Jost B. Jonas,Rahul A. Jonas,Shefali B. Jonas,Songhomitra Panda-Jonas
Myopic Versus Glaucomatous Parapapillary Beta Zone in Myopic Eyes Versus Eyes With Secondary Angle-Closure Glaucoma
parapapillary beta zone,parapapillary alpha zone,bruch´s membrane,retinal pigment epithelium,axial myopia,glaucoma
Purpose To search for histologic differences in the beta zone between myopic eyes versus eyes with secondary angle-closure glaucoma. Methods The histomorphometric study consisted of human eyes enucleated due to uveal melanomas or secondary angle-closure glaucoma. Results The study included 100 eyes (age: 62.1 ± 15.1 years; axial length: 25.6 ± 3.1 mm; range: 20.0–35.0 mm). In non–highly myopic glaucomatous eyes compared with non–highly myopic nonglaucomatous eyes, the parapapillary alpha zone was longer (223 ± 168 µm vs. 125 ± 128 µm; P = 0.03), beta zone prevalence (15/20 vs. 6/41; P < 0.001) and length (277 ± 245 µm vs. 44 ± 150 µm; P = 0.001) were higher, and RPE cell density in the alpha zone and alpha zone border was lower (all P < 0.05). In highly myopic nonglaucomatous eyes compared with non–highly myopic glaucomatous eyes, parapapillary RPE drusen prevalence (2/19 vs. 10/10; P = 0.01) and alpha zone prevalence (2/19 vs. 16/20; P < 0.001) and length (23 ± 68 µm vs. 223 ± 168 µm; P < 0.001) were lower. In non–highly myopic glaucomatous eyes, Bruch's membrane (BM) thickness decreased (P < 0.001) from the beta zone (6.0 ± 3.1 µm) to the alpha zone (5.1 ± 4.3 µm) and peripheral to it (3.0 ± 0.9 µm). In highly myopic nonglaucomatous eyes, BM thickness did not differ (P > 0.10) between all three regions. In the total study population, RPE cell density in the alpha zone (24.5 ± 9.3 cells/240 µm) was higher than at the alpha zone border (19.2 ± 4.8 cells/240 µm; P < 0.001) or peripheral to it (19.0 ± 3.6 cells/240 µm; P < 0.001). Conclusions The glaucomatous beta zone in eyes with chronic angle-closure glaucoma (with the alpha zone, parapapillary RPE drusen, thickened BM, and higher RPE cell count in the adjacent alpha zone) differs histologically from the myopic beta zone (characterized by the absence of the alpha zone and parapapillary RPE drusen, unremarkable BM thickness, and unremarkable parapapillary RPE). The differences suggest different etiologies of the glaucomatous versus myopic beta zone.
Myopic Versus Glaucomatous Parapapillary Beta Zone in Myopic Eyes Versus Eyes With Secondary Angle-Closure Glaucoma

To search for histologic differences in the beta zone between myopic eyes versus eyes with secondary angle-closure glaucoma.

The histomorphometric study consisted of human eyes enucleated due to uveal melanomas or secondary angle-closure glaucoma.

The study included 100 eyes (age: 62.1 ± 15.1 years; axial length: 25.6 ± 3.1 mm; range: 20.0–35.0 mm). In non–highly myopic glaucomatous eyes compared with non–highly myopic nonglaucomatous eyes, the parapapillary alpha zone was longer (223 ± 168 µm vs. 125 ± 128 µm; P = 0.03), beta zone prevalence (15/20 vs. 6/41; P < 0.001) and length (277 ± 245 µm vs. 44 ± 150 µm; P = 0.001) were higher, and RPE cell density in the alpha zone and alpha zone border was lower (all P < 0.05). In highly myopic nonglaucomatous eyes compared with non–highly myopic glaucomatous eyes, parapapillary RPE drusen prevalence (2/19 vs. 10/10; P = 0.01) and alpha zone prevalence (2/19 vs. 16/20; P < 0.001) and length (23 ± 68 µm vs. 223 ± 168 µm; P < 0.001) were lower. In non–highly myopic glaucomatous eyes, Bruch's membrane (BM) thickness decreased (P < 0.001) from the beta zone (6.0 ± 3.1 µm) to the alpha zone (5.1 ± 4.3 µm) and peripheral to it (3.0 ± 0.9 µm). In highly myopic nonglaucomatous eyes, BM thickness did not differ (P > 0.10) between all three regions. In the total study population, RPE cell density in the alpha zone (24.5 ± 9.3 cells/240 µm) was higher than at the alpha zone border (19.2 ± 4.8 cells/240 µm; P < 0.001) or peripheral to it (19.0 ± 3.6 cells/240 µm; P < 0.001).

The glaucomatous beta zone in eyes with chronic angle-closure glaucoma (with the alpha zone, parapapillary RPE drusen, thickened BM, and higher RPE cell count in the adjacent alpha zone) differs histologically from the myopic beta zone (characterized by the absence of the alpha zone and parapapillary RPE drusen, unremarkable BM thickness, and unremarkable parapapillary RPE). The differences suggest different etiologies of the glaucomatous versus myopic beta zone.

The parapapillary beta zone has clinically and histologically been defined by the presence of Bruch’s membrane (BM) and absence of RPE., Cross-sectional and longitudinal, clinical, and population-based studies have shown its association with glaucomatous optic neuropathy.– The studies also revealed a spatial relationship: the sector in which the intrapapillary neuroretinal rim loss was most marked, the beta zone in the parapapillary region was relatively most pronounced., Both neuroretinal rim loss and enlarged beta zone were spatially correlated with the longest distance to the central retinal vessel trunk in the lamina cribrosa. Besides in glaucomatous eyes, the beta zone is found in axially elongated, myopic eyes, in which it has to be differentiated from the parapapillary gamma zone.,– The latter is characterized by the absence of BM and is often located, as the beta zone, predominantly in the temporal and temporal inferior parapapillary regions., Since the beta zone in glaucomatous eyes may differ from the beta zone in myopic eyes with respect to its etiology and clinical importance, we histologically examined eyes with glaucoma and axially elongated eyes to find histologic differences between the glaucomatous beta zone and the myopic beta zone.

Human eyes enucleated due to reasons such as malignant uveal melanomas or painful secondary angle-closure glaucoma were included in this histomorphometric study. Following the guidelines published by the World Medical Association Declaration of Helsinki, the Medical Ethics Committee II of the Medical Faculty Mannheim of the Heidelberg University approved the study. The requirement of informed written consent signed by the study participants was waived because the enucleations had taken place approximately 25 to 60 years prior to the start of the study. The globes had been examined already in previous investigations addressing different topics., As described in detail previously, the globes had been fixed immediately after enucleation in a solution of 4% formaldehyde and 1% glutaraldehyde and had been kept in that solution at room temperature for 1 week. After the anterior-posterior, horizontal, and vertical diameters of the eyes had been measured, the globes were opened. A middle part with a thickness of about 8 mm and running through the optic nerve head, the pupil, and the posterior pole region were removed and dehydrated in alcohol, embedded in paraffin, and sectioned for light microscopy. In the case of eyes with tumors, the tumor was included in the section. After staining the sections with hematoxylin and eosin or using the periodic acid–Schiff method, we assessed a histologic slide, measuring about 5 to 8 µm in thickness and running through the central part of the pupil and optic nerve head. Using a millimeter scale built into the objective of the microscope, we determined the following histomorphometrically (Figs. 1–5): • the length of the parapapillary alpha zone (defined by the presence of the BM and an irregularly structured RPE,,; • the length of the beta zone; • the RPE cell density in the alpha zone (if present); • the RPE cell density adjacent to the alpha zone (if present) or adjacent to the beta zone (if there was no alpha zone); • the RPE cell density in a region between 240 µm and 480 µm peripheral to the border of the alpha zone (if present) or peripheral to the beta zone (if alpha zone was not present); • BM thickness in the beta zone, alpha zone (if present), and a distance of 240 µm to the peripheral border of the alpha zone (if present) or beta zone (if alpha zone was not present); and • presence of parapapillary drusen of the RPE (defined by a localized and nodular thickening of BM covered by a layer of RPE cells), located at the parapapillary end of BM.  High myopia was defined by an axial length of ≥26.0 mm. Using a statistical software program (SPSS, version 27.0; IBM-SPSS, Chicago, IL, USA), we determined in the statistical analysis the mean values, standard deviations, and 95% confidence intervals (CIs) of the outcome measures. Using the Mann–Whitney nonparametric test for the comparison of unpaired samples, we compared the outcome parameters between subgroups and applied the Wilcoxon test for paired samples for the comparison of outcome parameters within the same eyes. After carrying out Bonferroni’s correction for performing multiple comparisons, a two-sided P value ≤0.01 was regarded to be statistically significant.

The study included 100 eyes (100 patients; 61 [61%] men) with a mean age of 62.1 ± 15.1 years (median: 61.0 years; range: 30–89 years). The mean axial length was 25.6 ± 3.1 mm (median: 24.25 mm; range: 20.0–35.0 mm). An axial length of ≥25 mm, ≥26 mm, and ≥29 mm was present in 49 (49%) eyes, 39 (39%) eyes, and 15 (15%) eyes, respectively. The study population included 41 non–highly myopic eyes without glaucoma, 20 non–highly myopic eyes with glaucomatous optic nerve damage, 19 highly myopic eyes without glaucoma, and 20 highly myopic eyes with glaucomatous optic nerve damage. In non–highly myopic glaucomatous eyes compared with non–highly myopic nonglaucomatous eyes, the length of the alpha zone was longer, prevalence and length of the beta zone were higher, and RPE cell density in the alpha zone, just at the border of the alpha zone and peripheral to it, was lower (Table). The number of parapapillary RPE drusen, prevalence of the alpha zone, and BM thickness in the beta zone, alpha zone, and peripheral to it did not differ between both groups (Table). In highly myopic nonglaucomatous eyes compared with non–highly myopic glaucomatous eyes, the prevalence of parapapillary RPE drusen and prevalence and length of alpha zone were lower (Table). In highly myopic nonglaucomatous eyes compared with non–highly myopic nonglaucomatous eyes, the prevalence of parapapillary RPE drusen and the prevalence and length of the alpha zone were lower, the prevalence and size of the beta zone were higher, and BM thickness in the beta zone was lower (Table). Highly myopic nonglaucomatous eyes and highly myopic glaucomatous eyes did not differ significantly in any parameter measured (Table). In non–highly myopic glaucomatous eyes, BM thickness decreased significantly (P < 0.001) from the beta zone (6.0 ± 3.1 µm) to the alpha zone (5.1 ± 4.3 µm) and finally to the region peripheral to the alpha/beta zone (3.0 ± 0.9 µm), with significant differences (all P < 0.05) between each of the three zones/regions (Table). In a similar manner, in the non–highly myopic nonglaucomatous eyes, BM thickness decreased significantly (P < 0.001) from the beta zone (9.4 ± 3.0 µm) to the alpha zone (6.9 ± 2.4 µm) and finally to the region peripheral to the alpha/beta zone (3.2 ± 0.5 µm) (Table). In highly myopic nonglaucomatous eyes, BM thickness did not differ significantly (P = 0.16) between the beta zone and alpha zone (if present), between the alpha zone and the region peripheral to the alpha zone (P = 0.28), or between the beta zone and the region peripheral to the alpha/beta zone (P = 0.16) (Table). In the region peripheral to the beta/alpha zone, BM thickness did not differ significantly (all P > 0.05) between any of the four groups. In the total study population, RPE cell density in the alpha zone (24.5 ± 9.3 cells/240 µm) was significantly higher than at the border of the alpha zone (19.2 ± 4.8 cells/240 µm; P < 0.001), and it was higher than in the region at a distance of 240 µm from the alpha zone border (19.0 ± 3.6 cells/240 µm; P < 0.001).

In this histomorphometric study on enucleated human globes, the parapapillary region in highly myopic eyes as compared to the parapapillary region in non–highly myopic eyes with end-stage secondary angle-closure glaucoma was characterized by a lower prevalence and smaller size of the alpha zone, lower prevalence of parapapillary RPE drusen, and a thinner BM in the beta zone. Within the subgroup of non–highly myopic eyes, BM was thicker in the beta zone than in the alpha zone, where it was thicker than in the region peripheral to the alpha zone. In the highly myopic eyes, BM did not differ significantly between the various regions. In the total study population, RPE cell density in the alpha zone was higher than at the alpha zone border. The observations made in our histomorphometric study cannot directly be compared with the results of other investigations, since differences between the beta zone in glaucomatous nonmyopic eyes (“glaucomatous beta zone”) and the beta zone in myopic eyes (“myopic beta zone”) have not been examined yet. So far, the histologic characteristics of the parapapillary region have been described, with the alpha zone being characterized by the presence of BM covered by an irregular RPE, the beta zone showing a BM devoid of RPE cells, and the gamma zone being characterized by the absence of BM.,, In the myopic eyes in our study, the parapapillary region showed a mostly normal RPE morphology without irregularities, that is, a low prevalence of the alpha zone or no alpha zone at all, a normal RPE cell density at the border of the beta zone (i.e., the RPE cell density did not differ significantly between the beta zone border and a region peripheral to it), and a normal thickness of BM (i.e., no difference in BM thickness between the beta zone, the region adjacent to the beta zone, and the region peripheral to it). These findings fit with the notion that the beta zone in myopic eyes may be associated with a myopic axial elongation-related widening of the physiologic opening in the RPE layer of the optic nerve head canal. The optic nerve head canal includes the RPE layer opening, BM opening, opening in the choroid (separated from the intrapapillary compartment by the peripapillary border tissue of the choroid), and the opening in the peripapillary scleral flange, covered by the lamina cribrosa., Myopic axial elongation leads to an increase in the distance between the ora serrata and posterior pole by about 0.73 mm for each millimeter increase in axial length. Correspondingly, the surface area of the BM enlarges. It leads to a strain on the overlying RPE layer and its basal membrane. Clinical observations describing the detachment of the RPE cell layer (and presumably its basal membrane) from BM in the case of an RPE detachment in eyes with exudative age-related macular degeneration or in eyes with a central serous retinochoroidopathy suggest that the RPE cell basal membrane is not firmly connected to BM. It opens the possibility for the RPE and its basal membrane to slide on BM in the parapapillary region. The axial elongation-induced strain on the RPE layer may thus lead through a sliding on BM to a widening of the RPE opening, in an attempt to reduce the strain within the RPE layer. If the RPE opening enlarges, the parapapillary region will be free of RPE cells. If the enlargement of the RPE opening is larger than or occurs earlier than an axial elongation-induced enlargement of the Bruch's membrane opening (BMO), the parapapillary BM will be free of RPE cells, that is, the (myopic) beta zone will develop. Interestingly, axial elongation is associated also with a BMO enlargement, usually in highly myopic eyes with an axial length of >26 mm. A (myopic) beta zone can usually be found also in eyes with moderate myopia (i.e., an axial length of less than 26 mm). The reason for the difference between the enlargement of the RPE opening and the BMO enlargement with respect to axial elongation may be the difference in the biomechanical properties between both layers. BM showed in uniaxial tensile tests an average elastic (tangent) modulus of 1.60 ± 0.81 MPa and 2.44 ± 1.02 MPa at a strain of 0% and 5%, respectively. These results suggest that BM can withstand an average intraocular pressure of 82 mm Hg before rupture and that the elastic modulus of BM is higher than that of the sclera (0.2–0.5 MPa for an intraocular pressure of approximately 15 mm Hg), cornea (0.1–0.5 MPa), retina (0.011 MPa), and iris (0.004 MPa).– Although the biomechanical properties of the RPE and its basal membrane have not been examined yet, the much lower thickness of the RPE basal membrane (50 nm) versus BM thickness (2–5 µm) indicates a considerably lower biomechanical strength of the RPE layer as compared to BM. Based on clinical observations made in eyes with narrow anterior chamber angles and an acute intraocular pressure (IOP) rise, the glaucomatous beta zone has been associated with an uprolling and backward push of the parapapillary RPE in association with an IOP rise and a reflattening of the RPE after IOP reduction. This mechanical burden may lead to damage of the RPE, first in the form of structural irregularities (alpha zone) with an increased RPE cell density in the alpha zone (due to the backward push of the RPE layer) as found in the present study. It may eventually be followed by a complete loss of the RPE cells, that is, the development of the (glaucomatous) beta zone. The finding of a normal RPE cell density at the border of the beta zone in myopic eyes and the observation of an increased RPE cell density and RPE cell irregularities in the alpha zone at the border of the beta zone in glaucomatous eyes (with end-stage secondary angle-closure glaucoma) agree with the notion. The observations support the idea that the beta zone in myopic eyes differs in its etiology from the beta zone in glaucomatous eyes. Correspondingly, the increased RPE cell density in the alpha zone in glaucomatous eyes may be due to the pushing of the RPE end away from the optic disc border, while in myopic eyes, the RPE layer may be pulled away from the optic disc border. One may discuss that the stress to the RPE cells in the alpha zone may lead to and explain the thickening of BM in the alpha zone as found in the present study. It may also explain the BM thickening in the beta zone in glaucomatous eyes, considering that the beta zone in glaucomatous eyes is a former alpha zone. In the clinical differentiation between a myopic beta zone and a glaucomatous beta zone, the findings of the present histologic study suggest that the presence of an ophthalmoscopically detectable alpha zone may increase the probability of a glaucomatous beta zone versus a myopic beta zone. Vice versa, the absence of the alpha zone may point to a higher probability of a myopic beta zone. Using optical coherence tomography (OCT) images, detection of an upward rolling of the parapapillary RPE or of parapapillary RPE drusen may increase the likelihood of a glaucomatous beta zone, while the absence of these two signs may increase the probability of a myopic beta zone. If a myopic beta zone can clinically be differentiated from a glaucomatous beta zone, the diagnostic precision of a glaucomatous beta zone for the diagnosis of glaucomatous optic neuropathy will increase.– Future studies may explore and discuss whether the myopic beta zone as an axial elongation-induced widening of the RPE opening of the ocular optic nerve canal has similarities with secondary RPE openings in the macular region in the form of patchy atrophies and with cobblestones located in the fundus periphery of highly myopic eyes.– The histology of patchy atrophies and cobblestones fits with the notion that they may be axial elongation-induced newly developed expansion holes in the RPE layer, while the myopic beta zone would be a widening of a physiologically occurring opening in the RPE layer. The etiology of the parapapillary RPE drusen, found predominantly in our study in association with an alpha zone and a glaucomatous beta zone, has remained unclear so far. It has remained elusive whether they can be compared to RPE drusen in the macular region. In the highly myopic groups, BM thickness did not differ between the beta zone, the alpha zone, and the region peripheral to it, nor did BM thickness, as measured in the region peripheral to the beta/alpha zone, differ between the highly myopic groups and the non–highly myopic groups. It fits with the results of previous studies in which BM thickness measured from the ora serrata to the posterior pole (except for the parapapillary region) was not significantly associated with axial length and thus did not vary between eyes with normal axial length, moderately myopic eyes, and highly myopic eyes.– It also fits with the results of the study on the biomechanical properties of BM with a low degree of elasticity. It supports the notion of BM playing a biomechanical role in the process of axial elongation. The thickening of BM in the alpha zone observed in our study confirms findings made by Curcio and colleagues, who examined the parapapillary region of human eyes without optic nerve damage and without high myopia. They reported that the RPE degeneration and atrophy in the parapapillary region were associated with a thickening of the inner part of BM and thickening and reduplication of the basal lamina of the RPE. In contrast to Curcio et al., we did not differentiate between a thickening of BM and a thickening of the RPE basal membrane but included both parameters into one variable of “BM thickening.” In the non–highly myopic eyes, the prevalence of RPE drusen and the alpha zone did not differ significantly between the glaucomatous eyes and the nonglaucomatous eyes (Table). It shows that low prevalences of parapapillary RPE drusen and the alpha zone serve as features of the myopic beta zone for the differentiation of the myopic beta zone from the glaucomatous beta zone, but both parameters are not useful to separate glaucomatous eyes from nonglaucomatous eyes within the non–highly myopic group. The reason is that both the alpha zone and parapapillary RPE drusen often occur relatively in normal eyes, so they are not a glaucomatous characteristic. Correspondingly, clinical studies have not shown a significant difference in the prevalence of the alpha zone between glaucomatous eyes and nonglaucomatous eyes. The length of the alpha zone, however, was significantly longer in the glaucomatous eyes as compared to the nonglaucomatous eyes within the non–highly myopic group in the present study (Table). It fits with the results of clinical studies and with the notion that in glaucomatous eyes, the alpha zone is a precursor of the glaucomatous beta zone, as discussed in detail above. The RPE cell density in the alpha zone, just at the border of the alpha zone and peripheral to it, was significantly lower in the glaucomatous eyes versus nonglaucomatous eyes within the non–highly myopic group in the present study (Table). One may discuss that the glaucomatous process had already damaged and reduced the number of RPE cells before the glaucomatous beta zone had eventually developed (or enlarged). Limitations of our study should be discussed. First, the eyes included in our study were affected by various disorders as reasons for their enucleation. In particular, the glaucomatous group consisted of eyes with painful secondary angle-closure glaucoma, probably with high IOP. It has remained unclear whether the observations made in this histologic study on eyes with high-pressure (angle-closure) glaucoma can be transferred to patients with open-angle glaucoma and only moderately elevated or normal IOP. This concern holds true also since the clinical observation of an upward rolling of the parapapillary RPE was made in patients with an acute marked IOP rise due to an occlusion of the anterior chamber angle. Future studies may thus assess whether the findings of the present study can be transferred to patients with open-angle glaucoma with various IOP levels, in particular since some investigations have reported that the parapapillary beta zone was associated more with normal-pressure glaucoma than with high-pressure glaucoma. Second, the patients included in the study were not recruited in a population-based manner, so the measurements cannot be generalized with respect to the prevalence of the structures and zones explored. Third, histologic artifacts due to the processing of the histologic sections may have affected the tissue dimensions. Fourth, the glaucomatous group in our study consisted of eyes with secondary angle-closure glaucoma, so the study results cannot directly be transferred to eyes with primary open-angle glaucoma. Fifth, we did not differentiate between a thickening of the core part of BM (i.e., the collagenous, elastic, and collagenous layers) and a thickening of the basal membrane of the RPE or RPE basal laminar deposits. In conclusion, the glaucomatous beta zone in eyes with chronic angle-closure glaucoma (characterized by the presence of the alpha zone, parapapillary RPE drusen, thickened BM, and higher RPE cell count in the adjacent alpha zone) differs histologically from the myopic beta zone (characterized by the absence of the alpha zone and parapapillary RPE drusen, unremarkable BM thickness, and unremarkable parapapillary RPE). The observations agree with the notion of an axial elongation-associated enlargement of the RPE opening leading to the development of the myopic beta zone.