Glossary

Aberrations - An imperfection in image formation by an optical system. The image is blurred because the light rays do not come together at a single point. Aberrations are divided into two main categories: higher order and lower order. Lower order make up 85% of all the aberrations in the eye, and include myopia, hyperopia and astigmatism. Higher order aberrations in the eye include coma, spherical, trefoil and secondary astigmatism, the higher the order of the aberration the more complex the shape. No one's eye is perfect, but the more aberrations the eye has, the worse the individual's eye sight is, especially at night. This commonly occurs in individuals with large pupils, and from such conditions as scarring of the cornea and cataracts.

Ablation - The surgical removal of tissue from the eye, commonly using a cool beam laser, which vaporises excess tissue without heating the area.

Ablation Zone - The area on the surface of the eye where a laser has created full refractive error correction through the removal of corneal tissue.

Accommodation - The eyes ability to change its focus from distance to close up; this occurs by contraction of the Ciliary muscle, releasing tension in the ligaments, allowing the lens to become rounder and to focus the eye on near objects. Natural loss of accommodation comes with age and is called presbyopia, causing the individual to need reading glasses to assist the eye in focusing at different distances.

Acuity - Sharpness, clarity in vision. This is normally measured by a letter chart at 20 feet or 6 metres, giving the expression of 'normal' vision being 20/20 or 6/6.

Amblyopia - Lazy Eye - It is characterised by poor vision in an otherwise healthy eye. It is caused by a period of poor transmission of images to the brain usually during early childhood. Amblyopia normally only affects one eye, but it is possible to be amblyopic in both eyes, if both are similarly deprived of a good, clear visual image. Commonly it can be caused by a squint, cataract or abnormalities of the optics of the eye.

AMD - Age-related Macular Degeneration  This refers to the deterioration of photoreceptors in the macula, the part of the retina which is responsible for central vision. It is the leading cause of deterioration in eyesight and, in some cases, blindness for the elderly population. There are two types of AMD: dry and wet. Dry AMD is the more common type, affecting 90% of all cases, where the cells in the macula waste away and become debris, known as drusen, around the area of the macula; this causes gradual central vision loss. Wet AMD affects about 10% of cases, it usually has the same signs as dry AMD, but also the individual has new, weak blood vessels growing behind the retina, which leak fluid and lead to blind spots in central vision. Common risk factors are smoking, hereditary, hypertension, ageing, obesity and lighter eye colour.

Amsler Grid - This is a grid composed of horizontal and vertical black lines on a white background, with a central dot for the eye to focus on. It is used to detect AMD (age-related macular degeneration), through defects and distortion in the central vision. If an individual is suffering from the early symptoms of AMD, the lines may be distorted, wavy, blurred, have dark areas within the grid or the centre may be missing.

Analyzer - A diagnostic machine used to help determine the suitability of an individual before refractive surgery. Detailed and precise scans are taken of an individuals eyes to obtain data to map out where the higher order aberrations, imperfections in the eye, are situated. This provides essential data used in wavefront treatment, which reduces or even eliminates night vision problems after laser surgery.

Anisometropia - The condition where the eyes have an unequal myopic (short-sighted) or hyperopic (long-sighted) prescription; where one eye is considerably weaker than the other. In extreme cases an individual is considered antimetropic, where they have one eye which is myopic and one eye which is hyperopic.

Anterior Chamber - Is the area between the cornea and iris. It incorporates the front surface of the lens and is filled with aqueous humour; the watery fluid inside the eye.

Anterior Chamber Angle - Is the junction between the back of the cornea and the front of the iris, where the aqueous humour, watery fluid inside the eye, filters out.

Anti VEGF Drugs - Anti Vascular Endothelial Growth Factor Drugs  Are used to treat wet AMD. They target vascular endothelial growth; a protein that helps the formation of new blood vessels. In wet AMD the new blood vessels are weak and leak fluid and blood under the retina, causing an irreversible loss of sight. The anti-VEGF drugs help to prevent the growth of new blood vessels; treatment requires regular injections in the eye.

Antioxidants - A molecule capable of preventing oxidation, destruction, of free radicals in the body. Free radicals are highly reactive unpaired electrons that start chain reactions which damage cells, leading to cellular damage and cancer. Antioxidants prevent or slow down these chain reactions and help to keep an individual healthy. They can be taken as supplements.

Aphakia - Is the absence of the eye's natural lens, due to surgical removal; such as after a cataract operation.

Aqueous Humour - Is the watery fluid inside the eye occupying the anterior chamber; from the back of the cornea to the front of the vitreous. Its purpose is to nourish the cornea, iris and lens; and the process of the production and dispersion of the fluid maintains pressure in the eye.

A-Scan - A-Scan Ultrasound Biometry - Is a routine diagnostic test which gathers data on the length of an individuals eye. This information is most commonly used to calculate what type of intra-ocular lens would best suit an eye after a cataract operation.

Astigmatism - Is an irregularity in the shape of the cornea. Instead of being round, like a football, it is oval shaped, like a rugby ball. Most astigmatic corneas have two curves, a flatter curve and a steeper curve. As a result of this the light is focused at more than one point in the eye which blurs images at all distances, all the time. Many short and long-sighted people often have a degree of astigmatism in their eyes.

Avastin - Bevacizumab  By its chemical name is an Anti-VEGF Drug used in the treatment of wet AMD by preventing the new growth of weak, leaking blood vessels which leads to a loss of sight. It is a cancer treatment drug that is currently licensed for use in lung and bowel cancer, but in recent research it has been discovered that a small dose has a powerful impact on wet AMD development.

Axis - In an optical system, the optical axis is an imaginary line that defines the path along which light propagates through the eye or spectacle lens. For a system composed of simple lenses, the axis passes through the centre of curvature of each surface and coincides with the axis of rotational symmetry.