Optic disc

The optic disc (a.k.a. the optic nerve head) is the point on the retina where retinal ganglion cell axons converge to form the optic nerve and where the central retinal artery and vein enter or leave the eye. It is located medial (nasal) to the macula and appears as a pale circular or oval area on fundoscopy. Histologically, the optic disc lacks photoreceptors, including rods and cones, because it represents a site of nerve fibre exit rather than sensory retina.

Physiological blind spot

Because the optic disc contains no photoreceptors, it corresponds to the physiological blind spot in the visual field. Objects whose images fall on this area cannot be perceived. Normally, the blind spot is not noticed because the visual fields of the two eyes overlap and the brain compensates for the missing information.

Optic cup-to-disc ratio

At the centre of the optic disc is a small depression called the physiological cup, produced by the arrangement of nerve fibres as they converge posteriorly. The relationship between the diameter of the cup and the overall optic disc is described by the cup-to-disc ratio (CD ratio), which is normally less than about 0.5. An increased cup-to-disc ratio suggests loss of optic nerve fibres and is an important clinical feature of glaucoma.

Papilledema

Sometimes, the optic disc can swell up due to a raised intracranial pressure, which is called papilledema. Increased cerebrospinal fluid pressure within the subarachnoid space surrounding the optic nerve impairs axoplasmic flow and venous drainage, leading to optic disc edema. On fundoscopy, papilledema is characterized by a swollen hyperaemic optic disc with blurred margins, venous congestion, and sometimes retinal haemorrhages. Persistent papilledema can lead to optic nerve damage and visual impairment.