“D” selections in the Glossary of Terms for
Anatomy, Physiology & Pathology of the Human Eye

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deficiency, color
see color deficiency

degeneration, macular
see macular degeneration, dry and wet

a small, indented region of the corneal surface resulting from constant dryness, usually due to the inability of the eye lid to make contact with that area of the cornea while blinking

Descemet’s membrane
posterior elastic lamina; fourth layer of the cornea; a transparent, highly elastic apparently structureless membrane that covers the posterior surface of the cornea and is lined with endothelium

detachment, posterior vitreous (PVD)
a normal separation of the vitreous humor from the retina due to gradual liquification, with age, of the gel-like vitreous substance; often accompanied by floaters and flashes of light; infrequently leads to a retinal detachment

detachment, retinal (RD)
separation of the retina from the choroid beneath it, often initiated by a retinal tear resulting in the seepage of vitreous fluid underneath the retina; can be associated with the observation of a shower of floaters, flashes of light, and/or a “web” or “veil” in front of the eye or in the periphery; usually treated with a laser, freezing (“cryotherapy” or “cryopexy"), or a scleral buckle

a type of anomalous trichromatism color deficiency in which there is a partial loss of green color vision, so that an increased intensity of this color is required in a mixture of red and green to match a given yellow

a type of anomalous dichromatism color deficiency marked by confusion of purplish red and green, due to a lack of (or lack of function of) “M-cone” photoreceptors sensitive to medium (greenish) wavelengths of light

diabetes insipidus
a disorder of the pituitary gland characterized by intense thirst and by the excretion of large amounts of urine

diabetes mellitus
a variable disorder of carbohydrate metabolism caused by a combination of hereditary and environmental factors and usually characterized by an inadequate secretion of insulin by the pancreas or proper utilization of the available insulin, by excessive urine production, by excessive amounts of sugar in the blood and urine, and by thirst, hunger, and loss of weight; type I (juvenile) diabetes mellitus: insulin-dependent diabetes; type II (adult-onset) diabetes mellitus: non-insulin-dependent diabetes; can cause neovascularization (new blood vessel growth) in the retina (retinitis proliferans), resulting in fragile blood vessels which can bleed into the vitreous humor and lead to severe visual impairment or blindness

diabetic retinopathy, background
changes in the retina due to diabetes, eventually leading to proliferative diabetic retinopathy if the diabetes is not controlled; caused by adverse changes in the endothelial cells and pericyte cells in the retinal blood vessels, causing blood vessel weakening and leading to microaneurysms, venous beading, ischemic areas of the retina, cotton wool spots, intraretinal hemorrhages, and macular edema

diabetic retinopathy, proliferative
neovascularization (growth of new blood vessels), usually out of the retina and along the surface of the vitreous or into the vitreous cavity, leading to the formation of scar tissue on the surface of the retina (which can cause retinal tears and detachment) and leaking of blood by the new fragile vessels into the eye (vitreous hemorrhage), all capable of causing severe vision loss or blindness

dichromatism, anomalous
see anomalous dichromatism

dilation or dilatation, pupil
see pupil dilation or dilatation

diopter (D)
a unit of measure of the refractive power of a lens or of an eye, where the dioptric power is the inverse (reciprocal) of the focal length of the lens or of the eye’s optical system in meters; a +1.00 D convex lens focuses light, from an infinite source, 1 ÷ 1.00 D = 1 meter away from the lens (and corrects an eye which is 1.00 D hyperopic); a +2.00 D lens focuses infinite light 1 ÷ 2.00 D = ½ meter away (and corrects an eye which is 2.00 D hyperopic); a +0.25 D lens focuses infinite light 1 ÷ 0.25 D = 4 meters away (and corrects an eye which is 0.25 D hyperopic); etc.

double vision, usually caused by a strabismus

discrimination perimetry, pattern (PDP)
see pattern discrimination perimetry

dry eye
a chronic lack of sufficient lubrication and moisture in the eye(s), causing sensations of dryness, scratchiness, burning; can be caused by conditions such as rheumatoid arthritis, Sjogren’s syndrome, keratoconjunctivitis sicca, xerophthalmia, lupus erythematosis, Grave’s disease, diabetes, or scleroderma

dura mater
the outer meningeal layer of the optic nerve; fuses with the sclera where the optic nerve enters the eye

dystrophy, corneal
one of a number of inherited diseases—all potentially blinding disorders which frequently require corneal transplantation to improve visual acuity—characterized by the accumulation of abnormal material in the cornea, typically classified depending on which of the three levels of the cornea they primarily affect, including anterior corneal dystrophies (such as map-dot-fingerprint dystrophy and Reis-Bückler’s dystrophy), stromal dystrophies (such as granular, lattice, and Avellino dystrophy), and posterior corneal dystrophy (such as Fuch’s dystrophy and posterior polymorphous dystrophy or PPMD)

dystrophy, macular
one of a number of progressive, genetically determined degenerations of the retina or choroid, typically occurring at an early age, primarily affecting the macular area by causing anything from minimal visual acuity loss and color vision abnormality to profound loss of reading ability with night vision difficulty; includes Stargardt’s disease, fundus flavimaculatus, Best’s vitelliform dystrophy, cone or cone/rod dystrophy, juvenile retinoschisis, and juvenile macular dystrophy

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