Optic disc drusen

Anatomy
The optic nerve is a cable connection that transmits images from the retina to the brain. It consists of over one million retinal ganglion cell axons. The optic nerve head, or optic disc is the anterior end of the nerve that is in the eye and hence is visible with an ophthalmoscope. It is located nasally and slightly inferior to the macula of the eye. There is a blind spot at the optic disc because there are no rods or cones beneath it to detect light. The central retinal artery and vein can be seen in the middle of the disc as it exits the scleral canal with the optic nerve to supply the retina. The vessels send branches out in all directions to supply the retina.

Optic disc drusen (ODD) are globules of mucoproteins and mucopolysaccharides that progressively calcify1. They are thought to be the remnants of the axonal transport system of degenerated retinal ganglion cells2. ODD have also been referred to as congenitally elevated or anomalous discs, pseudopapilledema, pseudoneuritis, buried disc drusen, optic nerve head drusen and disc hyaline bodies3.

Epidemiology
Optic disc drusen are found in about 1% of the population but this increases to 3.4% in individuals with a family history of ODD1. An autosomal dominant inheritance pattern with incomplete penetrance is suspected3. Males and females are affected at equal rates. Caucasians are the most susceptible ethic group. Certain conditions have been associated with disc drusen such as retinitis pigmentosa, angioid streaks, Usher syndrome, and Alagille syndrome1. Optic disc drusen are not related to drusen of the retina which have been associated with age-related macular degeneration4.

Pathophysiology
In children, optic disc drusen are usually buried and undectectable by funduscopy except for a mild or moderate elevation of the optic disc. With age, the overlying axons become atrophied and the drusen become exposed and more visible. They may become apparent with an ophthalmoscope at the end of adolescence. ODD can compress and eventually compromise the vasculature and retinal nerve fibers. Rarely, choroidal neovascularization may develop as the juxtapapillary nerve fibers are disrupted, with subsequent subretinal hemorrhage and retinal scarring3. Even more rarely, vitreous hemorrhage may develop5.

Clinical Features
On funduscopic examination, the optic disc is small and often missing the optic cup. The optic disc also appears elevated and is classically described as having lumpy-bumpy or scalloped borders. The pattern of retinal vessel branching may be deranged. Superficial optic disc drusen are seen as round white-yellow granules located in any part of the disc, most often nasally3. Drusen buried within the disc are more difficult to detect without an imaging study. In 70% of cases, drusen affect the optic disc bilaterally1. If there is significant asymmetric optic disc involvement, an afferent papillary defect may be obvious3.

Optic nerve compression is progressive and insidious. Eventually 75% of patients will develop some peripheral field defects. These can include nasal step defects, enlarged blind spots, arcuate scotomas, sectoral field loss and altitudinal defects3. Central vision loss is rare.

Management
In most patients, optic disc drusen are an incidental finding. It is important to differentiate them from other conditions that present with optic disc elevation to avoid excessive worry and expensive workups. True papilledema may present with exudates or cotton-wool spots, unlike ODD. The optic disc margins are characteristically irregular in ODD but not blurred as there is no swelling of the retinal nerve fibers. Spontaneous venous pulsations are present in about 80 percent of patients with ODD, but absent in cases of true disc edema3.

Superficial optic disc drusen have a characteristic appearance under direct funduscopic examination. Superficial ODD are also autofluorescent on fluorescein angiography and can be seen with red-free photography1. Buried drusen calcifications can be detected by B-scan sonography or CT6. False negative results are more common in young children in whom ODD have not yet calcified.

Patients with optic disc drusen should be monitored periodically for Snellen acuity, contrast sensitivity, color vision, and threshold visual fields3. Associated conditions such as angioid streaks and retinitis pigmentosa should be screened for. There is no widely accepted treatment for ODD, although some clinicians will prescribe eye drops designed to decrease the intra-ocular pressure and theoretically relieve mechanical stress on fibers of the optic disc.