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Dictionary of Eye Terms

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A

Ablate

In surgery, to remove.

Ablation

The vaporisation of tissue with the excimer laser.

Ablation zone

The area of tissue that is removed during laser surgery.

ALK

See Automated lamellar keratoplasty.

Aniseikonia

A difference in imaging size between the two eyes.

Anisometropia

A difference in refractive power of the two eyes in which the variance is at least one diopter.

Anterior chamber

The fluid-filled area between the cornea and the lens.

Aqueous humor

The fluid in the anterior chamber.

Eye Anatomy Diagram - Eye Diagram - AstigmatismAstigmatism

A refractive error caused by an irregular shape of the cornea, rather like the surface of a rugby ball.This gives a blurred image. Astigmatism can occur by itself, but more usually it is associated with short or long-sightedness. Astigmatism is measured in terms of diopters, cylinder meridian or axis. Uncorrected astigmatism may produce ghosting or double images.

Automated lamellar keratoplasty (ALK)

A procedure in which the surgeon first creates a flap in the uppermost layer of the cornea using a device called a microkeratome. Then the surgeon makes an optical cut after removing additional tissue with a second pass of the microkeratome.

Axis

In ophthalmology, a line that is the symmetrical centre of a curved optical surface. Measure of astigmatism.

B

BCVA

See best corrected visual acuity below.

Best corrected visual acuity (BCVA)

The best possible vision a person can achieve with corrective lenses measured in terms of Snellen lines on an eye chart.

Botox®

See our oculoplastics page for use.

Bowman's membrane

The non-regenerative layer of tissue between the epithelium and the stroma (5-10 microns thick - .005 to .01 millimetres).

Blepharoplasty

Eyelid (upper and lower) surgery for sagging eyelids or excessive skin. FOr more information see our oculoplastics page.

Broad beam laser

A medical instrument that produces a powerful beam of light that is focused at close range to remove corneal tissue. A broad beam laser uses a relatively large beam diameter (from 6.0 to 8.0 millimetres) which can be manipulated to ablate the cornea.

C

Centre islands

A manageable complication of LASIK related to ablation. The incidence of centre islands has been greatly reduced as more ophthalmic research has been devoted to its contributing factors.

Cornea

The outer part of the eye that provides 70% of the eye's refractive power. The cornea is approximately 500 microns thick (.5 millimetre) and consists of 5 layers epithelium, Bowman's membrane, stroma, Descemet's membrane and endothelium.

Cylinder meridian

In ophthalmology, a line that is the symmetrical centre of a curved optical surface. Measure of astigmatism.

D

Decentration

A complication caused by movement of the pupil that can be corrected with an enhancement procedure.

Descemet's membrane

The layer of the cornea between the stroma and endothelium. Five microns thick (.005 millimetres), this membrane provides an adhesion layer for the endothelium.

Deturgescence

The balance of hydration in the eye.

Diopters

A measurement of refractive error. Hyperopia is measured in terms of positive diopters. Myopia is measured in terms of negative diopters. The most common refractive errors ranged between +6 to -6 diopters.

Dry eye

A Syndrome characterised by corneal dryness due to deficient tear production.

E

Ectasia

A progressive corneal thinning and bulging.

Ectropion / Entropion

Eyelids can turn outward (ectropion) or inward (entropion). See our oculoplastics page for more information.

Emmetropes

People who have no refractive error.

Eye Anatomy Diagram - Eye Diagram - Emmetropia (Ideal Sight)Emmetropia

The ophthalmic term for a perfect refractive state - no nearsightedness, farsightedness or astigmatism.

Endothelium

The innermost layer of the cornea. The endothelium is one cell layer thick (5-10 microns or .005-.01 millimetres) and provides hydration balance to maintain the cornea's transparency. The endothelium serves three main purposes it regulates the stroma's water content, provides a barrier to ingress of several constituents of the aqueous humour, and actively transports glucose.

Enhancement

A secondary refractive procedure performed after the initial one in an attempt to achieve better visual acuity.

Epithelial ingrowth

A complication of LASIK wherein epithelial cells grow underneath the corneal flap.

Epithelium

The outermost layer of cells of the cornea. Six cells thick (20 microns), the epithelium is the eye's first defence against infection.

Excimer laser

A "cold" laser used in refractive surgery to remove corneal tissue.

F

Farsightedness

See "Hyperopia"

G

Glare

A complication of refractive surgery in which the patient sees additional luster around lights. Glare is a subjective experience that often decreases with time.

Ghosting

A distortion of image due to irregular healing of the corneal surface.

Globe

The eyeball.

H

Halos

A complication of refractive surgery in which the patient sees additional rings around lights at night. Halos are subjective experiences that often decrease with time.

Haze

A relatively rare complication of LASIK caused by the deposition of ground substance in the cornea. An ophthalmologist can measure the haze response of a patient's eye under a slit lamp, but patients' experiences of haze vary. Outcomes for the patient include decreased night vision, halos or loss of best corrected visual acuity. Caused by the eye's reaction to the laser, haze often decreases with time and is more common with PRK.

Hyperopes

People who are farsighted.

Eye Anatomy Diagram - Eye Diagram - Hyperopia (Long-sightednessHyperopia

The ophthalmic term for longsightedness (or farsightedness). In the hyperopic eye, images are focused in back of the retina. The hyperopic eye is often described as being too flat or too short.

I

Intraocular pressure

The pressure the fluid contained within the eye exerts on the globe.

Irregular astigmatism

A refractive error caused by an irregular shape of the cornea in which the curve on one side of the meridian or axis is not symmetrical with the curve on the other side.

J

 

K

Keratectomy

The portion of the LASIK procedure in which the surgeon raises a thin layer of the cornea - creates a corneal flap - with an instrument called a microkeratome, to expose the layer of the cornea called the stroma.

Keratoconous

A rare inherited condition of the cornea in which the cornea is steepened to the point of being cone-shaped.

Keratomileusis

The carving of the cornea formerly done with a lathe and blade, now done with an excimer laser.

Keratoplasty

The replacement (transplantation) of the cornea. Keratoplasty can be lamellar (replacement of superficial layers) or penetrating (replacement of the full thickness of the cornea).

Keratotomy

A surgical incision (cut) of the cornea.

L

Lamellar keratoplasty

The replacement of superficial layers of the cornea with the layers of another donor cornea.

Laser

An acronym for light amplification by stimulated emission of radiation. A medical instrument that produces a powerful beam of light and can produce intense heat or cool vaporisation when focused at close range. Lasers are often used in surgery to remove tissue. For more information, see Laser Eye Surgery.

LASIK

The acronym for laser assisted in situ keratomileusis. The name refers to the use of a laser to reshape the cornea without invading the adjacent cell layers. For more information, see Lasik Eye Surgery.

LenSx

The LenSx FDA-approved femtosecond laser brings an incomparable level of accuracy and reproducibility to cataract surgery.

Leucomas

Scars that are dense and white.

Long sightedness

See Hyperopia.

M

Meibomian secretions

Oily secretions from the eyelid glands that supply the outer portion of tear film, prevent rapid tear evaporation and tear overflow.

Micron

One thousandth of a millimetre. The symbol for a micron is µm.

Microkeratome

The instrument a surgeon uses to create the corneal flap in the uppermost layer of the cornea during the LASIK procedure.

Monovision

The purposeful adjustment of one eye for near vision and the other for distance vision accomplished with either corrective lenses or surgery.

Myopes

People who are nearsighted.

Eye Anatomy Diagram - Eye Diagram - Myopia (Short-sightednes)Myopia

The medical term for shortsightedness (or nearsightedness). Eye is too steep, too long, image is focused in front of the retina.

N

Nearsightedness

See "Myopia" above.

Nebulas

Scars that are translucent.

Nomogram

A surgeon's adjustment to the laser's computer calculation to further refine his or her own results.

O

Off label use

The permissible use of an approved drug or instrument in a way that has not been specifically sanctioned.

Optic nerve

The millions of optical nerve fibres connecting to the eye and terminating in the brain where images are created and processed.

Overcorrection

The result achieved when the change to refractive error exceeds the attempted correction.

P

Pachymetry

The process of measuring corneal thickness, usually using an ultrasonic probe.

Photorefractive keratotomy

A procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma. Acronym is PRK.

Plano

Characterised by no refractive error. Practice of medicine. A regulatory body's allowance of practitioners to make decisions to best serve their patients.

Presbyopia

The natural deterioration of near vision caused by loss of flexibility in the eye's lens as one ages. See our page on long sightedness.

PRK

The acronym for photorefractive keratotomy. A procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma.

Ptosis

Droopy upper eyelid. For more information see our oculoplastic surgery page.

Q

 

R

Radial keratotomy

A surgical procedure designed to correct myopia (nearsightedness) by flattening the cornea with incisions. The procedure is called a radial keratotomy because the incisions resemble the spokes in a wheel. Acronym is RK.

Refraction

The bending of light waves as they pass from one medium to another

Refractive surgery

Any surgical procedure that attempts to decrease the patient's refractive error. Typically the surgeon alters the shape of the cornea in order to change the angle at which an image is projected onto the retina.

Regression

A backwards shift from the initial visual outcome.

Regular astigmatism

A refractive error caused by an irregular shape of the cornea (usually a football shape) in which the curvature is symmetrical across one or more meridians or axes.

Retina

Light processing membrane; converts light into electrical impulses that are transmitted to the optic nerve.

RK

Acronym for radial keratotomy. A surgical procedure designed to correct myopia (nearsightedness) by flattening the cornea with incisions. The procedure is called a radial keratotomy because the incisions resemble the spokes in a wheel.

S

Short sightedness

See Myopia.

Slit lamp

Table-top microscope for examining the eye.

Snellen chart

An eye chart used to test a patient's vision.

Snellen line

A line of same-sized letters on an eye chart that is used to test a patient's vision.

Spot scanning laser

A medical instrument that produces a powerful beam of light that is focused at close range to remove corneal tissue. Spot scanning lasers use radar technology to track the eye's movement.

Stroma

Thickest part of the cornea (450-600 microns - approximately .5 millimetres). Between Bowman's membrane and Decemet's membrane.

Symmetry of refractive error

The refractive errors in both eyes are close to the same value.

T

Tear film

A very thin film of water and other chemicals riding on top of the epithelium that lubricates the front of the eye.

Topical proparacain hydrochloride

Anaesthetic eye drops.

U

UCVA

See "uncorrected visual acuity" below.

Uncorrected visual acuity (UCVA)

A person's vision without corrective lenses measured in terms of Snellen lines on an eye chart. Acronym is UCVA.

Undercorrection

The result achieved when desired change in refractive error is not fully achieved.

V

Vitrectomy

In a vitrectomy, the vitreous (a clear, gel-like fluid inside your eye) is removed to enable your surgeon to carry out treatments that cannot be performed with the fluid in place. At many clinics, vitreoretinal surgery is still an in-patient procedure that requires you to stay overnight. But when you come to Eye Institute you’ll enjoy the very highest standards of care, along with the latest vitreoretinal surgery techniques, meaning you’ll be able to go home almost straight after your surgery.

Vitreous humor

The gel-like fluid in the main cavity of the eye behind lens and pupil.

W

Watering eyes

See our page on watery eyes for causes and solutions.

X

Y

Z

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