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Fig. 8.1: Noncontact lenses: +60D, +78D and Volk superfield lenses

It is important to draw the appearance of the optic nerve head based on these methods. Though drawing of the optic disk suffers from the disadvantage of being subjective in nature, this does offer a quick and inexpensive method of evaluation of the optic nerve head in patients with glaucoma during follow-up. In addition, photographs may not be possible in all cases such as patients with rigid miotic pupils and those with significant media opacities. However, wherever possible, photographs are an indispensable adjunct to clinical evaluation.

Features of Glaucomatous Disk Damage

Cup-Disk Ratio

Early studies by Armaly et al have reported that the vertical and horizontal cup-disk diameter ratios are useful for the quantification of glaucomatous optic neuropathy and for early detection of glaucoma.12 The ratio has limited value in the identification of glaucomatous damage, because of the wide variability in the size of the optic cup in the normal population.

Diskmarginisdefinedbyinneredgeofwhite scleral ring (outer arrows), and the optic cup isthelevelatwhichneuroretinalrim(NRR)steeps (inner arrow) (Figs 8.2 and 8.3). A large cupdisk ratio can be normal if the optic disk is large13 and a small cup-disk ratio may be glaucomatous if the optic disk is small14 (Fig. 8.4). The problem with estimating cup-disk ratio as a measure of

Fig. 8.2: Disk margin (black arrow) and cup margin (white arrow)

A

B

Figs 8.3A and B: Vertical disk diameter and horizontal disk diameter

A

B

Figs 8.4A and B: Cup-disk ratio in relation to optic disk size. A Optic disk is small with small cup and still has inferior notch (white arrow) with nerve fiber layer defect (black arrows) B Cup-disk ratio in a large disk

glaucomatous damage is that it is difficult to decide if the cup is physiological in a large disk or pathological in a small or normal-sized disk. In a recent study by Garway-Heath et al, vertical cup-disk diameter ratio corrected for the optic disksizewasthebestvariabletoseparatebetween normal subjects and patients of ocular hypertensionwithretinalnervefiberlayerdefect.15 Therefore, in the clinical description of the optic nerve head, it is important to state the vertical cup-disk diameter ratio in combination with the estimated disk size. The disk diameter can be easily measured by adjusting the slit-lamp beam height to the edges of the disk while viewing

Optic Disk Assessment in Glaucoma 117

Fig. 8.5: Measurement of disk diameter with slitlamp biomicroscopy with use of noncontact lenses

the disk with a 60D lens (Fig. 8.5).16 The measurement by this method is roughly equal to the measurementobtainedbytheplanimetryofdisk photographs by Litmann’s correction. Measurements can also be made with other lenses by multiplying the measured value with the appropriate magnification factor, Goldmann contact lens X1.26 and Volk superfield lens X1.5.16

It is important to differentiate contour cupping from color cupping. The margin of the cup should be determined by the bend of the small vessels

Fig. 8.6: Asymmetry of cupping in relation to asymmetry of disk size. The left optic disk is larger than right optic disk and has a larger optic cup

damage. In the glaucomatous optic disk, the pale and translucent atrophic tissue may replace the normal pink color of the neuroretinal rim which can result in a field defect in the corresponding opposite hemisphere.

Vascular Changes

Splinter hemorrhages on the optic disk are a common finding in glaucoma patients (Fig. 8.11). Various studies have shown that disk hemorrhages in association with localized nerve fiber layer defects and notches of the neuroretinal rim are more common among patients of normal tension glaucoma.18, 19 A possible explanation for the difference in frequency has been suggested by Jonas et al. They stated that the amount of blood leaking out of a vessel into the surrounding tissue depends on the intraocular pressure when the bleeding occurs.19 High transmural pressure gradient in normal pressure glaucoma leads to larger disk hemorrhages. Also, since the absorption rate of disk hemorrhages depends on the size of the disk bleed, the hemorrhages in patients of normal pressure glaucoma may take a longer time to disappear and thus have a higher chance to be detected than the disk

Fig. 8.11: Disk hemorrhage

Optic Disk Assessment in Glaucoma 121

hemorrhages in patients of high pressure glaucoma.20

Hemorrhages in glaucoma usually appear as splinter-shaped or flame-shaped hemorrhages on the disk surface21 (Fig. 8.11). They usually precede neuroretinal rim changes and visual field defects. The defects corresponding to the location of the hemorrhage may be expected to appear weeks to year later.22 The presence of disk hemorrhages is considered an indication for the enhancement of treatment of glaucoma.

Configuration of Vessels

The retinal vessels on the optic nerve head can provide clues about the topography of the disk. Nasalization of the vessels and baring of circumlinear vessels can be seen in glaucoma as well as in other diseases of the optic nerve. Bayoneting of the vessels can be seen if the rim is absent or very thin. This causes the vessels to pass under the overhanging edge of the cup and then make a sharp bend as they cross the disk surface. This convoluted appearance of the vessels is called ‘bayoneting’.

Peripapillary Atrophy

The zone closer to the optic nerve head with retinal pigment epithelium (RPE) and choroidal atrophy and baring of sclera is called zone β. The more peripheral zone with only RPE atrophy is called zone α (Fig. 8.12). A highly significant correlation has been reported between the location of peripapillary atrophy and visual field defects.23 Sometimes, these changes may represent a congenital anomaly, especially in myopic eyes. However, appearance of these changes de novo or their presence in small, nonmyopic disks should be viewed with suspicion. Peripapillary atrophy may be focal or circumferential (Figs 8.13 and 8.14).

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Fig. 8.12: Peripapillary atrophy. The diagram shows atrophic zone closer to the optic nerve head called zone β and the more peripheral zone called zone α

Fig. 8.13: Peripapillary atrophy: Localized in the temporal area of the disk

Retinal Nerve Fiber Layer Abnormalities

Examination of the nerve fiber layer is often useful in detecting early glaucomatous damage among patients of ocular hypertension with normal disk appearance and normal visual fields. The neuroretinal rim is formed by axons converging from the retina to the scleral canal. Since the axons are spread out in a thin layer in the retina, even minor losses of the axons can be observed in the retinal nerve fiber layer. In healthy eyes, the nerve fiber layer appears opaque with

Fig. 8.14: Peripapillary atrophy: generalized

radially oriented striations. The small retinal blood vessels have a blurred and crosshatched appearance, as they lie buried in the nerve fiber layer. The best way to see the nerve fiber layer defect is through a dilated pupil with a stereoscopic lens, at the slit-lamp, using white or green light and a wide-slit beam. In the presence of nerve fiber layer atrophy, the small retinal blood vessels become more clearly visible and appear unusually sharp, clear and well focused (Fig. 8.15). The fundus in the affected area appears darker and deeper red in contrast to the silvery or opaque hue of the intact nerve fiber layer. Defects may be in the form of a wedge shape arising from the disk margin and widening towards the periphery, are pathological (Fig. 8.16), while slit-like defects narrower than the adjacent blood vessels may be physiological. Diffuse areas of atrophy are less common in early glaucoma and more difficult to identify.

Myopic Changes vs Glaucoma

Myopic disks can present difficulty in evaluation for glaucoma due to the tilted disks, peripapillary atrophy and shallow cupping. One needs to

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Optic Nerve Coloboma

Optic nerve colobomas typically demonstrate enlargement of the papillary region, partial or complete excavation, blood vessels entering and exiting from the border of the defect and a glistening white surface. The visual field defects can be in the form of generalized constriction, centrocecal scotomas, altitudinal defects, arcuate scotomas, enlargement of the blind spot and ring scotomas that can mimic those found in glaucomatous eyes.

Morning glory syndrome is a variant of optic disc coloboma and is characterized by a large excavated disk, central core of white or gray glial tissue surrounded by an elevated annulus of variably pigmented subretinal tissue (Fig. 8.18). The retinal vessels appear to enter and exit from the margins of the disk, are straightened and often sheathed.

Fig. 8.18: Optic disk photograph showing characteristic morning glory syndrome

Congenital Optic Disk Pit

Congenital optic disk pits appear gray or yellowish-white, round or oval, localized depression within the optic nerve (Fig. 8. 19). They are located within the temporal aspect of the disk in over half of the cases and centrally

Fig. 8.19: Optic disk photograph showing congenital optic disk pits

in about one-third. Involvement is usually unilateral in about 80% cases and the optic disk is larger on the involved side. Approximately 55-60% of the eyes have a field defect in the form of arcuate scotomas, paracentral scotoma, altitudinal defect, generalized constriction and nasal or temporal steps.24

In the absence of other indicators of congenital anomaly (like associated fundus coloboma, the differential diagnosis may be difficult and the absence of progression on follow-up may be the only indicator that the patient has a congenital anomaly and not glaucoma.

Anterior Ischemic Optic Neuropathy

A history of acute visual loss, initial swelling of the optic disk, absence of marked cupping, rise in ESR, presence of centrocecal scotoma or altitudinal defects can help differentiate it from glaucoma (Fig. 8.20). In the late stages the cupping in some cases may be exactly the same as is seen in glaucoma.

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Neurological Causes

Pallor disproportionate to cupping, normal intraocular pressure or unusual history of onset, progression and age should arouse suspicion of a neurological disorder causing optic disk damage (Fig. 8.21).

Presence of visual field defects that respect vertical midline and the pattern of the field defects should be able to suggest the possible site of the intracranial lesion.

Summary

In summary, the optic disk evaluation in glaucoma is best done stereoscopically at the slitlamp with a dilated pupil using one of the 60D, 78D or 90D lenses. Changes in the neuroretinal rim, optic disk hemorrhages, peripapillary atrophy and nerve fiber layer defects are more important features than the cup-disk ratio. The cup-disk ratio is to be documented and interpreted along with the disk size and not in isolation. The diagnosis of glaucoma will depend on the presence of a visual field defect that correlates with the anatomic changes on the optic nerve head and the peripapillary retina.

References

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4.Bowd C, Zangwill LM, Berry CC, Blumenthal EZ, et al. Detecting early glaucoma by assessment of retinal nerve fibre layer thickness and visual functions. Invest Ophthalmol Vis Sci

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9.Quigley HA, Dunkelberger GR, Baginski TA, et al. Chronic human glaucoma causing selectively greater loss of larger optic nerve fibers. Ophthalmology 1988;95:357-63.

10.Sommer A, Pollack I, Maumenne AE. Optic disc parameters and onset of glaucomatous field loss: I Methods and changes in disc morphology. Arch Ophthalmol 1979;97:1444-48.

11.Greaney MJ, Hoffman DC, Garway-Heath DF, et al. Comparison of optic nerve imaging methods to distinguish normal eyes from those with glaucoma. Invest Ophthalmol Vis Sci 2002; 43(1):140-45.

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