Distinct iris gene expression profiles of primary angle closure glaucoma and primary open angle glaucoma and their interaction with ocular biometric parameters

Region-related and layer-specific permeability of the iris vasculature with morphological mechanism: A novel understanding of blood-aqueous barrier

The permeability of iris blood vessels has an important role in maintaining aqueous humor (AH) homeostasis, contributing to variation in iris volume and probably the pathogenesis of angle closure glaucoma. This study investigates the permeability of the iris microvasculature to plasma-derived protein and correspond it with the morphologic characteristics of vascular mural cells (MCs). Twenty-two enucleated porcine eyes were used in this study. 12 eyes were micro-perfused with vehicle alone as control or with FITC-albumin as a marker of protein leakage and histological sections subsequently made to examine for FITC-albumin presence. The other 10 eyes were immunolabeled via micro-perfusion for αSMA and VE-cadherin to investigate their topographic distribution in the porcine iris vasculature, and to cross correspond with the locations of FITC-albumin deposits. Distribution of FITC-signals exhibited a site-dependent pattern and time-dependent change in the iris. Fluorescence was initially detected around capillaries in the superficial and deep layer of the iris microvascular network. The pupillary region and the iris root retained more fluorescent signal than the iridal ciliary region. At low magnification, αSMA labelling displayed a regional variation which was inversely correlated with vascular permeability. At the cellular level, αSMA labeling corresponded with vascular MCs distribution in the iris vascular network. The correspondence between iris microvascular permeability to FITC-albumin and the pattern of αSMA distribution and MCs coverage adds to the understanding of the elements comprising the blood-aqueous barrier with implications for the bio-mechanics of iris volume change.

Application of advanced magnetic resonance imaging in glaucoma: a narrative review

Glaucoma is a group of eye diseases characterized by progressive degeneration of the optic nerve head and retinal ganglion cells and corresponding visual field defects. In recent years, mounting evidence has shown that glaucoma-related damage may not be limited to the degeneration of retinal ganglion cells or the optic nerve head. The entire structure of the visual pathway may be degraded, and the degradation may even extend to some non-visual brain regions. We know that advanced morphological, functional, and metabolic magnetic resonance technologies provide a means to observe quantitatively and in real time the state of brain function. Advanced magnetic resonance imaging (MRI) techniques provide additional diagnostic markers for glaucoma, which are related to known potential histopathological changes. Many researchers in China and globally have conducted clinical and imaging studies on glaucoma. However, they are scattered, and we still need to systematically sort out the advanced MRI related to glaucoma. We reviewed literature published in any language and included all studies that were able to be translated into English from 1 January 1980 to 31 July 2021. Our literature search focused on emerging magnetic resonance neuroimaging techniques for the study of glaucoma. We then identified each functional area of the brain of glaucoma patients through the integration of anatomy, image, and function. The aim was to provide more information about the occurrence and development of glaucoma diseases. From the perspective of neuroimaging, our study provides a research basis to explain the possible mechanism of the occurrence and development of glaucoma. This knowledge gained from these techniques enables us to more clearly observe the damage glaucoma causes to the whole visual pathway. Our study provides new insights into glaucoma-induced changes to the brain. Our findings may enable the progress of these changes to be analyzed and inspire new neuroprotective therapeutic strategies for patients with glaucoma in the future.

Changes in Iris Stiffness and Permeability in Primary Angle Closure Glaucoma

Purpose

To evaluate the biomechanical properties of the iris by evaluating iris movement during pupil constriction and to compare such properties between healthy and primary angle-closure glaucoma (PACG) subjects.

Methods

A total of 140 subjects were recruited for this study. In a dark room, the anterior segments of one eye per subject were scanned using anterior segment optical coherence tomography imaging during induced pupil constriction with an external white light source of 1700 lux. Using a custom segmentation code, we automatically isolated the iris segments from the AS-OCT images, which were then discretized and transformed into a three-dimensional point cloud. For each iris, a finite element (FE) mesh was constructed from the point cloud, and an inverse FE simulation was performed to match the clinically observed iris constriction in the AS-OCT images. Through this optimization process, we were able to identify the elastic modulus and permeability of each iris.

Results

For all 140 subjects (95 healthy and 45 PACG of Indian/Chinese ethnicity; age 60.2 ± 8.7 for PACG subjects and 57.7 ± 10.1 for healthy subjects), the simulated deformation pattern of the iris during pupil constriction matched well with OCT images. We found that the iris stiffness was higher in PACG than in healthy controls (24.5 ± 8.4 kPa vs. 17.1 ± 6.6 kPa with 40 kPa of active stress specified in the sphincter region; P < 0.001), whereas iris permeability was lower (0.41 ± 0.2 mm2/kPa s vs. 0.55 ± 0.2 mm2/kPa s; p = 0.142).

Conclusions

This study suggests that the biomechanical properties of the iris in PACG are different from those in healthy controls. An improved understanding of the biomechanical behavior of the iris may have implications for the understanding and management of angle-closure glaucoma.

In vivo non-contact measurement of human iris elasticity by optical coherence elastography

Quantifying the mechanical properties of the iris can offer valuable insights into the pathophysiology of primary angle closure glaucoma. However, current techniques for iris elastography remain ex vivo with limited clinical applications. This article describes a proposition for a non-contact and non-invasive air-puff optical coherence elastography (OCE) system that can evaluate iris elasticity in vivo. Ten eyes recruited from seven subjects underwent OCE imaging acquisition under three different illumination conditions. The Young's modulus of each eye was detected and shown to be inversely proportional to the iris length, indicating a relationship between mechanical properties and morphology of the iris. With its noninvasive and high-resolution features, this air-puff system shows great potential for applications in clinical ophthalmology.

Effect of valproic acid on functional bleb morphology in a rabbit model of minimally invasive surgery

Purpose

To determine the effect of valproic acid (VPA) on bleb morphology and scar characteristics in a rabbit model of minimally invasive glaucoma surgery (MIGS).

Methods

Nine New Zealand white rabbits were subjected to MIGS with intraoperative implantation of the PreserFlo MicroShunt. Rabbits were then administered with subconjunctival injections of phosphate buffered saline (PBS) (n=4) or with VPA (n=5). Bleb morphology was examined by slit-lamp biomicroscopy and in vivo confocal microscopy. Postoperative day 28 tissues were examined by immunohistochemical evaluation and label-free multiphoton microscopy to visualise the collagen matrix, by terminal deoxynucleotidyl transferase dUTP nick-end labelling assay and immunofluorescent labelling for Ki67 expression to detect apoptosis and cell growth, and by real-time quantitative PCR to measure Col1a1, Fn, and Smad6 transcript expression.

Results

VPA-treated blebs were detectable on day 28, while the PBS-treated blebs were not detectable by day 14. VPA-treated blebs were diffuse, extended posteriorly with near normal conjunctival vascularity and featured a combination of reticular/blurred stromal pattern with evidence of relatively large stromal cysts. Instead of the deposition of thick, disorganised collagen fibres characteristic of the PBS bleb, the VPA bleb contained conspicuously thinner collagen fibres which were associated with similarly thinner fibronectin fibres. In corroboration, Col1a1 and Fn mRNA expression was reduced in the VPA blebs, while increased Smad6 expression implicated the disruption of the transforming growth factor beta pathway. Apoptosis and cell growth profiles appeared similar with both treatments.

Conclusions

The results support the application of VPA to enhance bleb morphology associated with good bleb function in MIGS with no apparent cytotoxicity.

Updates on Genes and Genetic Mechanisms Implicated in Primary Angle-Closure Glaucoma

Primary angle-closure glaucoma (PACG) is estimated to affect over 30 million people worldwide by 2040 and is highly prevalent in the Asian population. PACG is more severe and carries three times the higher risk of blindness than primary open-angle glaucoma, thus representing a significant public health concern. High heritability and ethnic-specific predisposition to PACG suggest the involvement of genetic factors in disease development. In the recent past, genetic studies have led to the successful identification of several genes and loci associated with PACG across different ethnicities. The precise cellular and molecular roles of these multiple loci in the development and progression of PACG remains to be elucidated. Nonetheless, these studies have significantly increased our understanding of the emerging cellular processes and biological pathways that might provide more significant insights into the disease’s genetic etiology and may be valuable for future clinical applications. This review aims to summarize and update the current knowledge of PACG genetics analysis research.

Altered Iris Aquaporin Expression and Aqueous Humor Osmolality in Glaucoma

Purpose

Aquaporins (AQPs) facilitate transmembrane osmotic water transport and may play a role in iris fluid conductivity, which is implicated in the pathophysiology of glaucoma. In this study, we compared the iris expression of AQPs and aqueous osmolality between primary angle closure glaucoma (PACG), primary open-angle glaucoma (POAG), and nonglaucoma eyes.

Methods

AQP1-5 transcripts from a cohort of 36 PACG, 34 POAG and 26 nonglaucoma irises were measured by quantitative real-time PCR. Osmolality of aqueous humor from another cohort of 49 PACG, 50 POAG, and 50 nonglaucoma eyes were measured using an osmometer. The localization of AQP1 in both glaucoma and nonglaucoma irises was determined by immunofluorescent analysis.

Results

Of the five AQP genes evaluated, AQP1 and AQP2 transcripts were significantly upregulated in both PACG (3.48- and 8.07-fold, respectively) and POAG (3.12- and 11.58-fold, respectively) irises relative to nonglaucoma counterparts. The aqueous osmolalities of PACG (303.68 mmol/kg) and POAG (300.79 mmol/kg) eyes were significantly lower compared to nonglaucoma eyes (312.6 mmol/kg). There was no significant difference in expression of AQP transcripts or aqueous osmolality between PACG and POAG eyes.

Conclusions

PACG and POAG eyes featured significant increase in AQP1 and AQP2 expression in the iris and reduced aqueous osmolality compared to nonglaucoma eyes. These findings suggest that the iris may be involved in altered aqueous humor dynamics in glaucoma pathophysiology. Because PACG did not differ from POAG in both properties studied, it is likely that they are common to glaucoma disease in general.

Iris stromal cell nuclei deform to more elongated shapes during pharmacologically-induced miosis and mydriasis

Nuclear shape alteration in ocular tissues, which can be used as a metric for overall cell deformation, may also lead to changes in gene expression and protein synthesis that could affect the biomechanics of the tissue extracellular matrix. The biomechanics of iris tissue is of particular interest in the study of primary angle-closure glaucoma. As the first step towards understanding the mutual role of the biomechanics and deformation of the iris on the activity of its constituent stromal cells, we conducted an ex-vivo study in freshly excised porcine eyes. Iris deformation was achieved by activating the constituent smooth muscles of the iris. Pupillary responses were initiated by inducing miosis and mydriasis, and the irides were placed in a fixative, bisected, and sliced into thin sections in a nasal and temporal horizontal orientation. The tissue sections were stained with DAPI for nucleus, and z-stacks were acquired using confocal microscopy. Images were analyzed to determine the nuclear aspect ratio (NAR) using both three-dimensional (3D) reconstructions of the nuclear surfaces as well as projections of the same 3D reconstruction into flat two-dimensional (2D) shapes. We observed that regardless of the calculation method (i.e., one that employed 3D surface reconstructions versus one that employed 2D projected images) the NAR increased in both the miosis group and the mydriasis group. Three-dimensional quantifications showed that NAR increased from 2.52 ± 0.96 in control group to 2.80 ± 0.81 and 2.74 ± 0.94 in the mydriasis and miosis groups, respectively. Notwithstanding the relative convenience in calculating the NAR using the 2D projected images, the 3D reconstructions were found to generate more physiologically realistic values and, thus, can be used in the development of future computational models to study primary angle-closure glaucoma. Since the iris undergoes large deformations in response to ambient light, this study suggests that the iris stromal cells are subjected to a biomechanically active micro-environment during their in-vivo physiological function.

[Iris shape change and risk of anterior chamber angle closure]

Potential obstruction of the anterior chamber angle by iris root is a key factor of the pathogenesis of angle-closure glaucoma. Development and adoption of ultrasound biomicroscopy and optical coherence tomography in clinical practice have significantly expanded the capabilities of studying the structures of the anterior eye segment in terms of angle closure risk factors through improving visualization depth and quality of examination, including different variants of iris structure and potential changes of its shape in mydriasis. The article reviews various studies dedicated to the progression of primary angle-closure glaucoma and its dependence on the biometric parameters of the eye and changes in pupil size.

Iris and its relevance to angle closure disease: a review

Glaucoma is a leading cause of irreversible visual impairment, and primary angle closure glaucoma (PACG) affects Asians disproportionately. Whereas advances in ocular imaging have identified several anatomical risk factors, our ability to predict PACG still requires considerable improvement. The iris plays a crucial role in the pathophysiology of angle closure disease, either through a mechanical or vascular mechanism. Irises of closed-angle eyes inhibit vastly different structural constituents as compared with those of open-angle eyes, thereby effecting variations in biomechanical properties and iris fluid conductivity. The clinical consequences include a smaller change in iris volume on pupil dilation in closed-angle eyes, thereby bringing the iris and trabecular meshwork closer in apposition. In this review, we summarise the potential role of the iris in the pathogenesis of angle closure disease.

Microsurgical scleral drainage and trabeculectomy-scleral flap adjustable suture combination technique in the treatment of primary glaucoma

Objective:

To investigate the clinical effect of microsurgical scleral drainage and trabeculectomy combined with scleral flap adjustable suture technique in the treatment of primary glaucoma.

Methods:

One hundred primary glaucoma patients (120 eyes) in Xinyu People’s Hospital of Jiangxi province were selected from July 2014 to June 2016. The patients were randomly divided into control group and study group. The control group was treated with compound trabeculectomy, while the study group was treated with microsurgical scleral drainage and trabeculectomy combined with scleral flap adjustable suture technique. In both groups of patients, intraocular pressure, functional filtering bleb formation, and complications before and after surgery were monitored for three days, one week, one month, three months, six months and one year, while anterior chamber depth was determined one week after operation. The extent of success of operation was compared between the two groups.

Results:

At three days, one week, one month, three months, six months and one year after surgery, intraocular pressure of study group was significantly lower than that of the control group (P<0.05). There was 93.33% formation of functional filtering blebs in the study group, which was significantly higher than that in the control group (60.00%, P<0.001). Moreover, normal anterior chamber formation was significantly higher in the study group (91.67%) than in the control group (71.67%, P<0.01). There was 95.00% operation success in the study group, relative to 68.33% success in the control group (P<0.001).

Conclusion:

Microsurgical scleral drainage and trabeculectomy combined with scleral flap adjustable suture technique has better curative effect on primary glaucoma than compound trabeculectomy. Moreover, it does not exacerbate complications. Therefore, the combination treatment technique merits clinical application.

Correlation of iris collagen and in-vivo anterior segment structures in patients in different stages of chronic primary angle-closure in both eyes

Purpose

To compare the density of iris collagen and anterior segment parameters in eyes with chronic primary angle closure glaucoma (CPACG) and their fellow eyes with confirmed or suspected primary angle closure (PAC/PACS).

Methods

Nineteen patients with CPACG in one eye and PAC/PACS in the fellow eye requiring trabeculectomy in the CPACG eye and iridectomy in the fellow eye were recruited. Anterior segment optical coherence tomography (AS-OCT) measurements were conducted under light and dark conditions. Iris specimens, obtained by iridectomy/trabeculectomy, were analyzed by sirius red polarization for quantifying type I/III collagen density. AS-OCT parameters and type I/III collagen densities were compared between the two eyes.

Results

The iris curvatures were flatter in CPACG eyes in light and dark conditions (P < 0.05). The iris areas in light condition and iris thicknesses in dark condition were smaller in CPACG eyes (P < 0.05). The density of collagen type I in CPACG eyes was lower (P = 0.048). The light-to-dark changes in CPACG eyes and PAC/PACS eyes, respectively, were -0.679 ± 0.701 and -1.627 ± 0.802 mm for pupil diameters, and 0.069 ± 0.113 and 0.258 ± 0.157 mm² for iris areas, which differed significantly (P < 0.001). With the decrease of type I collagen, anterior chamber width increased and iris areas decreased in dark condition.

Conclusion

Dynamic changes in pupil diameter and iris areas differed significantly between CPACG eyes and their fellow PAC/PACS eyes. Decreased type I collagen density in iris tissue was associated with decreased iris area and increased anterior chamber width, which may contribute to disease progression.

Cholinergic nervous system and glaucoma: From basic science to clinical applications

The cholinergic system has a crucial role to play in visual function. Although cholinergic drugs have been a focus of attention as glaucoma medications for reducing eye pressure, little is known about the potential modality for neuronal survival and/or enhancement in visual impairments. Citicoline, a naturally occurring compound and FDA approved dietary supplement, is a nootropic agent that is recently demonstrated to be effective in ameliorating ischemic stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, cerebrovascular diseases, memory disorders and attention-deficit/hyperactivity disorder in both humans and animal models. The mechanisms of its action appear to be multifarious including (i) preservation of cardiolipin, sphingomyelin, and arachidonic acid contents of phosphatidylcholine and phosphatidylethanolamine, (ii) restoration of phosphatidylcholine, (iii) stimulation of glutathione synthesis, (iv) lowering glutamate concentrations and preventing glutamate excitotoxicity, (v) rescuing mitochondrial function thereby preventing oxidative damage and onset of neuronal apoptosis, (vi) synthesis of myelin leading to improvement in neuronal membrane integrity, (vii) improving acetylcholine synthesis and thereby reducing the effects of mental stress and (viii) preventing endothelial dysfunction. Such effects have vouched for citicoline as a neuroprotective, neurorestorative and neuroregenerative agent. Retinal ganglion cells are neurons with long myelinated axons which provide a strong rationale for citicoline use in visual pathway disorders. Since glaucoma is a form of neurodegeneration involving retinal ganglion cells, citicoline may help ameliorate glaucomatous damages in multiple facets. Additionally, trans-synaptic degeneration has been identified in humans and experimental models of glaucoma suggesting the cholinergic system as a new brain target for glaucoma management and therapy.

Distinct iris gene expression profiles of primary angle closure glaucoma and primary open angle glaucoma and their interaction with ocular biometric parameters

Background

This study aimed to evaluate differences in iris gene expression profiles between primary angle closure glaucoma (PACG) and primary open angle glaucoma (POAG) and their interaction with biometric characteristics.

Design

Prospective study.

Participants

Thirty‐five subjects with PACG and thirty‐three subjects with POAG who required trabeculectomy were enrolled at the Singapore National Eye Centre, Singapore.

Methods

Iris specimens, obtained by iridectomy, were analysed by real‐time polymerase chain reaction for expression of type I collagen, vascular endothelial growth factor (VEGF)‐A, ‐B and ‐C, as well as VEGF receptors (VEGFRs) 1 and 2. Anterior segment optical coherence tomography (ASOCT) imaging for biometric parameters, including anterior chamber depth (ACD), anterior chamber volume (ACV) and lens vault (LV), was also performed pre‐operatively.

Main Outcome Measures

Relative mRNA levels between PACG and POAG irises, biometric measurements, discriminant analyses using genes and biometric parameters.

Results

COL1A1, VEGFB, VEGFC and VEGFR2 mRNA expression was higher in PACG compared to POAG irises. LV, ACD and ACV were significantly different between the two subgroups. Discriminant analyses based on gene expression, biometric parameters or a combination of both gene expression and biometrics (LV and ACV), correctly classified 94.1%, 85.3% and 94.1% of the original PACG and POAG cases, respectively. The discriminant function combining genes and biometrics demonstrated the highest accuracy in cross‐validated classification of the two glaucoma subtypes.

Conclusions

Distinct iris gene expression supports the pathophysiological differences that exist between PACG and POAG. Biometric parameters can combine with iris gene expression to more accurately define PACG from POAG.