Clinical Examination of the Optic Nerve Head

Clinical examination of the ONH is preferably performed with a slit-lamp biomicroscope combined with a high-magnification posterior pole lens (60, 78, or 90 diopter [D] lens). The slit beam, rather than diffuse illumination, is useful for determining subtle changes in the contour of the nerve head. This system provides high magnification, excellent illumination, and a stereoscopic view. In addition, adjusting the height of the slit beam enables quantitative measurement of the diameter of the ONH. The disc is viewed through the handheld lens and the height of the slit beam is adjusted so that it is the same as the vertical diameter of the disc. The disc diameter can then be calculated, adjusting for the magnification of the lens used as follows:

• with a 60 D lens, the height of the slit equals the disc diameter, in millimeters, read directly from the scale

• with a 78 D lens, multiply the height of the slit by 1.1

• with a 90 D lens, multiply the height of the slit by 1.3

The normal ONH ranges from approximately 1.5 to 2.2 mm in diameter. Note that the patient’s refractive error will affect this measurement, resulting in underestimation in patients with myopia and overestimation in those with hyperopia.

The direct ophthalmoscope also may be used for clinical examination of the ONH. However, this instrument may not provide sufficient stereoscopic detail to detect subtle changes in ONH topography. The head-mounted indirect ophthalmoscope can be used for examination of the ONH in young children and in patients who are unable to cooperate with slit-lamp biomicroscopy. Lower-power lenses (15 D) can also be helpful when slit-lamp biomicroscopy is not possible and a more detailed view of the optic nerve is needed. Although cupping of the optic nerve can be detected with the indirect ophthalmoscope, in general, optic nerve cupping and pallor appear less pronounced than with slit-lamp methods, and the magnification offered by the indirect ophthalmoscope is often inadequate for detecting subtle or localized details important in the evaluation of glaucoma. Thus, the indirect ophthalmoscope is not recommended for routine use in examining the ONH.

The ONH 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. 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 is, the larger the cup will be. The cup–disc ratio alone is not an adequate assessment of the ONH for possible glaucomatous damage. For example, a 0.7 cup–disc ratio in a large ONH may be normal, whereas a 0.3 cup–disc ratio in a very small disc is likely pathologic. Thus, assessment of the disc size is important; in healthy subjects, a small disc (vertical disc diameter < 1.5 mm) will have a small cup, whereas a large disc (vertical disc diameter > 2.2 mm) will have a large cup. Individuals of African ancestry, on average, have larger disc areas and larger cup–disc ratios than do white individuals, although substantial overlap exists.

Differentiating a large physiologic or normal cup from acquired glaucomatous cupping of the ONH can be difficult. The early changes of glaucomatous optic neuropathy are subtle and include the following (Table 5-1):

• generalized enlargement of the cup

• focal rim thinning

• superficial disc hemorrhage

• retinal nerve fiber layer (RNFL) atrophy

• asymmetry of cupping

• peripapillary atrophy (PPA) of the beta (β) zone

Diffuse neuroretinal rim thinning associated with generalized enlargement of the cup may be an early sign of glaucomatous damage. However, diffuse loss may be difficult to appreciate unless previous objective documentation of the ONH (eg, photographs) is available. Comparing 1 eye with the fellow eye may be helpful, because cup–disc ratio asymmetry greater than 0.2 is unusual in healthy eyes in the absence of disc size asymmetry (Fig 5-5). The vertical cup–disc ratio typically ranges between 0.1 and 0.4, although up to 5% of individuals without glaucoma will have cup–disc ratios larger than 0.6. Asymmetry of the cup–disc ratio of more than 0.2 occurs in fewer than 1% of individuals without glaucoma. This asymmetry may be related to disc size asymmetry. Increased size of the physiologic cup may be a familial trait, and it is also observed with high myopia. An oblique insertion of the optic nerve into the globe of individuals with high myopia may also cause the ONH to appear tilted. Examination of other family members may clarify whether a large cup is inherited or acquired.

Localized loss of the neuroretinal rim usually occurs first at the inferior and superior temporal poles of the optic nerve in early glaucomatous optic neuropathy (Fig 6-5). The preferential loss of rim tissue in the superior and inferior poles leads to a vertically elongated cup in glaucomatous nerves (Fig 7-5). To help identify subtle thinning of the neuroretinal rim, a convention referred to as the ISNT rule may be useful. In healthy eyes, the Inferior neuroretinal rim is generally the thickest, followed by the Superior rim, the Nasal rim, and finally the Temporal rim. Therefore, if the rim widths do not follow this pattern, there should be increased concern for focal loss of rim tissue. However, violation of the ISNT rule is not highly specific and may also be observed in healthy eyes. Deep localized notching, in which the lamina cribrosa is visible at the disc margin, is sometimes termed an acquired optic disc pit (Fig 8-5). Patients with acquired pits are at especially high risk for progression. Even in the healthy eye, laminar trabeculations or pores may be visible as grayish dots in the base of the physiologic cup. In glaucomatous optic neuropathy, optic disc excavation is characterized by extensive exposure of the underlying lamina cribrosa and its pores or striations in the optic nerve cup (Fig 9-5). The backward bowing, strain, and compression of the lamina cribrosa cause damage to the laminar pores and beams. Glaucomatous damage to the lamina may also cause tearing of the connective tissue bundles between the pores, leading to coalescence of small pores and formation of larger ones. As the cup enlarges, nasal migration of the central retinal artery and central retinal vein is often observed.

Table 5-1 Ophthalmoscopic Signs of Glaucoma

Figure 5-5 ONH photographs showing asymmetry of optic nerve cupping. Note the generalized enlargement of the cup in the right eye (A) as compared with the normal left eye (B).

(Courtesy of G.A. Cioffi, MD.)

Figure 5-6 Progressive optic disc excavation in a patient with uncontrolled open-angle glaucoma over a 3-year period. The overall size of the optic disc is small. A, The inferior overall cup size is larger than expected for a small optic disc and the inferior neuroretinal rim is thinner than the superior rim, suggestive of early glaucomatous optic neuropathy. B, There is progressive thinning of the inferior neuroretinal rim at 5 o’clock, and an optic disc hemorrhage is present in the same location. C, Further inferior neuroretinal rim loss is evident at the 5-o’clock position, where a notch is developing.

(Courtesy of Angelo P. Tanna, MD.)

Figure 5-7 ONH photograph showing vertical elongation of the cup with localized thinning of the inferior and superior neuroretinal rim in the left eye of a patient with moderately advanced glaucoma.

(Courtesy of Felipe A. Medeiros, MD, PhD.)

Figure 5-8 Acquired optic disc pit located in the inferior temporal region. Note the absence of rim tissue.

(Courtesy of Felipe A. Medeiros, MD, PhD.)

Retinal nerve fiber layer hemorrhages can be a sign of glaucoma and usually appear as a linear red streak on or near the disc surface (Fig 10-5). However, their appearance can be highly variable, and hemorrhages may be difficult to detect unless the clinician actively searches for them. At some time during the course of their disease, one-third of patients with glaucoma may develop hemorrhages, which typically clear over several weeks to a few months. Some glaucoma patients have repeated episodes of optic disc hemorrhage, whereas others have none. Disc hemorrhages are an important prognostic sign of the development or progression of visual field loss, and the presence of a disc hemorrhage in any patient warrants detailed evaluation and follow-up. In many cases, disc hemorrhages are followed by localized notching of the rim and visual field loss. Disc hemorrhages may also be caused by posterior vitreous detachments, diabetes mellitus, retinal vein occlusions, and anticoagulation therapy.

Figure 5-9 Striations of the lamina cribrosa in an optic nerve with severe glaucomatous damage.

(Courtesy of Felipe A. Medeiros, MD, PhD.)

Figure 5-10 Flame-shaped optic disc hemorrhage in the left ONH (arrow) at the 5-o’clock position in a patient with open-angle glaucoma.

(Courtesy of Felipe A. Medeiros, MD, PhD.)

The RNFL of the healthy eye is best visualized with red-free (green) illumination. As the nerve fibers extend from the peripheral retina to converge at the ONH, they appear as fine striations created by the bundles of axons. In the healthy eye, the brightness and striations of the NFL are more easily visible superiorly and inferiorly. In an eye with progressive glaucomatous optic neuropathy, the NFL thins and becomes less visible. The loss may be diffuse (generalized) or localized (Fig 11-5). With diffuse loss, there is a general decrease in the RNFL brightness, with a reduction of the usual difference between the superior and inferior poles in comparison to the temporal and nasal regions. Localized RNFL loss appears as wedge-shaped dark areas emanating from the ONH in an arcuate pattern. Nonspecific slitlike defects may also be observed in the NFL; however, these do not usually extend to the disc margin. Diffuse nerve fiber loss is more common in glaucoma than is focal loss but is also more difficult to observe. The RNFL can be visualized clearly in high-contrast black-and-white photographs, and with good-quality photographs, experienced clinicians can recognize even early disease. However, such photographs are difficult to obtain and have been largely abandoned in clinical practice due to the availability of imaging methods to quantify RNFL thickness, which are discussed later in this chapter. Slit-lamp techniques and direct ophthalmoscopy can be successfully employed to observe the RNFL. The combination of a red-free filter, a wide slit beam, and posterior pole lens at the slit lamp affords the best view.

Figure 5-11 Nerve fiber layer photograph shows a nerve fiber bundle defect (arrowheads).

(Courtesy of Louis B. Cantor, MD.)

PPA can be classified into 2 general types: alpha (α) zone and beta (β) zone. Alpha zone is present in most nonglaucomatous eyes as well as in eyes with glaucoma and is characterized by a region of irregular hypopigmentation and hyperpigmentation of the retinal pigment epithelium (RPE). The more important type of PPA with respect to glaucoma is β zone, which results from atrophy of the RPE and choriocapillaris, leading to increased visibility of the large choroidal vessels and sclera (Fig 12-5). Beta zone is more common and extensive in eyes with glaucoma than in healthy eyes. The area of PPA is spatially correlated with the area of neuroretinal rim loss; the atrophy is largest in the corresponding area of thinner neuroretinal rim. Therefore, an area of β-zone atrophy signals the need for a search for glaucomatous loss in the adjacent neuroretinal rim.

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.

Conditions with associated optic nerve changes that can be confounded with glaucoma include congenital pits of the ONH, coloboma, morning glory disc anomaly, arteritic anterior ischemic neuropathy, and compressive optic neuropathies. Pallor of the neuroretinal rim itself is an indication of a nonglaucomatous optic neuropathy and necessitates further investigation (see BCSC Section 5, Neuro-Ophthalmology). With rare exceptions, glaucoma results in increased cupping and pallor within the cup, but not pallor of the remaining rim tissue. However, rim pallor that is out of proportion to the degree of cupping may sometimes occur following previous episodes of very high IOP, such as following an episode of acute angle closure. Optic nerve drusen or coloboma are other possible causes of glaucomatous-appearing visual field loss. Finally, the myopic optic disc represents a challenge in the assessment of possible glaucomatous damage. The size, tilting, and associated structural changes often preclude the ability to definitively determine the presence of glaucomatous damage.

Figure 5-12 In β zone (black arrows), there is marked atrophy of the retinal pigment epithelium and the large choroidal vessels can be easily seen. Alpha zone (white arrows) is peripheral and adjacent to β zone and is characterized by a region of irregular hypo- and hyperpigmentation.

(Courtesy of Felipe A. Medeiros, MD, PhD.)

• Jonas JB, Budde WM, Panda-Jonas S. Ophthalmoscopic evaluation of the optic nerve head. Surv Ophthalmol. 1999;43(4):293–320.