How is optic nerve damage detected?
The normal cup to disc ratio (the diameter of the cup divided by the diameter of the whole nerve head or disc) is about 1/3 or 0.3. There is some normal variation here, with some people having almost no cup (thus having 1/10 or 0.1), and others having 4/5ths or 0.8 as a cup to disc ratio. If someone has a cup/disc ratio larger than 1/3, then doctors get suspicious that the cup could be getting larger than it used to be. Glaucoma can cause the cup to enlarge (actually little nerve fibers are being wiped out along the rim of the optic nerve in glaucoma). Some doctors refer to an enlarged cup/disc ratio as cupping or a cupped nerve. Glaucoma typically causes the cup to get bigger in a vertical oval type pattern. To discern whether a large cup is glaucomatous or normal requires the doctor to pay close attention to the rim of the nerve on the temporal side (the side closest to the temple or ear side of the head). Stereo photos of the optic nerve are extremely valuable for documentation of t
