Causes of optic nerve damage in glaucoma. Robert N. Shaffer lecture

In vivo assessment of changes in corneal hysteresis and lamina cribrosa position during acute intraocular pressure elevation in eyes with markedly asymmetrical glaucoma

Purpose

To investigate the biomechanical response of the cornea, lamina cribrosa (LC), and prelaminar tissue (PT) to an acute intraocular pressure (IOP) increase in patients with markedly asymmetrical glaucoma and in healthy controls.

Patients and methods

A total of 24 eyes of 12 patients with markedly asymmetrical primary open-angle glaucoma (POAG) and 12 eyes of 12 healthy patients were examined with spectral-domain optical coherence tomography (SD-OCT) and ocular response analyzer (ORA) at baseline and during acute IOP elevation by means of an ophthalmodynamometer. The displacement of the LC and PT and the change in corneal hysteresis (CH) and corneal resistance factor (CRF) were evaluated.

Results

Following a mean IOP increase of 12.3±2.4 mmHg, eyes with severe glaucoma demonstrated an overall mean anterior displacement of the LC (−6.58±26.09 µm) as opposed to the posterior laminar displacement in eyes with mild glaucoma (29.08±19.28 µm) and in healthy eyes (30.3±10.9; p≤0.001 and p=0.001, respectively). The PT displaced posteriorly during IOP elevation in all eyes. The CH decreased in eyes with severe glaucoma during IOP elevation (from 9.30±3.65 to 6.92±3.04 mmHg; p=0.012), whereas the CRF increased markedly in eyes with mild glaucoma (from 8.61±2.30 to 12.38±3.64; p=0.002) and in eyes with severe glaucoma (from 9.02±1.48 to 15.20±2.06; p=0.002). The increase in CRF correlated with the anterior displacement of the LC in eyes with severe glaucoma.

Conclusion

Eyes with severe glaucoma exhibited a mean overall anterior displacement of the anterior laminar surface, while eyes with mild glaucoma and healthy eyes showed a posterior displacement of the LC during IOP elevation. The CH decreased significantly from baseline only in eyes with severe glaucoma, but the CRF increased significantly in all glaucomatous eyes. The CRF increase correlated with the anterior displacement of the LC in eyes with severe glaucoma.

Morphological and morphometric changes in rat optic nerve microvessels in a glaucoma experimental model

AIM

To study the morphological and morphometric changes produced in the capillaries of the optic nerve (ON) head and initial portion after the experimental increase in intraocular pressure (IOP).

MATERIAL AND METHODS

Wistar rats underwent cauterization of three episcleral veins, which produced an immediate increase in the IOP, and was maintained for 3 months. Sagittal sections of the eyeball were studied with immunohistochemical techniques, using a primary antibody to GLUT-1. The GLUT-1 positive capillaries were counted, and measurements were made of the area, perimeter and mean diameter.

RESULTS

Microscopic examination of sections of the ON of control rats revealed a lower density and larger caliber of capillaries in the prelaminar region as compared with the other regions of the ON (P<.05). Comparison between the control and the experimental groups showed a reduction in capillary density (except in the prelaminar region) and a smaller size in all the areas of the ON studied, but less evident in the initial portion (P<.05).

CONCLUSIONS

The increase in IOP was associated with significant qualitative and quantitative changes in the capillaries of the laminar and poslaminar regions of the ON head. These changes appear to return towards parameters compatible with normality in the initial portion of the ON, an area where the vascular collapse was less evident. These findings might explain the significant reduction in ocular blood flow seen in patients with primary open-angle glaucoma.

Optic nerve head blood flow response to reduced ocular perfusion pressure by alteration of either the blood pressure or intraocular pressure

PURPOSE

To test the hypothesis that blood flow autoregulation in the optic nerve head has less reserve to maintain normal blood flow in the face of blood pressure-induced ocular perfusion pressure decrease than a similar magnitude intraocular pressure-induced ocular perfusion pressure decrease.

MATERIALS AND METHODS

Twelve normal non-human primates were anesthetized by continuous intravenous infusion of pentobarbital. Optic nerve blood flow was monitored by laser speckle flowgraphy. In the first group of animals (n = 6), the experimental eye intraocular pressure was maintained at 10 mmHg using a saline reservoir connected to the anterior chamber. The blood pressure was gradually reduced by a slow injection of pentobarbital. In the second group (n = 6), the intraocular pressure was slowly increased from 10 mmHg to 50 mmHg by raising the reservoir. In both experimental groups, optic nerve head blood flow was measured continuously. The blood pressure and intraocular pressure were simultaneously recorded in all experiments.

RESULTS

The optic nerve head blood flow showed significant difference between the two groups (p = 0.021, repeat measures analysis of variance). It declined significantly more in the blood pressure group compared to the intraocular pressure group when the ocular perfusion pressure was reduced to 35 mmHg (p < 0.045) and below. There was also a significant interaction between blood flow changes and the ocular perfusion pressure treatment (p = 0.004, adjusted Greenhouse & Geisser univariate test), indicating the gradually enlarged blood flow difference between the two groups was due to the ocular perfusion pressure decrease.

CONCLUSIONS

The results show that optic nerve head blood flow is more susceptible to an ocular perfusion pressure decrease induced by lowering the blood pressure compared with that induced by increasing the intraocular pressure. This blood flow autoregulation capacity vulnerability to low blood pressure may provide experimental evidence related to the hemodynamic pathophysiology in glaucoma.

Longitudinal alterations in the dynamic autoregulation of optic nerve head blood flow revealed in experimental glaucoma

PURPOSE

To use a novel dynamic autoregulation analysis (dAR) to test the hypothesis that the optic nerve head (ONH) blood flow (BF) autoregulation is disrupted during early stages of experimental glaucoma (EG) in nonhuman primates.

METHODS

Retinal nerve fiber layer thickness (RNFLT, assessed by optical coherence tomography) and ONH BF (assessed by laser speckle imaging technique) were measured biweekly before and after unilateral laser treatment to the trabecular meshwork. Each nonhuman primate was followed until reaching either an early stage of damage (RNFLT loss < 20%, n = 6) or moderate to advanced stages of damage (RNFLT loss > 20%, n = 9). At each test, dAR was assessed by characterizing ONH BF changes during the first minute of rapid manometrical intraocular pressure (IOP) elevation from 10 to 40 mm Hg. The dAR analysis extracted the following parameters: baseline BF, average BF 10 seconds before IOP elevation; BFΔmax, maximum BF change from baseline BF; Tr, time from baseline BF to the BFΔmax; Kr, average descending BF rate.

RESULTS

Mean postlaser IOP was 20.2 ± 5.9 and 12.3 ± 2.6 mm Hg in EG and control eyes, respectively (P < 0.0001). Compared with prelaser values, baseline BF was higher in early EG, but lower in moderate to advanced EG (P = 0.01). Tr was increased and Kr was reduced in both stages (P < 0.01). BFΔmax was smaller in the early EG (P = 0.05) and remained low in the moderate to advanced EG (P = 0.15). No changes in the parameters were observed in control eyes.

CONCLUSIONS

Chronic IOP elevation causes ONH autoregulation dysfunction in the early stage of EG, characterized by a disrupted BF response and delayed Tr, revealed by dAR analysis.

Static blood flow autoregulation in the optic nerve head in normal and experimental glaucoma

PURPOSE

To characterize the static blood flow autoregulation in the optic nerve head (ONH), and to investigate its role in hemodynamic changes in experimental glaucoma (EG).

METHODS

Unilateral elevation of intraocular pressure (IOP) was induced in 15 adult rhesus macaques by laser treatment to the trabecular meshwork. Prior to and after laser treatment, retinal nerve fiber layer thickness (RNFLT) was assessed, biweekly, by spectral-domain optical coherence tomography. Optic nerve head static autoregulation was assessed by determining the percentage blood flow (BF) change after the IOP was acutely increased from 10 to 30, 40, or 50 mm Hg manometrically, utilizing a laser speckle flowgraphy device.

RESULTS

Postlaser IOP (measured during average 7.7 ± 2.6 months) was 20.2 ± 5.9 mm Hg in EG eyes and 12.3 ± 2.6 mm Hg in control eyes (P < 0.0001). Retinal nerve fiber layer thickness was reduced by 33 ± 22% of the baseline values (P < 0.001) on average in EG eyes and by 0.4 ± 2.3% in control eyes (P > 0.05). The ONH BF remained at a constant level within a range of ocular perfusion pressure (OPP), 41 mm Hg and above. The autoregulation curves, created by all 723 tests in control and 352 tests in EG, were not significantly different (P = 0.71).

CONCLUSIONS

Optic nerve head BF in normal nonhuman primate (NHP) eyes is effectively regulated within a range of OPP approximately 41 mm Hg and above. Chronic IOP elevation causes no remarkable change to the static autoregulation within the ONH of EG eyes.

Longitudinal hemodynamic changes within the optic nerve head in experimental glaucoma

PURPOSE

To characterize longitudinal changes in basal blood flow (BF) of the optic nerve head (ONH) during progression of structural damage in experimental glaucoma (EG).

METHODS

Unilateral elevation of IOP was induced in 15 adult rhesus macaques by laser treatment to the trabecular meshwork. Prior to and after laser, retinal nerve fiber layer thickness (RNFLT) and ONH BF were measured biweekly by spectral-domain optical coherence tomography and a laser speckle flowgraphy device (LSFG), respectively.

RESULTS

Average postlaser IOP was 20.2 ± 5.9 mm Hg in EG eyes and 12.3 ± 2.6 mm Hg in control eyes (P < 0.0001). Longitudinal changes in basal ONH BF were strongly associated with changes in RNFLT as EG progressed from early through moderately advanced stages of damage, with Pearson correlation coefficients ranging from 0.64 to 0.97 (average = 0.81) and an average slope of 1.0. During early stage (RNFLT loss < 10%), basal ONH BF was mildly increased (9% ± 10%, P = 0.004) relative to baseline and compared with fellow controls (P = 0.02). Basal ONH BF declined continuously throughout subsequent stages in EG eyes reaching 25.0% ± 9.6% (P < 0.0001) below baseline at the final stage studied (RNFLT loss > 40%). In fellow control eyes, there was no significant change in basal ONH BF over time (P = 0.27).

CONCLUSIONS

In EG based on chronic mild-to-moderate IOP elevation, a two-phase pattern of ONH BF alteration was observed. ONH BF increased during the earliest stage (while RNFLT was within 10% of baseline) followed by a linear decline that was strongly correlated with loss of RNFLT.

Factors associated with topographic changes of the optic nerve head induced by acute intraocular pressure reduction in glaucoma patients

PURPOSE

To investigate factors associated with changes in optic nerve head (ONH) topography after acute intraocular pressure (IOP) reduction in patients with primary open-angle glaucoma (POAG).

METHODS

Untreated POAG patients (IOP >21 mm Hg) were prospectively enrolled. Systemic and ocular information were collected, including central corneal thickness (CCT) and corneal hysteresis (CH). All patients underwent confocal scanning laser ophthalmoscopy and tonometry (Goldmann) before and 1 h after pharmacological IOP reduction. The mean of three measurements was considered for analysis. Changes in each ONH topographic parameter were assessed (one eye was randomly selected), and those that changed significantly were correlated with patient's systemic and ocular characteristics.

RESULTS

A total of 42 patients were included (mean age, 66.7 ± 11.8 years). After a mean IOP reduction of 47.3 ± 11.9%, significant changes were observed in cup area and volume, and in rim area and volume (P < 0.01), but not in mean cup depth (P = 0.80). Multiple regression analysis (controlling for baseline IOP and magnitude of IOP reduction) showed that CH (r(2) = 0.17, P < 0.01) and diabetes diagnosis (r(2) ≥ 0.21, P < 0.01) were negatively correlated with the magnitude of changes in ONH parameters, whereas the cup-to-disc ratio was positively correlated (r(2) = 0.30, P < 0.01). Age, race, disc area, and CCT were not significant (P ≥ 0.12). Including all significant factors in a multivariable model, only the presence of diabetes remained significantly associated with all ONH parameters evaluated (P < 0.01).

CONCLUSIONS

Different systemic and ocular factors, such as diabetes, CH, and the relative size of the cup, seem to be associated with the magnitude of changes in ONH topography after acute IOP reduction in POAG patients. These associations partially explain the ONH changes observed in these patients and suggest that other factors are possibly implicated in an individual susceptibility to IOP.

Laminar and prelaminar tissue displacement during intraocular pressure elevation in glaucoma patients and healthy controls

OBJECTIVE

To determine the response of the anterior lamina cribrosa and prelaminar tissue to acute elevation of intraocular pressure (IOP) in glaucoma patients and healthy subjects.

DESIGN

Prospective case-control series.

PARTICIPANTS AND CONTROLS

Patients with open-angle glaucoma (n = 12; mean age ± standard deviation [SD], 66.8 ± 6.0 years), age-matched healthy controls (n = 12; mean age ± SD, 67.1 ± 6.2 years), and young controls (n = 12; mean age ± SD, 36.1 ± 11.7 years).

METHODS

One eye was imaged with spectral-domain optical coherence tomography to obtain 12 high-resolution radial scans centered on the optic disc. Imaging was repeated at precisely the same locations with an ophthalmodynamometer held perpendicular to the globe via the inferior lid to raise the IOP. A line joining Bruch's membrane opening in 4 radial scans was used as reference in the baseline and elevated IOP images. The vertical distance from the reference line to the anterior prelaminar tissue surface and anterior laminar surface was measured at equidistant points along the reference line in the 2 sets of images. The difference between the 2 sets of corresponding measurements were used to determine laminar displacement (LD) and prelaminar tissue displacement (PTD).

MAIN OUTCOME MEASURES

Laminar displacement and PTD.

RESULTS

Intraocular pressure elevation among patients, age-matched controls, and young controls was similar (mean ± SD, 12.4 ± 3.2 mmHg). The mean ± SD LD and PTD were 0.5 ± 3.3 μm and 15.7 ± 15.5 μm, respectively. The LD was not statistically different from 0 (P = 0.366), but PTD was (P < 0.001). The mean ± SD LD was similar among the groups (-0.5 ± 3.7 μm, 0.2 ± 2.0 μm, and 2.0 ± 3.6 μm, respectively; P = 0.366), whereas the mean ± SD PTD was different (6.8 ± 13.7 μm, 20.8 ± 17.5 μm, and 19.6 ± 11.8 μm, respectively; P = 0.045). In all subjects, the PTD was greater than LD. In multivariate regression analyses, LD was negatively associated with optic disc size (P = 0.007), whereas PTD was positively associated with the degree of IOP elevation (P = 0.013).

CONCLUSIONS

In glaucoma patients and controls, the anterior laminar surface is noncompliant to acute IOP elevation. Acute optic disc surface changes represent compression of prelaminar tissue and not laminar displacement.

Mouse genetic models: an ideal system for understanding glaucomatous neurodegeneration and neuroprotection

Here we review how mouse studies are contributing to understanding glaucoma. We include discussion of aqueous humor drainage and intraocular pressure elevation, because new treatments to avoid exposure to high pressure will indirectly protect neurons from glaucoma, and complement direct neuroprotective strategies. We describe how mouse models are adding to both the understanding of glaucomatous neurodegeneration and the development of neuroprotective strategies.

Axons of retinal ganglion cells are insulted in the optic nerve early in DBA/2J glaucoma

Here, we use a mouse model (DBA/2J) to readdress the location of insult(s) to retinal ganglion cells (RGCs) in glaucoma. We localize an early sign of axon damage to an astrocyte-rich region of the optic nerve just posterior to the retina, analogous to the lamina cribrosa. In this region, a network of astrocytes associates intimately with RGC axons. Using BAX-deficient DBA/2J mice, which retain all of their RGCs, we provide experimental evidence for an insult within or very close to the lamina in the optic nerve. We show that proximal axon segments attached to their cell bodies survive to the proximity of the lamina. In contrast, axon segments in the lamina and behind the eye degenerate. Finally, the Wld(s) allele, which is known to protect against insults to axons, strongly protects against DBA/2J glaucoma and preserves RGC activity as measured by pattern electroretinography. These experiments provide strong evidence for a local insult to axons in the optic nerve.

Pathological optic-disc cupping

PURPOSE OF REVIEW

Pathological optic-disc cupping is most often caused by glaucoma, but may be seen in many less-common neuro-ophthalmic conditions. The goal of this article is to examine a host of entities causing optic-disc cupping, present key differentiating characteristics and pathophysiologies, and outline diagnostic approaches.

RECENT FINDINGS

Multiple entities not associated with elevated intraocular pressure or glaucomatous optic-nerve disease may result in pathologic optic-nerve excavation. Even with the photography and imaging of today, it is still difficult for the clinician to accurately diagnose other causes of optic-disc cupping. Up to 20% of patients may be misdiagnosed and treated for glaucoma due to misinterpretation of the optic-disc cupping. Newer forms of imaging including optical coherence tomography may assist the clinician in decision making. A scrutinizing history, close observation of disc appearance, and the vasculature will aid in the diagnosis of glaucoma or other entity of optic-disc cupping.

SUMMARY

Optic-disc cupping is a consequence of myriad disorders. Knowledge of the anatomy and vasculature of the disc is quintessential to the understanding of how, why, when, and what type of optic-disc cupping occurs in various conditions. Cupping can be seen with neurological processes, including benign tumors, which are treatable. Patient history, visual fields assessment, and funduscopic findings are the key to unlocking the diagnosis of glaucomatous versus nonglaucomatous optic-disc cupping. As clinicians, we must remain vigilant and receptive to the findings of potentially ominous forms of nonglaucomatous optic-disc cupping.

Effects of glaucoma drugs on ocular hemodynamics in normal tension glaucoma: a randomized trial comparing bimatoprost and latanoprost with dorzolamide [ISRCTN18873428]

Background

Reduced choroidal perfusion is hypothesized to play a role in the pathogenesis of normal tension glaucoma. Thus the impact of antiglaucomatous eye drops on ocular perfusion has been the focus of recent research and the subject of intensive investigations. The present study investigates whether topically applied latanoprost or bimatoprost influence ocular perfusion in patients with normal tension glaucoma and compares these effects with that changes detected after the treatment with dorzolamide.

Methods

Ocular hemodynamics were assessed by color Doppler imaging (CDI) shortly before and after a one-month treatment with either latanoprost, bimatoprost or dorzolamide. Primary end-points of the study were peak systolic and end-diastolic blood flow velocities in the short posterior ciliary artery (SPCA) under the new therapy. Intraocular pressure (IOP) and additional perfusion parameters in the SPCA and other retrobulbar vessels were tracked as observational parameters. n = 42 patients with normal tension glaucoma were enrolled in the study.

Results

Systolic and diastolic blood flow velocities in the SPCA showed no significant alteration after the treatment with latanoprost or bimatoprost. Dorzolamide lead to increase of peak systolic velocity. IOP was reduced by all three agents in a range reported in the literature.

Conclusion

Topically applied latanoprost and bimatoprost act in a hemodynamically neutral manner and have the capability to lower IOP even in patients with normal tension glaucoma and low initial IOP level. Dorzolamide accelerates blood flow in systole. None of the tested compounds has a negative impact on hemodynamics in the short posterior ciliary arteries.

Reproducibility of blood flow velocity measurements using colour decoded Doppler imaging

BACKGROUND

It is taken for granted that glaucomatous damage is caused by changed haemodynamics of the retrobulbar vessel system besides other factors such as, for example, an elevated intraocular pressure. This was proven by various studies in which glaucoma patients were shown to have a changed retrobulbar blood flow velocity. In this study, the reliability of measurements of retrobulbar vessel perfusion by colour decoded Doppler imaging (CDI) was evaluated.

PATIENTS AND METHODS

A total of 18 healthy volunteers and 15 patients with various glaucoma types were enrolled in this study. Using a CDI system, type Siemens Sonoline Elegra with a combined applicator (7.5L40), retrobulbar vessel perfusions of the ophthalmic artery, the short posterior ciliary arteries, and the long posterior ciliary arteries of each patient were measured six times. In each measurement, pulse amplitude, end-diastolic velocity, maximum systolic velocity, pulsatility index, and resistivity index of the vessels were determined. The reproducibility of measurements was evaluated by the calculation of the intraclass correlation coefficient (ICC) for each parameter.

RESULTS

The ICCs for the ophthalmic artery varied from 0.89 to 0.98, for the short posterior ciliary artery from 0.75 to 0.91, and for the long posterior ciliary artery from 0.77 to 0.99 in both the groups.

CONCLUSIONS

The ICCs of the repeated measurements reflect a good reproducibility for both the groups with assumed different retrobulbar perfusion. These findings are prerequisites for the use of CDI in clinical practice and research.

Estimation of choroid perfusion by colour Doppler imaging vs. other methods

Ocular haemodynamics play a prominent role in several ocular diseases. Recently, new methods for the determination of ocular perfusion were developed. Colour Doppler imaging (CDI) of orbital vessels has come up in the past decade and was shown to be useful in ophthalmological diagnostics. Little is known about measurement of choroid perfusion by CDI in comparison with other methods. Therefore, 49 eyes were examined with CDI, laser Doppler flowmetry (LDF) and the method of Langham (LOBF). Correlations between the methods were identified by the Spearman correlation coefficient (r). LDF readings correlated with time-averaged mean velocity assessed by CDI in the long posterior ciliary artery (r = 0.47; p = 0.039; n = 20), but not in the short posterior ciliary artery. LOBF measurements correlated with pulsatility index (PI) of CDI in short (r = 0.50; p = 0.005; n = 30) and long posterior ciliary arteries (r = 0.41; p = 0.024; n = 30). Methods strengthened each other by partial correlation. The study demonstrates that CDI allows a more detailed insight into ocular perfusion.

Drug delivery to the posterior segment from drops

The published evidence that instilled drugs can affect the blood supply to the retina and optic nerve head in humans is examined. As a background, seven techniques that have been used to measure flow are briefly described and criticized. For timolol, the corresponding measurements, obtained by a number of investigators are evaluated. The outcome is very erratic and does not allow any conclusion as to the effect of this drug on flow. Consideration is then given to the possible mechanism whereby a drug could affect blood flow; directly, by diffusion to receptors on the vessels, or indirectly, through more anterior receptors. The question is raised whether the small changes in circulation induced by drugs would not be swamped by those resulting from natural alterations in the ambient light level. The literature was analyzed in the hope of identifying discrete entry pathways, for example, through the lens or the suprachoroidal space, that are sufficiently permeable to allow a significant quantity of drug to pass. There was an indication that a drug might diffuse through the lens cortex in sufficient quantity to cause a measurable rise in its concentration in the vitreous. In general, however, there was insufficient quantitative data to allow any meaningful predictions to be made. Stimulated by recent evidence, it is suggested that drug penetration from the tear fluid takes place by direct diffusion across the conjunctiva into the sclera and orbit when the head is supine.

Use of a mathematical model to estimate stress and strain during elevated pressure induced lamina cribrosa deformation

BACKGROUND

High intraocular pressure (IOP), which is generally associated with glaucoma, causes lamina cribrosa retrodisplacement and deformation. Shear stress and strain resulting from lamina cribrosa deformation have been implicated in tissue remodeling, changes in retinal astrocyte function and retinal ganglion cell (RGC) death observed in vivo during glaucoma.

METHODS

A mathematical model was developed to describe the lamina cribrosa exposed to elevated intraocular pressure (IOP). The model is based on the bending theory of plates, incorporates anatomical properties of the lamina cribrosa, and provides estimates of its biomechanical properties. The model relates IOP, the parameter normally correlated with glaucoma, and lamina cribrosa retrodisplacement to stress and strain experienced by cells, parameters that may be more closely associated with cell injury.

RESULTS

We estimate that shear strains of 0.05 occur at the edge of a 200 microm thick lamina cribrosa at an IOP of 25 mm Hg. We estimate greater lamina cribrosa deformation and higher shear stress and strain for thinner lamina cribrosa and lamina cribrosa of larger radii.

CONCLUSION

These results may provide better estimates of the stress and strain experienced by cells in the lamina cribrosa and may further our understanding of the forces that contribute to optic nerve degeneration during glaucoma.

Nonglaucomatous cupping of the optic disc

Optic disc cupping is a consequence of myriad disorders. The anatomy and vasculature of the disc provide great insight into why, how, and when ODC occurs in various conditions. Approaches to distinguish glaucomatous from nonglaucomatous causes of ODC should rely on patient history, visual fields assessment, and funduscopic findings, as described. Cupping can be seen with neurological processes, including benign tumors, that are treatable. The clinician must remain vigilant to detect uncommon but potentially threatening forms of nonglaucomatous optic disc cupping.

Assessment of human ocular hemodynamics

Vascular abnormality and altered hemodynamics play important roles in many ophthalmic pathologies. Much of our knowledge of ocular hemodynamics was gained from invasive animal research, although a number of noninvasive methods suitable for in vivo use in humans have been developed. Data from these methods now produce a significant literature of their own. Understanding the origins of the data and appreciating their limitations can be difficult. Modern hemodynamic assessment techniques each examine a unique facet of the ocular circulation. No single facet provides a complete description of the hemodynamic state of the eye. These methods have contributed a great deal to our understanding of normal hemodynamics. More importantly, they continue to add to our understanding of altered hemodynamics found in disease. Some have found their way into limited clinical practice. The predominant ocular hemodynamic assessment techniques are reviewed with the aims of introducing the fundamental principles behind each, highlighting their inherent advantages and limitations, highlighting their contributions to understanding ocular physiology, and considering their potential to provide signs for diagnosis.

In vivo morphometry of the lamina cribrosa and its relation to visual field loss in glaucoma

PURPOSE

The lamina cribrosa has been proposed as a site of origin of the optic nerve damage in glaucoma. The purpose of this study was to investigate, in vivo, the clinical features of the lamina cribrosa pores of glaucomatous patients and to relate their morphometric characteristics to the extent of their visual field loss.

METHODS

Images of the internal lamina cribrosa surface of 60 glaucomatous patients and 15 normal subjects were acquired, in vivo, using a scanning laser ophthalmoscope (SLO). A purposely developed technique of image processing was employed to objectively evaluate pore morphometry, with particular regard to their geometrical characteristics (circularity and elongation). Visual function was assessed by automated perimetry (Humphrey Field Analyser).

RESULTS

Normal subjects showed approximately round lamina pores. In glaucomatous patients, pores become more elongated and less circular with increasing field loss (p = 0.009 and p < 0.001, respectively).

CONCLUSIONS

Scanning laser ophthalmoscopy and a new technique of image processing were employed, for the first time, to the investigation in vivo of the lamina cribrosa of glaucomatous patients, in relation to the extent of visual field loss. The results indicated differences in the lamina cribrosa pore morphometry associated with increasing severity of the disease. These changes may represent the result of compressing and shearing forces applied to the laminar plates.

Pattern electroretinogram and spatial contrast sensitivity in primary congenital glaucoma

OBJECTIVE

The authors investigated the temporal and spatial characteristics of pattern electroretinogram (PERG) and spatial contrast sensitivity (CS) in primary congenital glaucoma (PCG) to determine whether the PERG and CS could be useful tools in the diagnosis of childhood glaucoma, especially PCG.

PARTICIPANTS

The PERGs were evaluated in eyes from ten patients with PCG and nine age-matched visually normal subjects.

INTERVENTION

All patients received complete ophthalmologic evaluations including visual field testing.

MAIN OUTCOME MEASURES

The PERGs were recorded using phase-alternating (2, 4, and 16 reversals per second [rps]) checkerboard patterns (30' and 60' checks).

RESULTS

The patients with PCG exhibited decreased CS when compared with that of control subjects. Significant PERG deficits also were detected in these patients. However, PERG amplitude in patients with PCG almost reached control subject levels at high (16 rps) temporal frequency. This was true for both 30' and 60' checks. Taken together, these observations on PERG amplitude suggest a more important deficiency of the neural response of the retinal cells at lower temporal frequency (rps) in patients with PCG. This is unlike primary open-angle glaucoma (POAG) in which significant PERG deficits are observed at high temporal frequencies.

CONCLUSIONS

The PERG amplitude is reduced in patients with PCG, and this is consistent with a loss of CS and visual field changes in these patients. However, the spatiotemporal characteristics of the PERG deficits in PCG differ from those of POAG. This could suggest a difference in the mechanisms mediating retinal ganglion cell dysfunction in the two types of glaucoma.

Deformation of the lamina cribrosa by elevated intraocular pressure

The purpose of this study was to determine the mechanical response of the lamina cribrosa (LC) to elevated intraocular pressure (IOP) so as to identify possible mechanisms of optic nerve damage in early glaucoma. Ten pairs of normal human eyes were fixed after 24 hours' exposure to 50 mm Hg pressure (experimental eyes) or 5 mm Hg pressure (contralateral control eyes). Photomicrographs of the central region of the optic nerve head (ONH) were taken to examine the LC morphologically and to measure the dimensions of the LC. It was found that elevated IOP caused the LC to deflect posteriorly without affecting its thickness. The majority of the posterior displacement in the LC occurred near the periphery of the ONH. This shape change is consistent with a model of force distribution within the LC in which shear stresses are dominant; such stresses are maximal at the periphery and minimal at the centre of the ONH. These findings support a model in which mechanical forces, specifically shearing stresses within the peripheral lamina, play a direct role in the pathology of glaucomatous optic neuropathy.

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.

Preservation of nerve fiber layer by retinal vessels in glaucoma

The authors evaluated the correlation between various parameters and the local preservation of the retinal nerve fiber layer in 156 glaucomatous eyes. A vessel-associated preservation of the nerve fiber layer was observed in 45 of the 156 glaucomatous eyes. The presence of "straight" retinal vessels (either arterioles or large venules) and "tortuous" retinal vessels (large or small venules) inside of the scleral ring was correlated with the local preservation of the nerve fiber layer (P less than 0.001 and P less than 0.05, respectively). A local elevation of the floor of the cup was also correlated with the preservation of the nerve fiber layer (P less than 0.01). However, no correlation existed between either the preservation of the nerve fiber layer and the type of glaucoma, sex or age of patient, tilting of the disc, cilioretinal vessel, vertical cup-to-disc ratio, refractive error, disc size, distance between the disc and foveola, or the index of ovalness of the disc. These results suggest that retinal vessels in the disc significantly influence the vulnerability of the nerve fibers to glaucomatous damage.

Visual impairment of open angle glaucomas at first presentation and after a five to ten year follow-up

Of all patients attended to in a clinic during 1986, 441 had open angle glaucoma diagnosed during 1974-1986 on the basis of either a verified visual field defect, a glaucomatous disc, or a repeated intraocular pressure value of at least 35 mm Hg. At first presentation of recent cases 1984-1986 (N = 128) 65 per cent were more than 70 years old. Capsular glaucomas were twice as common as simple glaucomas (low tension cases included). 62 per cent of capsular but only 26 per cent of simple glaucomas had an initial pressure of 35 mm Hg or more (p < 0.001). One third of both capsular and simple glaucomas had an advanced visual field defect with breakthrough to the periphery in the worse eye already at first presentation. This was more common if the initial pressure was 35 mm Hg or more (p < 0.05). Almost half remained unilateral cases, and the rate of severely impaired visual function in the better eye did not exceed 15 per cent. While generally 30-50 per cent of glaucomatous field defects had progressed in five years, the progression in early detected cases was only three per cent (p < 0.05). Visual field defects with breakthrough to the periphery already at first presentation progressed more often than circumscribed scotomas (p < 0.02).

Acquired pits of the optic nerve. Increased prevalence in patients with low-tension glaucoma

The authors studied 232 glaucoma patients in an attempt to shed light on the implications of the pit-like localized cupping of the optic nerve often referred to as an "acquired pit of the optic nerve" (APON) both in low-tension glaucoma and in glaucoma associated with elevated intraocular pressure (IOP). Twenty (74%) of the 27 patients with low-tension glaucoma had APONs, whereas 31 (15%) of the 232 patients with elevated pressure did (P less than 0.001). Overall, there was no difference between the degree of field loss in patients with APONs and in those without. We suggest that APONs may signal an abnormal susceptibility of the optic nerve to the damaging effects of IOP.

Reversible optic disk cupping and visual field improvement in adults with glaucoma

To assess change over time in the optic disk and the associated change in visual field, we retrospectively reviewed perimetry results and optic disk photographs of adult patients treated for glaucoma. Three glaucoma specialists masked to the clinical course independently reviewed optic disk stereophotographs and visual fields taken before and after treatment. Each member of each pair of stereophotographs or perimetry results, which were presented in a random sequence, were graded as better, worse, or unchanged. Of the 75 eyes, 16 (21%) showed a reversal of optic disk cupping; 20 of the 63 visual fields (31%) were classified as improved. In six of 17 eyes (35%) with a change in the disk (better or worse), a corresponding change was also noted in the visual field. The frequency of improvement in optic disk and visual field appearance was significantly (P less than .05) associated with the degree of intraocular pressure reduction.

Results of a filtering procedure in low tension glaucoma

Twenty-six eyes of twenty patients with established low tension glaucoma, who had either a 'double flap' Scheie filtering operation or a trabeculectomy, were followed over a period ranging from one to ten years, with a median of three years. These 26 eyes include two second eyes that served as a contralateral control eye until the visual field deteriorated. All 26 eyes showed progression of visual field defects preoperatively, while postoperatively only 2 eyes showed further progression. Of the 16 nonoperated contralateral eyes, 7 showed progression of visual field defects over the same follow-up period. Two of these 7 eyes had to be operated during the study-period. The difference in progression between the operated and the non-operated eyes was significant (P less than 0.01). The filtering procedure provided a reduction in IOP of 20% or more in 21 of 26 eyes. There was a significant difference in the mean intraocular pressure (IOP) between the operated and the non-operated contralateral eye of 6.8 mmHg (37%) at one year postoperatively. The diurnal variation decreased highly significantly from 4.3 to 2.1 mmHg (P less than 0.001). It is concluded that filtering surgery in low tension glaucoma may result not only in a significant lowering of IOP, but is also effective in slowing further deterioration of the visual fields.

Pattern reversal electroretinogram (PRERG) abnormalities in ocular hypertension: correlation with glaucoma risk factors

The indices employed commonly for the diagnosis of glaucoma (tonometry, ophthalmoscopy and perimetry) do not always identify which patients with ocular hypertension (OHT) will develop primary open-angle glaucoma (POAG) before irreversible visual field loss is manifest (1). The human pattern reversal electroretinogram (PRERG) is a bioelectric response reflecting neural activity of the proximal retina. PRERG amplitude reductions have been observed in POAG and other diseases affecting the optic nerve and retinal ganglion cells. This study was designed to determine whether OHT patients exhibit PRERG amplitude reductions and whether PRERG results are correlated with routinely evaluated clinical parameters. Steady-state PRERG (16 rps) were elicited by high contrast (76%), phase alternating checkerboard patterns (15-20 min checks) from one eye of 130 patients with ocular hypertension and 47 age matched visual normals (AMVNs). A significant (p less than 0.05) reduction in PRERG amplitude was noted for the OHT patients and 11.5% of those patients exhibited PRERG amplitudes more than 2.0 standard deviations below the AMVN mean. PRERG amplitude was found to be positively correlated with diastolic blood pressure (DBP) and negatively correlated with age, but no correlation between PRERG amplitude and either IOP, C/D ratio, or systolic blood pressure was evident. The lack of correlation between PRERG amplitude and the commonly used clinical indices may suggest a complementary role for this neurophysiologic test in determining which OHT patients will develop glaucoma.

Optic disc cupping in arteritic anterior ischemic optic neuropathy resembles glaucomatous cupping

Five cases of anterior ischemic optic neuropathy secondary to biopsy-proven giant cell arteritis are presented. In each case, cupping of the optic disc, which closely resembled glaucomatous cupping, was observed in the affected eye. The presence of glaucoma was ruled out on the basis of normal intraocular pressures and normal tonographic measurements of facility of outflow. These cases indicate that arteritic ischemic optic neuropathy can result in optic disc cupping, which closely resembles glaucomatous cupping. The similarities in the appearance of cupping of these discs with that seen in eyes with glaucoma suggest that the pathogenesis of cupping in glaucoma and in arteritic ischemic optic neuropathy may share some common mechanisms.

Inter-individual variation in blood supply of the optic nerve head. Its importance in various ischemic disorders of the optic nerve head, and glaucoma, low-tension glaucoma and allied disorders

There is no one standard pattern of the blood supply of the optic nerve head in all human eyes. There is a marked inter-individual variation in the blood supply of the optic nerve head, and the various factors which produce this include variations in (I) the anatomical pattern of blood supply, (II) the pattern of posterior ciliary artery (PCA) circulation (the main source of blood supply to the optic nerve head), and (III) the blood flow. The variations in the pattern of PCA circulation include the variations in (a) number of PCAs supplying an eye, (b) area of supply to the optic nerve head by each PCA, (c) location of the watershed zones between the various PCAs in relation to the optic nerve head, and (d) blood pressure in various PCAs as well as short PCAs. The variations in the blood flow in the optic nerve head can be produced by changes in (i) the intraocular pressure, (ii) mean blood pressure in the capillaries of the optic nerve head and (iii) peripheral vascular resistance. These variations are discussed in detail. A lack of appreciation of these complexities of the blood supply of the optic nerve head in health and disease is responsible for many of the current problems in the understanding of the role of vascular disturbances in anterior ischemic optic neuropathy, glaucoma, low-tension glaucoma and various ischemic disorders of the optic nerve head.