2-4a - Disc Anatomy

The optic disc appearance is determined by: the size of the eye, the size of the scleral canal, how the nerve is inserted into the globe, the appearance of the lamina cribrosa, where myelination stops, and what is left behind in normal development. Even though this is a disc with a very large cup, it clearly demonstrates all of the major features that we must attend to when viewing the optic disc. The cup to disk ratio is determined by dividing the diameter of the cup by the diameter of the disc. The cup diameter can be measured either horizontally or vertically and should be roughly equal in both eyes. It may be difficult to identify the margin of the cup. Use the turn or bend of small vessels going over the neuroretinal rim as an indicator of cup margin when in doubt. Pair with 2-7a.