Glossary of Terms

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Ablate – In surgery, to remove.

Ablation – The vaporization of tissue with the Excimer laser.

Ablation zone – The area of tissue that is removed during laser surgery.

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

Aqueous humor – 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 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 removing additional tissue with a second pass of the microkeratome. This is the predecessor of LASIK.

Axis – In ophthalmology, a line that is the symmetrical center of a curved optical surface. It gives the location of astigmatism.


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.

Bowman’s membrane – The non-regenerative layer of tissue between the epithelium and the stroma (5-10 microns thick – .005 to .01 millimeters).


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

Cornea – The front, transparent, outer-part of the eye that provides 75% of the eye’s refractive power. The cornea is approximately 500 microns thick (.5 millimeter) and consists of 5 layers epithelium, Bowman’s membrane, stroma, Descemet’s membrane and endothelium.


Descemet’s membrane – The layer of the cornea between the stroma and endothelium. Five microns thick (.005 millimeters), this membrane provides an adhesion layer for the endothelium. This is the 2nd to the last layer between the endothelium and stroma.

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 condition characterized by corneal dryness due to deficient tear production.


Endothelium – The innermost layer of the cornea. The endothelium is one cell layer thick (5-10 microns or .005-.01 millimeters) 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 humor, and actively transports glucose.

Enhancement – A secondary refractive procedure performed 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.


Farsightedness – Also known as “Hyperopia”. People with farsightedness see things better at distance than at near. In the hyperopic eye, images are focused behind the retina. The hyperopic eye is often described as being too flat or too short.

Femtosecond Lase – An extremely precise laser that delivers pulses in ultra short durations of time, 10 to the -13 (femtosecond).Compare the fact that light travels around the world 7.5 times in one second and across a human hair in 100 femtoseconds.


Ghosting – A secondary image often with faint overlapping edges.

Glare – A condition where the patient sees additional luster around lights.


Halos – A condition where the patient sees additional rings around lights at night. Halos are subjective experiences that often decrease with time.

Haze – A uncommon complication of PRK 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.

Hyperopia – The ophthalmic term for farsightedness. People with hyperopia see things better at distance than near. In the hyperopic eye, images are focused behind the retina. The hyperopic eye is often described as being too flat or too short.


Intraocular pressure – The pressure inside the eye.

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. This condition cannot be corrected with glasses.


There are no terms in the Glossary for the Letter J


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

Keratomileusis – The reshaping of the cornea, formerly done with a lathe and blade, by first creating a corneal flap. LASIK is also a form of keratomileusis.

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).


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.

LASIK – The acronym for laser assisted in situ keratomileusis. The name refers the use of a laser to reshape the cornea without invading the adjacent cell layers.


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

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

Micron – One thousandth of a millimeter.

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

Myopia – The medical term for nearsightedness. The eye is too steep, or too long, and the image is focused in front of the retina.


Nearsightedness – Clinically known as “Myopia”. The eye is too steep, too long, image is focused in front of the retina.

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


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 fibers 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.


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

Photorefractive keratotomy (PRK) – A procedure involving the removal of the surface layer of the cornea (epithelium) followed by use of an Excimer laser to reshape the stroma. Acronym is PRK.

Plano – Indicated there is no refractive error.

Presbyopia – The natural deterioration of near vision caused by loss of flexibility in the eye’s lens as one ages.


There are no terms in the Glossary for the Letter Q


Radial keratotomy (RK) – A surgical procedure designed to correct myopia (nearsightedness) by flattening the cornea with incisions. The procedure is called 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. This is measured in an eye exam to determine the optimal lenses for vision.

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. Such procedures include RK, PRK, and LASIK.

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.


Slit lamp – Table-top microscope for examining the eye.

Snellen chart – An eye chart used to test a patient’s vision.

Stroma – Thickest part of the cornea (450-600 microns – approximately .5 millimeters). Between Bowman’s membrane and Decemet’s membrane.

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


Tear film – A very thin film of oil, salt water, and mucin riding on top of the epithelium that lubricates the front of the eye.


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.


Vitreous humor – The gel-like fluid in the main cavity of the eye behind lens and pupil that accounts for 80% or the eye’s volume.


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