The optic disc is the round spot on the retina formed by the passage of the axons of the retinal ganglion cells. The ganglion cells receive the final impulse created by the reception of light by the retinal photoreceptors. Their axons form the nerve fiber layer of the retina. The nerve fibers flow together at this spot to form the beginning of the optic nerve.
This is called the optic disc, or the optic nerve head, and it forms a slightly raised round whitish spot slightly to the nasal side of the center of the retina. At this place, there are no photoreceptors, and so it is the blind spot of the eye. The optic disc has a slight cup-shaped depression, called the physiologic cup. It is about 1.5 mm in diameter. The central retinal artery and vein also pass through the retina at this point.
Swelling of the optic disc may be confined to one side or occur on both sides.
Causes of Optic Disc Swelling
The optic disc swells in reaction to a variety of insults. The most common causes include:
- Non-arteritic anterior ischemic optic neuropathy (NA-AION) in over a third of all patients
- Optic neuritis in slightly less than a third
- Intracranial disease in one in six patients
- Other disease conditions such as diabetes
Unilateral swelling is observed in the following disorders, among others:
- Demyelinating optic neuritis
- Compressive optic neuropathy
- Occlusion of the retinal vein
- Diabetic papillopathy
Bilateral swelling of the optic disc may be due to causes such as:
- Infiltrative neuropathy of the optic nerve
- Toxic optic neuropathy
- Malignant hypertension
Swelling of the optic disc presents with symptoms based on the underlying causative condition. In the majority, therefore, patients complain of a sudden diminution of visual acuity and visual field. Visual field defects in NA-AION are mostly inferonasal in location. Altitudinal field defects, affecting only half of the vertical field of vision, are more common in optic neuritis, but scotomas of different types or an enlargement of the blind spot are also often present.
Pain on eye movement is seen more frequently in optic neuritis, but is typically absent in ischemic neuropathy. In compressive forms of optic neuropathy, various intracranial tumors should be ruled out. A relative afferent pupillary defect is characteristic, but pain is usually absent.
Diagnosis and Treatment
The following signs indicate the presence of non-papilledema optic disc swelling:
- The cupping of the disc is lost
- The margins of the disc are blurred
- No peripapillary hemorrhages are seen
- No infarcts, exudates or edema of the nerve fiber layer
Diagnosis depends on a careful history of symptoms related to intracranial hypertension, other systemic conditions and visual symptoms related to the current episode.
Tests include dilated fundoscopy and imaging findings obtained with fluorescein angiography and optical coherence tomography (OCT). Treatment of optic disc swelling is based on the cause of the condition.