Common pathological basis of the nervous ocular symptoms in chronic glaucoma

Photonic crystal-based smart contact lens for continuous intraocular pressure monitoring

Glaucoma is a very common disease after cataracts and is dangerous enough to cause irreversible blindness. However, often the main symptom of glaucoma is difficult to recognize because it may be absent or appear late, so the risk of blindness is greater. Intraocular pressure (IOP) is a well-known primary factor indicating glaucoma. In this study, we demonstrate a smart IOP sensor embedded in a contact lens that works through visual color changes without an external power source such as a battery or RF-based wireless power transfer. A microhydraulic amplification mechanism is adopted to enhance the range of color change from a photonic crystal (PC)-based flexible membrane whose lattice distance between nanostructures varies according to the morphology changes of an eyeball caused by IOP. The performance of the sensor is quantitatively demonstrated using an artificial silicone eye model for in vitro evaluation and a porcine eyeball for ex vivo verification. It has a limit of detection (LOD) of 3.2 and 5.12 mmHg, which was measured and evaluated using a spectrometer and a smartphone camera, respectively. The results prove that our sensor embedded in the contact lens can continuously monitor the IOP change using color change, and a smartphone camera can be used as a quantitative IOP measurement system in a noninvasive manner without an expensive optical spectrometer.

Microvascular Compromise Develops Following Nerve Fiber Layer Damage in Normal-Tension Glaucoma Without Choroidal Vasculature Involvement

PURPOSE

To investigate capillary densities in patients with unilateral normal-tension glaucoma (NTG) and normal controls using optical coherence tomography-angiography.

MATERIALS AND METHODS

We recruited 13 patients with unilateral NTG and 13 healthy controls. Optical coherence tomography-angiography was performed and pure peripapillary retinal capillary density was calculated after manually excluding large vessels and the optic disc. Paired comparison of capillary densities was performed among 3 categories of eye: NTG eye, fellow eye, and normal eye. Through vascular layer stratification we investigated differences in the retinal and choroidal circulation.

RESULTS

In total, 33 eyes of 11 unilateral NTG patients and 11 controls were analyzed. Capillary densities of NTG eyes were significantly lower than those of fellow eyes or control eyes (both P=0.013). No significant differences were found between fellow eyes and control eyes (P=0.328). Area of capillary compromise was identical to the area of retinal nerve fiber layer (RNFL) defect in all 11 eyes. In layer analysis, a decrease in capillary plexus was demonstrated only in the inner retina and no definite changes were found in the outer retina and choroid. Optic nerve head microvasculature did not show areas of capillary dropout.

CONCLUSIONS

In patients with unilateral NTG, we could observe significant retinal capillary compromise in the area of RNFL defect. No changes were demonstrated in the choroid and optic disc area. We speculated on a possibility of secondary microvascular changes in the retina to nerve damage from the wedge-shaped but not geographic shaped capillary compromise corresponding to RNFL defect area. Further studies on the optic nerve head microvasculature and blood flow are warranted to assess their relationship to glaucoma pathogenesis.

Clinicopathological correlation of parapapillary atrophy in monkeys with experimental glaucoma and temporary central retinal artery occlusion

Objective:

To investigate the clinicopathological correlation of parapapillary atrophy.

Materials and Methods:

The study included 16 eyes of rhesus monkeys (Macaca mulatta) – 4 eyes with experimental glaucoma, 11 eyes after experimental temporary occlusion of the central retinal artery, and 1 normal eye. On histological sections, we measured zones with different histological characteristics. On fundus photographs, alpha zone and beta zone of parapapillary atrophy were measured and correlated with the histological data.

Results:

The size of the clinical alpha zone of parapapillary atrophy was significantly correlated with the size of the histological region with irregularities of the retinal pigment epithelium (P = 0.05; correlation coefficient r = 0.49) and with the size of the histological region with a decreased density of retinal photoreceptors (P = 0.01; r = 0.60). The size of clinical beta zone of parapapillary atrophy significantly correlated with the size of the histological region with complete loss of the retinal pigment epithelium (P <0.001; r = 0.91), with the size of the histological zone with a complete loss of photoreceptors (P <0.001; r = 0.81), and with the size of the histological zone with a closed choriocapillaris (P <0.001; r = 0.89).

Conclusions:

The clinically seen alpha zone of parapapillary atrophy correlates with histological parapapillary irregularities of the retinal pigment epithelium and decreased density of retinal photoreceptors. The clinically seen beta zone of parapapillary atrophy correlates with histological complete loss of the retinal pigment epithelium and of the photoreceptors, and a closure of the choriocapillaris.

Fluorescein filling defects of the optic nerve head in normal tension glaucoma, primary open-angle glaucoma, ocular hypertension and healthy controls

BACKGROUND

To evaluate fluorescein filling defects of the optic nerve head in normal tension glaucoma (NTG), primary open-angle glaucoma (POAG), ocular hypertension (OHT) and controls.

METHODS

Forty patients with NTG (mean age 55 +/- 10 years), 40 patients with POAG (mean age 55 +/- 11 years), 40 patients with OHT (mean age 53 +/- 13 years), and 40 age-matched controls (mean age 54 +/- 11 years) were included in a prospective study. Video fluorescein angiograms were performed by means of a scanning laser ophthalmoscope. The extent of absolute filling defects of the optic nerve head was assessed (as a percentage of the disc area) using digital image analysis. Visual fields were tested by automatic static perimetry (Humphrey Field Analyzer, programme 24-2).

RESULTS

Absolute filling defects were significantly larger in patients with NTG (12.2 +/- 15.5%) and POAG (12.9 +/- 13.1%) compared to patients with OHT (1.2 +/- 3.6%) and healthy controls (0.1 +/- 0.5%) (p < 0.0001). The area under the receiver operating characteristic (ROC) curve was 0.806 for NTG vs healthy controls, and 0.812 for POAG vs OHT. Absolute filling defects are significantly correlated to the global indices mean deviation (r = -0.63, p < 0.0001), pattern standard deviation (r = 0.61, p < 0.0001), and corrected pattern standard deviation (r = 0.62, p < 0.0001) and significantly correlated to horizontal (r = 0.50, p < 0.0001) and vertical (r = 0.53, p < 0.0001) cup-to-disc-ratios.

CONCLUSIONS

Fluorescein filling defects of the optic disc representing capillary dropout are present in NTG and POAG. The extent of these filling defects is correlated to visual field loss and morphological damage. Fluorescein angiography may be useful in the diagnosis and management of NTG and POAG.

Inhibition of endothelin-1 and KCL-induced increase of [CA2+]i by antiglaucoma drugs in cultured A7r5 vascular smooth-muscle cells

Over contraction of vascular smooth muscle may result in ischemia to ocular neuronal cells and deteriorate the glaucoma. The purpose of this study was to investigate the inhibitory effects of various commercial antiglaucoma drugs including brimonidine, dipivefrin, betaxolol, timolol, levobunolol, carteolol, brinzolamide, dorzolamide, unoprostone, latanoprost, pilocarpine, and preservative benzalkonium chloride on endothelin-1(ET-1) and KCl-induced increase of intracellular free Ca2+ ([Ca2+]i) in cultured rat A7r5 vascular smooth muscle cells. These drugs were diluted from original concentrations to 1/100, 1/1000, and 1/10000. [Ca2+]i mobility was analyzed by spectrofluorometry after loading with fura-2-AM. Betaxolol, timolol, levobunolol, and carteolol were found to inhibit KCl-induced release of [Ca2+]i in a dose-dependent manner. High concentrations of betaxolol, timolol, levobunolol, carteolol, and unoprostone also inhibited ET-1-induced increase of [Ca2+]i in A7r5 cells. However, ET-1- and KCl-induced increase of [Ca2+]i was not diminished by other drugs including brimonidine, dipivefrin, brinzolamide, dorzolamide, latanoprost, pilocarpine, and benzalkonium chloride. These results indicate that high concentrations of unoprostone and beta-adrenergic blocking agents including betaxolol, timolol, levobunolol, and carteolol may inhibit ET-1-induced increase of [Ca2+]i. The mechanism may be mediated by inhibition of extracellular calcium influx via blocking of L-type voltage-dependent Ca2+ channel in A7r5 cells.

Colour Doppler imaging and fluorescein filling defects of the optic disc in normal tension glaucoma

AIM

To investigate the relation between blood flow parameters of the retrobulbar vessels measured by means of colour Doppler imaging (CDI) and fluorescein filling defects of the optic nerve head in patients with normal tension glaucoma (NTG) and control subjects.

METHODS

29 patients with NTG and 29 age and sex matched control subjects were included in this study. Blood flow velocities-peak systolic velocity (PSV), end diastolic velocity (EDV), and resistive indices (RI) of the ophthalmic artery (OA), the central retinal artery (CRA), and of the temporal and nasal short posterior ciliary arteries (TPCA, NPCA)-were measured with CDI. Fluorescein angiograms were performed with a scanning laser ophthalmoscope. The extent of absolute fluorescein filling defects of the optic nerve head in relation to the optic nerve head was assessed.

RESULTS

The PSV of the OA, the PSV and EDV of the CRA, and of the TPCA and NPCA were significantly reduced in NTG (p<0.05). The RI of the CRA, the TPCA and NPCA were significantly increased in NTG (p<0.01). The optic nerve head fluorescein filling defects were significantly larger in NTG (p<0.01). The filling defects were significantly negatively correlated (p<0.05) with the PSV and EDV of the CRA (PSV(CRA): r = -0.41; EDV(CRA): r = -0.34), with the PSV and EDV of the NPCA (PSV(NPCA): r = -0.34; EDV(NPCA): r = -0.38), and with the EDV of the TPCA (r = -0.29). A significant positive correlation (p<0.05) was found with the RI of both PCAs (RI(NPCA): r = 0.28; RI(TPCA): r = 0.29).

CONCLUSION

Patients with NTG had reduced blood flow velocities and higher resistive indices in most retrobulbar vessels. Optic nerve head fluorescein filling defects were larger compared to controls. The filling defects were correlated with end diastolic velocities and resistive indices of the PCAs and with blood flow velocities of the CRA. Capillary loss of the optic nerve head may be related to higher downstream resistance and reduced blood flow velocities of the retrobulbar vessels.

Blood flow in the optic nerve head and factors that may influence it

In the recent past there has been great interest in the blood supply of the optic nerve head (ONH), how to evaluate ONH blood flow, and what factors influence it, in health and disease. This is because evidence has progressively accumulated that there is vascular insufficiency in the ONH in both anterior ischemic optic neuropathy (AION) and glaucomatous optic neuropathy (GON)-two major causes of blindness or of seriously impaired vision in man. For the management and prevention of visual loss in these two disorders, a proper understanding of the factors that influence the blood flow in the ONH is essential. The objective of this paper is, therefore, to review and discuss all these factors. The various factors that influence the vascular resistance, mean blood pressure and intraocular pressure are discussed, to create a better basic understanding of the ONH blood flow, which may help us toward a logical strategy for prevention and management of ischemic disorders of the ONH.

Optic disk morphology in experimental central retinal artery occlusion in rhesus monkeys

PURPOSE

To evaluate longitudinally the optic disk morphology of nonglaucomatous optic nerve damage secondary to retinal nerve fiber damage, using experimental central retinal artery occlusion in rhesus monkey eyes as a model.

METHODS

This prospective study included 24 eyes of 16 monkeys. In eight eyes of eight animals, central retinal artery occlusion was produced by clamping the central retinal artery in the retrobulbar space. Occlusion was verified by fluorescein fundus angiography. The same eyes at baseline as well as the eight contralateral healthy eyes and eight monkey eyes with experimental high-pressure glaucoma served as control groups. Serially taken optic disk photographs were morphometrically evaluated.

RESULTS

The area and shape of the neuroretinal rim and alpha zone and beta zone of parapapillary chorioretinal atrophy of eyes after central retinal artery occlusion did not vary significantly (P > .30) from the same eyes before central retinal artery occlusion nor from the normal contralateral eyes. In the glaucomatous eyes, the neuroretinal rim was significantly (P < .001) smaller and parapapillary atrophy significantly (P = .01) larger than in the eyes after central retinal artery occlusion.

CONCLUSIONS

Experimental central retinal artery occlusion, in contrast to glaucoma, does not markedly change the size and shape of parapapillary atrophy and neuroretinal rim; this confirms previous clinical studies. Thus, assessment of parapapillary atrophy and neuroretinal rim may be helpful to differentiate between glaucomatous optic neuropathy and nonglaucomatous optic neuropathy secondary to retinal nerve fiber damage. Parapapillary atrophy is independent of decreased retinal blood perfusion and development of nonglaucomatous optic nerve atrophy following experimental central retinal artery occlusion.

Distribution and regulation of the optic nerve head tissue PO2

We investigated the distribution and regulation of the optic nerve head (ONH) tissue partial pressure of oxygen (PO2) under various stimuli and the role of the nitric oxide in the ONH circulation. Tissue PO2 was measured using double-barreled recess microelectrodes in the intact eyes of miniature pigs during normoxia, hyperoxia, hypoxia, variations of systemic blood pressure, and after inhibition of the endothelial nitric oxide synthesis by the administration of nitro-L-arginine. Measurements were performed in front of the ONH at intervascular and juxta-arteriolar areas and at a depth of 50 and 200 microm within the ONH at the center and the rim. During normoxia, PO2 was heterogeneously distributed in the ONH, higher close to the arterioles than in intervascular areas. Hyperoxia induced a significant increase of juxta-arteriolar tissue PO2, while in intervascular areas no change was noticed. Hypoxia did not modify intervascular tissue PO2 at 200 microm depth within the ONH. Variations of the systemic blood pressure did not induce any significant change in ONH tissue PO2. Similarly, no modification was noticed after the administration of nitro-L-arginine. There is a remarkable autoregulatory capacity of the ONH circulation that may compensate for parameters such as hyperoxia, hypoxia, and variations of the systemic blood pressure. Endothelially derived nitric oxide inhibition does not modify the ONH tissue PO2, probably because the tissue PO2 is stabilized by compensatory regulation.

Hemodynamic parameter estimation from ocular fluorescein angiograms

BACKGROUND

A method is proposed for parameterizing choroidal blood flow from fluorescein angiograms.

METHODS

After digitizing and aligning the angiographic sequence, the intensity build-up curves of fluorescence are analysed per pixel (approx. 10 microns in fundo). Two models are compared. A one-compartment model predicts an exponential build-up curve, from which the following parameters are estimated: maximum fluorescence, dye appearance time and local perfusion rate (reciprocal of the time constant of the exponential). To account for the contribution of the systemic circulation to the shape of the build-up curve, a two-compartment model is used which predicts a bi-exponential curve.

RESULTS

Introduction of the second (systemic) compartment resulted in a significant improvement of fit in 37 of 48 patients studied. The rate constants of the systemic compartment found were mainly in the range of 0.30-1.00 s-1.

CONCLUSION

For the individual patient, the local perfusion rates may vary strongly, with lower perfusion rates possibly being of prognostic value for ocular diseases such as glaucoma or diabetic retinopathy.

Mechanisms of optic nerve damage in primary open angle glaucoma

Several mechanisms have been postulated to explain the optic nerve damage that occurs in primary open angle glaucoma (POAG). No single mechanism can adequately explain the great variations in susceptibility to damage and the patterns of damage seen in this syndrome. The etiology of POAG is likely to be multifactorial. Mechanical, vascular and other factors may influence individual susceptibility to optic nerve damage. An enhanced understanding of the nature of the optic nerve damage in POAG and improved methods of study may result in earlier diagnosis or may allow us to distinguish among different pathological processes all currently grouped under the diagnosis of POAG. As we gain a better understanding of the neuropharmacology and cellular biology of injury and repair of the visual system we will undoubtedly refine the concepts of glaucomatous optic neuropathy.

Effects of dopamine agonists and antagonists on pulsatile blood flow of ocular hypertensive rabbits

It has been reported that some dopamine antagonists can suppress the intraocular pressure and can increase the blood flow in the retina and choroid. Therefore, several dopamine antagonists and agonists were studied to determine if they can increase the ocular pulsatile blood flow in ocular hypertensive rabbits with the intraocular pressure raised artificially to 40 mm Hg. It was found that numerous dopamine antagonists including loxapine, moperone, domperidone, haloperidol and metoclopramide increased pulsatile blood flow for 49-110%, 95-155%, 72-86%, 60-114%, and 39-139%, respectively. Floropipamide reduced the ocular pulsatile blood flow for 18% at 90 min and 10% at 120 min. Chlofluperol produced biphasic action on pulsatile blood flow by significantly reducing it initially at 60 min (-49%) and then markedly increasing it at 180 min (91%). In case of dopamine agonists, neither dopamine nor bromocriptine affected the pulsatile blood flow significantly. These results indicate that some of dopamine antagonists could be used to lower the intraocular pressure and to increase the ocular pulsatile blood flow as well.

Pulsatile ocular blood flow in patients with low tension glaucoma

Measurements of the intraocular pressure (IOP) pulse and pulsatile ocular blood flow (POBF) have been made in 22 patients with bilateral low tension glaucoma (LTG) and 29 healthy subjects matched as closely as possible for age, refractive error, IOP, systemic pulse pressure, and heart rate. Recordings were made in both the standing and supine positions. The amplitude of the intraocular pressure pulse was significantly lower in patients with LTG (1.2, SEM 0.1 mmHg standing, and 1.3, SEM 0.1 mmHg lying) than in healthy subjects (1.9, SEM 0.1 mmHg standing, and 2.0 SEM 0.1 mmHg lying): p less than 0.001 standing and p less than 0.002 lying. Measurement of POBF also showed a significant reduction between the healthy subjects (428 (31) SEM microliters/min standing and 345 (28) SEM microliters/min lying) and subjects with LTG (301 (27) SEM microliters/min standing and 249 (24) SEM microliters/min lying), p less than 0.005 standing and p less than 0.02 lying. This represents a difference of approximately 30% between the two groups in either posture. A close non-parametric correlation existed between the level of IOP and the POBF (r = 0.75, p less than 0.001 standing, and r = -0.55, p less than 0.02 lying). Such a correlation was not present in the healthy subjects. A reduction in POBF occurred in both groups on assuming the supine posture (healthy subjects 83 (16) SEM microliters/min, LTG subjects 52 (17) SEM microliters/min). These figures represent reductions of 19% and 17% respectively in comparison with the standing value. The results lend further confirmation to the hypothesis that vascular factors are associated with low tension glaucoma.

Pallor of the optic disc in glaucoma patients with artificial hypertension

Measurement of four parameters, namely brightness of the unaffected optic disc, brightness of the optic disc under artificially elevated eye pressure, baseline eye pressure, and systolic blood pressure, gives the possibility of discerning glaucomatous eyes from healthy ones, with an accuracy of about 90%. Measuring eye pressure and systolic blood pressure is very simple. If measurement of the optical density could be simplified, an additional practical test for glaucoma could be developed.

Ocular hypertension in primary glaucoma: a new hypothesis

Some considerations of the vascular events occurring the uvea during the course of glaucoma are reported and a mathematical model is adopted to clarify the interdependence between intraocular pressure, rate of blood flow, blood pressure gradient, and the number of unobstructed capillary vessels in the uveal membrane. By means of this mathematical model the physiopathological vascular cause of the rise in intraocular pressure is confirmed (also independently of an obstacle to aqueous outflow), and the clinical implications in relation to visual sensory impairment are emphasized.

The histology of human glaucoma cupping and optic nerve damage: clinicopathologic correlation in 21 eyes

We have examined by light and electron microscopy the retina, optic nervehead, and optic nerves of 21 human eyes from glaucoma patients in whom clinical information was available for comparison. In several cases it was possible to correlate the degree and distribution of optic nerve damage with the clinical appearance of the optic disc and visual field studies. There was no selective loss of astrocytes of the optic nervehead in early glaucoma cupping. Acquired increases in optic disc cup size prior to detectable visual field loss probably represent loss of ganglion cell axonal fibers which is not yet significant enough to produce field defects. It is unlikely that the mechanism of axonal damage in chronic human glaucoma involves early loss of astrocytic glial cells at the optic nervehead. At the level of the retrobulbar optic nerve, the ganglion cell axonal fibers of the superior and inferior quadrants seem to be lost earlier than the fibers of the nasal and temporal nerve periphery. Since the superior and inferior poles of the optic nerve may contain the fibers of arcuate area ganglion cells, these data confirm the presumption from visual field testing that arcuate area ganglion cell fibers are selectively more susceptible to damage in chronic glaucoma.

Pressure lowering effect of timolol with reference to its topical vascular action

The hemodynamic effect of timolol maleate in open angle glaucoma was studied rheo-oculographically. Increase in the ocular pulse which reaches maximal amplification 2 h after the instillation of the drug was observed. The maximal pressure lowering effect appeared 3 h after instillation, as if the hemodynamic effect were the trigger for the pressure lowering effect.

Low-tension glaucoma

A retrospective study of 45 patients with low-tension glaucoma revealed the mean age at diagnosis to be 66 years. Seventeen patients had follow-up visual field examinations, the average follow-up period being 6.4 years. There was no significant difference in prognosis of the ocular course between patients with Po/C equal to or greater than 100 and those with Po/C less than 100. The presence of splinter hemorrhages at the optic disk (10% of affected eyes) or of systemic arterial hypertension (diastolic blood pressure greater than 100 mm Hg) was associated with progression of visual field defects. Patients with sudden visual loss or associated hemodynamic events (33% of the total patients) had a more favorable prognosis regarding stability (lack of progression) of visual field defects than those without such an event. Extension of visual field defects across the macula was a common finding (25% of affected eyes). No firm evidence was obtained to indicate that treatment of the low-tension glaucoma improved the prognosis of the ocular course.

Fluorescein angiography: its contributions towards understanding the mechanisms of visual loss in glaucoma

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Pathogenesis of visual field defects. Role of the ciliary circulation

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Blood supply of the optic nerve head and its role in optic atrophy, glaucoma, and oedema of the optic disc

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The blood supply of the optic nerve and chiasma in man

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Implications of the vascular anatomy of the optic nerve and chiasma

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