Cup-to-disc ratio
Encyclopedia
The cup-to-disc ratio is a measurement used in ophthalmology
Ophthalmology
Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...

 and optometry
Optometry
Optometry is a health care profession concerned with eyes and related structures, as well as vision, visual systems, and vision information processing in humans. Optometrists, or Doctors of Optometry, are state licensed medical professionals trained to prescribe and fit lenses to improve vision,...

 to assess the progression of glaucoma
Glaucoma
Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...

. The optic disc is the anatomical location of the eye's "blind spot", the area where the optic nerve and blood vessels enter the retina. The optic disc can be flat or it can have a certain amount of normal cupping. But glaucoma, which is due to an increase in intra-ocular pressure, produces additional pathological cupping of the optic disc. The pink rim of disc contains nerve fibers. The white cup is a pit with no nerve fibers. As glaucoma advances, the cup enlarges until it occupies most of the disc area.

The cup-to-disc ratio compares the diameter of the "cup
Optic cup (ophthalmology)
The optic cup is the white, cup-like area in the center of the optic disc.The ratio of the size of the optic cup to the optic disc is measured to diagnose glaucoma....

" portion of the optic disc with the total diameter of the optic disc. A good analogy to better understand the cup-to-disc ratio is the ratio of a donut hole to a donut. The hole represents the cup and the surrounding area the disc. If the cup fills 1/10 of the disc, the ratio will be 0.1. If it fills 7/10 of the disc, the ratio is 0.7. A large cup-to-disc ratio may imply glaucoma or other pathology. However, cupping by itself is not indicative of glaucoma. Rather, it is an increase in cupping as the patient ages that is an indicator for glaucoma. Deep but stable cupping can occur due to hereditary factors without glaucoma.
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