heatmaps were generated for 3 different convolutional layers and quantitatively validated by occluding the regions with the highest grad-CAM weights (12.5% of original input volumes) and then evaluating the performance drop. Further, 8-retinal layers segmentation method was used to compute the average heatmap weights for each segmented layer separately, and used to identify the layers that were deemed as important for the task. Results : The model achieved an AUC of 0.97 for the test set (110 scans). Occlusion resulted in a 40% drop in performance (Fig.1). The RNFL and photoreceptors showed the highest median weights for grad-CAM heatmaps (0.1 and 0.2, respectively). The retinal pigment epithelium (RPE) and photoreceptors showed higher weights in the glaucomatous scans (Fig.2-a). RNFL had wider range of weights in healthy cases versus POAG ones. Analysis of the B-scans showed that central part around the optic disc (# 85-135) had the highest contribution to the network decision and the heatmap weights were much higher in glaucoma cases than healthy ones across all B-scans (Fig.2-b). Conclusions : The occlusion experiment indicates that the regions identified by the grad-CAMs are in fact pertinent to the glaucoma detection task. The increased emphasis on the photoreceptors in the glaucoma cases may be attributed to the atrophy in the superficial layers which in turn increased the brightness of this structure. This technique can be used to identify new biomarkers learned for other ocular diseases

Purpose : To compare the discrimination accuracy for glaucoma diagnosis using the OCT RNFL clock hours compared with average RNFL. Methods : In a large, ongoing, longitudinal cohort of healthy subjects and subjects with glaucoma, all subjects underwent visual field (VF) and OCT testing. Principal component (PC) analysis was used to reduce the dimensionality of clock hour measurements while maintaining maximum information variability for diagnostic performance. The first four PCs with linear regression were used as predictors of VF mean deviation (MD) and to classify glaucoma diagnosis. The prediction accuracy and discrimination power using cross validation were compared to the models using only average RNFL as a predictor. All models were adjusted for age, signal strength, and intra-subject correlation. Results : 1317 healthy and glaucomatous eyes (717 subjects) were included in the study. A PC analysis was built on the 9 clock hours while excluding non-informative sectors (clock hours 3, 4, and 9). The first PC explained 51% of the total variance, and the first four PCs explained 82% of the total variance and thus were used for subsequent regression models. A PC regression for glaucoma discrimination showed that clock hours 1, 5, 6, 7, 10, 11, 12 were significantly association with diagnosis. The PC showed better glaucoma diagnosis performance compared to average RNFL, with 10-fold cross-validation AUCs of 0.898 and 0.877, respectively (p<0.001). The PC regression for MD improved the model fit measured by R² by 9% compared to a regression using average RNFL. PC showed that clock hours 2, 5, 6, 7, 10, 11, 12 were significantly associated with MD. Conclusions : Using PCs with RNFL clock hours improved classification performance for glaucoma diagnosis and model fit for MD, compared to using average RNFL. This method improves discrimination performance by both considering all sectoral RNFL information and removing locations with low diagnostic yield

Purpose : Open-angle glaucoma (OAG) is a leading cause of blindness globally, yet the relative contributions of genetic background and the environment to the development of this disease are unclear. As a first step in determining these contributions, we document posterior pole pathology and investigate the association between optic nerve head glaucomatous features, intraocular pressure (IOP), and demographic information, in an exceptionally large cohort of free-ranging rhesus macaques. Methods : We administered ophthalmologicl exams under sedation to 55 female and 54 male animals aged 0-21 years (mean age = 6.08; SD = 4.27). IOP was measured using TonoPen and TonoVet Plus tonometers and measurements adjusted using published calibration equations (McAllister et al Optom Vis Sci. 2018). Cup/Disk ratio (CDR) was calculated from clinical examination aided by optical coherence tomography (OCT; Bioptigen Envisu). Posterior pole pathology was documented using fundus imaging. Association between CDR and age, sex, and IOP was assessed using generalized linear mixed models (GLMM) and likelihood ratio tests (LRTs). Results : The mean (+/-standard deviation) IOP in the 218 eyes measured was 19.32 (+/-6.24) mmHg. Mean CDR was 0.37 (+/-0.15). We detected elevated IOP (> 22mmHg) in 78 eyes (36.62%) and CDR > 0.7 was detected in 13 eyes (5.96 %). CDR values were highly concordant in eyes of the same animal (CDR of left and right eyes within 0.2 for all animals. IOP was a significant predictor of CDR (p<0.001) in models that either included or excluded age, and animal sex. The best fitting model included only IOP as a predictor variable (AIC =-196.3). This model was significantly better than models containing age (AIC =-185.8, p = 0.004) or both age and sex (AIC =-183.4). Additional posterior pole pathology included pigmentary macular changes (8 eyes), macular scars (2 eyes), vessel tortuosity (19 eyes), and retinal hemorrhages (5 eyes). Conclusions : IOP is a significant predictor of CDR in this cohort. Age did not appear to correlate with CDR, but controlling for relatedness may further elucidate impacts of individual biology on OAG. Similarities between rhesus and human glaucomatous phenotypes and the presence of additional retinal pathology in our population may make these animals valuable in the study of other complex human disease, such as age-related macular degeneration