Optic disc drusen

Optic disc drusen (ODD), or hyaline bodies, are a relatively common entity usually found incidentally on CT or on follow-up of abnormal fundoscopy.

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

A number of entities have been described associated with optic disc drusen ^3-5:

• retinitis pigmentosa
• pseudoxanthoma elasticum
• angioid streaks

Clinical presentation

Most cases of drusen are asymptomatic, although patients may report an episodic visual loss. The blind spot may also 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 swelling 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 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.

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-anterior ischaemic optic neuropathy (NAION), central retinal artery occlusion (CRAO), central retinal vein occlusion (CRVO) ⁵. 

Optic pressure lowering-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 there are typical appearances, the differential is small. In general, the differential includes other causes of calcification of the globe including:

• retinoblastoma
  • usually a mass with calcification
  • protruding into the vitreous body
  • usually not located at the optic disc
• astrocytic hamartomas
  • seen in patients with tuberous sclerosis or neurofibromatosis
  • usually unilateral
  • usually larger
  • associated with retinal detachment ¹
• chorioretinitis
• choroidal osteoma