optic cup

The optic disc (optic nerve head) is the site where retinal ganglion cell axons exit the eye to form the optic nerve; it contains no photoreceptors, creating the physiologic blind spot. The optic cup is the central depression within the disc, lacking nerve fibers, through which the central retinal artery and vein pass. The cup-to-disc (C/D) ratio compares the diameter of the cup to the diameter of the entire disc, normally ranging from 0.3 to 0.4, with symmetry expected between the two eyes.

In glaucoma, particularly primary open-angle glaucoma, chronically elevated intraocular pressure causes progressive loss of retinal ganglion cell axons, leading to thinning of the neuroretinal rim and enlargement of the optic cup (increased C/D ratio, often >0.6, or significant asymmetry >0.2 between eyes). This is classically described as 'cupping' of the optic disc and is a hallmark finding on fundoscopic exam. Notching, bayoneting of vessels, and nasalization of vessels may also be seen with advanced cupping.

Other causes of optic disc cupping include normal-tension glaucoma, ischemic optic neuropathy, and congenital anomalies. Clinically, optic disc cupping correlates with visual field defects, typically starting with paracentral scotomas and arcuate defects, progressing to peripheral field loss while central vision is often preserved until late stages (tunnel vision). Fundoscopic examination and optical coherence tomography (OCT) are used to monitor the optic cup and disc for glaucoma progression, making this an essential concept for both diagnosis and monitoring of glaucoma on USMLE exams.