Optic disc drusen (ODD), or optic nerve head drusen or hyaline bodies, are a relatively common entity usually found incidentally on CT or on follow-up of abnormal fundoscopy.
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Epidemiology
Optic disc drusen are identified radiographically in up to 0.3-3.7% of the population and are frequently bilateral 1,4,5. They are typically seen in patients with age-related macular degeneration 7. They can also be familial and inherited as a dominant trait 1,4. In general, they are more common in Caucasians than African-Americans 4.
Associations
In addition to age-related macular degeneration, optic disc drusen have been associated with: 3-5,10
aneurysm of the ophthalmic artery
combined hamartoma of the retina and retinal pigment epithelium
familial macular dystrophy
optic nerve tumours
severe early childhood onset retinal dystrophy (SECORD)
tubulointerstitial nephritis and uveitis (TINU) syndrome
Clinical presentation
Most cases of drusen are asymptomatic, although patients may report an episodic visual loss. The blind spot may be enlarged, but this is subtle and is only usually found following formal visual field mapping 4.
Fundoscopic findings depend on how deeply the drusen is located in the optic disc. Superficial deposits exhibit characteristic autofluorescence making the diagnosis in such cases obvious. Deeper deposits have less-specific appearances with elevation of the optic disc that mimics papilloedema (pseudopapilloedema) 1,4,9.
Pathology
Optic disc drusen are small collections of hyaline material located within the head of the optic nerve (CN II) at variable depths 1-3. The earliest signs of drusen formation histologically are small globular or plaque-like deposits in Bruch membrane of hyaline material, present in the majority of individuals, and perhaps congenital in nature 3.
Small hard drusen (<63 micrometres), visible on graded fundus photography are seen in a high proportion of individuals. They are not thought to be a risk factor for the development of age-related maculopathy, and their prevalence is not age-dependent 2. They are not visible on imaging.
Over time in some individuals, multiple micro-drusen (<25 micrometres) coalesce 2,3. These larger deposits increase in prevalence with age and are associated with age-related maculopathy 2. As they mineralise, it is these drusen that are visible radiographically.
Radiographic features
Optic disc drusen vary in size from less than 1 mm to up to 4 mm in diameter and 3 mm in thickness, most of which are calcified in adults 1.
Ultrasound
Ocular ultrasound demonstrates an echogenic focus within or on the surface of the optic nerve head. Posterior acoustic shadowing may be present with larger lesions 1.
CT
Although drusen can be identified in childhood, they are small and non-mineralised. As such, drusen are rarely identified in the paediatric patient, although a region of slightly increased density and swelling may be evident on thin-section scans 3. In adults, drusen appear as small calcific foci.
Treatment and prognosis
In the vast majority of cases, no therapy is required 5. Occasionally, especially when drusen are large, vascular complications may occur, e.g. non-arteritic anterior ischaemic optic neuropathy (NAION), central retinal artery occlusion (CRAO), central retinal vein occlusion (CRVO) 5.
Intraocular pressure-lowering agents may be of benefit if pressures are elevated, although whether elevated pressures represent glaucoma superimposed on drusen, or a direct complication of drusen cannot be distinguished 5.
Differential diagnosis
When appearances are typical, a differential diagnosis is not required. In general, the differential includes other causes of calcification of the globe including:
-
usually a mass with calcification
protruding into the vitreous body
usually not located at the optic disc
-
seen in patients with tuberous sclerosis or neurofibromatosis
usually unilateral
usually larger
associated with retinal detachment 1
-
choroidal naevus
within the choroid and not the optic disc