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The optic nerve contains over one million axons of the retinal ganglion cells. At the optic nerve head, the axons are predominantly located at the margins, known as the neuro-retinal rim (NRR).
Retinal axons tend to be grouped into superior and inferior nerve fibre bundles, causing a thicker NRR superiorly and inferiorly at the optic nerve head and giving the appearance of a horizontally oval cup in the younger healthy eye. Changes in the NRR width and regularity, signifying a loss of the retinal ganglion cell axons, are among the most characteristic changes in glaucoma.
Signs
? The acronymn ISNT can be helpful in detecting diffuse thinning of the NRR in the normal eye the greatest rim width is usually as follows: Inferior (I), Superior (S), Nasal (N) and Temporal (T). A loss of the ISNT rule, where the thinnest NRR is inferior or superior, represents a significant and relatively specific risk factor for glaucoma
? Notching of the NRR: represents a focal loss of the nerve fibres and is especially of concern if the notch is located infratemporal or supratemporal. Notching is less common in open angle glaucoma than diffuse losses of NRR (see ISNT above)
? Asymmetry of NRR. Glaucoma is typically an asymmetric disease, whereas healthy eyes usually show enantiomorphism (mirror-image symmetry). Early optic nerve head changes may produce a right-left eye asymmetry. An interocular difference in the C/D ratio of =0.2 is suspicious for glaucoma
? Cup to disc (C/D) ratio: The C/D ratio is usually evaluated vertically, in order to detect loss of the superior or inferior NRR. Documented change in the C/D ratio over time is considered a likely sign of glaucoma. As a single measure, a large C/D ratio is usually considered to be only a risk factor, since large C/D ratios can be physiologically normal in some individuals and a small C/D ratio may be associated with glaucoma when the optic disc is small. The presence of peripapillary atrophy can also be confusing in assessing C/D ratio. A criterion of C/D ratio = 0.6 may be used as risk factor for glaucoma
? Exceptions occasionally occur: Visual field or retinal nerve fibre layer losses have been reported without optic disc cupping, for example with small optic discs, in association with optic disc drusen, or in conditions associated with pressure spikes such as angle closure or pigmentary glaucoma.
Pupil dilation and the utilisation of binocular stereoscopic examination techniques are essential. Monocular ophthalmoscopic assessment of the NRR relies, in part, on the colour difference between cup and rim, which does not always correspond to the actual shape of the optic nerve head (see Pallor of the NRR in the next condition).
Prevalence, Differential diagnosis, Management
See the individual glaucoma conditions listed in: Glaucoma classification.