Eye Terms Dictionary

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Ablate. In surgery, to remove.
Ablation. Removal of tissue with the excimer laser.
Ablation zone. Area of tissue removed during laser surgery.
Accommodation. Ability of the eye to change its focus from objects in the distance to near objects. Achieved by changing the shape of the lens.
Acuity. Measure of the quality of vision.
Allergy. Exaggerated response to a substance that is usually well tolerated.
Ametropia. Presence of a refractive error; myopia, hyperopia or astigmatism. Light from a distant object does not focus on the retina.
Aniseikonia. Difference in image size between the two eyes. Caused by a difference in the refractive state of each eye.
Anisometropia. Difference in refractive power of the two eyes. Usually reserved for when the difference is at least one diopter.
Anterior chamber. Space between the cornea and the lens. Filled with aqueous humour.
Anterior chamber lens. Intraocular lens placed in the anterior chamber of the eye. May be fixed to the iris or fitted into the iridocorneal angle.
AquaLase®. Method of cataract extraction in which pulses of water are used to break the lens of the eye into fine particles that can then be aspirated through a tiny wound.
Aqueous humor. Clear fluid that fills the anterior and posterior chambers of the eye.
Astigmatism. Error of focus in which light entering the eye in different planes is brought to a focus at different points. Usually arises due to the cornea being ‘rugby ball shaped’ rather than ‘soccer ball shaped’.


BCVA. See best corrected visual acuity.
Best corrected visual acuity (BCVA). Best possible vision a person can achieve with corrective lenses. Measured in terms of Snellen lines on an eye chart.
Blepharitis. Inflammation of the eyelids. Divided into anterior blepharitis and posterior blepharitis, the latter also known as meibomian gland disease.
Bowman’s membrane. Non-regenerative layer of tissue between the epithelium and the stroma of the cornea.


Cataract. Clouding of the natural lens of the eye.
Cataract extraction or surgery. Removal of cataract. Usually performed by phacoemulsification or AquaLase®.
Chalasia. Meibomian gland cysts.
Concave lens. Lens that can diverge light. Used to correct myopia. Also known as a minifying, minus or -ve lens.
Conjunctiva. Thin clear mucous membrane that covers the sclera of the eye and the inner surface of the eyelids.
Contact lenses. Lenses that sit directly on the surface of the eye.
Convex lens. Lens that can converge light. Used to correct hypermetropia. Also known as a magnifying, plus or +ve lens.
Cornea. Transparent part of the outer eye, or clear window of the front of the eye. The cornea provides the majority of the eye’s refractive power, is approximately 0.5 millimetres thick and consists of 5 layers (epithelium, Bowman’s membrane, stroma, Descemet’s membrane and endothelium).
Corneal graft. Replacement or transplantation of the cornea. Grafting can be lamellar (replacement of isolated layers) or penetrating (replacement of the full thickness of the cornea). Also known as keratoplasty.
Crystalline lens. Transparent ‘magnifying glass’ shaped structure suspended behind the pupil. Transmits and focuses light. Ability to change focus known as accommodation.
Cylindrical lens. Lens having different power in different planes. Also known as a toric lens. Used to correct astigmatism.


Dendritic. Literally ‘branching’. Commonly-used term to describe pattern of epithelial ulceration in herpes simplex corneal disease.
Descemet’s membrane. The layer of the cornea between the stroma and endothelium. Five microns thick (.005 millimetres), this membrane provides an adhesive layer for the corneal endothelium.

Diopter. A measurement of refractive error. Hyperopia is measured in terms of positive diopters, eg +3.00, myopia is measured in terms of negative diopters, eg -2.50.

Dry eye. Syndrome characterized by corneal dryness and discomfort due to either deficient tear production or increased tear evaporation.


Emmetropia. Ophthalmic term for a perfect refractive state – no myopia, hypermetropia or astigmatism.
Endothelium. Innermost layer of the cornea. The endothelium is one cell layer thick. Its main function is the maintainance of the cornea’s transparency.
Enhancement. Secondary refractive procedure performed to attempt to achieve better unaided visual acuity.
Epithelial ingrowth. Complication of LASIK in which epithelial cells grow underneath the corneal flap.
Epithelium. Outermost layer of cells of the cornea. Five cell layers thick (20 microns), the epithelium is the eye’s first defense against infection and the most important optical surface in the eye.
Excimer laser. Ultraviolet laser used in refractive surgery to remove corneal tissue.


Farsightedness. Common term for hyperopia.
Fuchs’ corneal dystrophy. Slow failure of the corneal endothelium leading to swelling and loss of transparency. Can be considered a premature aging. Characterised by corneal guttata.


Glare. Scatter of light that decreases vision.
Ghosting. Visual distortion in which a patient sees additional blurred images around the object they are viewing.
Globe. The eyeball.
Guttata. Mushroom shaped bodies in the corneal endothelium. Most commonly seen in Fuchs’ corneal dystrophy.


Halos. ‘Rings’ seen around lights due to imperfections in the eyes optical system.
Haze. Loss of corneal transparency following laser refractive surgery. Relatively rare complication of LASIK. More common after PRK.
Herpes simplex. One of the herpes virus family. The most common cause of recurrent viral infection of the cornea.
Herpes keratitis. Reactivation of herpes simplex virus in cornea. May result in epithelial ulceration (dendritic ulcer) or stromal inflammation (disciform keratitis).
Herpes zoster. Disease caused by varicella zoster virus, one of the herpes virus family. Primary infection results in chicken pox; reactivation causes shingles.
Herpes zoster ophthalmicus. Shingles affecting the fifth cranial nerve, the ophthalmic nerve. May affect almost any part of the eye, most commonly the conjuctiva, cornea and iris.
Hyperopia. Ophthalmic term for farsightedness. The hyperopic eye may be considered as having too flat a cornea or too short an axial length so that light entering the eye comes to a focus behind the retina.


Intraocular lens. Lens placed inside the eye after cataract surgery. May be a simple sphere, toric or multifocal lens.
Intraocular pressure. Pressure inside the eye.
Iridectomy. Surgical removal of iris tissue.
Iridotomy. Openings in the iris, usually created with the YAG laser.
Iris. Coloured ring of tissue behind the cornea. The pupil is the central apperture in the iris.
Irregular astigmatism. Refractive error caused by an irregular shape of the cornea.


Keratectomy. Removal of corneal tissue, usually with a laser.
Keratitis. Inflammation of the cornea.
Keratoconjunctivitis sicca. Inflammation of the cornea and conjunctiva due to dry eye.
Keratoconous. Condition in which the cornea has a cone-shaped profile.
Keratopathy. Usually bullous keratopathy. Swelling and blistering of the cornea due to endothelial cell dysfunction.
Keratoplasty. Replacement or transplantation of the cornea. May be be lamellar (replacement of isolated layers) or penetrating (replacement of the full thickness of the cornea). Also known as corneal grafting.
Keratotomy. Surgical incision or cut in the cornea.


Lacrimal gland. Gland that secretes the majority of the watery components of the tear film. Located in the upper outer aspect of the eye socket.
Lamellar keratoplasty. Replacement of individual layers of the cornea with donor cornea.
Laser. Acronym for Light Amplification by Stimulated Emission of Radiation. Medical instrument that produces an intense beam of light.
LASIK. Acronym for Laser Assisted In-situ Keratomileusis. The use of an excimer laser to reshape the cornea to reduce refractive error.
Lens. Structure that has the ability to converge (plus or +ve lens) or diverge (minus or -ve lens) light. Also the ‘crystalline’ lens of the eye, a transparent biconvex structure suspended behind the pupil.
Lensectomy. Removal of the crystalline lens of the eye to correct refractive error.
Limbus. Junction of the cornea and sclera.


Meibomian glands. Sebaceous glands in the edge of the eyelid that produce the oily layer of the tears.
Meibomianitis. Inflammation of the meibomian glands.
Melanoma. Malignant tumour. Usually pigmented.
Melanosis. Abnormal deposits of melanin or pigment. May be a pre-cancerous condition.
Micron. One thousandth of a millimetre.
Microkeratome. Instrument a surgeon uses to create the corneal flap during the LASIK procedure.
Monocular. Pertaining to or affecting one eye.
Monovision. Intentional treatment of one eye for near vision and the other for distance vision. Can be accomplished with either corrective lenses or surgery.
Myopia. Medical term for nearsightedness. The myopic eye may be considered as having too steep a cornea or too long an axial length so that light from a distant object is brought to a focus in front of the retina.


Nearsightedness. Common term for myopia. The nearsighted eye may be considered as having too steep a cornea or too long an axial length so that light from a distant object is brought to a focus in front of the retina.
Near vision. Ability to see objects at a close distance. Commonly tested at 40cms.


Ophthalmologist. Medical doctor specialising in the medical and surgical management of eye disease and visual disorders.
Ophthalmoscope. Instrument capable of examining the internal structures of the eye.
Optic nerve. Cord of nerve fibres that pass from the eye to the brain.
Optician. Expert in the manufacture and dispensing of spectacles and contact lenses.
Optometrist. Professional trained to provide primary care of eye disorders; in particular the treatment of refractive error with spectacles and contact lenses.
Overcorrection. Change of refractive error exceeds the planned correction.


Pachymetry. Process of measuring corneal thickness, usually using an ultrasonic probe.
Pannus. Infiltration of the cornea by blood vessels.
Peripheral vision. ‘Round about vision’; as opposed to central vision.
Phacoemulsification. Method of cataract extraction in which ultrasound is used to break the lens of the eye into fine particles that can then be aspirated through a tiny wound.
Phakic. Eye that still possesses its natural crystalline lens.
Phakic intraocular lens (Phakic IOL). Lens placed in an eye which still possesses its natural crystalline lens, to reduce refractive errors.
Photorefractive keratotomy (PRK). Reshaping of the the surface layers of the cornea with the excimer laser.
Photophobia. Abnormal sensitivity to light.
Plano. No refractive error.
Posterior capsule. Posterior part of the transparent capsule that surrounds the crystalline lens.
Posterior capsular opacification (PCO). Loss of transparency of the posterior capsule following cataract surgery.
Posterior capsulotomy. Opening of the posterior capsule, usually with the YAG laser.
Posterior chamber. Space between the back of the iris and front of the crystalline lens. Filled with aqueous.
Posterior chamber lens. Intraocular lenses placed in the space where the natural lens of the eye is, or was previously, located.
Presbyopia. Literally ‘old sight’. The natural deterioration of near vision with increasing age. Caused by loss of flexibility of the crystalline lens.
PRK. Acronym for Photo Refractive Keratotomy. A procedure that uses excimer laser to reshape the surface layers of the cornea.
Pseudophakia. Presence of an artifical lens (intraocular lens, IOL or ‘implant’) in the eye following cataract surgery.
Pterygium. Triangular growth of tissue that extends from the conjunctiva onto the cornea.
Puncta. Openings on the upper and lower eyelid that lead into the canaliculi of the tear drainage system.
Pupil. Opening in the center of the iris that changes size to allow more or less light to enter the eye.


Red reflex. The orange glow observed in the pupil when a light is shone directly into the eye. Often seen as ‘red eye’ in photographs taken with simple cameras.
Refraction. Bending of light waves as they pass from a transparent material to another of different density. Also the process of testing an eye to determine the presence and magnitude of refractive errors such as myopia, hypermetropia and astigmatism.
Refractive errors. Imperfections in the optical state of the eye (myopia, hyperopia and astigmatism) that prevent light entering the eye coming to a single focus point on the retina. Also known as ametropia.
Refractive surgery. Surgical procedure that attempts to decrease the patient’s refractive error.
Regression. Backwards shift toward a patients preoperative refractive error.
Regular astigmatism. 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.
Relaxing incisions. Corneal incisions to reduce astigmatism.
Retina. Innermost layer of the eye. Receives the optical images and begins the process of creation of ‘vision’.


Schirmers’ test. Filter paper test of tear flow.
Sclera. Tough white outer coat of the eye.
‘Six six vision’. Measure of central vision. Ability to identify a letter on the eyechart that an average person can seen at a distance of six metres.
Slit lamp biomicroscope. Microscope for examining the eye.
Snellen acuity. Measure of central vision. Compares patient’s vision to that of normal population.
Snellen chart. Eye chart used to test a patient’s central vision.
Specular microscopy. A technique to obtain high magnification photographic images of the endothelial cells of the cornea.
Stroma. Central, thickest layer of the cornea (approximately 0.5 millimetres).
Stye. Infection of an eyelash follicle.


Toric lens. Lens having different power in different planes. Also known as a cylindrical lens. Used to correct astigmatism.
Topical anaesthesia. Anaesthesia of the eye achieved with eye drops. Also known as ‘needleless surgery’.
Topography. A technique to obtain a surface ‘map’ of the cornea; similar to topography of a geographical landscape.
‘Twenty twenty vision’. Measure of central vision. Ability to identify a letter on the eyechart that an average person can seen at a distance of 20 feet.


UCVA. See uncorrected visual acuity.
Uncorrected visual acuity (UCVA). Snellen vision without corrective lenses.
Undercorrection. Desired change in refractive error is not fully achieved.
Ulcer. Loss of surface tissue or epithelium. May eventually progress into deeper tissue layers.
Uvea. Middle, vascular layer of the eye. Consists of iris, ciliary body and choroid.


Visual acuity. Measure of the ability of the eye to resolve fine detail.
Visual axis. Imaginary line connecting the object of regard and the central retina.
Vitreous humor. Gel-like fluid which fills the main cavity of the eye behind lens and pupil.


YAG capsulotomy. Creation of an opening in the posterior capsule with the YAG laser. The treatment of posterior capsule opacification.
YAG laser. Strictly Nd:YAG; acronym for Neodymium-doped Yttrium Aluminium Garnet. Infrared laser used to perform capsulotomy or iridotomy.


Zonules. Fine guy-wires that suspend the crystalline lens.