The importance of assessing the Optic Disc size along with the C/D Ratio in Optometry

It is very important to assess the size of the optic disc along with the optic cup to optic disc ratio. In some patients, the optic disc can be round but it is typically vertically oval in shape. The overall size of the optic disc will influence the cup to disc ratio (C/D ratio), so it is important that both measurements should be considered together to gain a clearer insight into whether the optic nerve is outside the norm. A normal, large optic disc will have a larger cup to disc ratio when compared to a normal, smaller optic disc and a normal, small optic disc will have a smaller cup to disc ratio than a normal, larger optic disc, for example,  If all other glaucomatous risk factors were equal between two patients, and both patients had a cup to disc ratio of 0.6, the patient with the smaller optic nerve head would be of greater risk.

Assessing IOPs (Intraocular pressures) is the classic test for glaucoma, but if we don't think about other factors such as cup to disc ratio and optic nerve head size, we are missing vitally important patient information. In the neuroretinal rim of each optic nerve head, there are an estimated 1.2 million nerve fiber axons, larger optic nerve heads have more nerve fiber axons than smaller ones. The number of nerve fiber axons decreases with age. The reason why a large cup to disc ratio (C/D ratio) raises a red flag in optometry is because we are worried about the decline of neuroretinal rim tissue.

The average horizontal size of the optic disc is 1.77mm and the average vertical size of a typical optic disc is 1.88mm. The normal optic nerve head diameter varies in size from 1.2mm to 2.5mm. There is some inconsistency in the literature as to what cutoffs to use for a small or large optic disc, but in general an optic disc is considered small if <1.2mm and large if >1.8mm. The key is to review the cup to disc ratio in collaboration with the optic disc size.

Another method to assess the size of the optic disc is to calculate how many times the CNV (Central Retinal Vein) which has an average diameter of 0.125mm as it crosses the inferior rim of the optic disc, would fit side by side into the margins of the optic disc in order to determine the optic disc size.