Section 1-B. The Meaning of Cupping
Figure 1-5. The optic nerve is divided into tenths and the cup is compared to the entire optic nerve (optic disc) to obtain the cup-to-disc ratio. This C/D ratio here is 0.4.
Figure 1-6. Normal optic nerve - 0.3 C/D ratio
Figure 1-7. Glaucomatous optic nerve – 0.9 C/D ratio
The C/D ratio attempts to quantify the extent of axonal (nerve fiber) loss. It compares the diameter of the cup to the entire diameter of the optic nerve head (disc). The normal C/D ratio is typically less than 0.5 (meaning that the diameter of the cup is 50% the diameter of the optic nerve head). This number is not a uniform number, because eyes with glaucoma tend to have more cupping in the vertical orientation than the horizontal orientation due to a difference in axonal support structures. Therefore, a measurement in just one orientation may not fully describe the extent of cupping. Eye doctors will often draw pictures and take photographs to accurately document the appearance of a glaucomatous optic nerve. Increases in cupping or nerve fiber loss indicate poorly controlled glaucoma.
In glaucoma the position of the blood vessels within the optic nerve can shift with the progressive cupping, and this can be an important clue that the glaucoma is continuing to cause optic nerve damage. Other exam findings suggestive of glaucoma include hemorrhages (bleeding) on or near the optic nerve. This is commonly seen in poorly controlled glaucoma and is indicative of ongoing optic nerve damage or unstable glaucoma.
If glaucoma affects only one eye, there may be asymmetry of the optic nerve cupping between the two eyes. The unaffected optic nerve will look normal while the affected eye will look cupped. Asymmetry of the optic nerves is another clue that can aid in the diagnosis of glaucoma.
Figures 1-8, 1-9. There is a slight asymmetry of the cup between these eyes. The right eye (pictured at left) has a slightly larger cut than the left eye (pictured at right).
In end stage disease, the nerve may be completely cupped, with no nerve fibers left. In this case, the nerve may appear pale and vision would be poor. While most patients that receive treatment will not progress to end-stage glaucoma, those that do may eventually become totally blind.
It is rare that other optic neuropathies result in cupping of the optic nerve. Most of the time, optic nerve head cupping is caused by glaucoma, and the amount of vision loss corresponds to the extent of cupping.