Drusen
Encyclopedia
Drusen 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).
, 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.
'Hard drusen' may coalesce into 'soft drusen' which is a manifestation of macular degeneration.
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 photoreceptor cells 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
.
Bruch's membrane
Bruch's membrane is the innermost layer of the choroid. It is also called the vitreous lamina, because of its glassy microscopic appearance.It is 2–4 μm thick.-Layers:Bruch's membrane consists of five layers :...
of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. 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
Macula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
is a common early sign of age-related macular degeneration
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...
(AMD).
Classification
Drusen associated with aging and macular degeneration are distinct from another clinical entity, optic disc drusenOptic disc drusen
Optic disc drusen or optic nerve head drusen are globules of mucoproteins and mucopolysaccharides that progressively calcify in the optic disc...
, which is present on the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
head. Both age-related drusen and optic disc drusen can be observed by ophthalmoscopy
Ophthalmoscopy
Ophthalmoscopy is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope . It is done as part of an eye examination and may be done as part of a routine physical examination...
. 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.
'Hard drusen' may coalesce into 'soft drusen' which is a manifestation of macular degeneration.
Pathophysiology
Drusen were initially described by Franciscus DondersFranciscus Donders
-External links:* B. Theunissen. , F.C. Donders: turning refracting into science, @ History of science and scholarship in the Netherlands.* in the Virtual Laboratory of the Max Planck Institute for the History of Science* P. Eling, , Geneeskundige en fysioloog....
who called them "Colloidkugeln" (colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
spheres). Later, Heinrich Müller
Heinrich Müller (physiologist)
Heinrich Müller was a German anatomist and professor at the University of Würzburg. He is best known for his work in comparative anatomy and his studies involving the eye.He was a native of Castell, Lower Franconia...
named them for the German word for geode
Geode
Geodes are geological secondary sedimentary structures which occur in sedimentary and certain volcanic rocks. Geodes are essentially spherical masses of mineral matter that were deposited sygenetically within the rock formations they are found in. Geodes have a Chalcedony shell containing...
, based on their glittering appearance. In view of their location between the retinal pigment epithelium (RPE) and its vascular supply, the choriocapillaris
Choriocapillaris
The capillary lamina of choroid or choriocapillaris is a layer of capillaries that is immediately adjacent to Bruch's membrane in the choroid....
, it is possible that drusen deprive the RPE and photoreceptor cells of oxygen and nutrients. Interestingly, drusen always develop above the so called pillars of choriocapillaris
Choriocapillaris
The capillary lamina of choroid or choriocapillaris is a layer of capillaries that is immediately adjacent to Bruch's membrane in the choroid....
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
Choroid
The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...
. 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
Complement system
The complement system helps or “complements” the ability of antibodies and phagocytic cells to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime...
. 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
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
in drusen has led some investigators to conclude that these deposits are product of the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
.