Arcuate defects (nerve fiber bundle defects)

The arcuate scotoma represents a complete nerve fiber bundle defect. It begins at the blind spot, arcs around fixation, and ends at the horizontal nasal raphe.The defect may break through into the

periphery nasally and then expand further to ultimately become an altitudinal defect (Fig. 10-11). The arcuate defect as described by Bjerrum is a classic finding in middleto late-stage glaucoma.

End-stage defects

Central and temporal islands

In the later stages of glaucoma, most of the axons at the superior and inferior poles of the disc are destroyed, leaving only the papillomacular bundle and some nasal fibers.This destruction produces the characteristic end-stage field, with a small central island and a larger temporal crescent remaining. The central island may split fixation so that only fibers from half of the papillomacular bundle remain (Fig. 10-12). Kolker10 found that patients with split fixation are more susceptible to central vision loss at surgery, although this is still a very rare outcome.11 These patients may need to have their pressures controlled in the mid teens or below to slow further progression.

Reversal of visual field defects

Fluctuation and increasing familiarity with the test or random chance may cause subsequent visual field examinations to appear improved.12,13 Nevertheless, at least slight reversibility of visual field defects seems to be a real phenomenon in occasional patients following therapy for glaucoma.14–17 The rule, unfortunately, is that glaucoma patients do not regain visual function under treatment, but rather they continue to lose field even when controlled. The rate of loss varies, and about 1 in 5 patients are stable over 20 years, but in general some degree of loss is usual. The rate and degree of loss in treated eyes are less than that reported in rare studies of untreated glaucoma.18

Fig. 10-5  Central 10° field from the right eye of a patient with advanced glaucoma. The nasal horizontal step (left side in this figure) runs all the way to fixation.

ANALYSIS OF VISUAL FIELD LOSS

CHRONIC OPEN-ANGLE GLAUCOMA

Any of the preceding types of visual field loss may be seen in chronic open-angle glaucoma.19 In the early stages there may be

Fig. 10-6  Note the inferior nasal step present on the peripheral and central isopters in this patient. There is an inferior arcuate scotoma present also.

Fig. 10-8  Visual field chart from the right eye of a 56-year-old woman with early glaucoma. Generalized blind spot enlargement is not necessarily a glaucomatous defect. This blind spot is roughly 14° 14°, with sloping margins. The normal blind spot is about 7° vertically and 5.5° horizontally, with sharp borders.

a generalized depression that progresses gradually or sometimes in steps from paracentral scotomata to arcuate to altitudinal to endstage defects. Defects usually become denser and then increase in area in one hemifield before progressing to the next hemifield (Fig. 10-13). Scotomata may show episodic (stepwise), linear, or curvilinear progression.20–22

Many recent investigations have suggested that the two forms of glaucomatous visual field loss, diffuse and localized, may have different pathogenic origins.23 It has been speculated that increased intraocular pressure (IOP) may cause diffuse loss24 but have less influence on the development of localized defects.25 Observer bias may have some influence on these findings, however, because patients with mild diffuse loss and normal pressure are often not identified as abnormal. Conversely, patients with elevated IOPs are examined closely because of the pressure, and, because suspicion is high, mild diffuse defects are recognized. Patients with dense localized defects tend to have localized optic nerve changes and may have visual field studies based on the appearance of the optic nerve. If the IOP is normal, a diagnosis of glaucoma is more likely when the field defect is local and dense rather than diffuse or nonspecific. Drance,26 however, found that patients with increased IOP with localized defects in one hemifield had nearly double the amount of generalized reduction in sensitivity in the other hemifield compared with a similar group of patients with normaltension glaucoma. Many others have investigated this issue, and there is general agreement that early glaucomatous field loss may appear in different forms.

A study by Gazzard and colleagues found that the pressure level at diagnosis correlated with the amount of visual field loss measured by Advanced Glaucoma Intervention Study (AGIS) score and by mean deviation (MD), but not by pattern standard deviation (PSD) or corrected pattern standard deviation (CPSD).27 In other words, higher presenting pressures were associated with the degree of diffuse damage but not with the degree of localized damage.The association was stronger for patients with primary angle-closure glaucoma (PACG) than those with primary open-angle glaucoma (POAG).This supports the concept that increased IOP is the proximal cause of damage in PACG, but that other factors may predominate in at least some patients with POAG. In both circumstances, the amount of field loss correlated well with the amount of optic nerve damage.27

Fig. 10-10  Test grid pattern from the Octopus G-1 program. Twenty-one test points are placed in the central 12° rather than the 16 test points in the more standard Octopus 31 or 32 program. Humphrey has a similar program, the 10-2, which is very useful for identifying and monitoring paracentral scotomata.

Fig. 10-12  Central island of a patient’s left eye with field defects encroaching on fixation. The temporal field is partially spared.

Fig. 10-13  The superior field (left eye) exhibits similar, but more advanced, loss compared with the inferior field. If the disease is unchecked, both fields will continue to progress.

Fig. 10-11  Field from the right eye of patient with advanced glaucoma. The superior arcuate defect has expanded to include the entire superonasal quadrant and includes half of the superotemporal quadrant as well. The inferior half of the field is much less severely affected.

Angle-closure glaucoma

During the acute phase of angle-closure glaucoma in patients with high IOP, corneal edema and retinal ischemia can produce bizarre field defects that have little clinical value for following disease progression. After the pressure has been normalized, field defects may remain and may sometimes be extensive if ischemic atrophy of the nerve has occurred. In such cases, pallor of the nerve may be more severe than cupping. This is one situation in which glaucomatous field defects may not correspond well to the amount of cupping of the nerve head.

Other causes

Other diseases may cause arcuate nerve fiber bundle visual field defects (Box 10-1) that may be confused with glaucomatous damage. Generally, if excavation of the optic nerve does not correspond with the appearance of the field, other causes must be sought to explain the defect. If visual field defects occur or progress with normal pressures, normal-tension glaucoma may be the cause (see Ch. 17), but the examiner must be sure that other retinal or visual pathway lesions are not present, especially if the process is occurring unilaterally. Glaucoma is a jigsaw puzzle in which all the ‘pieces’ of the disease should fit. If a piece does not fit properly, the physician should be suspicious that it may belong to some other puzzle (disease). Generally, the configuration of the optic nerve and the appearance of the visual field correspond. Superior visual field defects are accompanied by erosion of the inferior portion of the optic disc and vice versa.The nerve in a patient with a temporal visual field defect should have a thinned nasal rim. Although normal-tension glaucoma may account for 10% or more of glaucoma patients, depending on definitions and the patient