Astigmatism

Causes

The cause of congenital astigmatism is not fully understood but is thought to involve genetic factors, though twin studies as of 2007 suggest genetics play only a small role. Genome-wide association studies have identified several candidate genes, but results have been inconsistent. A 2011 study in Asian populations linked variants in the PDGFRA gene on chromosome 4q12 to corneal astigmatism, but a 2013 European study found no significant association at the genome-wide level.

A 2018 study including both Asian and Northern European ancestries replicated the PDGFRA locus and identified three novel candidate genes: CLDN7, ACP2, and TNFAIP8L3. Other studies have pointed to the VAX2 gene on chromosome 2p13.3, while some found no strong genetic signals for refractive astigmatism, suggesting possible co-susceptibility with spherical errors. Additional loci such as ZC3H11B, NPLOC4, LINC00340, and HERC2 have also been reported.

Astigmatism can also develop after cataract surgery or corneal injury, as scar contraction flattens the cornea in one direction. In keratoconus, progressive corneal thinning and steepening cause irregular astigmatism.

Pathophysiology

In a non-astigmatic eye, the lens and cornea are spherical. An astigmatic eye has principal meridians—the steepest and flattest axes. Regular astigmatism occurs when these meridians are perpendicular. With-the-rule astigmatism has the vertical meridian steepest, making the cornea wider than tall; against-the-rule has the horizontal meridian steepest, making the cornea taller than wide.

Oblique astigmatism has the steepest curve between 120–150 degrees and 30–60 degrees. Irregular astigmatism has non-perpendicular principal meridians. In with-the-rule astigmatism, the eye has excessive plus cylinder in the horizontal axis relative to the vertical, so vertical light beams focus anterior to horizontal beams. Correction uses a minus cylinder placed on the horizontal axis, causing vertical light to diverge slightly before entering the eye, compensating for the stronger vertical convergence.

In against-the-rule astigmatism, a plus cylinder is added on the horizontal axis (or a minus cylinder on the vertical). Axis is recorded in degrees from 0 to 180 counterclockwise, with 0 and 180 on a horizontal line at pupil center; 0 lies on the right of both eyes as seen by an observer.

Treatment

Astigmatism can be corrected with eyeglasses, contact lenses, or refractive surgery. Glasses are the simplest and safest option, while contact lenses offer a wider field of vision. Refractive surgery permanently reshapes the eye but carries greater risk and expense. In keratoconus, certain contact lenses often provide better visual acuity than glasses.

Toric lenses are available in both rigid gas-permeable and soft forms. Common prescriptions follow two equivalent conventions; for example, −0.75 diopters at 45° equals +0.75 diopters at 135°.

In older patients, astigmatism can be corrected during cataract surgery with a toric intraocular lens or limbal relaxing incisions; toric lenses probably yield better outcomes. Toric intraocular lenses are also effective in patients with complex ophthalmic history. Laser eye surgery has become a common method with high success rates according to peer-reviewed research.

Epidemiology

The WHO reports 2.2 billion people have vision impairment, 88.4 million from refractive error. A compilation of systematic reviews found astigmatism prevalence among adults ranges from 8% to 62%, with an estimated worldwide prevalence of 40%. China has the highest reported prevalence at 62%. Prevalence increases with age due to changes in refractive index gradients.

Infants in the first few months often have astigmatism due to a steep cornea, especially those with low birth weight and post-conceptional age. By age four, prevalence decreases as the cornea flattens. The cornea remains stable during adulthood, then steepens again after age 40. Mild astigmatism is most common, comprising about 82% of cases, and its prevalence increases until age 70; moderate and significant astigmatism increase after 70.

With-the-rule astigmatism has a global prevalence range of 4–98%, against-the-rule 1–58%, and oblique 2–61%. With-the-rule is more prevalent in young adults, shifting to against-the-rule with age. A 2005 Polish study suggested with-the-rule astigmatism may lead to myopia onset. Untreated astigmatism reduces vision-related quality of life, increasing glare, haloes, and difficulty with night driving.

Did You Know?

• Astigmatism can be asymptomatic, but higher degrees may cause blurred vision, double vision, squinting, eye strain, fatigue, or headaches.
• George Biddell Airy independently discovered astigmatism in his own eye and presented his observations in February 1825 at the Cambridge Philosophical Society.