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Editors Selection IGR 9-2

Examination methods: Health of the optic nerve

Comment by George Spaeth on:

13789 'What controls aqueous humour outflow resistance?', Johnson M, Experimental Eye Research, 2006; 82: 545-557

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The glaucomatous process is best recognized by the damage it causes to the optic nerve in so-called 'characteristic' ways. Valid determination of the health of the optic nerve is, then, one of the most important and direct methods of evaluating and managing patients with glaucoma. One of the 'characteristic' aspects of the changes that occur in the optic nerve of patients with glaucoma, a finding that has been noted for over 100 years, is 'cupping'. Because physicians rely so heavily on the presence of 'cupping', as a sign of a disc damaged by the glaucomatous process, critical consideration of the validity of this sign is appropriate and important.

The most widely-utilized method of detecting and describing 'cupping' of the optic nerve is to look for a bowl-shaped depression in the surface of the optic nerve, comparing the width of the bowl in a plane parallel to that of the retina, with the width of the optic disc in the same plane. This 'cup/disc ratio' is used by many ophthalmologists to determine whether glaucoma is present or whether the glaucoma is getting worse. It has been generally believed that small cup/disc ratios (say 0.1 to 0.3) are typical of nerves without glaucoma damage, whereas large cup/disc ratios (say 0.7 to 0.9) are indications of nerves that have been damaged by glaucoma. However, there are fatal flaws in the cup/disc ratio system, and indeed all systems that use the width of the cup as a measure of the presence of glaucoma damage. One of those fatal flaws, not recognized until remarkably recently, was that the size of the cup is strongly affected by the size of the optic disc.

The study by Mardin et al. (332) evaluated the accuracy of two current, frequently-used methods of evaluating the health of the optic nerve in patients with glaucoma: the GDxFCC and the HRT I and noted that a high percentage of normal, large discs were considered by both instruments to be damaged by glaucoma. That is, both the GDxFCC and the HRT I falsely diagnosed large, normal discs as being glaucomatous. The implications of this study are clear and important: the GDxFCC, the HRT I, and by extension the cup/disc ratio system, are not valid ways of distinguishing between discs that are large but healthy, and discs that have been damaged by the glaucomatous process. A large cup does not mean that the patient has glaucoma! Not evaluated by this study, but of equal importance, is the recognition that small discs tend to have small cups, and that systems that do not take this into account will also yield misleading results when considering small discs, indicating that discs are normal, when in fact they are actually pathologic.

Using the MRA and RB/FSM LDFs, specificity fell dramatically with increasing disc size, particularly in males. This may reflect the finding of significantly larger cups in males than in females in the normal elderly population. This divergence was not predicted by these diagnostic functions, which were developed on samples of younger subjects (424)

See also BJO 2006; 90: 395-396, Editorial by Henderer on disc damage likelihood scale.

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