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). Irregular astigmatism – principal meridians are not perpendicular.Oblique astigmatism – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.Against-the-rule astigmatism – the horizontal meridian is steepest (a rugby ball or American football standing on its end).With-the-rule astigmatism – the vertical meridian is steepest (a rugby ball or American football lying on its side).(The steepest and flattest meridians of the eye are called principal meridians.) Regular astigmatism – principal meridians are perpendicular.Illustration of astigmatism Axis of the principal meridian In keratoconus progressive thinning and steepening of the cornea cause irregular astigmatism. Contraction of the scar due to wound or cataract extraction causes astigmatism due to flattening of the cornea in one direction. Acquired Īstigmatism may also occur following a cataract surgery or a corneal injury. 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). They also found that the TOX gene region previously identified for spherical equivalent refractive error was the second most strongly associated region. 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 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 TNFAIP8元. Facing the inconsistency, a study by Shah and colleagues in 2018 included both populations with Asian and Northern European ancestry. 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). 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.
Although no conclusive result has been shown, various candidates have been identified. Genome-wide association studies (GWAS) have been used to investigate the genetic foundation of astigmatism. Genetics, based on twin studies, appear to play only a small role in astigmatism as of 2007. The cause of congenital astigmatism is unclear, however it is believed to be partly related to genetic factors. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches. Īlthough astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as blurred vision, double vision, squinting, eye strain, fatigue, or headaches. Astigmatism was first reported by Thomas Young in 1801. People of all ages can be affected by astigmatism. In Europe and Asia, astigmatism affects between 30 and 60% of adults. Refractive surgery permanently changes the shape of the eye. Contact lenses can provide a wider field of vision. Three treatment options are available: glasses, contact lenses, and surgery. Diagnosis is by an eye examination, through objective and subjective refraction. The underlying mechanism involves an irregular curvature of the cornea or abnormalities in the lens of the eye. The cause of astigmatism is unclear however, it is believed to be partly related to genetic factors. If it occurs in early life and is left untreated, it can result in amblyopia. Astigmatism often occurs at birth and can change or develop later in life. Other symptoms can include eyestrain, headaches, and trouble driving at night. This results in distorted or blurred vision at any distance. Blur from astigmatic lens at different distancesĭistorted or blurred vision at all distances, eyestrain, headaches Īstigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power.