Optic Nerve Head Surface Research Articles

3-D Histomorphometry of the Normal and Early Glaucomatous Monkey Optic Nerve Head: Lamina Cribrosa and Peripapillary Scleral Position and Thickness

To delineate three-dimensionally the anterior and posterior surfaces of the lamina cribrosa, scleral flange, and peripapillary sclera, to determine the position and thickness of these structures within digital three-dimensional (3-D) reconstructions of the monkey optic nerve head (ONH). The trephinated ONH and peripapillary sclera from both eyes of three monkeys with early glaucoma (EG; one eye normal, one eye given laser-induced EG) were serially sectioned at 3-mum thickness, with the embedded tissue block's face stained and imaged after each cut. Images were aligned and stacked to create 3-D reconstructions, within which Bruch's membrane opening (BMO) and the anterior and posterior surfaces of the lamina cribrosa and peripapillary sclera were delineated in 40 serial radial (4.5 degrees interval) digital sagittal sections. For each eye, a BMO zero reference plane was fit to the 80 BMO points, which served as the reference from which all position measurements were made. Regional laminar, scleral flange, and peripapillary scleral position and thickness were compared between the normal and EG eyes of each monkey and between treatment groups by analysis of variance. Laminar thickness varied substantially within the normal eyes and was profoundly thicker within the three EG eyes. Laminar position was permanently posteriorly deformed in all three EG eyes, with substantial differences in the magnitude and extent of deformation among them. Scleral flange and peripapillary scleral thickness varied regionally within each normal ONH with the scleral flange and peripapillary sclera being thinnest nasally. Overall, the scleral flange and peripapillary sclera immediately surrounding the ONH were posteriorly displaced relative to the more peripheral sclera. Profound fixed posterior deformation and thickening of the lamina are accompanied by mild posterior deformation and thinning of the scleral flange and peripapillary sclera at the onset of confocal scanning laser tomography (CSLT)-detected ONH surface change in young adult monkey eyes with early experimental glaucoma.

Three-Dimensional Histomorphometry of the Normal and Early Glaucomatous Monkey Optic Nerve Head: Neural Canal and Subarachnoid Space Architecture

To delineate three dimensionally the neural canal landmarks-Bruch's membrane opening (BMO), anterior sclera canal opening (ASCO), anterior laminar insertion (ALI), posterior laminar insertion (PLI), and posterior scleral canal opening (PSCO)-and the anterior-most aspect of the subarachnoid space (ASAS), within digital three-dimensional (3-D) reconstructions of the monkey optic nerve head (ONH). The trephinated ONH and peripapillary sclera from both eyes of three early glaucoma (EG) monkeys (one eye normal, one eye with laser-induced EG) were serial sectioned at 3-microm thickness, with the embedded tissue block face stained and imaged after each cut. The images were aligned and stacked in a 3-D volume, within which the BMO, ASCO, ALI, PLI, PSCO, and ASAS were delineated in 40 digital, radial, and sagittal sections. An ellipse was fitted to the 80 BMO points to establish a BMO zero reference plane, on which all other points were projected. The distance from each projected point to the BMO centroid (offset) and BMO zero reference plane (depth) were calculated and compared regionally between normal and EG eyes, both overall and within each monkey, by analysis of variance. BMO was the clinically visible optic disc margin in all six eyes. The neural canal architecture was highly variable in the three normal eyes. Radial expansion of the neural canal was greatest posteriorly in the EG eyes. Axial elongation of the canal was less pronounced overall but was regionally present within all three EG eyes. ASAS was regionally radially expanded and anteriorly displaced within two of the three EG eyes. Profound deformation of the neural canal and ASAS architecture are present in young adult monkey eyes at the onset of ONH surface change in early experimental glaucoma.

Three-Dimensional Reconstruction of Normal and Early Glaucoma Monkey Optic Nerve Head Connective Tissues

To introduce high-resolution, digital three-dimensional (3-D) reconstruction of the connective tissues of the optic nerve head (ONH). Trephinated ONH and peripapillary sclera from both eyes of three monkeys with early glaucoma (EG; one eye normal, one eye given laser-induced EG) were embedded in paraffin and serial sectioned at 3-mum thickness from the vitreous surface through the orbital optic nerve, with the embedded tissue block face stained and imaged after each cut. Each image was aligned, and then the scleral canal wall, sclera, border tissue of Elschnig, Bruch's membrane, lamina cribrosa, optic nerve septa, pial sheath, and vasculature were delineated as unique objects. Delineated images were stacked, color mapped, and volume rendered and then serial sagittal and transverse digital sections of the resultant voxel geometries were viewed and measured. Substantial differences in the 3-D architecture of the peripapillary sclera, scleral canal wall, and lamina cribrosa were present among the three normal eyes. All three EG eyes displayed permanent posterior deformation of the central lamina cribrosa, as well as expansion of the anterior and posterior neural canal openings in comparison with their respective contralateral normal control eyes. Peripherally, whereas laminar deformation was greatest inferiorly or superiorly in all three EG eyes, statistically significant deformation was present in all four quadrants of all three eyes. High-resolution, digital 3-D reconstructions of the load-bearing connective tissues of the monkey ONH confirm that the ONH connective tissues are profoundly altered at the onset of detectable ONH surface change in experimental glaucoma.

Image analysis of optic nerve disease

Existing methodologies for imaging the optic nerve head surface topography and measuring the retinal nerve fibre layer thickness include confocal scanning laser ophthalmoscopy (Heidelberg retinal tomograph), optical coherence tomography, and scanning laser polarimetry. For cross-sectional screening of patient populations, all three approaches have achieved sensitivities and specificities within the 60-80th percentile in various studies, with occasional specificities greater than 90% in select populations. Nevertheless, these methods are not likely to provide useful assistance for the experienced examiner at their present level of performance. For longitudinal change detection in individual patients, strategies for clinically specific change detection have been rigorously evaluated for confocal scanning laser tomography only. While these initial studies are encouraging, applying these algorithms in larger numbers of patients is now necessary. Future directions for these technologies are likely to include ultra-high resolution optical coherence tomography, the use of neural network/machine learning classifiers to improve clinical decision-making, and the ability to evaluate the susceptibility of individual optic nerve heads to potential damage from a given level of intraocular pressure or systemic blood pressure.

Deformation of the lamina cribrosa and anterior scleral canal wall in early experimental glaucoma.

To test the hypothesis that pathophysiologic deformation of the lamina cribrosa and anterior scleral canal wall underlies the onset of confocal scanning laser tomography (CSLT)-detected optic nerve head (ONH) surface change in early experimental glaucoma. Both eyes of four normal (two normal eyes) monkeys and four with early glaucoma (one eye with laser-induced IOP elevation, observed until the onset of CSLT-detected ONH surface change) were enucleated immediately after death and immersion fixed at IOP 0 mm Hg. In an additional four normal monkeys and five with early glaucoma, both eyes were cannulated, and IOP set to 10 mm Hg in one normal eye and either 30 or 45 mm Hg in the other (normal or early-glaucoma) eye. After 15 to 80 minutes of acute IOP elevation, these nine monkeys were perfusion-fixed. Within images of serial sagittal sections of the ONH tissues in all 17 monkeys, anterior lamina cribrosa position, laminar thickness, and scleral canal diameter were measured. For each parameter, differences between the two eyes of each monkey and between treatment groups were assessed by ANOVA. Within the eyes of the eight monkeys with IOP 0 mm Hg, the lamina cribrosa was posteriorly displaced and thicker and the scleral canal was enlarged at Bruch's membrane and at the anterior laminar insertion in the early-glaucoma eyes compared with the contralateral normal eyes (plastic deformation). Within the high-IOP normal eyes, the lamina cribrosa was posteriorly displaced compared with that in the low-IOP normal eyes, but there were no significant differences in laminar thickness or scleral canal diameter (normal compliance). Within the high-IOP early-glaucoma eyes, the lamina cribrosa was posteriorly displaced and thicker and the scleral canal enlarged, compared with both low-IOP normal eyes and high-IOP normal eyes (hypercompliant deformation). Differences in laminar position between the high-IOP early-glaucoma eyes and the contralateral low-IOP normal eyes (hypercompliant plus plastic deformation) were more than eight times greater than the differences between the high-IOP normal eyes and the contralateral low-IOP normal eyes (normal compliance). Both plastic (permanent) and hypercompliant deformation of the lamina cribrosa and anterior scleral canal wall are present in young adult monkey eyes with early experimental glaucoma. These findings suggest that damage to the ONH connective tissues occurs early in the monkey model of experimental glaucoma.

Clinician change detection viewing longitudinal stereophotographs compared to confocal scanning laser tomography in the LSU Experimental Glaucoma (LEG) Study

PurposeTo compare optic nerve head (ONH) surface change detection by confocal scanning laser tomography (CSLT) within the LSU Experimental Glaucoma (LEG) study to expert clinicians viewing the LEG stereophotographs. DesignExperimental study. ParticipantsFour fellowship-trained glaucoma specialists. MethodsIn the LEG study, six 15° CSLT images (TopSS, Laser Diagnostics Technologies, San Diego, CA) and four 2× optic disc stereophotographs were obtained from both eyes of 12 monkeys on 3 separate days and then every 2 weeks after laser to one eye (study eye) to elevate intraocular pressure. ONH surface change detection within the CSLT images is described in our companion report. In this report, the preliminary study compared change detection by the CSLT multivariate strategy with that of a single clinician viewing stereophotograph pairs on three separate occasions as the “gold standard.” The main study compared change detection by three additional clinicians viewing a subset of LEG stereophotograph pairs on three separate occasions with that of the CSLT multivariate strategy as the “gold standard.” Clinician change detection was assessed for partial (two of three occasions) or complete (three of three occasions) agreement. Three comparison groups within the main study are emphasized: 44 group A comparisons assessed false-positive change detection (specificity); 38 group B comparisons assessed change detection within 38 instances (11 onset and 27 progression events) of CSLT-detected study eye change; and 30 group C comparisons assessed change detection within 30 instances in which the CSLT failed to detect change in study eyes. Main outcome measuresClinician change detection within each comparison with either partial or complete agreement. ResultsWithin the main study, intrareading (−.29 to 0.67) and interreading (0.24–0.56) session agreement for each clinician was slight to moderate by kappa test. Good specificity (less than 10% false-positive change detection) was achieved within the 44 group A comparisons by two of the three clinicians, but only when the more stringent criterion (change detection on three of three occasions) was applied. Of the 38 group B comparisons (in which the CSLT detected change), the two clinicians who achieved good specificity in group A failed to detect change in 25 and 16 instances, respectively, using the more stringent (three of three) criterion. Similarly, of the 30 group C comparisons (in which the CSLT failed to detect change), these two clinicians detected change in three and seven instances, respectively, but in only one comparison did they both detect change. ConclusionsThis study provides the first direct evidence that an existing CSLT may reasonably meet or exceed the ONH surface change detection performance of fellowship-trained glaucoma specialists in at least those eyes with good CSLT reproducibility.

Change detection in regional and volumetric disc parameters using longitudinal confocal scanning laser tomography

PurposeTo develop a clinically specific strategy for optic nerve head (ONH) surface change detection within longitudinally acquired confocal scanning laser tomographic (CSLT) images. DesignExperimental study. Participants and/or controlsTwelve monkeys, each with one glaucomatous and one contralateral normal eye. Main outcome measuresDetection of ONH surface change within the CSLT images of each monkey’s glaucomatous eye. MethodsSix 10° and six 15° CSLT images and four stereo optic disc photographs were obtained from both eyes of 12 monkeys on 3 separate days (imaging sessions) and then every 2 weeks after laser to one eye (study eye) to elevate intraocular pressure. For each set of 10° and 15° images per eye (and for each of the 7–25 imaging sessions per monkey), 37 CSLT parameters were calculated. Univariate change required an individual parameter to change in excess of its analysis of variance-determined minimum detectable change (MDC) value. Multivariate change required groups of three parameters, considered together, to demonstrate significant change as determined by a multivariate analysis of variance. The rate of false-positive change detection for each individual parameter and for a group of three-parameter combinations was determined using both a one-in-a-row strategy (change at a single session) and a two-in-a-row strategy (change at two successive sessions) within the postlaser images of the 12 unchanging contralateral normal eyes. Change detection within the study eye images was then assessed for only those individual parameters and three-parameter combinations that were clinically specific (i.e., showed less than 10% false-positive change detection in the normal eyes). ResultsA total of 36 prelaser (three per monkey) imaging sessions and 158 postlaser (4–22/monkey) imaging sessions was performed. Clinically specific change detection (low rate of false positives) was achieved only with the two-in-a-row strategy. Overall, multivariate ONH surface change detection performed best; the best-performing three-parameter combination detected only 8 change events (4 onset and 4 progression) in 139 imaging sessions within the postlaser imaging sessions of the contralateral normal eyes and a total of 47 change events (11 onset and 36 progression) within the postlaser imaging sessions of the 12 study eyes. Counterintuitive (anterior) change occurred in most parameters within the late postlaser imaging sessions of the study eyes followed to end-stage damage. ConclusionsClinically specific detection of the onset and progression of glaucomatous ONH surface change is possible within longitudinally acquired CSLT images.

Optic disc surface compliance testing using confocal scanning laser tomography in the normal monkey eye.

To determine the effect of acute, experimentally increased intraocular pressure on deformation of the surface of the optic nerve head (optic nerve head surface compliance testing) in normal monkey eyes using confocal scanning laser tomography. A total of 156 compliance tests were performed on 48 normal eyes of 30 monkeys in three separate studies. Compliance testing involved obtaining confocal scanning laser tomographic images using a 10 degrees and/or 15 degrees and/or 20 degrees scan angle at various times after intraocular pressure was raised from 10 to 30 or 45 mm Hg. At each point, six images were analyzed to provide a value for a parameter, called mean position of the disc, which was used to express the amount of deformation the surface of the optic nerve head had undergone at that point. Statistical analysis (ANOVA) was performed to evaluate differences in the amounts of deformation in individual eyes at different intraocular pressures and at different compliance testing sessions (studies 1 and 2) and in the two eyes of individual monkeys under the same conditions (study 3). The majority of eyes showed posterior deformation of the surface of the optic nerve head ranging from 15 to 86 microm as early as 10 minutes after intraocular pressure was increased from 10 to 30 mm Hg. When pressure was increased from 30 to 45 mm Hg in a subset of these eyes, most showed additional deformation. Of the 12 eyes for which both 15 degrees and 20 degrees images were obtained at the same compliance test, 7 showed larger amounts of deformation in the 20 degrees images. Of the 18 monkeys tested in both eyes, 12 showed some differences and 4 showed substantial differences between the two eyes. In the normal monkey eye, the surface of the optic nerve head deforms rapidly (in as few as 10 minutes) in response to increased intraocular pressure. The amount of deformation varies between subjects and even within the two eyes of individual monkeys. Increasing the scan angle from 15 degrees to 20 degrees frequently increases the amount of deformation detected, suggesting that the peripapillary sclera and the optic nerve head may be involved in the deformation in some eyes.

Displacement of the Optic Nerve Head by Acute Changes in Intraocular Pressure in Monkey Eyes

The authors used the Humphrey Retinal Analyzer to study the effect of acute changes in intraocular pressure (IOP) on the topography of the optic nerve head in normotensive and hypertensive eyes of cynomolgus monkeys. Chronically elevated IOP was produced monocularly in monkeys by argon laser angle treatment. In 8 hypertensive eyes, the mean IOP of 51 mmHg was lowered acutely to 15 mmHg with a needle placed in the anterior chamber. In 12 normotensive eyes, the mean IOP of 16 mmHg was similarly increased to 45 mmHg. Simultaneous stereophotographs were taken both before and within 15 minutes after the IOP change. The surface of normotensive optic nerve heads moved posteriorly a mean of 17.8 microns after IOP elevation (P < 0.02). The surface of hypertensive optic nerve heads moved anteriorly 47.4 microns after IOP lowering (P = 0.1); this change was significantly less in nerve heads with larger and deeper cups (P < 0.02). Significant changes were not detected in disc diameter or vein diameter with acute changes in IOP. Measurable shifts in the position of the optic nerve head surface when IOP is altered may provide a future prognostic test for glaucomatous optic neuropathy.