optic disc

The optic disc is located nasal to the fovea and represents the exit point of retinal ganglion cell axons (cranial nerve II) as they leave the eye through the lamina cribrosa of the sclera. Because it contains no rods or cones, it corresponds to the physiologic blind spot in the visual field. The central retinal artery and vein pass through the center of the disc, creating a normal 'cup' (physiologic cupping) with a cup-to-disc ratio typically <0.3–0.4. Clinical significance: (1) Papilledema - bilateral optic disc swelling due to increased intracranial pressure (e.g., from mass lesions, idiopathic intracranial hypertension, malignant hypertension); disc margins become blurred, venous pulsations are lost, and the optic cup may be obliterated. (2) Glaucoma - progressive optic disc cupping (increased cup-to-disc ratio >0.6, asymmetry between eyes) due to retinal ganglion cell axon loss from elevated intraocular pressure, leading to peripheral visual field loss. (3) Optic neuritis - disc may appear swollen (papillitis) or normal (retrobulbar neuritis) with pain on eye movement and afferent pupillary defect; associated with multiple sclerosis. (4) Optic atrophy - pale, well-demarcated disc from axonal loss due to various causes (compression, ischemia, toxic/nutritional, hereditary). (5) Central retinal artery/vein occlusion - can cause disc pallor or swelling with characteristic fundoscopic findings ('cherry-red spot' for CRAO). Fundoscopic examination of the optic disc is a key component of the neurologic and ophthalmologic exam, often tested on boards in the context of increased ICP, glaucoma, and demyelinating disease.