Astigmatism (eye)

Astigmatism (eye)

DiseaseDisorder infobox
Name = Astigmatism
ICD10 = ICD10|H|52|2|h|49
ICD9 = ICD9|367.2

Astigmatism is an optical defect, whereby vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism, regular and irregular. Irregular astigmatism is often caused by a corneal scar or scattering in the crystalline lens and cannot be corrected by standard spectacle lenses. Irregular astigmatism of the cornea can be corrected by contact lenses. Regular astigmatism arising from either the cornea or crystalline lens can be corrected by a toric lens. A toric surface resembles the surface of a foot ball or a doughnut where there are two regular radii, one smaller than another. This optical shape gives rise to regular astigmatism in the eye. [ [http://www.mayoclinic.com/health/astigmatism/DS00230 Astigmatism - MayoClinic.com ] ] The first spectacle lenses that corrected astigmatism were made in Philadelphia in 1841 and shortly after in London.

The refractive error of the astigmatic eye stems from a difference in degree of curvature refraction of the two different meridians (i.e., the eye has different focal points in different planes.) For example, the image may be clearly focused on the retina in the horizontal (sagittal) plane, but not in front of the retina in the vertical (tangential) plane. Astigmatism causes difficulties in seeing fine detail, and in some cases vertical lines (e.g., walls) may appear to the patient to be tilted. The astigmatic optics of the human eye can often be corrected by spectacles, hard contact lenses or contact lenses that have a compensating optic.cylindrical lens (i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery.

Types

Based on axis of the principal meridians

*Regular astigmatism – principal meridians are perpendicular
**With-the-rule astigmatism – the vertical meridian is steepest (an American football lying on its side).cite web | title= Astigmatism at Buzzle.com | url= http://www.buzzle.com/articles/astigmatism.html | publisher=Buzzle.com | accessdate=21 June | accessyear=2008 |]
**Against-the-rule astigmatism – the horizontal meridian is steepest (an American football standing on its end).
**Oblique astigmatism – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.
*Irregular astigmatism – principal meridians are not perpendicularAlso known as Murdoch Syndrome (Ref: glastonbury Medics)

In With-the-rule astigmatism, the eye sees vertical lines sharper than horizontal lines. Against-the-rule astigmatism reverses the situation. In With-the-rule astigmatism a plus cylinder is placed in the vertical axis to correct the refractive error. Adding a plus cylinder in the vertical axis makes the horizontal axis "steeper" which makes both axes equally "steep." In Against-the-rule astigmatism a plus cylinder is added in the horizontal axis.

Children tend to have With-the-rule astigmatism and adults tend to have Against-the-rule astigmatism.

Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. 0 and 180 lie on a horizontal line at the level of the centre of the pupil, and as seen by an observer 0 lies on the right of both eyes.

Based on focus of the principal meridians

*Simple astigmatism
**Simple hyperopic astigmatism – first focal line coincides with the retina while the second is located behind the retina
**Simple myopic astigmatism – first focal line is located in front of the retina while the second focal line is located on the retina
*Compound astigmatism
**Compound hyperopic astigmatism – both focal lines are located behind the retina
**Compound myopic astigmatism – both focal lines are located in front of the retina
*Mixed astigmatism – focal lines are on both sides of the retina (straddling the retina)

Prevalence

According to an American study published in "Archives of Ophthalmology", nearly 3 in 10 children between the ages of 5 and 17 have astigmatism [http://archopht.ama-assn.org/cgi/content/abstract/121/8/1141] . A recent Brazilian study found that 34% of the students in one city were astigmatic [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16059562&query_hl=7] . Regarding the prevalence in adults, a recent study in Bangladesh found that nearly 1 in 3 (32.4%) of those over the age of 30 had astigmatism [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15177965&query_hl=20] .

A recent Polish study revealed that "with-the-rule astigmatism" may lead to the onset of myopia [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16052807&query_hl=6] .

A number of studies have found that the prevalence of astigmatism increases with age [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15838729&query_hl=26] .

Diagnosis

ymptoms

Although mild astigmatism may be asymptomatic, higher amounts of astigmatism may cause symptoms such as blurry vision, squinting, asthenopia, fatigue, or headaches. [ [http://www.eyetopics.com/articles/45/1/Astigmatism Astigmatism ] ] [ [http://www.medicinenet.com/astigmatism/article.htm Astigmatism symptoms and treatment on MedicineNet.com ] ] [ [http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20symptoms.htm HIPUSA ] ]

igns and tests

There are a number of tests used by ophthalmologists and optometrists during eye examinations to determine the presence of astigmatism and to quantify the amount and axis of the astigmatism. [ [http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20treatment.htm HIPUSA ] ] A Snellen chart or other eye charts may initially reveal reduced visual acuity. A keratometer may be used to measure the curvature of the steepest and flattest meridians in the cornea's front surface. [ [http://www.stlukeseye.com/eyeq/Keratometry.asp Keratometry ] ] A corneal topographer may also be used to obtain a more accurate representation of the cornea's shape. [ [http://www.emedicine.com/OPH/topic711.htm eMedicine - Corneal Topography and Imaging : Article by Peter Fedor, MD ] ] An autorefractor or retinoscopy may provide an objective estimate of the eye's refractive error and the use of Jackson cross cylinders in a phoropter may be used to subjectively refine those measurements [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=13900989&dopt=Abstract] [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3808608&dopt=Abstract] [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8320415&dopt=Abstract] . An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the astigmatic axis and power. [ [http://www.quantumoptical.com/onlinecourses/nysso/brp/slide1.asp?courses=19 Basic Refraction Procedures ] ] [ [http://www.nova.edu/hpd/otm/nbeo/refract1.htm Introduction to Refraction ] ]

Another refraction technique that is rarely used involves the use of a stenopaic slit (a thin slit aperture) where the refraction is determined in specific meridians - this technique is particularly useful in cases where the patient has a high degree of astigmatism or in refracting patients with irregular astigmatism.

Treatment

Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another. In those with keratoconus, rigid gas permeable contact lenses often enable patients to achieve better visual acuities than eyeglasses. If the astigmatism is caused by a problem such as deformation of the eyeball due to a chalazion, treating the underlying cause will resolve the astigmatism. Should the person suffer from severe astigmatism, they will be advised to wear hard rigid gas permeable lenses rather than the more comfortable soft lenses.

References

See also

Related conditions

*Hyperopia
*Keratoconus
*Myopia
*Presbyopia

Other

*Eyeglass prescription
*Refractive surgery
*Lens (optics)
*Ophthalmology
*Optician
*Optometry

External links

*


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Eye surgery — Intervention Eye surgery in the Middle Ages. ICD 10 PCS 08 ICD 9 CM …   Wikipedia

  • Astigmatism — A*stig ma*tism, n. [Gr. a priv. + ?, ?, a prick of a pointed instrument, a spot, fr. ? to prick: cf. F. astigmatisme.] (Med. & Opt.) A defect of the eye or of a lens, in consequence of which the rays derived from one point are not brought to a… …   The Collaborative International Dictionary of English

  • astigmatism — ► NOUN ▪ a defect of the eye or a lens, resulting in distorted images. DERIVATIVES astigmatic adjective. ORIGIN from Greek stigma point …   English terms dictionary

  • astigmatism — [ə stig′mə tiz΄əm] n. [< Gr a , without + stigma (gen. stigmatos) a mark, puncture (see STICK) + ISM] 1. an irregularity in the curvature of a lens, including the lens of the eye, so that light rays from an object do not meet in a single focal …   English World dictionary

  • Astigmatism — An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp… …   Wikipedia

  • Astigmatism — A common form of visual impairment in which part of an image is blurred, due to an irregularity in the curvature of the front surface of the eye, the cornea. The curve of the cornea is shaped more like an American football or a rugby ball rather… …   Medical dictionary

  • astigmatism — /euh stig meuh tiz euhm/, n. 1. Also called astigmia /euh stig mee euh/. Ophthalm. a refractive error of the eye in which parallel rays of light from an external source do not converge on a single focal point on the retina. 2. Optics. an… …   Universalium

  • eye disease — Introduction  any of the diseases or disorders that affect the human eye (eye, human).       This article briefly describes the more common diseases of the eye and its associated structures, the methods used in examination and diagnosis, and the… …   Universalium

  • Eye disease — This is a partial list of human eye diseases and disorders. The World Health Organization publishes a classification of known diseases and injuries called the International Statistical Classification of Diseases and Related Health Problems or ICD …   Wikipedia

  • eye — eyeable, adj. eyelike, adj. eyer, n. /uy/, n., pl. eyes, (Archaic) eyen or eyne; v., eyed, eying or eyeing. n. 1. the organ of sight, in vertebrates typically one of a pair of spherical bodies contained in an orbit of the skull and in humans… …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”