Drusen

Drusen (singular, "druse") are tiny yellow or white accumulations of extracellular material that build up in Bruch's membrane of the eye. The presence of a few small ("hard") drusen is normal with advancing age, and most people over 40 have some hard drusen. However, the presence of larger and more numerous drusen in the macula is a common early sign of age-related macular degeneration (AMD).

Classification
Drusen associated with aging and macular degeneration are distinct from another clinical entity, optic disc drusen, which is present on the optic nerve head. Both age-related drusen and optic disc drusen can be observed by ophthalmoscopy. On CT scans of the orbits or head, calcification at the head of the optic nerve without change in size of globe strongly suggests drusen in a middle-age or elderly patient.

Whether drusen promote AMD or are symptomatic of an underlying process that causes both drusen and AMD is not known, but they are indicators of increased risk of the complications of AMD.

Pathophysiology
Drusen were initially described by Franciscus Donders who called them "Colloidkugeln" (colloid spheres). Later, Heinrich Müller named them for the German word for geode, based on their glittering appearance. In view of their location between the retinal pigment epithelium (RPE) and its vascular supply, the choriocapillaris, it is possible that drusen deprive the RPE and photoreceptors of oxygen and nutrients. Interestingly, drusen always develop above the so called pillars of choriocapillaris that is the area between two microvessels.

The source of the proteins and lipids in drusen is also not clear, with potential contributions by both the RPE and the choroid. Several trace elements are present in drusen, probably the most concentrated being zinc. The protein composition of drusen includes apolipoproteins and members of the complement system. Zinc in drusen have been suggested to play a role in drusen formation by precipitating and inhibiting the elements of the complement cascade, especially complement factor H.

The presence of molecules that regulate inflammation in drusen has led some investigators to conclude that these deposits are product of the immune system.