T

Eye Health Glossary

A &sdot B &sdot C &sdot D &sdot E &sdot F &sdot G &sdot H &sdot I &sdot J &sdot K &sdot L &sdot M &sdot N &sdot O &sdot P Q &sdot R &sdot S &sdot T &sdot U &sdot V &sdot W &sdot X &sdot Y &sdot Z &sdot #

tapetum lucidum

Highly reflective thin membrane at the back of the eye of many nocturnal animals (including cats & raccoons). By reflecting light back to the retina, more light is absorbed and vision is therefore better in the dark. The unfortunate consequence to this is poorer vision during the day time due to excess light. Many animals accommodate for this by having pupils that can constrict to tiny slits.

tay sachs disease

Severe inherited disease of the central nervous system that is fatal by around age 5. Newborns appear healthy for an average of 4-6 months, after which development slows and symptoms of Tay-Sachs begin to appear. Progressive nerve death continues as the child becomes blind, paralyzed, and loses all awareness of his/her surroundings.

tear duct

Canal that begins at the inside corned of each eyelid at the punctum. The tear duct is the route through which tears drain away from the eyes, into the lacrimal sac.

tear film

A layer of moisture-rich tears that spreads out across the surface of the eye. The tear film is made up of three layers, each of which is necessary for a stable tear film to keep the eyes moist and healthy. An unstable tear film results in dry eye syndrome.

tonic pupil

Adie's tonic pupil; condition in which the pupils are of different sizes (anisocoria). Typically Adie's tonic pupil occurs in young patients with the wider pupil (more dilated) being the problematic pupil. The tonic pupil also tends to react sluggishly to light (constriction). Generally there is no treatment for Adie's tonic pupil, but if the cosmetics of the different-sized pupils bothers the patient, opaque contact lenses can be considered.

tonometry

Measurement of the pressure inside the eye (intraocular pressure). There are a few different ways to examine IOP, most common of which being NCT (the puff test). Goldmann tonometry (see Figure) is accomplished by using an attachment on the slit-lamp biomicroscope and a Perkins tonometer is a handheld instrument for measuring IOP.

toric

Lens design that combines two different powers in perpendicular meridians for the correction of astigmatism.

trabecular meshwork

Network of fibers contained within the anterior angle that is responsible for draining the aqueous humour from the eye.

trachoma

Extremely contagious and persistent infection of the conjunctiva caused by a bacteria. If left untreated, extensive scarring can occur, which leads to entropion and subsequent scarring of the cornea, which clouds over the cornea and leads to blindness. While treatment can be as simple as antibiotics, trachoma is raging in developing countries where people are not receiving treatment.

Transitions

Plastic photochromic lenses with a patented design.

trichiasis

Inward turning of the eyelashes, which can lead to persistent irritation of the cornea.

trichotillomania

Compulsive pulling out of one's hair (including eyelashes and eyebrows).

trichromat

A person who is able to make use of all three types of cone photoreceptors and can therefore see more than two hues (as is the case with dichromats).

trifocal

A lens that is designed to have three separate focal lengths and therefore three different powers. Trifocals are most often designed with the main part of the lens being used for distance vision and the two small areas of the lens being used for intermediate (looking at objects that are about arm's length away) and near vision.

tritan

Missing or altered S (blue) cone, which is the cone photoreceptor that responds maximally to short wavelengths in the visible spectrum (shades of blue and violet light).

tritanopia

A color vision defect in which the S cone (blue cone) is either absent or non-functional. As a result, only shades of red and greenish-blue can be seen and shades of blue are difficult to distinguish.

tropicamide

Anesthetic, cycloplegic and mydriatic drug that is commonly used to pharmaceutically dilate the pupil.

tunnel vision

Loss of peripheral vision such that the only useful sight remaining is straight ahead (central vision). Glaucoma and retinitis pigmentosa are both eye diseases that can eventually lead to tunnel vision.

<< T >>

Back to Eye Health Glossary Home