Astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors present can be corrected with simple glasses. In an eye with astigmatism, either the lens or the cornea is slightly egg-shaped, with higher curvature in one direction than the other. This gives distorted or blurred vision at any distance and requires corrective lenses that apply different optical powers at different rotational angles. Astigmatism can lead to symptoms that include eyestrain, headaches, and trouble driving at night. Astigmatism often is present at birth, but can change or develop later in life. If it occurs in early life and is left untreated, it may result in amblyopia.
The cause of astigmatism is unclear, although it is believed to be partly related to genetic factors. The underlying mechanism involves an irregular curvature of the cornea and protective reaction changes in the lens of the eye, called lens astigmatism, that has the same mechanism as spasm of accommodation. Diagnosis is by an eye examination called autorefractor keratometry and subjective refraction.
Three treatment options are available: glasses, contact lenses, and surgery. Glasses are the simplest. Contact lenses can provide a wider field of vision and fewer artifacts than even double aspheric lenses. Refractive surgery aims to permanently change the shape of the eye and thereby cure 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, double vision, squinting, 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, although 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 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. 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, ACP2, 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 ; 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, NPLOC4, LINC00340 and HERC2.
Acquired
Astigmatism may also occur following a cataract surgery 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
Axes of the principal meridians
In eyes without astigmatism, the lens and cornea are spherical in shape. An astigmatic eye can be described by defining principal meridians, which are the steepest and flattest axes of the eye. There are several types of astigmatism, depending on the orientation of the principal meridians:- Regular astigmatism – the principal meridians are perpendicular to one another.
- * With-the-rule astigmatism – the vertical meridian is steepest, and the cornea is wider than it is tall.
- * Against-the-rule astigmatism – the horizontal meridian is steepest, and the cornea is taller than it is wide.
- * Oblique astigmatism – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.
- Irregular astigmatism – the principal meridians are not perpendicular to one another.
In against-the-rule astigmatism, a plus cylinder is added in the horizontal 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 ; and/or the two steep hemimeridians may be asymmetrically steep—that is, one may be significantly steeper than the other. 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.
Throughout the eye
Diagnosis
A number of tests are used during eye examinations to determine the presence of astigmatism and to quantify its amount and axis. 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. Corneal topography may also be used to obtain a more accurate representation of the cornea's shape. 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 or trial frame 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 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 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, 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 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.Common eyeglass prescriptions follow one of two conventions. E.g., a cylindrical astigmatic correction of would be equivalent to one of.
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. 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 seem to be as effective as in non-complex cases for correction of concurrent corneal astigmatism.
With advancement in laser science and the increasingly prevalent use of surgical laser systems in ophthalmology, laser eye surgery has become one of the most commonly used methods for treating astigmatism, with success rates among the highest of all the currently available treatment methods according to peer-reviewed research studies and comparative analyses.