D

D-15 test See test, Farnsworth.

dacryoadenitis Inflammation of the lacrimal gland. The acute type is characterized by localized pain, swelling and redness over the upper temporal area of the eye. The chronic type is painless and develops slowly. A frequent cause is an associated systemic infection such as mumps, infectious mononucleosis, influenza, or it can be due to a local condition such as trachoma, herpes zoster or staphylococcal infection. The chronic type may be due to any of the granulomatous diseases (tuberculosis, syphilis, sarcoidosis). Treatment consists mainly of warm compresses and antibiotics.

See syndrome, Mikulicz’s.

dacryocele, congenital A congenital condition in which the infant is born with a swollen lacrimal sac filled with mucoid material. Physical examination reveals a bluish mass located in the nasal canthal region, probably due to an obstruction of the lower end of the nasolacrimal duct, with associated blockage of the canaliculi and puncta. Treatment includes antibiotics as well as nasolacrimal probing and irrigation in many cases. Syn. dacryocystocele.

dacryocystectomy Surgical removal of the lacrimal sac.

dacryocystitis Inflammation of the lacrimal sac. It is a rare condition, which may occur when there is a blockage of the nasolacrimal drainage system. The acute type gives rise to redness, tenderness and swelling below the lid margin, while in the chronic type there is epiphora and with pressure on the lacrimal sac pus will come out of the punctum. Treatment includes broad-spectrum antibiotics and warm compresses but surgery may be needed in the chronic type.

See fistula, lacrimal; lacrimal apparatus.

dacryoliths Concretions found in the lacrimal apparatus, in the puncta or canaliculi which it may occlude. The concretions are usually composed of epithelial cells, lipid, nonspecific debris as well as calcium.

dacryoma 1. A tumour or swelling anywhere within the lacrimal apparatus. 2. A blockage of a lacrimal punctum.

dacryops 1. A chronic watery eye. 2. A cyst in a tear duct of the lacrimal gland.

See epiphora.

Dalen–Fuchs nodules See Dalen– Fuchs nodules.

Dalrymple’s sign See sign, Dalrymple’s.

daltonism Term used formerly to designate colour blindness, usually deutan, so named because John Dalton (1766–1844) was the first to describe his own anomaly.

See colour vision, defective.

dapiprazole See alpha-adrenergic antagonist.

dark adaptation See adaptation, dark.

dark current See electrooculogram.

dark filter test See test, neutral density filter.

dark focus See accommodation, resting state of.

dark room test See test, provocative.

dark vergence See vergence, tonic.

dark wedge test See test, Bielchowsky’s phenomenon.

day blindness See hemeralopia.

daylight, artificial Illumination produced by a source of artificial light having a spectral distribution similar to that of daylight. CIE Illuminant C is considered to almost fulfill this criterion.

daylight, natural Illumination dependent on the sun and the extent of clear sky.

daylight vision See vision, photopic.

deaf-blind A person who has a severe hearing impairment in addition to a visual defect. It is usually congenital but it may result from ageing or some systemic disease or as part of a syndrome (e.g. Usher’s syndrome which accounts for about half of all cases of deaf-blind people; rubella syndrome).

debility The state of being feeble or without strength.

debridement Removal of dead or infected tissue or foreign material until surrounding healthy tissue is exposed. This is done to facilitate healing. Corneal debridement is usually performed with a cotton-tipped applicator, a spatula or with a sharp instrument. Example: debridement of some of the corneal epithelium in dendritic keratitis or in corneal erosion.

decentration (dec) A displacement, horizontal and/or vertical, of the centration point of a spectacle lens from the standard optical centre position (British Standard).

See centre, standard optical position; point, centration.

decibel (dB) 1. Unit used for the measurement of the intensity of a sound. 2. Light intensities are often presented on a logarithmic (rather than linear) scale. This is done, in particular, to abbreviate large numbers. Moreover, it has become common, especially in perimetry, to use decibels rather than log units. A decibel scale is a logarithmic scale where 10 decibels are equal to 1 log unit; 20 decibels, to 2 log units, etc. In perimetry, decibels are used to indicate the attenuation of brightness of the stimulus. Thus, a 20 dB stimulus is equal to one-tenth the brightness of a 10 dB stimulus.

decompensation Failure of an organ to fulfill its function adequately. Examples: corneal decompensation following years of extended contact lens wear; a failure of the eye movement system to overcome a heterophoria.

decongestant, ocular A pharmaceutical agent used to reduce hyperaemia in the eye, usually by vasoconstriction. Decongestants are weak concentrations of sympathomimetic or alpha-adrenergic agonists. Examples: adrenaline (epinephrine); naphazoline hydrochloride; phenylephrine hydrochloride, tetrahydrozoline hydrochloride. A decongestant can also be used to differentiate between conjunctival and ciliary injection; if the instillation of a decongestant alleviates eye redness the injection is primarily conjunctival, otherwise the redness is of ciliary origin.

decussation Crossing of nerve fibres passing through the mid-sagittal plane of the central nervous system and connecting with structures on the opposite side. Partial decussation occurs at the optic chiasma.

defocus To put or go out of focus.

hyperopic d. State of the eye in which the retinal image is focused behind the retina. It may occur when placing a negative lens in front of an emmetropic presbyopic eye; in an uncorrected presbyopic hyperope; in a high hyperope unable to overcome the ametropia by accommodating; or as an accommodative lag in an uncorrected or corrected myope, as well as in an emmetrope viewing a near object.

See accommodation, lag of; over-correction.

myopic d. State of the eye in which the retinal image is focused in front of the retina. It may occur when placing a positive lens in front of an emmetropic eye; in an uncorrected or undercorrected myopic eye; or if there is a lead of accommodation, as is the case in some individuals viewing a distant object.

See accommodation, lead of; undercorrection.

degeneration Deterioration of tissue or organ resulting in reduced efficiency. Examples: degeneration of the cornea; degeneration of the retina.

See dystrophy, corneal.

age-related macular d. See macular degeneration, age-related.

cobblestone d. See degeneration, paving-stone.

cone d. See dystrophy, cone.

Doyne’s honeycombed d. See drusen, familial dominant.

lattice d. of the retina See retina, lattice degeneration of the.

lipid droplet d. See keratopathy, actinic.

paving-stone d. Discrete, yellowish round areas of retinal thinning and depigmentation located near the ora serrata. The underlying choroid may be seen. It is a benign degeneration occurring with advancing age. Syn. cobblestone degeneration; peripheral chorioretinal degeneration.

pellucid marginal d. A rare condition characterized by bilateral, slowly progressive thinning and protrusion of the inferior peripheral cornea. The involved area is clear (hence the word pellucid), but the condition may be complicated by hydrops and the central cornea typically develops against the rule astigmatism. Treatment usually consists of gas permeable scleral lenses, but keratoplasty may be necessary.

See ectasia, corneal; hydrops; keratoconus.

peripheral chorioretinal d. See degeneration, paving-stone.

peripheral cystoid d. A degenerative process in the peripheral retina that occurs almost universally in the elderly. It consists of numerous, discrete cystic spaces in the outer plexiform or inner nuclear layer presenting a frothy appearance. The degeneration starts at the ora serrata and slowly progresses to the peripheral retina. If the cysts should join together, degenerative retinoschisis develops. It is not usually associated with retinal tears. The condition does not require any treatment.

Salzmann’s nodular d. A degenerative condition characterized by bluish-white, elevated nodules on the surface of the cornea. It may occur in people previously affected by trachoma, phlyctenular keratitis, vernal keratitis or keratoconjunctivitis sicca. Most cases are asymptomatic, but if the nodules impair vision, keratoplasty may be necessary.

senile macular d. See macular degeneration, age-related.

tapetoretinal d. A hereditary degeneration affecting the photoreceptors of the retina or the pigment epithelium layer. Some authors also include the choroid. Syn. tapetoretinopathy.

See choroideremia; retinitis pigmentosa.

Terrien’s marginal d. See ectasia, corneal.

vitreoretinal d. See disease, Wagner’s; syndrome, Stickler’s.

dellen A transient shallow depression in the cornea near the limbus which is caused by a local dehydration of the corneal stroma, leading to a compression of its lamellae. It can occur as a result of strabismus surgery, cataract surgery, swelling of the limbus (as in episcleritis or pterygium), rigid contact lens wear or senility.

DEM test See test, developmental eye movement.

demand line See line, demand.

demecarium bromide See anticholinesterase.

dendritic keratitis See keratitis, dendritic.

densitometry, retinal A technique used to study visual pigments in vivo. It consists of measuring the small fraction of light that is reflected by the pigment epithelium of the retina before and after bleaching with a bright source of light.

See pigment, visual.

density An indication of the compactness of a substance. It is expressed as the ratio of the mass of the substance to its unit volume. The common units are g/cm³ and kg/m³. This property is usually given by lens manufacturers, the greater the density of a material, the greater its weight, all other factors being equal.

density, optical A term applied to optical filters. It is equal to the logarithm to the base 10 of the reciprocal of the transmission factor T thus,

D=log101T

where D is the symbol for optical density. Syn. absorbance.

See absorbance; absorption; spectrophotometer; transmittance.

Denver Developmental Screening Test See test, developmental and perceptual screening.

deorsumduction See depression.

deorsumvergence See infravergence.

deorsumversion See version.

depolarization A change in the value of the resting membrane potential towards zero. The inside of the cell becomes less negative compared to the outside. This is due to a change in permeability and migration of sodium ions into the interior of the cell. Depolarization is excitatory because the membrane potential shifts towards the neuron’s threshold at which an action potential occurs.

See hyperpolarization; synapse.

depolished glass See glass, ground.

deposits, contact lens Accumulation of materials on or into the matrix of contact lenses. They are mainly tear components (proteins, lipids, mucin, calcium) but other materials can be found (e.g. mercurial or iron deposits, nicotine, hand cream). Deposits reduce comfort, vision, patient tolerance and discolour and spoil the lenses. They may act as antigens for the development of giant papillary conjunctivitis. Most of these deposits can be removed with a surfactant, an enzymatic system and a calcium-preventing solution.

depression Downward rotation of an eye. It is accomplished by the inferior rectus and superior oblique muscles. It can be induced by using base-up prisms. Syn. infraduction; deorsumduction.

depressors Extraocular muscles that move the eye downward, such as the inferior rectus and the superior oblique.

deprivation amblyopia See amblyopia, deprivation.

deprivation, sensory The condition produced by a loss of all or most of the stimulation from the visual, auditory, tactile and other sensory systems. Often, deprivation involves only one modality (e.g. vision). Methods used for deprivation include diffusing goggles, white noise, padded gloves, etc. Its effect has shown the necessity of continuous sensory activity to maintain the normal development and functioning of any sensory system.

deprivation, visual The condition produced by a loss of form vision. It may occur as a result of an anomaly within the eye (e.g. opacification of the cornea), or it can be artificially induced (e.g. by placing a transparent plastic occluder in front of the eye, as used in myopia research with animals).

depth of field For a given setting of an optical system (or a steady state of accommodation of the eye) it is the distance over which an object may be moved without causing a sharpness reduction beyond a certain tolerable amount. Depth of field increases when the diaphragm (or pupil) diameter diminishes as, for example, in old eyes (Fig. D1). Examples: viewing at infinity, the depth of field ranges between infinity and about 3.6 m for a pupil of 4 mm in diameter; and between infinity and about 2.3 m for a 2 mm pupil. At a viewing distance of 1 m, the depth of field ranges from about 1.4 m to 80 cm with a 4 mm pupil; and from about 1.8 m to 70 cm with a 2 mm pupil.

See distance, hyperfocal.

depth of focus For a given setting of an optical system (or a steady state of accommodation of the eye) it is the distance in front and behind the focal point (or retina) over which the image may be focused without causing a sharpness reduction beyond a certain tolerable amount. (The criterion could be as much as a line of letters on a Snellen chart.) The depth of focus D is represented by the total distance in front and behind (Fig. D1). As with depth of field, it is inversely proportional to the diameter of the diaphragm (or pupil). It can be calculated, expressed in dioptres, using the equation

D =(2b/p)Fe

where F_e is the power of the emmetropic eye, p the pupillary diameter and b the maximum size of the retinal image beyond which it is perceived as blurred. Example: Assuming a pupil of 3 mm and a retinal image spread over five cones, each 0.002 mm in diameter and spaces between the cones of 0.0005 mm, that is a total size = 0.012 mm, D = 2 × 0.012/3 × 60 = 0.48 D. Thus an object appears clear if the vergence at the eye varies in the range ±0.24 D. If the eye is focused at infinity, the equation becomes D = (b/p) F_e.

depth perception See perception, depth.

depth, vertex See vertex depth.

dermatitis See eczema.

dermatochalasis A condition in which there is a redundancy of the skin of the upper eyelids. It is often associated with a protrusion of fat through a defective orbital septum. The condition occurs usually in old people. The excess skin may cause pseudoptosis. In severe cases it may obstruct vision. Treatment is surgical. Syn. ptosis adiposa; ptosis atrophica.

See blepharochalasis.

dermoid cyst See cyst, dermoid.

desaturated D–15 test See test, Farnsworth.

Descartes’ law See law of refraction.

Descemet’s membrane See membrane, Descemet’s.

descemetocele A forward bulging of Descemet’s membrane due to either trauma or a deep corneal ulcer which has eroded the overlying stroma. Syn. keratocele.

See keratolysis.

desiccation The process of becoming dry. See eye, dry.

desmosome A site of adhesion between two adjacent cells, such as in the corneal epithelium. It consists of a small, dense body in which the two halves are separated by an intercellular gap filled with extracellular substance. The basal cells are attached at irregular intervals to the underlying basement membrane (adjacent to Bowman’s layer) by hemidesmosomes (one half of a desmosome). Thus, scraping off the epithelium usually leaves fragments of the basal cells attached to the basement membrane.

detachment, choroidal See choroidal detachment.

detachment, retinal See retinal detachment.

deturgescence State of relative dehydration maintained by the normal cornea that is necessary for transparency. It is maintained by the epithelium, which to a large extent is impermeable to water, and also by a metabolic transport system in the endothelium.

deutan A person who has either deuteranomaly or deuteranopia.

deuteranomal Person who has deuteranomaly.

deuteranomaly A type of anomalous trichromatism in which an abnormally high proportion of green is needed when mixing red and green light to match a given yellow. This is the most common type of colour vision deficiency, occurring in about 4.6% of males and 0.35% of the female population. Syn. deuteranomalous trichromatism; deuteranomalous vision; green-weakness.

See anomaloscope; colour vision, defective.

deuteranope Person who has deuteranopia.

deuteranopia Type of dichromatism in which red and green are confused, although their relative spectral luminosities are practically the same as in normals. In the spectrum, the deuteranope only sees two primary colours, the long wavelength portion of the spectrum (yellow, orange or red) appears yellowish and the short wavelength portion (blue or violet) appears bluish. There is, in between, a neutral point which appears whitish or colourless, at about 498 nm. It occurs in slightly over 1% of the male population and only rarely in females. Syn. green blindness (although this term is incorrect as green lights appear to a deuteranope as bright as to a normal observer).

See colour vision, defective; dichromatism.

developmental and perceptual screening test See test, developmental and perceptual screening.

deviating eye See eye, deviating.

deviation 1. In strabismus, the departure of the visual axis of one eye from the point of fixation. 2. A change in direction of a light ray resulting from reflection or refraction at an optical surface.

angle of d. See angle of deviation.

conjugate d. The simultaneous and equal rotations of the eyes in any direction. It may be physiological such as versions, or pathological, due to either muscular spasm or paralysis.

See movements, disjunctive; version.

dissociated vertical d. (DVD) A form of strabismus in which one eye apparently moves vertically without any compensatory movement from the other eye. Although initially felt to disobey Hering’s law, it is now felt that Hering’s law is observed if the horizontal, vertical and rotational aspects of the condition are considered together. This form of strabismus often accompanies infantile esotropia and is almost always noted from the period of infancy. The misalignment can be either latent or manifest, and may require operative intervention if of a great degree.

See procedure, Faden; test, Bielschowsky’s phenomenon.

Hering–Hillebrand d. The deviation of the apparent frontoparallel plane horopter from the Vieth–Müller circle (horopter) (Fig. D2).

minimum d. of a prism See prism, minimum deviation of a.

primary d. The deviation found in paralysis of an extraocular muscle when the unaffected eye is fixating.

secondary d. The deviation found in paralysis of an extraocular muscle when the eye with the paralytic muscle is fixating.

skew d. A form of strabismus, typically vertical, that does not follow any standard or typical pattern and is usually difficult to quantify. It may be due to a midbrain disorder, multiple sclerosis or myasthenia gravis.

vertical d. 1. Type of ocular deviation found in strabismus in which the deviating eye is rotated upward with respect to the fixating eye. 2. Upward ocular deviation of an occluded eye in the cover test, as found in hyperphoria or hypophoria.

Devic’s disease See disease, Devic’s.

dexamethasone See antiinflammatory drug.

dextroclination Rotation of the upper pole of the vertical meridian of an eye to the subject’s right. Syn. dextrocycloduction; dextrotorsion.

dextrocycloversion Rotation of the upper poles of the vertical meridians of both eyes towards the subject’s right.

See laevocycloversion.

dextrodeorsumversion Movement of the eyes down and to the right.

dextroduction Rotation of one eye to the right. See duction.

dextrophoria A tendency of the visual axes of both eyes to deviate to the right, in the absence of a stimulus to fusion.

See heterophoria; laevophoria.

dextrotorsion See dextroclination.

dextroversion Movement of both eyes to the right.

See version.

diabetes A chronic disease that occurs when the pancreas does not produce enough insulin, or alternatively, when the body cannot effectively use the insulin it produces. Insulin is a hormone that regulates blood sugar. Hyperglycaemia, or raised blood sugar, is a common effect of uncontrolled diabetes and over time leads to serious damage to many of the body’s systems, especially the nerves and blood vessels (definition of the World Health Organization). The most common types of diabetes are: Type 1 diabetes, which is characterized by a lack of insulin production and therefore dependent upon insulin administration and Type 2 diabetes, which is characterized by an ineffective use of insulin. Type 1 diabetes is the most common type in young people whereas type 2 is the most common diabetes and affects primarily but not exclusively adults and it is largely the result of obesity and physical inactivity. The main complications in the eye are retinopathy, cataract, rubeosis iridis, ocular motor nerve palsies, xanthelasma and ptosis.

See accommodative insufficiency; anisocoria; glaucoma, neovascular; glaucoma, open-angle; hypoxia; myopia, lenticular; paralysis of the sixth nerve; paralysis of the third nerve; pupil, Adie’s; tritanopia; vitrectomy; vitreous detachment.

diabetic maculopathy; retinopathy See under the nouns.

diagnosis 1. Term that indicates the disease (e.g. pulmonary tuberculosis) or the refractive error (e.g. compound myopic astigmatism) that a person has. 2. The art of determining a disease or visual anomaly based on the signs, symptoms and tests.

diagnostic positions of gaze See positions of gaze, diagnostic.

dialysis, retinal See retinal dialysis.

diameter, total (TD) The linear measurement (usually specified in millimetres) of the maximum external dimension of a contact lens. It is equal to the back optic zone diameter (BOZD) plus twice the width of each of the back peripheral optic zones (if any) or twice the width of the edge in a spherical lens. Formerly, it was called overall size (OS).

See optic zone diameter; v gauge.

diaphragm 1. In optics, an aperture generally round and of variable diameter placed in a screen and used to limit the field of view of a lens or optical system (field stop). It also limits stray light (light stop). Syn. stop; aperture-stop. 2. In anatomy, a dividing membrane.

diascope A projector used to project transparent objects.

dibrompropamidine See antibiotic.

dichlorphenamide See carbonic anhydrase inhibitors.

dichoptic Viewing a separate and independent field by each eye, in binocular vision, as for example in a haploscope.

See masking, dichoptic.

dichroism Property exhibited by certain transparent substances of producing two different colours depending upon the thickness of substance traversed, the directions of transmission of light and/or viewing, the concentration of the substance, etc. The most common example is that of crystals (e.g. tourmaline) that absorb unequally the ordinary and extraordinary rays.

See anisotropic; crystal, dichroic; pleochroism.

dichromat Person having dichromatism, i.e. a deuteranope, a protanope or a tritanope.

dichromatism A form of colour vision deficiency in which all colours can be matched by a mixture of only two primary colours. The spectrum appears as consisting of two colours separated by an achromatic area (the neutral point). There are several types of dichromatism: deuteranopia, protanopia and tritanopia. Syn. daltonism; dichromatopsia; dichromatic vision.

See colour vision, defective; pigment, visual.

diclofenac See antiinflammatory drug.

dicoria A condition in which there are two pupils in one iris. It may be congenital or the result of surgery or injury. Syn. diplocoria.

See polycoria.

differential threshold See threshold, differential.

diffraction Deviation of the direction of propagation of a beam of light, which occurs when the light passes the edge of an obstacle such as a diaphragm, the pupil of the eye or a spectacle frame. There are two consequences of this phenomenon. First, the image of a point source cannot be a point image but a diffraction pattern. This pattern depends upon the shape and size of the diaphragm as well as the wavelength of light. Second, a system of close, parallel and equidistant grooves, slits or lines ruled on a polished surface can produce a light spectrum by diffraction. This is called a diffraction grating.

See disc, Airy’s; fringes, diffraction; theory, Maurice’s.

diffractive contact lens See lens, contact.

diffuser A device used to scatter light. It can be a reflecting surface (e.g. matt paint) or a transmitting medium (e.g. ground glass).

diffusion 1. Scattering of light passing through a heterogeneous medium, or being reflected irregularly by a surface, such as a sandblasted opal glass surface. Diffusion by a perfectly diffusing surface occurs in accordance with Lambert’s cosine law. In this case, the luminance will be the same, regardless of the viewing direction. 2. The passive movement of ions or molecules through a medium or across a semi-permeable membrane (e.g. the ciliary epithelium) in response to a concentration gradient until equilibrium is reached. It is one of the three mechanisms that create aqueous humour.

See light, diffuse; reflection, diffuse; ultrafiltration.

diffusion circle See blur circle.

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