A to Z of the eye


Have a look through our dictionary of eye terms if you are unsure about the meaning of anything or are just keen to learn more:

Contents

A B C D E F G H I J K L M

N O P Q R S T U V W X Y Z

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.

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 dioptre.

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

Aqueous humour
The fluid in the anterior chamber.

Astigmatism
A refractive error caused by an irregular shape of the cornea (much like a football). Astigmatism is measured in terms of dioptres, cylinder meridian or axis. Uncorrected astigmatism may produce ghosting or double images.

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

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B

BCVA
See ‘best corrected visual acuity’.

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.

Bladeless
Refers to the creation of a corneal incision without the use of a metal microkeratome blade. The Ziemer Femtosecond LDV Laser is a bladeless technology that uses a laser to create the corneal flap in Step One of LASIK.

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) that can be manipulated to ablate the cornea.

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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.

Choroid
The middle layer of the eye between the retina and the sclera. It also contains a pigment that absorbs excess light so preventing blurring of vision.

Ciliary body
The part of the eye that connects the choroid to the iris.

Cone cells
Cone cells are the second type of light sensitive cells in the retina of the eye. The human retina contains between six and seven million cones; they function best in bright light and are essential for acute vision (receiving a sharp accurate image). It is thought that there are three types of cones, each sensitive to the wavelength of a different primary colour – red, yellow or blue. Other colours are seen as combinations of these primary colours.

Cornea
The outer part of the eye that provides 70% of the eye’s refractive power. The cornea is approximately 500 microns thick (0.5 millimetres) and consists of five layers – epithelium, Bowman’s membrane, stroma, Descemet’s membrane and endothelium.

Corneal Flap
A circular, hinged portion of the outer layer of the cornea, lifted in Step One of LASIK surgery. The corneal flap allows the surgeon to access the inner portion of the cornea in Step Two. Although LASIK complications are rare, most of those that occur result from improperly formed flaps.

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

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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 (0.005 millimetres), this membrane provides an adhesion layer for the endothelium.

Deturgescence
The balance of hydration in the eye.

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

Dry eye
A syndrome characterised by corneal dryness due to deficient tear production.

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E

Ectasia
A progressive corneal thinning and bulging.

Emmetropes
People who have no refractive error.

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 0.005-0.01 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 defense against infection.

Excimer laser
A “cold” laser used in refractive surgery to remove corneal tissue.

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F

Farsightedness
See “Hyperopia”

Femto-LASIK
Another term for Z-LASIK. The use of the Ziemer LDV femtosecond laser (precise thickness corneal flap creation) with the AMO VISX excimer laser (precise reshaping of the cornea to reduce your focusing error) – also known as blade-free LASIK.

Femtosecond laser
A silent, bladeless, computer-guided laser used in Step One of LASIK to safely create corneal flaps of exact size, shape, and depth. Ziemer engineering have produced the Leonardo Di Vinci (LDV) femtosecond laser for the current application of making the flap in laser vision correction. Faster speed and lower energy produce results that are significantly better than the Intralase system and other systems that use “amplification” for speed. The “oscillation” method unique to the LDV allows for speeds in the mega hertz range and complete smooth dissection at appropriate and very low energy. This translates into better vision for the patient.

Fovea
The fovea forms a small indentation at the centre of the macula and is the area with the greatest concentration of cone cells. When the eye is directed at an object, the part of the image that is focused on the fovea is the image most accurately registered by the brain.

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G

Glare
A possible complication of refractive surgery in which the patient sees additional lustre around lights. Glare is a subjective experience that often decreases with time. Other causes of glare include cataract, corneal scars or other corneal problems.

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

Globe
The eyeball.

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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.

Hyperopia
The ophthalmic term for farsightedness. In the hyperopic eye, images are focused behind the retina. The hyperopic eye is often described as being too flat or too short.

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I

Iris
The iris regulates the amount of light that enters your eye. It forms the coloured, visible part of your eye in front of the lens. Light enters through a central opening called the pupil.

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

IOL Master 500
The latest version of this instrument. It is used to measure the power of the cornea, the length of the eye and the distance between the cornea and the lens of the eye. By accurately measuring all these, the implant needed in cataract or refractive lens surgery can be calculated. The implant corrects the focus of the eye, allowing good distance or near vision without glasses in most situations.

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.

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J

No entries

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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, (or Femtosecond laser) to expose the layer of the cornea called the stroma.

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

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.

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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 vaporization when focused at close range. Lasers are often used in surgery to remove tissue.

LASEK
The acronym for laser assisted epithelial keratomileusis. The name refers to the use of a laser to reshape the cornea. In a LASEK operation, a very thin layer of cells from the surface of the cornea (the ‘epithelium’) is removed. This process is also called ‘surface laser treatment’ or ‘epi-LASEK’. . For more information, see LASEK 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.

Lens
A transparent structure situated behind your pupil. It is enclosed in a thin transparent capsule and helps to refract incoming light and focus it onto the retina. A cataract is when the lens becomes cloudy, and a cataract operation involves the replacement of the cloudy lens with an artificial plastic lens.

Leucomas
Scars that are dense and white.

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M

Macula
A yellow spot on the retina at the back of the eye which surrounds the fovea.

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 µ.

Microkeratome
The instrument a surgeon uses to create the corneal flap in the uppermost layer of the cornea during the LASIK procedure, (now mostly superseded by the Femtosecond laser).

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.

Myopia
The medical term for nearsightedness. Eye is too steep, too long, image is focused in front of the retina.

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N

Nearsightedness
See “Myopia”

Nebulas
Scars that are translucent.

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

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O

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

Optic disk
The visible (when the eye is examined) portion of the optic nerve, also found on the retina. The optic disc identifies the start of the optic nerve where messages from cone and rod cells leave the eye via nerve fibres to the optic centre of the brain. This area is also known as the ‘blind spot’.

Optic nerve
The 1.2 million 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.

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P

Pachymetry
The process of measuring corneal thickness, usually using the Oculus Pentacam or an ultrasonic probe. The Pentacam measures corneal thickness in thousands of points in your cornea.

Pentacam
A precision diagnostic tool that gives detailed information about the front part of the eye, including thickness of the cornea, shape of the front and back of the cornea, depth of the anterior chamber of the eye (between cornea and lens) and density of any cataract in the eye.

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. The 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.

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 eyelid.

Pupil
The circular opening circular opening in the centre of the iris through which light passes into the lens of the eye. The iris controls widening and narrowing (dilation and constriction) of the pupil.

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Q

No entries.

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R

Radial keratotomy (RK)
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. This was used before the excimer laser became available, and is no longer used now.

Refraction
The bending of light waves as they pass from one medium to another (such as from air to water).

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 rugby ball shape) in which the curvature is symmetrical across one or more meridians or axes. The steep curve and the flat curve are at 90 degrees from each other, as in the shape of a rugby ball.

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

Rod Cells
Rod cells are one of two types of light-sensitive cells in the retina of the eye. There are about 125 million rods which are necessary for seeing in dim light.

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S

Sclera
The white part of the eye, a tough covering with which the cornea forms the external protective coat of the eye.

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 0.5 millimetres). Between Bowman’s membrane and Descemet’s membrane.

Symmetry of refractive error
The refractive error in both eyes are close to the same value.

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T

Tear film
A very thin film of water and other chemicals riding on top of the corneal epithelium (surface) that lubricates the front of the eye.

Topical proparacain hydrochloride
Anaesthetic eye drops.

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U

UCVA
See ‘uncorrected visual acuity’.

Uncorrected visual acuity (UCVA)
A person’s vision without corrective lenses measured in terms of Snellen lines on an eye chart. The acronym is UCVA.

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

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V

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

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W

Wavescan (Wavefront aberrometer)
The machine used with the AMO VISX laser to discover the precise way your eye focuses light, and the various errors that cause a blurred image when you see.

Wavefront-guided LASIK
The use of the wavescan information from your eye to guide the AMO VISX excimer laser in reshaping your cornea, so minimizing any blurring of vision.

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X

No entries.

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Y

No entries.

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Z

Z-LASIK
The use of the Ziemer LDV femtosecond laser (precise thickness corneal flap creation) with the AMO VISX excimer laser (precise reshaping of the cornea to reduce your focusing error); also known as blade-free LASIK. Z-LASIK is also known as LASIK.

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