Keratoconus Glossary A-B


20/20 vision - 20/20 (6/6) vision is considered normal. A person with 20/20 vision can see at 20 ft (6 m) what people with normal vision can see at this distance.

Aberrations - Imperfections, irregularities, and/or distortions of the optical quality of the eye that cause unclear vision

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.

Accommodation - The ability of the lens inside the eye to change focus from distance to near range; begins to decrease in our 40s requiring the use of reading glasses.

Acute corneal hydrops
 - In advanced cases, spontaneous ruptures of Descemet's membrane can occur, causing a crescent-shaped tear in Descemet's and the endothelium near the apex of the cone. The rupture allows aqueous to pass into the stroma, resulting in significant corneal oedema and opacification.

The patient will report a sudden loss of vision, discomfort and a visible white spot on the cornea. Although the break usually heals within six to 10 weeks and the corneal oedema clears, a variable amount of stromal scarring may develop. Corneas that do not recover transparency may require keratoplasty surgery. Occasionally, hydrops can benefit patients with extremely steep corneas as the scar can flatten the cornea, making it easier to fit with contact lenses


AK (astigmatic keratotomy) - Procedure in which incisions are made in the cornea to make it more spherical, thus reducing astigmatism.


 - 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.


Antioxidant - Substance that inhibits oxidation and can guard the body from the damaging effects of free radicals. Molecules with one or more unpaired electrons, free radicals can destroy cells and play a role in many diseases. Antioxidant vitamins include B, C, and beta carotene. It has been theorized that antioxidants can help prevent macular degeneration and other serious eye diseases; many studies are being conducted in this area.


Aqueous humor
 - The fluid in the anterior chamber.


Atopy - Type of allergy where levels of the antibody immunoglobin E are increased; atopy includes rhinitis, asthma, hay fever and eczema.
A-scan. Type of ultrasound; very high frequency sound waves that are reflected by the ocular structures and converted into electrical impulses. Used for measuring length of eyeball (axial length) prior to cataract surgery, to help determine power of IOL to be implanted; also to help differentiate normal and abnormal eye tissue.

- Not quite spherical. Aspheric eyeglass lenses are popular among people who have strong prescriptions because they are thin and lightweight, and reduce distortion and eye magnification. Aspheric contact lenses can work as a multifocal, or to correct a single-vision problem like astigmatism.

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 after removing additional tissue with a second pass of the microkeratome.


Axis - In ophthalmoloy, a line that is the symmetrical center of a curved optical surface. Measure of astigmatism.

Best corrected visual acuity (BCVA) - The best possible vision a person can achieve with corrective lenses (glasses or contact lenses) measured in terms of Snellen lines on an eye chart.


Bifocal Lens - with one segment for near vision and one segment for far vision. The term can apply to both eyeglass lenses and contact lenses.

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


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 millimeters) which can be manipulated to ablate the cornea.


B-scan - Type of ultrasound; provides a cross-section view of tissues that cannot be seen directly. High frequency sound waves are reflected by eye tissues and orbital structures and converted into electrical pulses, which are displayed on a printout.