The optic nerve head, or optic disc, is usually round or slightly oval in shape and contains a central cup. The tissue between the cup and the disc margin is called the neural rim or neuroretinal rim. In normal individuals, the rim has a relatively uniform width and a color that ranges from orange to pink. The size of the physiologic cup is developmentally determined and is related to the size of the disc. For a given number of nerve fibers, the larger the overall disc area, the larger the cup. Cup–disc ratio alone is not an adequate assessment of the optic disc for possible glaucomatous damage. For example, a 0.7 ratio in a large optic disc may be normal whereas a 0.3 ratio in a very small disc could be pathologic. This shows the importance of assessing the disc size. The size of the cup may increase slightly with age. Nonglaucomatous black individuals, on average, have larger disc areas and larger cup–disc ratios than do whites, although a substantial overlap exists. On average, people with myopia have larger eyes and larger discs and cups than do those with emmetropia and those with hyperopia.
Differentiating physiologic or normal cupping from acquired glaucomatous cupping of the optic disc can be difficult. The early changes of glaucomatous optic neuropathy are very subtle (Table 3-2):
generalized enlargement of the cup
focal enlargement of the cup
superficial splinter hemorrhage
loss of nerve fiber layer
translucency of the neuroretinal rim
development of vessel overpass
asymmetry of cupping between the patient’s eyes
peripapillary atrophy (beta zone)
Generalized enlargement of the cup may be the earliest change detected in glaucoma. This enlargement can be difficult to appreciate unless previous photographs or diagrams are available. It is useful to compare one eye with the fellow eye because disc asymmetry is unusual in normal individuals (Fig 3-13). The vertical cup–disc ratio is normally between 0.1 and 0.4, although as many as 5% of normal individuals will have cup–disc ratios larger than 0.6. Asymmetry of the cup–disc ratio of more than 0.2 occurs in less than 1% of normal individuals. This asymmetry may be related to disc size asymmetry. Increased size of the physiologic cup may be a familial trait and it is also seen with high myopia. An oblong insertion of the optic nerve into the globe of individuals with high myopia may also cause a tilted appearance to the optic nerve head. Examination of other family members may clarify whether a large cup is inherited or acquired.
Focal enlargement of the cup appears as localized notching or narrowing of the rim. Focal atrophy most typically occurs at the inferior and superior temporal poles of the optic nerve in early glaucomatous optic neuropathy. Thinning of the neuroretinal rim with development of a focal notch or extension of the cup into the neuroretinal rim may be seen. To help identify subtle thinning of the neuroretinal rim, a convention referred to as the ISNT rule may be useful. In general, the Inferior neuroretinal rim is the thickest, followed by the Superior rim, the Nasal rim, and finally the Temporal rim. If the rim widths do not follow this progression, there should be increased concern for the presence of focal loss of rim tissue. Deep localized notching, where the lamina cribrosa is visible at the disc margin, is sometimes termed an acquired optic disc pit. If notching or acquired pit formation occurs at either (or both) the superior or the inferior pole of the disc, the cup becomes vertically oval (Fig 3-14). Even in the normal eye, laminar trabeculations or pores may be seen as grayish dots in the base of the physiologic cup. With glaucomatous optic neuropathy, neural atrophy results in more extensive exposure of the underlying lamina and may reveal more laminar pores in the optic nerve cup. Nasalization of the central retinal artery and central retinal vein is often seen as the cup enlarges.
Splinter, or nerve fiber layer, hemorrhages usually appear as a linear red streak on or near the disc surface (Fig 3-15). Nerve fiber layer hemorrhages may occur in the neuroretinal rim or in the peripapillary area in as many as one-third of glaucoma patients at some time during the course of their disease. Hemorrhages typically clear over several weeks to months but are often followed by localized notching of the rim and visual field loss. Some glaucoma patients have repeated episodes of optic disc hemorrhage; others have none. Individuals with normal-tension glaucoma are more likely to have disc hemorrhages. Optic disc hemorrhage is an important prognostic sign for the development or progression of visual field loss, and any patient with a splinter hemorrhage requires detailed evaluation and follow-up. Splinter hemorrhages may be caused by posterior vitreous detachments, diabetes mellitus, branch retinal vein occlusions, and anticoagulation therapy.
Axons in the nerve fiber layer of the normal eye may best be visualized with red-free illumination. The nerve fiber layer extending from the neuroretinal rim to the surrounding peripapillary retina appears as fine striations created by the bundles of axons. In the healthy eye, the nerve fiber layer bundles have a plush, refractile appearance. With progressive glaucomatous optic neuropathy, the nerve fiber layer thins and becomes less visible. The loss may be diffuse (generalized) or localized to specific bundles (Fig 3-16). Focal abnormalities can consist of slitlike grooves or wedge defects. Slitlike defects can be seen in normal retinal nerve fiber layer anatomy, although they usually do not extend to the disc margin. Early wedge defects are sometimes visible only at a distance from the optic disc margin. Diffuse nerve fiber loss is more common in glaucoma than focal loss but also more difficult to observe. The nerve fiber layer can be visualized clearly in high-contrast black-and-white photographs, and experienced observers can recognize even early disease if good-quality photographs are available. Direct ophthalmoscopy and slit-lamp techniques can both be successfully employed to observe the retinal nerve fiber layer. The combination of red-free filter, wide slit beam, and posterior pole lens at the slit lamp affords the best view.
In the early stages of nerve fiber loss, often before enlargement of the cup, existing neuroretinal rim tissue can be observed to become more translucent. The clinician can best observe this rim translucency by using a lens at the slit-lamp biomicroscope, employing a thin slit beam and confining the beam to the disc surface.
As the nerve fiber loss continues, the cup may begin to enlarge by progressive posterior collapse and compaction of the remaining viable nerve fibers. In circumstances where the neuroretinal tissue—but not the overlying nerve head vasculature—has collapsed, vessel overpass can often be observed. The blood vessels overlying the collapsed neural rim tissue look like a highway overpass suspended over—but not in contact with—the underlying tissue.
Peripapillary atrophy occurs as 2 types. Alpha-zone peripapillary atrophy is the typical temporal crescent often seen in myopia with areas of hyperpigmentation and hypopigmentation; it has no known impact on glaucoma. The second type, beta-zone peripapillary atrophy, is seen with greater frequency and is more extensive in eyes with glaucoma than in unaffected eyes. It represents loss of choriocapillaris and retinal pigment epithelium, leaving only large choroidal vessels and sclera and resulting in the characteristic white appearance adjacent to the disc margin. The location of the atrophy often correlates with the position of visual field defects. Other less specific signs of glaucomatous damage include nasal displacement of the vessels, narrowing of peripapillary retinal vessels, and baring of the circumlinear vessels. With advanced damage, the cup becomes pale and markedly excavated.
It is important to recognize that glaucomatous optic nerve damage is only one type of pathologic change of the optic nerve; other etiologies of optic nerve changes should be considered in the differential diagnosis. Optic discs where the remaining neuroretinal nerve tissue is pale may need to be evaluated for causes of nonglaucomatous optic atrophy (see BCSC Section 5, Neuro-Ophthalmology). Glaucoma results in increased cupping and pallor within the cup, but not pallor of the remaining rim tissue. In addition, a large cup may be physiologic in a large disc. This can best be assessed following measurement of the disc diameter. Consideration must also be given to a disc with optic disc drusen or coloboma, which can result in visual field loss but not on the basis of glaucoma. Finally, the myopic disc represents a challenge when attempting to assess possible glaucoma damage. The size, tilting, and associated structural changes often preclude the ability to definitively determine the likelihood of glaucoma damage.