Human Optic Nerve Fiber Count and Optic Disc Size

Transcription

1 Investigative Ophthalmology & Visual Science, Vol. 33, No. 6, May 1992 Copyright Association for Research in Vision and Ophthalmology Human Count and Disc Size Josr B. Jonas, Andreas M. Schmidt, Jens A. Muller-Bergh, Ursula M. Schldrzer-Schrehardr, and Gottfried O. H. Naumann In the optic nerve head, the optic nerve fibers are represented by the neuroretinal rim. The rim area showing a high interindividual variability is positively correlated with the optic disc size. This study was performed to address the question of whether, in addition to having a larger neuroretinal rim, eyes with large optic discs also have a higher count of optic nerve fibers compared to eyes with small optic nerve heads. Histologic semithin sections of 72 optic nerves of 56 cornea donors were histomorphometrically evaluated using a computerized image analyzer. The optic nerve fiber count increased significantly (P = 0.01) with enlarging optic disc size. The nerve fiber count was positively correlated with the retrobulbar optic nerve cross section area. It decreased with advancing age, with a mean annual loss of about 4,000 fibers. The nerve fiber density per disc area decreased with increasing optic disc area. Mean and median of the minimal nerve fiber diameter was larger in older subjects. The results may indicate that the optic nerve fiber count, and the anatomic reserve capacity in progressive optic neuropathies, are higher in eyes with large optic discs than in eyes with small optic nerve heads. The optic nerve fiber population decreased with advancing age. This is important for progression, pseudoprogression, and prognosis of optic neuropathies. nerve fiber crowding is more marked in eyes with small optic discs than in eyes with large optic nerve heads. The age-related loss of predominantly small optic nerve fibers can potentiate the optic nerve atrophy in glaucoma and Alzheimer's disease, with both damaging preferentially large axons. Invest Ophthalmol Vis Sci 33: ,1992 The neuroretinal rim is the intrapapillary equivalent of the retinal and optic nerve fibers. It is a target in the morphologic evaluation of optic nerve anomalies and diseases. 1 " 5 The rim area showing in previous studies a high interindividual variability is independent of age, refractive error, sex and right or left eye. It increases significantly with increasing optic disc area. 6 " 9 The positive relationship between the neuroretinal rim area and the optic disc size prompts the question of whether eyes with large optic discs have more optic nervefibers than eyes with small optic discs. A higher optic nerve fiber count possibly could be associated with a better data transfer between the eye and the brain. It could indicate a greater anatomic reserve capacity in progressive optic neuropathies such as glaucoma. Previous studies have not conclusively shown the existence or nonexistence of such a correlation. Mikelberg and colleagues 10 did not detect a statistically From the Department of Ophthalmology and Eye Hospital, University Erlangen-Nurnberg, FRG. Supported by Dr. Helmut und Margarete Meyer-Schwarting- Stiftung, Bremen, and Deutsche Forschungsgemeinschaft (Klinische Forschungsgruppe Glaukome, DFG Na 55/6-1/Jo). Submitted for publication: May 15, 1991; accepted December 13, Reprint requests: Dr. J. B. Jonas, University Eye Hospital, Schwabachanlage 6, D-8520 Erlangen, FRG. significant relation between axon count and area of the scleral canal. Applying a multiple regression analysis to correct the effect of age, they calculated that eyes with small scleral canals had more axons than eyes with large scleral canals, a finding difficult to explain. Another study performed previously on the optic nerves of eight subjects also did not reveal a clear correlation between optic nerve fiber count and optic disc size. 11 The sample size, however, was too small for a statistical analysis. Correspondingly, Balazsi, Drance, and coworkers 12 did not detect a relation between the fiber count in 16 optic nerves and the area of the optic disc and neuroretinal rim determined post mortem at the time of gross pathologic examination. Evaluating a larger number of optic nerves sufficient for a statistical work-up of data, the present investigation was performed to address the same question if eyes with large optic discs have a higher count of optic nerve fibers than eyes with small optic discs. Materials and Methods Seventy two eyes (43 right eyes, 29 left eyes) of 56 white cornea donors (33 men, 23 women) with a mean age of 54.7 ± 19.8 yr (mean ± standard deviation; minimum 19 yr, maximum 88 yr) were included in the study. Twenty two optic nerves of 19 subjects already had been evaluated in a recent study. For 8 of 2012

2 No. 6 HUMAN OPTIC NERVE FIBER COUNT AND OPTIC DISC SIZE / Jonos er ol these 19 subjects, the optic nerve scleral canal size had been measured.1' The remaining 50 optic nerves of 37 donors were evaluated for the first time using the same equipment and method as was used in the prestudy. For all of them, the area of the inner aperture of the optic nerve scleral canal had been measured. None of these eyes had any known disease. Gross or histologic examination revealed no significant abnormalities or pathologic changes in the anterior chamber angle or in the posterior fundus. Male and female donors did not differ significantly in age (53.6 ± 19.6 yr versus 56.4 ± 20.1 yr; P = 0.58) and in area of the inner aperture of the optic nerve scleral canal (2.33 ± 0.44 mm2 versus 2.28 ± 0.41 mm2; P = 0.85). The globes had been enucleated 3.2 ± 1.9 hr after blood circulation had been stopped (range hr). Total time between stoppage of circulation and fixation was 4.7 ± 2.3 hr (range hr), including the time between enucleation and preparation, when the globes were kept at a temperature of about 4 C. After trephining a corneoscleral button to obtain the corneal transplant, the globes werefixedin a solution of 1 % glutaraldehyde and 4% paraformaldehyde in 0.1 % phosphate buffer (ph 7.2) or in buffered 2.5% glutaraldehyde. The superior and nasal positions of each optic nerve were marked with a waterproof color marker and 7-0 nylon sutures through the optic nerve meninges. The optic nerve length ranged from 5-16 mm. Slices 2-3 mm in thickness were obtained from the fixed optic nerves at a position corresponding to 2-5 mm behind the globe. Specimens were postfixed in 2% osmium tetroxide, dehydrated in an ethanol 2013 series, and embedded in Epon. The specimens were prepared routinely, just as for electron microscopy. Semithin sections of 1 pm thickness were obtained at the specimen end that originally faced the globe. Stained with toluidine blue (Fig. 1), the slides were histomorphometrically evaluated with an automatic video image analysis system (IBAS 2000; KontronZeiss, Eching-Munich, Germany). The examination was performed directly from the slides, not from photographs. The optic nerve cross section was divided into 24 peripheral sectors of equal size and one central sector.11 Number, minimal diameter, and area of the optic nervefiberswere assessed at two randomly chosen places in each of the peripheral sectors and at one place in the central sector. The optic nerve cross section area, except for the leptomeninges, was plani metrically measured on photographs. The total nerve fiber count was calculated as a product of the number of all nerve fibers counted multiplied by the optic nerve cross section area and divided by the summed area covered by all 49 video images. Ten to fifteen thousand nerve fibers per optic nerve were counted. The method, including its reproducibility, was described in detail recently.10'11 To measure the area of the optic nerve scleral canal, the retina and choroid of thefixedglobes were mechanically removed from the sclera and the optic nerve head. Under retroillumination, macrophotographs were taken with a millimeter scale aligned. They were planimetrically analyzed. If both nerves of the same individual had been examined, only one randomly chosen nerve was taken for interindividual comparison. Fig. 1. Microphotograph of the retrobulbar part of a human optic nerve; Eponembedded, semithin {1 fim), toluidine stained; distance to the globe: 3 mm.

3 2014 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / May 1992 Vol. 33 Results Mean optic nerve fiber count was 1,158,000 ±222,000 fibers (range 777,000-1,679,000 fibers). The optic nerve fiber count did not vary significantly between males and females (1,184,000 ± 235,000 fibers versus 1,131,000 ± 200,000 fibers; P = 0.36) and right or left eye (1,177,000 ± 213,000 fibers versus 1,131,000 ± 236,000fibers;P = 0.38). Side differences of less than 100,000 nerve fibers between the right and left eye occurred in 4 of 16 subjects (25%); of less than 200,000 fibers in 9 of 16 subjects (56%); of less than 300,000 fibers in 13 subjects (81%); of less than 400,000 fibers in 15 subjects (94%); and of less than 500,000 in all 16 subjects bilaterally examined. The nerve fiber count decreased significantly with increasing age. The linear annual loss was about 4,000 fibers starting at a calculated original population of about 1,400,000 fibers at birth (Fig. 2). The optic nervefiber count correlated positively with increasing size of the inner aperture area of the optic nerve scleral canal. This indicated a plus of about fibers per every square millimeter of fixationcorrected optic nerve scleral canal area (Fig. 3). For the latter calculation, the data measured on the fixed specimen had been corrected for the fixation-related area shrinkage factor of 1:1.13 (see below). The correlation remained significant if all eyes evaluated were taken for statistical analysis and if the favorable extreme value in the upper right corner of the scattergram was excluded. Side differences in optic nerve fiber count evaluated bilaterally for 16 subjects were not significantly correlated with side differences in optic disc area. This may be a result of the small number of subjects examined bilaterally. The nerve fiber count was significantly related to the optic nerve cross section area (Fig. 4). For these parameters, the side differences were significantly correlated with each other (R = 0.92; P < ). The density of the nerve fibers calculated as a ratio of nerve fiber count to disc area was significantly correlated with the optic disc area (R = -0.50; P = ). The nerve fiber density was significantly higher in eyes with small optic nerve heads than in eyes with large optic discs. Minimal fiber diameter (mean: 1.18 ± 0.16 /im; median: 0.96 ± 0.17 ^ra) and fiber cross section area (mean: 1.75 ± 0.31 square ^m; median: 1.07 ±0.18 square ^m) were slightly larger than figures reported recently. 10 " 15 Mean and median of the minimal fiber diameter increased significantly with increasing age (Fig. 5). They were independent of the size of the optic nerve scleral canal, the optic nerve cross section area, and the nerve fiber count (P > 0.30). They did not Y = -4,021 X + 1,378,000 R = -0.36; P = Count Fig. 2. Scattergram showing the optic nerve fiber count and age in 56 normal eyes of 56 subjects. 1OOOOOO T Age (years)

4 No. 6 HUMAN OPTIC NERVE FIDER COUNT AND OPTIC DISC SIZE / Jonas er al f UVVWV Fig. 3. Scattergram showing the optic nerve fiber count and optic disc area in 45 normal eyes of 45 subjects. Correlation coefficient R = 0.37; slope of the regression line: Y = 175,000 X + 730,000; P = Count r. " OOOOOO- ^ " Disc Area (mm2) differ significantly between men and women (mean: 1.19 ± 0.17 Atm versus 1.14 ± 0.14 /*m;p = 0.24)and right or left eye (P = 0.53). The median of the nerve fiber cross section area showed a marginally significant correlation with age (correlation coefficient R = 0.32; P = 0.05). Similar to the mean and median of the minimal nerve fiber diameter, it was independent of sex (P = 0.66), right or left eye (P = 0.47), optic disc area (P = 0.55) and nerve fiber count (P = 0.67). Mean area of the inner aperture of the optic nerve scleral canal in the fixed specimens was 2.30 ± 0.42 mm 2. Compared to values obtained in unfixed specimens, 16 it indicates an area shrinkage factor of 2.59/ r Y = 114,000 X + 218,000 R = 0.82; P < Fig. 4. Scattergram showing the optic nerve fiber count and retrobulbar optic nerve cross-section area in 56 normal eyes of 56 subjects Count Retrobulbar Cross Section Area (mm2)

5 2016 INVESTIGATIVE OPHTHALMOLOGY G VISUAL SCIENCE / May 1992 Vol. 33 Median of the Minimal Diameter (mikrometer) Fig. 5. Scattergram showing the median of the minimal optic nerve fiber diameter and age in 56 normal eyes of 56 subjects. Correlation coefficient R = 0.33; slope of the regression line: Y = X ;/> = 0.0l. 50 Age (years) 2.30 mm 2 or 13%. To obtain correlations close to intravital conditions, the measured data of the optic nerve scleral canal were corrected for the shrinkage factor. The optic nerve scleral canal area was independent of age (P > 0.50). It was significantly correlated with the nerve fiber count (Fig. 3). It was correlated but not significantly correlated with the optic nerve cross section area (r = 0.27; P = 0.07). Mean value of the latter was 8.22 ± 1.60 mm 2. With a maximum time between death and fixation of 9.5 hr, the time between death and enucleation or fixation of the globe was not significantly correlated with the nerve fiber count (P = 0.88 and P = 0.17), mean minimal nerve fiber diameter (P = 0.76 and P = 0.61), nerve fiber area (P = 0.60 and P = 0.82), and retrobulbar optic nerve cross section area (P = 0.98 and P = 0.38). Discussion In this sample of 72 optic nerves of 56 human subjects, a significant correlation was found between the optic nerve fiber count and the area of the inner aperture of the optic nerve scleral canal (Fig. 3). The latter can be regarded as corresponding with the optic disc. In a recent study of unfixed specimens, the area of the inner aperture of the optic nerve scleral canal determined histologically post mortem did not vary significantly in size and form from the optic disc assessed intravitally in another series of eyes. 16 The optic nerve fiber count increased by a factor of 175,000 fibers or about 15% to 20% per every additional square millimeter of optic nerve scleral canal area or optic disc area. The regression line of this correlation (slope: 175,000 fibers/mm 2 optic disc area) is similar to the regression line in the scattergram showing the correlation between neuroretinal rim area and optic disc size in normal eyes with circular steep optic disc cupping (slope: 0.30 mm 2 of rim area per 1.00 mm 2 of optic disc area). 8-9 It could be interpreted that in normal optic nerve heads with circular steep cupping, an increase of 0.30 mm 2 of neuroretinal rim area may represent an increase of about 175,000 optic nerve fibers. It must be taken into account, however, that the neuroretinal rim does not change with age 6 " 9 despite a significant loss of optic nerve fibers with increasing number of years (Fig. 2). 1O " 13;17>18 The equation of about 175,000 nerve fibers per 0.30 mm 2 of neuroretinal rim area may be valid only for a certain period of life. The correlation between nerve fiber count and disc size resulting from this study agrees with a report of Quigley, Coleman, and Dorman-Pease. 19 It contradicts the finding of Mikelberg's investigation with a smaller sample size. 10 If Mikelberg's data, as shown in Figure 3 of his paper, and our data were put together, the optic nerve fiber count and the optic disc area would remain significant (R = 0.27; P = 0.04; slope: 141,000 fibers/mm 2 of disc area). Whether the smaller sample size of Mikelberg's study prevented a significant correlation or whether the nerve fiber count may be only marginally correlated with the optic disc area remains to be studied. The relation between nerve fiber count and disc area points to a higher anatomic reserve capacity in eyes with large optic nerve heads compared to eyes with small optic discs. This can have prognostic value

6 No. 6 HUMAN OPTIC NERVE FIDER COUNT AND OPTIC DISC SIZE / Jonos er ol 2017 for patients with progressive optic neuropathies, such as demyelinating diseases or glaucoma. Regarding the latter, the susceptibility to optic nerve damage in white patients with primary open-angle glaucoma has been reported to be independent of the optic disc size. 20 Several factors and mechanisms may compensate each other. These may include a higher proportion of interpore connective tissue on total lamina cribrosa tissue 21 and a higher optic nerve fiber count in large discs compared to small optic discs. Also possible is a less pronounced capacity of the lamina cribrosa to deform, 22 a possible inhibition of the orthograde axoplasmic flow, and a lack of space in small optic discs compared to large discs. 9 In the present study, the nerve fiber density per disc area was higher in eyes with small optic nerve heads than in those with large optic discs. This indicates a crowding of nerve fibers in small optic nerve heads. It remains to be studied whether the optic nerve fiber count increases with the optic disc area in races other than whites. The larger optic discs in blacks compared to whites 22 have been considered pathogenetically important in a higher glaucoma susceptibility in blacks. The optic nerve fiber count did not vary between males and females. This corresponded to a lack of difference in optic disc size between the two sexes. 6 " 9 The nerve fiber count was significantly correlated with the retrobulbar optic nerve cross section area (Fig. 4), indicating that a thin retrobulbar optic nerve can be taken as an indicator of an optic nerve atrophy. This may be useful information for patients with opaque optic media. In turn, the optic nerve cross section area was related to the optic disc size. This closes the circle of correlations between nerve fiber count, optic disc area, and optic nerve cross section area. As has been shown in many studies, 10 " 13 ' 1718 the optic nerve fiber count decreased with increasing age in the present study, indicating an age-related physiologic optic nerve atrophy. This was already pointed out recently 1 ' and is important for assessing a progression of optic neuropathies. A continuing loss of visual function need not be necessarily a result of progression of the disease but may be caused only by the natural ongoing age-related loss of optic nerve fibers. This warrants early therapy for diseases such glaucoma, because in addition the disease itself, age will diminish the optic nerve fiber population. Mean nerve fiber count was 1,158,000 ± 222,000 fibers. Thisfigureis comparable to those published in the literature. 10 " 13 ' 15 ' 18 ' 23 The high interindividual variability of the nerve fiber count, ranging from 777,000-1,679,000 fibers before, and from 890,000-1,856,000 fibers after statistical correction of the effect of age may be one of several factors responsible for a varying performance of visual function and susceptibility to optic neuropathies. It may be caused by a varying rate of regression of originally formed embryonic optic nerve fibers. 24 ' 25 Median and mean values of the minimal nerve fiber diameter increased with increasing age. This suggests a predominant loss of small optic nerve fibers. Similar results have been reported by Mikelberg and coworkers 10 and Repka and Quigley 14 but were not found by Johnson and coworkers. 18 This is important for patients with Alzheimer's disease and glaucoma in which the large axons have been shown to be lost preferentially. 26 " 28 Thus, age as well as glaucoma or Alzheimer's disease may potentiate their devastating effects on the optic nerve fiber population. Aging destroys the thin axons, and the remaining thick axons are damaged by the diseases themselves. Key words: optic nerve, glaucoma, optic disc size, neuroretinalrim,age References 1. Betz PH, Camps F, Collignon-Brach C, and Weekers R: Photographic stereoscopique et photogrammetrie de l'excavation physiologique de la papille. J Fr Ophthalmol 4:193, Drance SM and Balazsi G: Die neuroretinale Randzone beim friihen Glaukom. KJin Monatsbl Augenheilkd 184:271, Airaksinen PJ, Drance SM, and Schulzer M: Neuroretinal rim area in early glaucoma. 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7 2018 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / May 1992 Vol. 33 Weis MM: The normal human optic nerve: Axon count and axon diameter distribution. Ophthalmology 96:1325, Jonas JB, Gusek GC, Guggenmoss-Holzman I, and Naumann GOH: Size of the optic nerve scleral canal and comparison with intravital determination of optic disc dimensions. Graefes Arch Clin Exp Ophthalmol 226:213, Dolman CL, McCormick AQ, and Drance SM: Aging of the optic nerve. Arch Ophthalmol 98:2053, Johnson BM, Miao M, and Sadun AA: Age-related decline of human optic nerve axon populations. Age 10:5, Quigley HA, Coleman AL, and Dorman-Pease ME: Larger optic nerve heads have more nervefibersin normal monkey eyes. Arch Ophthalmol 109:1441, Jonas JB, Fernandez MC, and Naumann GOH: Correlation of the optic disc size to the glaucoma susceptibility. Ophthalmology 98:675, Jonas JB, Mardin CY, SchlOtzer-Schrehardt U, and Naumann GOH: Histomorphometry of the human lamina cribrosa surface. Invest Ophthalmol Vis Sci: 32:401, Chi T, Ritch R, Stickler D, Pitman B, Tsai C, and Hsieh FY: Racial differences in optic nerve head parameters. Arch Ophthalmol 107:836, Oppel O: Mikroskopische Untersuchung iiber die Anzahl und Kaliber der markhaltigen nfasern im Fasciculus opticus des Menschen. Graefes Arch Clin Exp Ophthalmol 166:18, Rajic P and Riley KP: Overproduction and elimination of retinal axons in the fetal rhesus monkey. Science 219:1441, Provis JM, van Driel D, Billson FA, and Russel P: Human fetal optic nerve: Overproduction and elimination of retinal axons during development. J Comp Neurol 238:92, Quigley HA, Sanchez RM, Dunkelberger GR, L'Hernault NL, and Baginski TA: Chronic glaucoma selectively damages large optic nervefibers.invest Ophthalmol Vis Sci 28:913, Quigley HA, Dunkelberger GR, and Green WR: Chronic human glaucoma causing selectively greater loss of large optic nervefibers.ophthalmology 95:357, Sadun AA and Bassi CJ: nerve damage in Alzheimer's disease. Ophthalmology 97:9, 1990.