8

Examination of the Optic Nerve

Ian Conner, MD, PhD; Joel S. Schuman, MD, FACS; and David L. Epstein, MD, MMM

METHODS OF EXAMINATION

The clinician should use all available techniques to examine and document the optic nerves in patients with glaucoma, both before and after pupillary dilation. One will find that the cup contour can appear different when examined using different tools. There are 3 primary tools for examining the optic nerve: the direct ophthalmoscope, the slit-lamp condensing lens, and stereo photographs.

Although the direct ophthalmoscope provides a non-stereoscopic view, the use of parallax and the attention to the bend of small vessels as they cross the disc margin can allow a remarkably accurate assessment of the contour of the rim tissue, the key parameter being assessed. The ophthalmologist in training must take advantage of the opportunity to critically evaluate cupping in the referral glaucoma center during training, when the pupils of most patients are dilated and the diagnosis of glaucoma has already been made. In practice, the general ophthalmologist must evaluate many patients through undilated pupils and often make the primary diagnosis of glaucoma. The general ophthalmologist must therefore be a true disc sleuth, and deserves our praise for detecting glaucomatous changes in the optic nerve.

Unfortunately, many senior clinicians have observed that recent residency graduates are not as skilled with the direct ophthalmoscope as the previous generation; new imaging techniques should supplement but not replace previous invaluable methods.

The condensing lens (60 D, 78 D, or 90 D) at the slit lamp is the next technique to master. The slit beam of the slit lamp is aligned coaxial with the oculars, the condensing lens is placed in front of the patient’s eye perpendicular to the beam, and the slit lamp is moved toward or away from the patient in order to bring the optic nerve head (ONH) into focus. This technique is easiest to perform with a dilated pupil, but can be mastered to allow an excellent view with all but the most miotic of pupils. It can be valuable to draw what one sees and later compare this to actual stereo photos. It is curious that contour changes accentuated with the condensing lens are sometimes less easily appreciated as with direct ophthalmoscopy or stereoscopic photography.

One should draw (at least once) the appearance of the optic nerve using the above methods, but stereoscopic disc photographs should be obtained after pupil dilation and later compared to the drawings. All subsequent evaluations will be asking the question: Has there been progression in the optic nerve cupping and visual field? Evaluation of the visual field alone is not sufficient. Detailed baseline information, as indicated in Chapter 5, is crucial to optimal patient management.

Indirect ophthalmoscopy provides notoriously inaccurate assessment of the contour changes in glaucoma and should be avoided as a technique for primary glaucoma evaluation (although it is important in all glaucoma patients to examine the retina using indirect ophthalmoscopy).

The process of glaucomatous optic neuropathy affects the contour of the rim tissue between the edge of the physiological cup and the edge of the disc (Figure 8-1). The process causes a backward bowing of this rim tissue that may result in a fairly discreet (deep) extension of the cup in one meridian, or a gentler posterior sloping, termed saucerization. With this loss of substance of the optic nerve, there is a change in color, presumably due to the accompanying loss of small blood vessels. However, the key parameter is the change in the contour of the rim tissue.

In nonglaucomatous optic atrophy, there is loss of color (atrophy), but there is usually no contour change in the rim tissue. For example, after vascular occlusion, the disc is pale, but there is usually no increase in size or contour of the cup (Figure 8-2). However, there are certain optic atrophies that can be accompanied by contour changes, and these occasionally present diagnostic challenges: giant cell arteritis and other ischemic optic neuropathies, intracranial neoplasms, methanol intoxication, hereditary optic atrophies, and optic cup pits or colobomas. Some of these have been termed pseudoglaucomatous because of the contour change, but assessment of the color of the intact rim tissue is key; the intact rim tissue is usually pale in pseudoglaucomatous cases.

The assessment of the contour of the remaining optic nerve rim tissue throughout its entire circumference is crucial both to the detection of glaucoma and its progression. We use all available diagnostic methods, including examination of the course of small vessels that cross the disc margin, to make this crucial assessment.

Figures 8-3 through 8-8 portray the “Chandler and Grant” interpretation of the changes in rim contour that occur in the progression of the glaucomatous process, based on their long-term clinical observations. The cross-sectional views in these figures help clarify what is observed, and we have used these in patient charts. Trainees in ophthalmology are encouraged to draw these cross-sectional diagrams in order to better learn careful assessment of the optic disc.

The key concept is detection of backward bowing and its relationship to the edge of the disc and the depth of this change. The following terms have been used to characterize the apparent depth of the contour change: saucerization (as in a saucer plate), connoting a mild change in depth; shelving, an intermediate change in depth; and excavation, a precipitous change in depth like the edge of the normal central cup. Chandler and Grant intended the term saucerization to indicate an uncommon, more generalized backward bowing like a saucer plate (see Figure 8-8) to contrast this with the more common excavated change of a tea cup, but the term saucerization has evolved to indicate a common subtle change in disc rim contour that appears to be acquired early in the glaucomatous process.

For practical purposes, the terminology is not as important as the detection of the contour change of shallow or deep backward bowing of disc rim tissue. The deeper changes, whether shelving or excavation, are difficult to fully differentiate from each other, but the more subtle posterior bowing of rim tissue commonly called saucerization and its progression to deeper and/or more extensive changes has become an important part of disc sleuthing.

Thus, although the disease process does produce optic atrophy and thereby a sectoral color change in the disc, it is the change in contour that distinguishes glaucoma from other optic neuropathies. The clinician must assess and record these contour changes to diagnose and evaluate follow-up in patients with glaucoma.

In assessing the contour of the rim tissue of the optic nerve, special attention should be placed on the temporal portion of the optic disc that receives the papillomacular bundle of retinal nerve layer fibers. In primary open-angle glaucoma associated with myopia and in normal tension glaucoma, there can be preferential early cupping in this area with corresponding visual field loss that can extend into or actually involve central fixation. Therefore, careful attention to the amount of rim tissue and its health in this area can be crucial in avoiding meaningful vision loss.

The myopic disc is notoriously difficult to evaluate for early glaucomatous changes because of its usual generalized sloping contour. Nevertheless, in primary open-angle glaucoma associated with myopia, one needs to carefully examine the temporal margin of rim tissue for early glaucomatous contour changes because of the possibility that the initial visual field loss could involve central fixation (see Chapter 19). This uncertainty in detecting the earliest possible disc contour change in myopic eyes suggests the need for earlier and more assertive treatment in these eyes.

Chandler and Grant were excellent observers of the optic nerve and emphasized the importance of asymmetry and verticalization of the cup, as well as the predictive value of the optic nerve changes for visual field changes. They wrote:

It is true that, in certain varieties of glaucoma, a decision that treatment is urgently needed can be made primarily on the basis of very high intraocular pressure (IOP), but in most cases the character of both the optic disc and visual field greatly influences the decision.

Change in the optic disc can forewarn of change in the visual field in many cases, and usually the nature of the abnormality of the optic disc can be correlated with abnormality of the visual field to help establish whether a given field defect is correctly attributable to glaucoma or to some other cause.

We will first describe the changes in appearance of the optic disc that are characteristic in glaucoma and their correlation with abnormalities in the visual field.

OPHTHALMOSCOPIC FINDINGS

Nearly all of the observations to be described were made originally with the direct ophthalmoscope, which, though monocular, permits valuable appreciation of depth and contour, confirmed by comparison with stereo photographs. However, considerable practice is required to learn how to make use of a number of monocular clues to depth perception.

A simple enlargement of the physiologic cup in all directions is commonly a sign of early glaucoma but is insufficient for diagnosis. Asymmetry of the physiologic cups in the 2 eyes is seldom normal, and should be considered a sign of early glaucoma until proven otherwise (Figure 8-9). The IOP is often slightly higher in the eye with the wider cup.

The size of the physiologic cup in normal eyes varies considerably from person to person, and a single exam is by definition insufficient to determine change over time. The only way the ophthalmologist can say that the cups are becoming larger is to compare the exams over time, ideally by using previous stereo photographs of the discs or previously prepared drawings. Because previous stereo photographs or accurate drawings are seldom available to the consulting ophthalmologist, one cannot be certain from the first examination whether the size of symmetric cups is normal or abnormal in any individual patient.

If the cups are asymmetric, then this must be considered likely to be an early glaucomatous change. A difference in the size of the cups in the 2 eyes is a common finding in early glaucoma.

There is considerable discussion in the ophthalmic literature concerning cup-to-disc ratio. We know that the size and depth of the physiologic cup vary greatly among different individuals. We feel that the most significant parameter is not the cup-to-disc ratio but the shape of the cup and its comparison to that of the cup in the fellow eye (see Figures 8-3 and 8-4).

Abnormal Cupping

In most patients for whom glaucomatous changes develop in the optic disc, the physiologic cup enlarges to reach the margin inferiorly and temporally. This is a definite glaucomatous change, yet at first there may not be a visual field defect (see Figure 8-4).

The shelving of the cup to the lower temporal margin gradually becomes deeper, not yet a true excavation. Gradually, the cup becomes deeper at the margin, and an excavation becomes evident (see Figure 8-5). It is now usually possible to find an upper field defect—typically a superonasal step—but there may be none if there is not yet atrophy of nerve fibers. At first, such pathologic cupping extends only to the major inferior vessels, but later may involve the entire lower pole of the disc.

In other cases, the physiologic cup first enlarges superiorly. As the disease progresses, the upwardly enlarged cup comes closer to the disc margin and is seen to be excavated at its upper border (see Figure 8-6). At this stage, although the cup still does not quite reach the upper outer margin, there is usually a lower field defect—typically an inferonasal step.

In some cases, the cup may enlarge toward both the upper and lower borders simultaneously (see Figure 8-7), and it is in this type of glaucomatous change that we eventually find both an upper and lower Bjerrum scotoma, isolating a central island of vision.

Eventually, the excavation reaches the margin above as well as below, and the disc may become totally cupped to the margins in all sectors; however, in some such cases, there is usually still a tiny rim of normal tissue on the nasal edge of the disc.

It is important to remember that not all visual field defects in a patient with elevated IOP or documented glaucoma are necessarily due to the glaucoma itself. It is important to correlate the optic cup appearance with the visual field. Dense field defects are to be regarded with suspicion if they are not explainable by the optic disc appearance—patients are unfortunately permitted to lose vision from more than one independent pathology (eg, macular disease, chiasmal lesions, neurovascular disease).

Saucerization

Sometimes the first detectable pathologic change may be a slight backward bowing in the periphery of a portion of the disc or of the whole disc, in a form that we have called saucerization (see Figure 8-8). The contour of such a disc closely resembles the contour of an ordinary saucer plate, in contrast to an excavation, which more closely resembles the contour of a teacup. Saucerization involves at least a quadrant of the disc, and may involve any portion of the disc, sometimes even the nasal portion or the entire disc. In some cases, one may see pathologic cupping at one border of the disc and saucerization at the other. Whether the saucerization involves the entire disc or only a portion, it represents a definite glaucomatous change and must be taken into account along with the other signs in the management of the case.

In some cases, there is saucerization of the entire disc, but there still may be no field defect until the saucerization becomes considerably deeper. Eventually, we find loss of field; with advanced saucerization of the whole disc, however, this may not be the typical nerve fiber bundle defect but rather a concentric contraction of the field. Such cases are much more difficult to follow by perimetry than those characterized by the ordinary type of cupping.

Saucerization of the disc is a relatively rare form of glaucomatous cupping, comprising no more than 10% of cases. Its importance seems to be that if we recognize saucerization of a portion or of the whole disc, even if there is no loss of field whatsoever, we can make a positive diagnosis of glaucoma no matter what level of IOP is found at the moment.

In many eyes, we find a crescent all around the disc, sometimes called a halo (Figure 8-10), which can confound identification of the border of the disc. Erroneous identification of the crescent as the disc border leads to the false conclusion that the cup, although large, does not reach the margin in any sector, whereas a more critical examination would correctly conclude that the cup does in fact reach the margin of the disc, well inside the borders of the crescent.

Atrophy

Thus far, we have spoken mainly of the changes in contour of the optic disc that occur in glaucoma and have only incidentally mentioned atrophy. We must remember that the ultimate cause of irreversible loss of function in glaucoma is atrophy of nerve fibers, whether or not there is pathologic cupping.

With the exception of cases of high IOP or acute glaucoma in which atrophy and loss of vision may develop with little or no pathologic cupping, and about which we will say more later, in most cases of chronic glaucoma without exceptionally high IOP, the first change in the optic disc is a change in the cupping that conforms to the various patterns described in Figures 8-3 through 8-8. Abnormal changes in the contour of the optic disc are characteristically recognized before there is an irreversible abnormality in the visual field. It is when atrophy of nerve fibers develops in association with cupping that characteristic changes in the visual field appear, corresponding anatomically to the location of visibly affected portions of the optic disc. In exceptional cases, there may be typical glaucomatous visual field defects that do not correspond to visible changes in the optic disc if the fields are measured when the IOP is considerably elevated, but these are commonly reversible when the IOP is reduced. Reversible field defects can be induced in normal eyes and in glaucomatous eyes by raising the IOP with external manipulation.

Our primary concern is with the irreversible loss of visual field, and it is here that ophthalmoscopic recognition of atrophy can be helpful in validating results of visual field testing. When atrophy and field loss are extensive, it is generally easy to recognize the degree of atrophy, but when atrophy is partial or limited to a small portion of the disc, it may be difficult to be certain. Characteristically, when there is localized pathologic cupping and corresponding field defect, one can appreciate that the affected portion of the disc is atrophic by its relative lack of color compared with the more normal portions of the disc. The affected area may even appear slightly gray.

In exceptional cases, one may find glaucomatous-type field defects, such as a Bjerrum-type scotoma or a Ronne step, with the IOP and the disc completely normal. We know that these defects must be secondary to another pathology. We have seen many patients who had glaucoma and were alleged to be losing field, or who had lost field despite apparently good IOP control, but the appearance of the disc did not correlate with the field defect. The inconsistencies triggered further causative investigation and were resolved by the findings of vascular occlusion, prior chorioretinitis, drusen of the optic disc, brain lesions, retinitis pigmentosa, retinal detachment, or retinoschisis (Figure 8-11).

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