(A)

(B)

Large open (white) arrows

=Zone beta Small black arrows

=Zone alpha Black diamonds

=Scleral ring

Fig. 13-11  (A) Peripapillary areas can sometimes be distinguished as zone beta (hypopigmented atrophic area that is most prominent temporally and that can correspond to sectors of the greatest disc cupping; large white arrows) and zone alpha (poorly demarcated hyperpigmented region irregularly surrounding zone beta; smaller black arrows).

(B) Halo glaucomatosus. With extensive glaucomatous cupping, zone beta (large white arrows) can surround the entire disc, with indistinct zone alpha (smaller black arrows). Note distinction between the scleral ring (thin white circumferential show of scleral tissue; black diamonds) and zone beta.

to zone alpha; internal to zone beta is the peripapillary scleral ring, which is often exaggerated in highly myopic eyes and with tilted discs.When zone beta completely surrounds the ONH, it has been called the halo glaucomatosus (Figs 13-11 through 13-14).

Histologically, zone alpha shows irregularities in the retinal pigment epithelium, consisting of an unequal distribution of melanin granules and partial atrophy of cells. In zone beta, adjacent to the optic disc, Bruch’s membrane is bare of retinal pigment epithelium cells, the photoreceptors are markedly reduced in density or completely missing, and the choriocapillaris is severely attenuated.165 Although there are slight methodological differences in measuring the more clearly distinguished zone beta, the increased area and increased frequency of the discrete locus of PPCA correlate both with variables of increasing severity (such as NRR area loss, loss of

RNFL, and correlation with disc hemorrhage) and with progressive field changes in some studies.154,157,158,163,166

Although some maintain that progressive changes in zone beta are independent of IOP and correlate with a relatively non-specific manifestation of advanced glaucomatous disc damage,157 others have associated pronounced PPCA alterations with two distinctive subtypes of glaucoma – normal-pressure disease156 and age-related POAG.59 Because non-glaucomatous optic atrophy is not associated

with PPCA any more than with normal eyes, PPCA recognition may contribute to the clinical discrimination between glaucomatous and other mechanisms of disc damage.143

Patterns of optic nerve changes and subtypes of glaucoma

With the wealth of clinical and morphometric studies of the ONH in all stages of glaucoma, a variety of classification schemes has

been proposed to clinically distinguish subtypes of glaucoma based on the appearance of the disc.7–11,13,58–60,74,169 Although these

‘archetypes’ are not universally accepted, and there is considerable overlap of features as they appear in clinical practice, their tabulation is useful. Intraand interobserver correlations for consistent identification have been reasonable.170 These patterns are summarized and collated in Table 13-1.

High myopia disc pattern

Highly myopic eyes with open-angle glaucoma have larger and often abnormally shaped optic discs (Fig. 13-15), and their

Fig. 13-12  Normal optic disc with physiologic scleral ring surrounding the optic disc. The scleral ring is limited centrally to the edge of the chorioscleral canal (black arrow) and peripherally to the edge of the retinal pigment epithelium (white arrow).

(From Airaksinen PJ, Tuulonen A, Werner EB: Clinical evaluation of the optic disc and retinal nerve fiber layer. In: Ritch R, Shields MB, Krupin T, editors: The glaucomas, 2nd edn, St Louis, Mosby, 1996.)

Fig. 13-14  Partial atrophy of peripapillary layers with disorganization, hypopigmentation, and hyperpigmentation of the retinal pigment epithelium, designated zone alpha. A small disc hemorrhage is located at 3 o’clock position. (From Airaksinen PJ, Tuulonen A, Werner EB: Clinical evaluation of the optic disc and retinal nerve fiber layer. In: Ritch R, Shields MB, Krupin T, editors: The glaucomas, 2nd edn, St Louis, Mosby, 1996.)

Fig. 13-13  Glaucomatous optic disc with diffuse thinning of the neural rim inferiorly with corresponding total atrophy of the retinal pigment epithelium and choriocapillaris, designated zone beta.

(From Airaksinen PJ, Tuulonen A, Werner EB: Clinical evaluation of the optic disc and retinal nerve fiber layer. In: Ritch R, Shields MB, Krupin T, editors: The glaucomas, 2nd edn, St Louis, Mosby, 1996.)

diagnosis­ represents a special problem in the management of glaucoma. Many myopic eyes have lost considerable vision from primary or secondary glaucoma before the ophthalmologist becomes aware of the diagnosis.The reasons for this difficulty are three-fold.

First, as Goldmann171 has pointed out, the distance between the level of the lamina cribrosa and the level of the retina is much less than in normal or hyperopic eyes. The average value of this distance in the normal eye is about 0.7 mm,172 whereas that of the myopic eye is between 0.2 and 0.5 mm. Therefore a completely cupped disc in a myopic eye will have only half the depth of the usual glaucomatous cup – and such a shallow excavation is difficult to clinically appreciate. Furthermore, the myopic ONH is masked by the usual myopic conus, tilting of the disc, and peripapillary atrophy. Often disc photographs are superior to drawings for surveillance of the subtle progression of the shallow cup with associated shifts in vessels or changes in PPCA.

Also, the ocular rigidity usually is lower than that of normal eyes. Therefore Schiøtz tensions, using the ordinary conversion tables, are lower than the actual IOP. As described in Chapter 4, applanation tonometry prevents this error.A less well characterized potential artifact is the influence of thin corneas causing falsely lower applanation IOP readings; this may be operative in some myopic eyes.

Staphylomas of the posterior pole or peripheral fundus may produce irregular refractive errors that affect visual field examination. These errors especially affect current methods of perimetry using reduced-intensity static targets. In high myopia, it is best to have patients wear their regular contact lenses during testing. Finally, an enlargement of the blind spot or cecocentral changes may be incorrectly attributed to the myopic conus and choroidal atrophy. The astute clinician must be on guard for glaucoma in myopic

patients because it occurs more frequently in these patients (Fig. 13-16).62

Table 13-1  Subtypes of glaucoma by optic nerve head appearance

Data from references 7–11, 13, 53, 59, 60, 74, 167–169.

RNFL, Retinal nerve fiber layer; PPCA, peripapillary choroidal atrophy; IOP, intraocular pressure; POAG, primary open-angle glaucoma; OAG, open-angle glaucoma.

165

chapter 13 head nerve optic the of evaluation Clinical

Fig. 13-15  High myopia disc pattern.

(A)

(B)

Fig. 13-16  (A) The myopic disc can be particularly difficult to interpret for glaucomatous change. In this case, the

disc edge is distinguished from the large temporal crescent. Myopic changes in the fundus can contribute to the visual field defects. (B) Visual field from the same patient. There is superior central loss caused by myopic degeneration. Peripheral contraction and apparent superior arcuate defect may be glaucoma related.