Primary Open-Angle Glaucoma Is Not Associated With ... - CiteSeerX

Primary Open-Angle Glaucoma Is Not Associated With Photoreceptor Loss Kurtis R. Kendell, Harry A. Quigley, Lisa A. Kerrigan, Mary E. Pease, and Erica N. Quigley

Purpose. To determine if photoreceptors die in primary open-angle glaucoma. Methods. Retinas were examined in a masked fashion from nine standard locations of 14 eyes with documented open-angle glaucoma and from nine age-matched control eyes. The number and density of photoreceptors, as well as the area and height of the outer nuclear layer, were calculated with an automated image analysis system. The number of photoreceptors per 0.1 mm of retina was determined. Results. No significant difference was seen between control and glaucomatous eyes in comparisons of photoreceptor density, outer nuclear layer height, or photoreceptors per 0.1 mm of retinal length in nine retinal zones. There was no detectable association between photoreceptor number and severity of glaucoma (denned as mild, moderate, or severe), visual field, and optic nerve fiber loss. In eyes in which damage predominated in the upper or lower visual field, no corresponding difference in photoreceptbr number in upper compared to lower retinal zones was observed. Conclusion. Photoreceptors are not lost in substantial numbers in primary open-angle glaucoma. Invest Ophthalmol Vis Sci. 1995;36:200-205.

1 he predominant damage to the retina in openangle glaucoma is loss of retinal ganglion cells and their axons.'~3The physical features of optic disc excavation and atrophy of the retinal nerve fiber layer support the concept that ganglion cell loss occurs.4'5 The pattern-evoked electroretinogram, a response that is dependent on ganglion cell presence,6 declines with damage from glaucoma, both in human eyes7 and in experimental monkeys.89 There have been no demonstrations of either primary or transsynaptic degeneration of mid-retinal or outer retinal cells in open-angle glaucoma. Indeed, the flash-evoked electroretinogram is normal even in advanced glaucoma,10 indicating no major effect on From the Glaucoma Service and the Dana Center for Preventive Ophthalmology, Wilmer Ophthalmological Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland. Suf/ported in part by Public Health Service research grants EY02I20 (HAQ), KY0I765 (Cure Facility Grant, Wilmer Institute), both awarded by the National Eye Institute, National Institutes of Health; and by National Glaucoma Research, a program of the American Health Assistance Foundation, Rockville, Maryland, and The Johns Hopkins School of Medicine Dean's Fund for Medical Research. Submitted for publication March 10, 1994; revised August 29, 1994; accepted September 6, 1994. Proprietary interest category: N. Reprint requests: Harry A. Ojiigley, Dana Center for Preventive Ophthalmology, Wilmer 120, Johns Hopkins Hospital, 600 N. Wolfe Street, Baltimore, MD 21287.

200

the rods, cones, and mid-retinal cells that generate this response. However, to our knowledge, no previous investigation has attempted to quantify the number of rods and cones in primary glaucomatous eyes. Recently, Panda and Jonas" reported a decrease in photoreceptor number in eyes with advanced secondary glaucoma caused by traumatic injury. Because potential photoreceptor loss in glaucoma may have clinical or pathophysiological significance, we designed the present investigation to compare the estimated number of photoreceptors in human eyes with a history of primary open-angle glaucoma to agematched, normal eyes.

METHODS The eyes for this study were obtained by postmortem donations to eye banks with informed consent of next of kin and in accordance with the tenets of the Declaration of Helsinki. We collected ophthalmic histories pertaining to glaucoma in all 14 eyes of 13 donors (Table 1). Either visual field studies (12 eyes) or optic nerve histology (11 eyes), or both, was available to provide staging and corroboration of damage from

Investigative Ophthalmology & Visual Science, January 1995, Vol. 36, No. 1 Copyright © Association lor Research in Vision and Ophthalmology

Photoreceptors Not Lost in Glaucoma

201

'o

- !

ddododooodo c 62

'•Si0 *> 2 | v

zzzzzzzzzzz>-zz cZZ

-s g 1 i i

Ills C

1> ui . .

ill? ii IT'Jj 2 II b

©ooomoooom

o o

OOOOOOOOOO

C3O rtO tuoQ

O II

a «-o S Si

ZZ

2 11 !

cZ c

o O

*

* O O O 03 rt

« S S

-a « «

glaucoma. Nine control eyes matched for age were selected from nine persons without a known history of eye disease (age: 72 ± 15 years for controls, 76 ± 18 years for those with glaucoma, mean ± 1 SD, P > 0.05, two-tailed Rest). As detailed below, control eyes also had normal retina and optic nerve head appearance on examination with the dissecting microscope, normal retinal and optic nerve examination by light microscopy, and an optic nerve area of neural tissue within the range we previously reported for identically prepared tissues in normal eyes. It is, therefore, unlikely that control eyes had glaucoma. All persons whose eyes were studied were white. There was a slightly higher proportion of male donors in the control group. The mean time to fixation of the glaucomatous eyes (8.5 ± 8.9 hours) was not significantly different from that of control eyes (9.2 ± 8.0 hours, two-tailed Rest, P = 0.85). The glaucomatous eyes were most often fixed in phosphate-buffered aldehydes, whereas the control eyes were typicallyfixedin aqueous aldehyde solutions (see last paragraph, Methods). All eyes were examined under the dissecting microscope for evidence of past ocular surgery or secondary ocular disease. One glaucomatous eye had a laser iridectomy but no signs of acute glaucoma (such as iris atrophy or peripheral anterior synechiae); three glaucomatous eyes had undergone cataract surgery. The axial lengths of the eyes were measured, because glaucomatous eyes may have a higher prevalence of myopia. This might be associated widi thinner retinas or a different distribution of photoreceptors. The mean axial length of the glaucomatous eyes (24.9 ± 1 . 4 mm) was slightly, but not significantly, greater than that of control eyes (23.8 ± 0.8 mm, two-tailed Rest, P= 0.07). A cross-section of the optic nerve was removed 1 to 3 mm posterior to the globe, and the superior and nasal locations were marked for orientation with one and two razor blade cuts, respectively. The samples were postfixed in 1% osmium tetroxide, dehydrated in graded ethanol, and embedded in epoxy resin. Onemicron sections were cut perpendicular to the major axis and stained with toluidine blue. The optic nerve cross-section was used to document whether fiber loss was present in glaucomatous and control eyes. Fiber loss was staged by comparison to previously published data on the neural area of normal optic nerves.3 From 0% to 25%, loss of fibers was denoted normal; from 25% to 50%, loss was denoted mild; from 50% to 75%, loss was denoted moderate; and 75% or more loss was denoted severe. There were four glaucomatous eyes in the normal neural area range, four in the mild range, and three each in the moderate and severe ranges. The location of the fiber loss was also recorded to correlate its location with the location of visual field defects and potential

5mm

5mm

RE 1. Diagram of the nine retinal locations from which ine samples were taken relative to the fovea and the nerve head.

oreceptor loss. Each control eye had a neural area in 25% of the normal mean value. The visual field tests were staged as follows: no ct = normal, small defect or nasal loss = mild, or loss in one hemifield = moderate, and major in upper and lower fields = severe. Evaluation of field test and of neural loss was performed indedently and in a masked fashion. The data were compared to give a final degree of injury estie; where the field and nerve estimates differed, nerve estimate was used (for example, field = erate, nerve = mild, final grade = mild). Nine samples of the retina were taken from each e eyes at the locations shown in Figure 1 using a m trephine. The samples were kept flat using a n screen in a filter during dehydration in graded nol. They were embedded in JB-4 resin (Polysces, Warrington, PA), and \-fim sections were ed with 0.5% hematoxylin for 30 minutes. The number of photoreceptor nuclei per unit (density) and the height of the outer nuclear (ONL) were measured, in a masked fashion, usthe VIDAS microscopic counting and measuret system (Roche Image Analysis Systems—Elon ege, NC, and Carl Zeiss, New York, NY). This sysacquires a digital video image of the tissue, with ei seen as dark areas whose size and number can stimated with appropriate software. The counts made in an area circumscribed by the ONL

boundaries in randomly chosen continuous reti strips (Fig. 2). We multiplied the density of photo ceptor nuclei within the ONL by the ONL height 0.1 mm to estimate the photoreceptor number i defined length of retina in each region. The 0.1-m length seemed an appropriate length to account local variation. We examined all three variables, d sity, ONL height, and nuclei per 0.1 mm, beca photoreceptor loss could be manifest in at least different ways. First, the ONL might remain the sa height with a decrease in its cell density as cells Or, the ONL might decrease in height as cells preserving the same cell density. Unless both possi ties were evaluated, a true difference between g coma and normal might be missed. Each of the th measurements was averaged from five different zo of each of the nine retinal locations. We did not tempt to differentiate between rod and cone nu (see Discussion). Four persons performed the counting proc Each evaluated 15 retinal areas on three separate o sions to estimate intraobserver and interobserver va tion. The coefficient of variation was approxima 5% for interobserver variation in nuclei per 0.1 m and ONL height counts. The total number of photoreceptors counted 67,697. The mean number of photoreceptor nu counted per eye was 2904 (SD = 776; n = 23); ran 1607 to 3606). These data were gathered from 45 l tions (five from each of nine retinal samples), e location comprising an area of approximately 5 square micrometers, for a total area of about 0. mm2 per retina. We measured both eyes of one person with g coma. Because intradonor variation may be less t that seen among donors, all summary calculations the groups with glaucoma were made with one of th eyes excluded and with both eyes included. No

FIGURE 2. Retinal photomicrograph of typical specimen retina stained with hematoxylin; outer nuclear layer (O is marked by lines. Original magnification, X320.

TABLE 2.

Mean Density, ONL Height, and Nuclei/0.1 mm for Control and Glaucoma Eyes (N)

Control All Glaucoma Glau: normal Glau: mild Glau: moder Glau: severe

(9)

(14) (4) (4) (3) (3)

Density

ONL Height

Nuclei/0.1 mm

187 (23) 219 (26) 225 (8) 321 (18) 237 (9) 178 (13)

40.3 (5.8) 37.1 (4.5) 38.8 (2.3) 36.1 (4.3) 36.0 (1.9) 37.1 (7.2)

75.3 (13.1) 80.8 (12.9) 86.1 (4.9) 82.4 (12.1) 85.1 (4.0) 67.4 (17.1)

ONL = outer nuclear layer; Glau = glaucoma; moder = moderate. Difference in density between control and all glaucoma significant (P = 0.007); other differences between these two groups not significant (P > 0.05). Differences between subgroups of glaucoma and normal not tested because of small samples. Units for mean (standard deviation) in density = nuclei per 100 square micrometers; in ONL height = micrometers; in nuclei/0.1 mm = estimated cell number in 100 micrometer length of retina.

ant difference was seen in any of the calculations; eyes were included in the analyses presented. Because the type of fixation might affect the nting of photoreceptor nuclei, we have evaluated etinal samples from each of five further control whose data are not included here. These were ined without prior fixation; half the retina of each was fixed in aqueous formalin and half in phos-buffered 4% glutaraldehyde/2% paraformaldee. Mean photoreceptor nuclei per 0.1 mm was not stically different between the fixation protocols.

SULTS

density of photoreceptors was actually higher in comatous than in control eyes (f-test, P — 0.007; e 2). There were no significant differences ben glaucomatous and control groups in the ONL ht or photoreceptor nuclei per 0.1mm (P > 0.05; e 2). The ONL height of the glaucomatous eyes somewhat less than the control ONL height. How, because of the higher density of photoreceptors laucomatous eyes, when the number of photoreors in a linear segment of retina was calculated, mean number of nuclei per 0.1 mm in the glaucoous eyes (81 ± 1 3 nuclei) was numerically greater the control mean (75 ± 1 3 nuclei), though not ficantly so (P > 0.05). These data have an 80% er to determine a mean difference between ps of 16 nuclei per 0.1 mm and a 95% power to ct a mean difference of 21 nuclei per 0.1 mm. With stratification of the glaucomatous eyes by rity of damage, all groups except the most severe values of density and nuclei per 0.1 mm that exed the normal mean (Table 2). The single excepwas the group with severe glaucoma, in which the es were lower. The value for nuclei per 0.1 mm he group with severe glaucoma can be attributed

solely to one outlier specimen, because the other t members in this group had values at or above normal range. One glaucomatous eye had selective superior sual field loss, whereas two others had selective in rior visual field loss. Comparisons of photorecep density and ONL height in superior versus infer zones show no tendency for photoreceptor numb to match the hemiretinal ganglion cell loss (Table The retinal zone, including the fovea (#9), had lowest density and the lowest number of photorecep nuclei per 0.1 mm in control and glaucomatous e (Table 4). None of the individual zones had a sign candy different number of photoreceptor nuclei tween glaucomatous and control eyes (Table 4). DISCUSSION

We found no evidence that there are fewer rods a cones in the retinas of persons with primary op angle glaucoma than in control eyes in persons similar ages. The mean estimate for photorecep number was actually higher in the group with gl coma, though the difference was not statistically nificant. There was no single region among the n zones that were studied in which the group with gl coma substantially differed from normal. There w no consistent trend toward fewer photoreceptors w increasing damage from glaucoma. The values of o retinal specimen with glaucoma differed dramatica from those of the others. The severe loss in mid-reti cells and in overall retinal thickness in this specim suggested the possibility that it had suffered a vascu occlusion, though we have no evidence of this in history or in its gross or microscopic appearan There was no trend toward fewer photoreceptors the more damaged half of three specimens with alti dinal glaucoma injury. In summary, we find no reas

TABLE 3.

Ratios of Upper to Lower Photoreceptor Numbers in Three Glaucomatous Eyes With Asymmetric Optic Nerve Damage Eye Number Nerve Damage

Inferior Superior Superior

Zones 1 and 2

0.98 0.74 0.85

Zones 3 and 4

1.12 1.07 1.75

Zones 5 and 6

0.87 1.25 0.99

Zones 7 and 8

0.89 0.86 1.12

Odd-numbered zones are from the superior retina. For nerve damage and photoreceptor loss to be concordant, ratios should be below 1.0 in eye 1 and above 1.0 for eyes 2 and 3.

onclude that photoreceptor atrophy is characterisof primary glaucoma. It is possible that detrimental cts on retinal neurons might occur that would not expressed as loss of cells, but such functional nges would not have been detected by this study. It has been speculated that loss of photoreceptors ght provide evidence for impaired choroidal vascufunction in glaucoma.11 Alternatively, loss of photoeptors might occur in glaucoma as a transsynaptic eneration after loss of ganglion cells and the invening bipolars. Our study does not support either hese as having a role in primary open-angle glauma. A previous report11 that found a loss of rods and es in eyes with secondary glaucoma had several erences from the present investigation. Perhaps st important, the glaucomatous eyes in that study e blind, painful eyes enucleated because of secondglaucoma after severe injury. Those eyes may have ered undetected vascular occlusion, despite any ect evidence for this at enucleation and histologic mination. Furthermore, concussive trauma rouly leads to loss of photoreceptors, as demonstrated experimental models.12 In addition, several of the s in that report had undergone glaucoma surgery

BLE 4.

Nuclei/0.1 mm of Retina in Nine dividual Zones Zone 1

2 3 4 5 6

7 8 9

All Glaucoma

90.8 85.0 97.4 81.4 80.8 79.0 77.6 74.0 62.8

(16.6) (14.3) (16.8) (20.9) (20.5) (19.3) (15.9) (15.2) (19.7)

Control

98.5 (14.4) 88.7 (6.8) 93.3 (10.7) 94.0 (7.7) 78.4 (15.5) 91.1 (21.6) 80.5 (16.1) 84.7 (11.7) 71.1 (13.9)

s are mean (±standard deviation) nuclei per 100 fj.m linear th of retina. In the group with glaucoma, (n = 14), zone 9 is ficantly lower (P < 0.05) than all zones except zone 8. In rol group (n = 9), zone 9 is significantly lower than all zones pt zones 5 and 7. None of the individual zone data are ficantly different between the group with glaucoma and the rol group (all P > 0.05).

that might have been associated with transient ch dal and/or serous retinal detachment, either of w also could lead to loss of rods and cones. There no estimates of the optic nerve fiber loss in the re of Panda and Jonas,11 though we might assume there were few ganglion cells remaining in eyes were considered appropriate for enucleation. T the difference in our results may be an accurat flection of differences between the consequence secondary glaucoma and their absence in eyes primary glaucoma, such as the ones we studied. We included only eyes with documented hist of primary glaucoma. We think that in every case mechanism was open angle, though one eye had h laser iridectomy. We used eyes with either an avai visual field test or a detailed estimation of optic n fiber number by histologic examination. A large n ber of photoreceptor nuclei were counted wit image analysis system from a variety of location the posterior retina in each specimen. Our me has acceptable reproducibility both within and am observers. The use of a plastic embedding med instead of paraffin improved the resolution of ind ual photoreceptor nuclei. Our study might have failed to detect differe between glaucoma and normal eyes because o modest sample size. However, a power calculatio dicates that we had a 95% chance to detect a 29% in photoreceptor density if it had occurred. This difference of the magnitude reported by Panda Jonas.11 Although there may be autolytic chang eye bank tissue that could obscure some finding matched the control tissues for all relevant par ters, including time from death to fixation. A s difference between some members of the co group and the group with glaucoma in the solu in which the aldehyde fixatives were used is unl to be of importance. Although retinal area is re to photoreceptor number, glaucomatous and no eyes in this study did not differ in axial length surements, suggesting that we controlled adequ for eye size.13 The finding that there are fewer photorece in our specimens in the zone centered on the f

205

Photoreceptors Not Lost in Glaucoma was initially puzzling to us. We then compared the available information on regional distribution of rods plus cones and realized that our data are consistent with the estimates of Osterberg14 and Curcio. ls Within the central 30° of the retina; the density of rods is relatively constant except in the centralmost 1° (the fovea), where there are only cones. The cone number is small relative to rods outside the fovea. Hence, the rod density largely determines the photoreceptor density in our data. Only in the foveal zone does the cone density reach a high level, but it still has a lower value than the density of rods (plus cones) elsewhere in the posterior retina. Because our study and the previous report" of histologic counts of photoreceptors concentrated largely on areas of the retina in which rods predominate, it is possible that a subtle loss of cones—for example, a selective decrease in the number of shortwavelength sensitive (blue) cones—might not have been detected. To approach this question, we have studied the number Of cones in monkey eyes with chronic experimental glaucoma (data not shown) in the area 900 fi to 1200 fim from the foveal center. The number of cones in glaucomatous.eyes was within 4% of that in the normal fellow eyes of these monkeys. It is unclear whether further investigation of photoreceptor differences between glaucomatous and normal eyes will be productive. We hope that continued study of the anatomic changes characteristic of glaucoma will stimulate correlative research into the improved functional testing of this disorder.

3.

4.

5.

6.

7.

8.

9.

10. 11.

12. Key Words glaucoma, pathology, photoreceptor, pathogenesis, image analysis

13.

References 1. Quigley HA, Green WR. The histology of human glaucoma cupping and optic nerve damage: Clinicopathologic correlation in 21 eyes. Ophthalmology. 1979; 10:1803-1827. 2. Quigley HA, Addicks EM, Green WR, Maumenee AE.

14.

15.

Optic nerve damage in human glaucoma: II: The site of injury and susceptibility to damage. Arch Ophthalmol. 1981;99:635-649. Quigley HA, Addicks EM, Green WR. Optic nerve damage in human glaucoma: III: Quantitative correlation! of nerve fiber loss and visual field defect in glaucoma, ischemic neuropathy, disc edema, and toxic neuropathy. Arch Ophthalmol. 1982; 100:135-146. Quigley HA, Katz J, Derick RJ, Gilbert D, Sommer A. An evaluation of optic disc and nerve fiber layer examinations in monitoring progression of early damage from glaucoma. Ophthalmology. 1992;99:19-28. Sommer A, Katz J, Quigley HA, et al. Clinically detectable nerve fiber atrophy precedes the onset of glaucomatous field loss. Arch Ophthalmol. 1991; 109:77-83. Maffei L, Fiorentini L. Electroretinographic responses to alternating gratings before and after section of the optic nerve. Science. 1981; 211:953-955. MayJG, Ralston JV, Reed JL, van Dyk HJL. Loss in pattern-elicited electroretinograms in optic nerve dysfunction. Am J Ophthalmol. 1982;93:418-422. Marx MS, Podos SM, Bodis-Wollner I, et al. Flash and pattern electroretinograms in normal and laser-induced glaucomatous primate eyes. Invest Ophthalmol VisSci. 1986; 27:378-386. Johnson MA, Drum B, Quigley HA, Sanchez RM, Dunkclbergcr GR. Pattern-evoked potentials and optic nerve fiber loss in monocular laser-induced glaucoma. Invest Ophthalmol Visual Sci. 1989; 30:897-907. Leydhecker G. The electroretinogram in glaucomatous eyes. BrJ Ophthalmol. 1950; 34:550-554. Panda S, Jonas JB. Decreased photoreceptor count in human eyes with secondary angle-closure glaucoma. Invest Ophthalmol Vis Sci. 1992;33:2532-2536. SipperleyJO, Quigley HA, GassJDM. Traumatic retinopathy in primates—die explanation of commotio retinae. Arch Ophthalmol. 1978;96:2267-2273. Panda-Jonas S, Jonas JB, Jakobczyk M, Schneider U. Retinal photoreceptor count, retinal surface area, and optic disc size in normal human eyes. Ophthalmology. 1994; 101:519-523. Osterberg GA. Topography of the layer of rods and cones in the human retina! Ada Opththal. 1935; 13(suppl):l-97. . Curcio C. Aging loss of photoreceptors in human eyes. Invest Ophthalmol Vis Sci. 1993;34:3278-3297.