|
|
||||||||
1From the Department of Ophthalmology, Faculty of Clinical Medicine Mannheim, University of Heidelberg, Mannheim, Germany; and the 2Department of Ophthalmology, University Erlangen-Nürnberg, Erlangen, Germany.
| Abstract |
|---|
|
|
|---|
METHODS. The histomorphometric study included 111 enucleated nonglaucomatous eyes of 111 white subjects. On anteriorposterior histologic sections through the pupil and the central optic disc region, the thickness of the cornea, lamina cribrosa, and peripapillary sclera and the shortest distance between the intraocular space and the cerebrospinal fluid space were measured. Axial length ranged between 20 and 32 mm.
RESULTS. Mean central corneal thickness (mean ± SD: 616.6 ± 108.3 µm) and mean central lamina cribrosa thickness (378.1 ± 117.8 µm) were statistically independent of each other (P = 0.15; correlation coefficient, r = 0.14). In a similar manner, lamina cribrosa thickness at the optic disc border was statistically independent of central corneal thickness (P = 0.51; r = 0.06) and peripheral corneal thickness (P = 0.34; r = 0.09). In a parallel way, peripapillary scleral thickness (P = 0.84) and the shortest distance between the prelaminar space and cerebrospinal fluid space (P = 0.10) were statistically independent of central corneal thickness.
CONCLUSIONS. In nonglaucomatous human globes, central corneal thickness may not correlate significantly with lamina cribrosa thickness, peripapillary scleral thickness, and shortest distance between intraocular space and cerebrospinal fluid space. Histologic artifact and sectioning methods could partially account for the lack of an association. The study results may suggest clinically that an assumed relationship between central corneal thickness and susceptibility to glaucoma cannot be explained by an anatomic correspondence between corneal thickness and histomorphometry of the optic nerve head.
Other studies have discussed the pathogenic role as a pressure barrier between the intraocular space and the cerebrospinal fluid space that the anatomy of the lamina cribrosa may play in optic nerve diseases such as the glaucomas.10 11 12 13 14 Histomorphometric investigations have shown that eyes with advanced glaucomatous optic nerve damage have a markedly thinner lamina cribrosa than do normal eyes, which may explain an increased risk of further progression in patients with chronic open-angle glaucoma in an advanced stage of the disease compared with patients with an early stage of glaucoma.15 16 17 Another histomorphometric investigation suggested that the lamina cribrosa is significantly thinner in highly myopic eyes than in nonhighly myopic eyes, which may be a reason for the presumably higher susceptibility to glaucoma of highly myopic eyes versus nonhighly myopic eyes.18
Because the lamina cribrosa forms the bottom of the optic disc, and because the dimensions and shape of the cornea are correlated with the dimensions of the optic disc,19 20 we, while looking for a possible explanation of the relationship between corneal thickness and susceptibility to glaucoma, conducted the present study to evaluate whether the central thickness of the cornea is related to the thickness of the lamina cribrosa in human eyes.
| Methods |
|---|
|
|
|---|
Immediately after enucleation, the globes were fixed in a solution of 4% formaldehyde and 1% glutaraldehyde. They remained in the fixation agent for
1 week, before they were further processed for histologic sectioning. The globes were prepared in a routine manner for light microscopy. An anteriorposterior segment going through the pupil and the optic nerve was cut out of the fixed globes. These segments were dehydrated in alcohol, imbedded in paraffin, and sectioned for light microscopy. Most of the eyes were stained by the periodic-acid-Schiff (PAS) method, the remaining eyes were stained by hematoxylin-eosin. For all eyes, one section running through the central part of the optic disc was selected for further histomorphometric evaluation. According to the scale in the eyepiece of the microscope, the magnification at which the measurements were performed was x100.
Axial length and horizontal and vertical diameter of the globes were measured macroscopically before sectioning of the globes. Based on axial length, the total study sample was divided into a nonhighly myopic study group with an axial length of <26 mm (n = 89 globes; 80.2%) and a highly myopic study group with an axial length of
26 mm (n = 22; 19.8%). Histomorphometrically, we measured the following: (1) the thickness of the lamina cribrosa in the center of the optic disc, at the optic disc border, and in the intermediary positions between the center and the border of the optic disc; (2) the shortest distance between the prelaminar space and the cerebrospinal fluid space; (3) the thickness of the peripapillary sclera at the optic disc border; (4) and the thickness of the cornea in the center, at the limbus, and in the intermediary positions between the center and the limbus.
The anterior and posterior border of the lamina cribrosa were outlined on the histologic specimens. For the anterior border, care was taken to differentiate the lamina cribrosa from overlying glial tissue. For the posterior border, care was taken to delineate the lamina cribrosa from the optic nerve. Staining of the histologic sections by the PAS method enhanced the difference between the lamina cribrosa and the surrounding tissue. The reproducibility of the technique was evaluated in a previous study in which 10 randomly selected histologic optic disc sections were reevaluated 10 times.18 The coefficient of variation, defined as the ratio of the mean of the standard deviations of the reevaluations divided by the mean of the means, was 0.143.
For statistical analysis, the means and standard deviations, medians, and ranges are presented. For the comparison of the study groups, statistical tests for unpaired samples were applied. The level of significance was 0.05 (two-sided) in all statistical tests. The statistical analysis was performed on computer (SPSSWIN, ver. 11.5; SPSS, Chicago, IL).
| Results |
|---|
|
|
|---|
The mean central corneal thickness measured 616.6 ± 108.3 µm (media, 612 µm). Comparison with the intravital pachymetric measurements of central corneal thickness published in the literature shows a correction factor for the histomorphometric determinations of central corneal thickness of approximately 1:1.1.21 The histomorphometric measurements of corneal thickness were independent of age (P = 0.88), gender (P = 0.43), right or left eye (P = 0.40), axial length (P = 0.80), and horizontal (P = 0.06) and vertical (P = 0.80) globe diameters.
The mean central lamina cribrosa thickness was 378.1 ± 117.8 µm (median, 360 µm). The measurement was independent of age (P = 0.82), gender (P = 0.60), right or left eye (P = 0.91), and horizontal (P = 0.44) and vertical (P = 0.32) globe diameter. The mean central lamina cribrosa thickness correlated significantly and negatively with axial length (P = 0.03; correlation coefficient, r = 0.21; Fig. 1 ). Correspondingly, the lamina cribrosa was significantly thinner in the highly myopic group than in the nonhighly myopic group (322.2 ± 144.3 µm vs. 391.9 ± 106.8 µm; P = 0.04). Within the nonhighly myopic group, central lamina cribrosa thickness was statistically independent of axial length (P = 0.14, r = 0.16).
|
|
|
|
|
| Discussion |
|---|
|
|
|---|
In view of the possible importance of the thickness of the lamina cribrosa at the bottom of the optic nerve head in the pathogenesis of glaucomatous optic nerve damage, considering that the dimensions of the cornea, such as horizontal and vertical diameter and anterior corneal curvature, correlate with the diameters and area of the optic nerve head, and since a thin central cornea has been described as a predictive factor for further progression of chronic open-angle glaucoma, the purpose of the present study was to evaluate whether the central corneal thickness correlates with the thickness of the lamina cribrosa. The findings showed that in nonglaucomatous human globes, the thicknesses of the central cornea and the lamina cribrosa do not correlate significantly (Figs. 2 3) , which suggests that an assumed relationship between central corneal thickness and susceptibility to glaucoma may not be explained by a corresponding anatomy between corneal thickness and thickness of the lamina cribrosa. This finding holds true in nonhighly myopic eyes and in highly myopic eyes, since for both study subgroups, central corneal thickness was unrelated to thickness of the lamina cribrosa. The data of the present study indirectly correspond with results of the Early Manifest Glaucoma Trial (EMGT), in which the corneal thickness was unrelated to the development of visual field defects in the study population.36 The data of the present investigations also correspond with another study in this issue of IOVS in which central corneal thickness did not have a major impact on the rate of the progression of chronic open-angle glaucoma.37
In addition to the lamina cribrosa thickness, thickness of the peripapillary sclera and the shortest distance between the intraocular space and the cerebrospinal fluid space did not correlate significantly with central corneal thickness (Figs. 4 5) . Even if it can be assumed that a short distance between the intraocular space and the cerebrospinal fluid space steepens the translamina-cribrosa gradient and increases susceptibility to glaucoma, and even if it can be assumed that a thin peripapillary sclera is associated with a weak suspension of the lamina cribrosa, again leading to a higher susceptibility to glaucoma, the data of the present study did not find a correlation between the optic nerve head parameters and central corneal thickness. This result may again suggest that an assumed relationship between central corneal thickness and susceptibility to glaucoma cannot be explained by a corresponding anatomy between corneal thickness and histomorphometry of the optic nerve head.
The finding that corneal thickness and thickness of the lamina cribrosa were not significantly associated with each other may be due to differences in embryonic development. The corneal stroma and corneal endothelium start to be formed by the fourth to sixth weeks of gestation by immigrating cells from the neural crest. The lamina cribrosa begins to develop as the lamina scleralis in the region of the primitive optic nerve head during the eighth week. Invading glial cells of the outer wall of the optic stalk form a sieve-like scaffolding around the pre-existing ganglion cell axons, followed by ingrowing of sclera-derived cells in the fourth month, which forms the mesenchymal part of the lamina cribrosa, including connective tissue fibers penetrating the glial lamina cribrosa and running between glia-covered ganglion cell axons and the centrally located hyaloid vessel. The lamina cribrosa is vascularized by the 13th to 14th weeks, and its mature structure is reached during the 7th month. It should be emphasized that, during formation of the lamina cribrosa, the ganglion cell axons and the hyaloid vessels are the first to be present. They form the contents of the future lamina cribrosa pores, followed by formation of the lamina cribrosa trabecula in two steps: In the first step, glial cells of the outer optic stalk wall create a provisional net that is stabilized and further strengthened in the second step by ingrowing sclera-derived mesenchymal cells. Consequently, the mature lamina cribrosa can be considered a result of the secondary development of vascularized scleral connective tissue penetrating the preformed glial lamina, including the ganglion cell axons and the hyaloid vessels.38 Because of the loss of approximately two thirds of the originally formed retinal ganglion cell axons in the primitive optic nerve, the primitive lamina cribrosa furthermore undergoes a continuous remodeling during the embryonic stage. These differences between the cornea and lamina cribrosa in their embryonic development may contribute to the result of the present study that central corneal thickness appeared to be unrelated to the thickness of the optic nerve head structures.
There are several weaknesses in this study. First, postmortem tissue swelling and artifact in either the cornea or lamina cribrosa have certainly introduced some bias in the correlation between corneal thickness and measured optic disc parameters. There is little reason to believe that histologic artifacts would be related to corneal thickness, and thus this bias is probably nondifferential. However, any nondifferential bias would be toward the null, which may explain the lack of any significant associations. In addition, histologic sections were not of a consistent orientation, and lamina cribrosa thickness in each eye was characterized from a single histologic section. Considering that the lamina is remarkably variable in its three-dimensional geometry, as shown in Bellezza et al.12 14 and Burgoyne and Morrison,13 the lamina cribrosa may be thinnest in different regions in each eye, and anything less than a three-dimensional reconstruction or serial sectioning of each eye may fail to find the thinnest portion of the lamina that in fact correlates. From that point of view, the present investigation may be regarded as a pilot study, with findings that must be confirmed by an investigation with three-dimensional reconstruction of the anatomy of the lamina cribrosa. Another limitation of the study is the relatively small number of eyes included in the investigations. The scattergrams show, however, that there is not even a tendency toward correlation between central corneal thickness and thickness of the lamina cribrosa (Figs. 2 3 4 5) . This finding may suggest that even had a larger number of eyes been included in the study, the correlation between central corneal thickness and thickness of the lamina cribrosa may not have become relevant.
In conclusion, the present study suggests that in nonglaucomatous human globes, corneal thickness does not correlate significantly with lamina cribrosa thickness, thickness of the peripapillary sclera, or the shortest distance between the intraocular space and the cerebrospinal fluid space. In the interpretation of the measurements, it has to be taken into account, that histologic artifact and sectioning methods led to corneal thickness measurements well over 600 to 800 µm in many eyes, which results in a nondifferential bias toward the null that may also explain the lack of any significant associations. Direct clinicalhistomorphometric comparisons of enucleated eyes may answer the question of how much the histomorphometric measurements in the present study were influenced by postmortem- and preparation-induced changes. If the results of the present study are confirmed by other studies, they suggest that an assumed relationship between central corneal thickness and susceptibility to glaucoma may not be explained by a correspondence between central corneal thickness and thickness of the lamina cribrosa and peripapillary sclera.
| Acknowledgements |
|---|
| Footnotes |
|---|
Disclosure: J.B. Jonas; None; L. Holbach, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Jost B. Jonas, Universitäts-Augenklinik, Theodor-Kutzer-Ufer 1-3, 68167 Mannheim, Germany; jost.jonas{at}augen.ma.uni-heidelberg.de.
| References |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
I. A. Sigal Interactions between Geometry and Mechanical Properties on the Optic Nerve Head Invest. Ophthalmol. Vis. Sci., June 1, 2009; 50(6): 2785 - 2795. [Abstract] [Full Text] [PDF] |
||||
![]() |
R. Ren, N. Wang, B. Li, L. Li, F. Gao, X. Xu, and J. B. Jonas Lamina Cribrosa and Peripapillary Sclera Histomorphometry in Normal and Advanced Glaucomatous Chinese Eyes with Various Axial Length Invest. Ophthalmol. Vis. Sci., May 1, 2009; 50(5): 2175 - 2184. [Abstract] [Full Text] [PDF] |
||||
![]() |
M. M. Altaweel, L. Freisberg, N. Pal, J. Gleiser, E. H. Ryan, D. Dawson, and D. Albert Radial Optic Neurotomy Using Nasal and Temporal Approach Incisions: Histopathologic Study in Human Cadaver Eyes Arch Ophthalmol, November 1, 2007; 125(11): 1553 - 1557. [Abstract] [Full Text] [PDF] |
||||
![]() |
M Pakravan, A Parsa, M Sanagou, and C F Parsa Central corneal thickness and correlation to optic disc size: a potential link for susceptibility to glaucoma Br. J. Ophthalmol., January 1, 2007; 91(1): 26 - 28. [Abstract] [Full Text] [PDF] |
||||
![]() |
I. A. Sigal, J. G. Flanagan, and C. R. Ethier Factors Influencing Optic Nerve Head Biomechanics Invest. Ophthalmol. Vis. Sci., November 1, 2005; 46(11): 4189 - 4199. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |