compound astigmatism


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Related to compound astigmatism: compound hyperopic astigmatism

astigmatism

 [ah-stig´mah-tizm]
an error of refraction in which a ray of light is not sharply focused on the retina, but is spread over a more or less diffuse area; it is due to differences in curvature in the refractive surfaces (cornea and lens) of the eye. adj., adj astigmat´ic. Its exact cause is not known; some common types of astigmatism seem to run in families and may be inherited. Probably everyone has some astigmatism, since it is rare to find perfectly shaped curves in the cornea and lens, but the defect is rarely serious. If the refractive error is troublesome, corrective lenses may be needed.
compound astigmatism that in which both principal meridians are either hyperopic (compound hyperopic astigmatism, with rays coming into focus behind the retina) or myopic (compound myopic astigmatism, with rays coming into focus in front of the retina).
corneal astigmatism that due to the presence of abnormal curvatures on the anterior or posterior surface of the cornea.
hypermetropic astigmatism hyperopic astigmatism.
hyperopic astigmatism that in which the light rays are brought to a focus behind the retina.
irregular astigmatism that in which the curvature varies in different parts of the same meridian or in which refraction in successive meridians differs irregularly.
lenticular astigmatism astigmatism due to defect of the crystalline lens.
mixed astigmatism that in which one principal meridian is hyperopic and the other myopic.
myopic astigmatism that in which the light rays are brought to a focus in front of the retina.
regular astigmatism that in which the refraction changes gradually in power from one principal meridian of the eye to the other, the two meridians always being at right angles; this condition is further classified as being against the rule when the meridian of greatest refractive power tends toward the horizontal, with the rule when it tends toward the vertical, and oblique when it lies 45 degrees from the horizontal and vertical.

compound astigmatism

Astigmatism in which both horizontal and vertical curvatures are involved.
See also: astigmatism

eye

The peripheral organ of vision, in which an optical image of the external world is produced and transformed into nerve impulses. It is a spheroidal body approximately 24 mm in diameter with the segment of a smaller sphere (of about 8 mm radius), the cornea, in front. It consists of an external coat of fibrous tissue, the sclera and transparent cornea; a middle vascular coat, comprising the iris, the ciliary body and the choroid; and an internal coat, the retina, which includes the cones and rods photoreceptors. Within the eye, there are the aqueous humour located between the cornea and the crystalline lens, the crystalline lens held by the zonule of Zinn and the vitreous body located between the crystalline lens and the retina. The movements of the eye are directed by six extraocular muscles (Fig. E9). Syn. organ of sight; visual organ.
amaurotic eye See amaurosis.
amblyopic eye An eye which has amblyopia. Syn. lazy eye (colloquial).
aphakic eye An eye without the crystalline lens.
artificial eye A prosthesis made of glass or plastic which resembles the eye and which is placed in the socket after enucleation. See ocularist; ocular prosthesis.
axial length of the eye See axial length of the eye.
eye bank An organization that collects, evaluates, stores and distributes eyes from donors. The eyes are used for corneal transplants and research. See keratoplasty.
black eye A colloquial term for a swollen or blue-black spot on the skin of the eyelid caused by effusion of blood as a result of a superficial injury in which the skin is not broken. The correct term is ecchymosis of the eyelid. The condition recovers by itself within 2-3 weeks while changing in colour to yellow. Immediately after the injury, application of ice helps minimize the haemorrhage and swelling. See haematoma.
bleary eye A red and watery eye, with a lacklustre appearance. Lack of sleep is a common cause. Syn. blear eye.e. blink See blink.
compound eye The eye of arthropods composed of a variable number of ommatidia. See corneal facet; ommatidium.
crossed eye's See convergent strabismus.
cyclopean eye Imaginary eye located at a point midway between the two eyes. When the two visual fields overlap and the impressions from the two eyes are combined into a single impression, the apparent direction of a fixated object appears in a direction that emanates from the cyclopean eye.
dark-adapted eye An eye that has been in darkness and is sensitive to low illumination. Syn. scotopic eye.
deviating eye The non-fixating eye in strabismus or under heterophoria testing. Syn. squinting eye.
dominant eye The eye that is dominant when ocular dominance exists. See manoptoscope; hole in the card test.
dry eye This term encompasses various tear film disorders ranging from a mild form causing discomfort, which is usually relieved with artificial tears, to the most common form keratoconjunctivitis sicca. See keratoconjunctivitis sicca.
equatorial plane of the eye See equatorial plane.
exciting eye See sympathetic ophthalmia.
fixating eye The eye that is directed towards the object of regard in strabismus. See deviating eye.
glass eye An artificial eye made of glass. See ocularist; ocular prosthesis.
eye impression See eye impression.
lazy eye See amblyopic eye.
eye lens See eyepiece.
light-adapted eye An eye that has been exposed to light and is insensitive to low illumination. Syn. photopic eye. See light adaptation; duplicity theory.
eye movements See eye movements.
eye patch A piece of material or plastic that is worn over the eye when it has been injured or over the socket when it is missing.
phakic eye An eye that contains the crystalline lens. See phakic.
photopic eye See light-adapted eye.
pink eye See contagious conjunctivitis.
eye position Position of the eye in the orbit, maintained by the extraocular muscles. See primary position; secondary position; tertiary position.
pseudophakic eye An eye fitted with an intraocular lens implant. See intraocular lens.
red eye A colloquial term often used for any condition in which the blood vessels of the conjunctiva or ciliary body are congested. Many conditions result in a red eye (e.g. subconjunctival haemorrhage, pterygium, conjunctivitis, episcleritis, corneal abrasion, corneal erosion, ulcerative keratitis, corneal dendritic ulcer, acute iritis, angle-closure glaucoma, orbital cellulitis, and possibly contact lens wear). See ciliary injection; con-junctival injection.
reduced eye A mathematical model of the optical system of the eye. It consists of a single refracting surface with one nodal point, one principal point and one index of refraction. In the first such model, proposed by Listing in 1853, the refracting surface had a power of 68.3 D and was situated 2.34 mm behind the schematic eye's cornea. It had an index of refraction of 1.35, a radius of curvature of 5.124 mm and a length of 20 mm. Donders' reduced eye was even more simplified. It has a power of 66.7 D, a radius of curvature of 5 mm, an index of refraction of 4/3 and anterior and posterior focal lengths of −15 and +20 mm, respectively, with a refracting surface situated 2 mm behind the schematic eye's cornea. Gullstrand's reduced eye has a radius of curvature of 5.7 mm, an index of refraction of 1.33, a power of 61 D with the refracting surface situated 1.35 mm behind the schematic eye's cornea. Emsley's reduced eye has a power of 60 D, an index of refraction of 4/3 and is situated 1.66 mm behind the schematic eye's cornea, with anterior and posterior focal lengths of −16.67 and +22.22 mm, respectively.
schematic eye A model consisting of various spherical surfaces representing the optical system of a normal eye based on the average dimensions (called the constants of the eye) of the human eye. There are many schematic eyes, although the most commonly used is that of Gullstrand. A great deal of variation among authors stemmed from the difficulty in giving an index of refraction that would represent the heterogeneous character of the crystalline lens. Gullstrand in fact proposed two schematic eyes, one which he called the exact schematic eye and the other which he called the simplified schematic eye in which the divergent effect of the posterior corneal surface is ignored and the cornea replaced by an equivalent surface; the crystalline lens is homogeneous and the optical system is free from aberrations.
scotopic eye See dark-adapted eye.
eye shield 1. See occluder.
2. A protective device to cover the eye against injury, glare or in radiotherapy of the face.
sighting-dominant eye The eye that is preferred in monocular tasks, such as looking through a telescope or aiming a firearm.
eye socket The bony orbit which contains the eyeball, the muscles, the nerves, the vessels, the orbital fat and the orbital portion of the lacrimal gland.
eye speculum An instrument designed to hold the eyelids apart during surgery. Syn. blepharostat.
squinting eye See deviating eye.
eye stone A small, smooth shell or other object that can be inserted beneath the eyelid to facilitate the removal of a foreign body from the eye.
sympathetic eye The uninjured eye in sympathetic ophthalmia that becomes secondarily affected. Syn. sympathizing eye.
wall eye A colloquial term referring to (1) a white opaque cornea or (2) a divergent strabismus.
watery eye See epiphora.
Fig. E9 Cross-section of the eyeenlarge picture
Fig. E9 Cross-section of the eye