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Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or
blurred vision Blurred vision is an ocular symptom where vision becomes less precise and there is added difficulty to resolve fine details. Temporary blurred vision may involve dry eyes, eye infections, alcohol poisoning, hypoglycemia, or low blood pressure. ...
at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at night. Astigmatism often occurs at birth and can change or develop later in life. If it occurs in early life and is left untreated, it may result in
amblyopia Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to fully process input from one eye and over time favors the other eye. It results in decreased vision in an eye that typically appears normal in other aspects. Amb ...
. The cause of astigmatism is unclear; however, it is believed to be partly related to genetic factors. The underlying mechanism involves an irregular curvature of the
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical powe ...
and protective reaction changes in the lens of the eye, called lens astigmatism, that has the same mechanism as spasm of accomodation. Diagnosis is by an
eye examination An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, op ...
called autorefractor keratometry (objective, allows to see lens and cornea components of astigmatism) and subjective refraction, but subjective methods are almost always inaccurate, if lens astigmatism is not fully removed first with a week of eye drops. Three treatment options are available: glasses,
contact lens Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...
es, and surgery. Glasses are the simplest. Contact lenses can provide a wider field of vision and less artifacts than even double aspheric lenses. Refractive surgery permanently changes the shape of the eye and thereby cures astigmatism. In Europe and Asia, astigmatism affects between 30 and 60% of adults. People of all ages can be affected by astigmatism. Astigmatism was first reported by Thomas Young in 1801.


Signs and symptoms

Although astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as
blurred vision Blurred vision is an ocular symptom where vision becomes less precise and there is added difficulty to resolve fine details. Temporary blurred vision may involve dry eyes, eye infections, alcohol poisoning, hypoglycemia, or low blood pressure. ...
, double vision,
squinting Squinting is the action of looking at something with partially closed eyes. Squinting is most often practiced by people who suffer from refractive errors of the eye who either do not have or are not using their glasses. Squinting helps momentar ...
, eye strain, fatigue, or headaches. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches.


Causes


Congenital

The cause of congenital astigmatism is unclear, however it is believed to be partly related to genetic factors. Genetics, based on twin studies, appear to play only a small role in astigmatism as of 2007. Genome-wide association studies (GWAS) have been used to investigate the genetic foundation of astigmatism. Although no conclusive result has been shown, various candidates have been identified. In a study conducted in 2011 on various Asian populations, variants in the PDGFRA gene on chromosome 4q12 were identified to be associated with corneal astigmatism. A follow-up study in 2013 on the European population, however, found no variant significantly associated with corneal astigmatism at the genome-wide level (single-nucleotide polymorphism rs7677751 at PDGFRA). Facing the inconsistency, a study by Shah and colleagues in 2018 included both populations with Asian and Northern European ancestry. They successfully replicated the previously identified genome-wide significant locus for corneal astigmatism near the PDGFRA gene, with a further success of identifying three novel candidate genes:
CLDN7 Claudin-7 is a protein that in humans is encoded by the ''CLDN7'' gene. It belongs to the group of claudin Claudins are a family of proteins which, along with occludin, are the most important components of the tight junctions ( zonulae occlu ...
,
ACP2 Lysosomal acid phosphatase is an enzyme that in humans is encoded by the ''ACP2'' gene. Lysosomal acid phosphatase is composed of two subunits, alpha and beta, and is chemically and genetically distinct from red cell acid phosphatase. Lysosomal ...
, and TNFAIP8L3. Other GWAS studies also provided inconclusive results: Lopes and colleagues identified a susceptibility locus with lead single nucleotide polymorphism rs3771395 on chromosome 2p13.3 in the VAX2 gene (VAX2 plays an important role in the development of the dorsoventral axis of the eye); Li and associates, however, found no consistent or strong genetic signals for refractive astigmatism while suggesting a possibility of widespread genetic co-susceptibility for spherical and astigmatic refractive errors. They also found that the TOX gene region previously identified for spherical equivalent refractive error was the second most strongly associated region. Another recent follow-up study again had identified four novel loci for corneal astigmatism, with two also being novel loci for astigmatism: ZC3H11B (associated with axial length), NPLOC4 (associated with myopia), LINC00340 (associated with spherical equivalent refractive error) and HERC2 (associated with eye color).


Acquired

Astigmatism may also occur following a
cataract surgery Cataract surgery, also called lens replacement surgery, is the removal of the natural lens of the eye (also called "crystalline lens") that has developed an opacification, which is referred to as a cataract, and its replacement with an intraocul ...
or a corneal injury. Contraction of the scar due to wound or cataract extraction causes astigmatism due to flattening of the cornea in one direction. In keratoconus, progressive thinning and steepening of the cornea cause irregular astigmatism.


Pathophysiology


Axis of the principal meridian

*Regular astigmatism – principal meridians are perpendicular. (The steepest and flattest meridians of the eye are called principal meridians.) **With-the-rule astigmatism – the vertical meridian is steepest (a rugby ball or American football lying on its side). **Against-the-rule astigmatism – the horizontal meridian is steepest (a rugby ball or 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 perpendicular. In with-the-rule astigmatism, the eye has too much "plus" cylinder in the horizontal axis relative to the vertical axis (i.e., the eye is too "steep" along the vertical meridian relative to the horizontal meridian). Vertical beams of light focus in front (
anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
) to horizontal beams of light, in the eye. This problem may be corrected using spectacles which have a "minus" cylinder placed on this horizontal axis. The effect of this will be that when a vertical beam of light in the distance travels towards the eye, the "minus" cylinder (which is placed with its axis lying horizontally – meaning in line with the patient's horizontal meridian relative to the excessively steep vertical meridian) will cause this vertical beam of light to slightly "diverge", or "spread out vertically", before it reaches the eye. This compensates for the fact that the patient's eye converges light more powerfully in the vertical meridian than the horizontal meridian. Hopefully, after this, the eye will focus all light on the same location at the retina, and the patient's vision will be less blurred. In against-the-rule astigmatism, a plus cylinder is added in the horizontal axis (or a minus cylinder in the vertical axis). Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. Both 0 and 180 degrees lie on a horizontal line at the level of the center of the pupil, and as seen by an observer, 0 lies on the right of both the eyes. Irregular astigmatism, which is often associated with prior ocular surgery or trauma, is also a common naturally occurring condition. The two steep hemimeridians of the cornea, 180° apart in regular astigmatism, may be separated by less than 180° in irregular astigmatism (called ''nonorthogonal'' irregular astigmatism); and/or the two steep hemimeridians may be asymmetrically steep—that is, one may be significantly steeper than the other (called ''asymmetric'' irregular astigmatism). Irregular astigmatism is quantified by a vector calculation called topographic disparity.


Focus of the principal meridian

With accommodation relaxed: *Simple astigmatism **Simple hyperopic astigmatism – first focal line is on the retina, while the second is located behind the retina. **Simple myopic astigmatism – first focal line is in front of the retina, while the second is 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).


Throughout the eye

Astigmatism, whether it is regular or irregular, is caused by some combination of external (corneal surface) and internal (posterior corneal surface, human lens, fluids, retina, and eye-brain interface) optical properties. In some people, the external optics may have the greater influence, and in other people, the internal optics may predominate. Importantly, the axes and magnitudes of external and internal astigmatism do not necessarily coincide, but it is the combination of the two that by definition determines the overall optics of the eye. The overall optics of the eye are typically expressed by a person's refraction; the contribution of the external (anterior corneal) astigmatism is measured through the use of techniques such as
keratometry A keratometer, also known as an ophthalmometer, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It was invented by the German physiologi ...
and
corneal topography Corneal topography, also known as photokeratoscopy or videokeratography, is a non-invasive medical imaging technique for mapping the anterior curvature of the cornea, the outer structure of the eye. Since the cornea is normally responsible for som ...
. One
method Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
analyzes vectors for planning refractive surgery such that the surgery is apportioned optimally between both the refractive and topographic components.


Diagnosis

A number of tests are used during
eye examination An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, op ...
s to determine the presence of astigmatism and to quantify its amount and axis. A
Snellen chart A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen, who developed the chart in 1862. Many ophthalmologists and vision scientists now use an improved ...
or other eye charts may initially reveal reduced visual acuity. A
keratometer A keratometer, also known as an ophthalmometer, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It was invented by the German physiologi ...
may be used to measure the curvature of the steepest and flattest meridians in the cornea's front surface.
Corneal topography Corneal topography, also known as photokeratoscopy or videokeratography, is a non-invasive medical imaging technique for mapping the anterior curvature of the cornea, the outer structure of the eye. Since the cornea is normally responsible for som ...
may also be used to obtain a more accurate representation of the cornea's shape. An autorefractor or
retinoscopy Retinoscopy (Ret) is a technique to obtain an objective measurement of the refractive error of a patient's eyes. The examiner uses a retinoscope to shine light into the patient's eye and observes the reflection (reflex) off the patient's retina. W ...
may provide an objective estimate of the eye's refractive error and the use of
Jackson cross cylinder The Jackson cross cylinder (JCC) is an instrument used by ophthalmologists, orthoptists and optometrists in their routine eye examination, particularly in determination of corrective lens power in patients with astigmatism. It is also used for tes ...
s in a
phoropter A phoropter or refractor is an ophthalmic testing device. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction of the eye during sight testing, to measure an individual's re ...
or
trial frame A trial frame is a tool used by ophthalmic professionals like ophthalmologists and optometrists. It is basically an adjustable spectacle frame with multiple cells, used to hold corrective lenses, and other accessories in subjective refraction (fin ...
may be used to subjectively refine those measurements. An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the astigmatic axis and power. A keratometer may also be used to estimate astigmatism by finding the difference in power between the two primary meridians of the cornea.
Javal's rule Javal's rule is a mathematical formula used to estimate astigmatism based on keratometry readings.Schwartz, S. Geometric and Visual Optics. pg 219-221. McGraw-Hill. 2002. The estimate is useful for high cylinder measures, generally over -2.00 diopt ...
can then be used to compute the estimate of astigmatism. A method of astigmatism analysis by Alpins may be used to determine both how much surgical change of the cornea is needed and after surgery to determine how close treatment was to the goal. Another rarely used refraction technique involves the use of a stenopaeic 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.


Classification

There are three primary types of astigmatism: myopic astigmatism, hyperopic astigmatism, and mixed astigmatism. Cases can be classified further, such as regular or irregular and lenticular or corneal.


Treatment

Astigmatism may be corrected with eyeglasses,
contact lenses Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...
, or refractive surgery. Glasses are the simplest and safest, although contact lenses can provide a wider field of vision. Refractive surgery can eliminate the need to wear corrective lenses altogether by permanently changing the shape of the eye but, like all elective surgery, comes with both greater risk and expense than the
non-invasive A medical procedure is defined as ''non-invasive'' when no break in the skin is created and there is no contact with the mucosa, or skin break, or internal body cavity beyond a natural or artificial body orifice. For example, deep palpation and pe ...
options. Various considerations involving eye health, refractive status, and lifestyle determine whether one option may be better than another. In those with keratoconus, certain contact lenses often enable patients to achieve better visual acuity than eyeglasses. Once only available in a rigid, gas-permeable form, toric lenses are now also available as soft lenses. In older people, astigmatism can also be corrected during cataract surgery. This can either be done by inserting a toric intraocular lens or by performing special incisions (limbal relaxing incisions). Toric intraocular lenses probably provide a better outcome with respect to astigmatism in these cases than limbal relaxing incisions. Toric intraocular lenses can additionally be used in patients with complex ophthalmic history, such as previous ophthalmic surgery. In such complex cases, toric intraocular lenses seems to be as effective as in non-complex cases for correction of concurrent corneal astigmatism.


Epidemiology

According to an American study, nearly three in ten children (28.4%) between the ages of five and seventeen have astigmatism. A Brazilian study published in 2005 found that 34% of the students in one city were astigmatic. Regarding the prevalence in adults, a recent study in
Bangladesh Bangladesh (}, ), officially the People's Republic of Bangladesh, is a country in South Asia. It is the eighth-most populous country in the world, with a population exceeding 165 million people in an area of . Bangladesh is among the mos ...
found that nearly 1 in 3 (32.4%) of those over the age of 30 had astigmatism. A Polish study published in 2005 revealed "with-the-rule astigmatism" may lead to the onset of myopia. A number of studies have found the prevalence of astigmatism increases with age.


History

As a student, Thomas Young discovered that he had problems with one eye in 1793. In the following years he did research on his vision problems. He presented his findings in a
Bakerian Lecture The Bakerian Medal is one of the premier medals of the Royal Society that recognizes exceptional and outstanding science. It comes with a medal award and a prize lecture. The medalist is required to give a lecture on any topic related to physical ...
in 1801. Independent from Young, George Biddell Airy discovered the phenomenon of astigmatism on his own eye. Airy presented his observations on his own eye in February 1825 at the Cambridge Philosophical Society. Airy produced lenses to correct his vision problems by 1825, while other sources put this into 1827 when Airy obtained cylindrical lenses from an optician from Ipswich. The name for the condition was given by William Whewell. By the 1860s astigmatism was a well established concept in ophthalmology, and chapters in books described the discovery of astigmatism. In 1849, Irish English physicist and mathematician George Stokes invented
Stokes lens Stokes lens also known as variable power cross cylinder lens is a lens used to diagnose a type of refractive error known as astigmatism. Lens design The Stokes lens also known as variable power cross cylinder lens, in its standard version, is a l ...
to detect astigmatism. In 1887 American ophthalmologist Edward Jackson revised the Stokes lens concept and made a cross cylinder lens to refine power and axis of astigmatism. In 1907 Jackson described determination of the axis of a correcting cylinder in astigmatism using a cross cylinder.


See also

* Near-sightedness * Far-sightedness


References


External links

* * * {{DEFAULTSORT:Astigmatism (Eye) Disorders of ocular muscles, binocular movement, accommodation and refraction Wikipedia neurology articles ready to translate Vision Wikipedia medicine articles ready to translate