Astigmatism

Astigmatism is a refractive error in which the cornea (most commonly) or, less often, the crystalline lens has unequal curvature across different meridians, so incoming light is focused at two separate focal points rather than one, producing blurred or distorted vision at both distance and near. Regular astigmatism (principal meridians perpendicular) is classified as with-the-rule (steepest curvature near vertical), against-the-rule (steepest near horizontal), or oblique, and is correctable with a spherocylindrical lens. Irregular astigmatism — where curvature varies unpredictably or the principal meridians are not perpendicular — is not fully correctable with a simple spherocylindrical lens and is most commonly caused by corneal ectasia (keratoconus, pellucid marginal degeneration), scarring, or irregular post-surgical healing.

Astigmatism is common across ages and populations, though prevalence varies by age, ethnicity, and diagnostic cylinder threshold. In a population-based study of rural Iran (0.50 D cutoff), overall prevalence was 35.6%, ranging from 13.6% (ages 6-20) to 82.5% (over 70), with age and education level independently associated.¹ In Chinese preschoolers (Changsha Children Eye Study, cylinder ≥1.50 D), astigmatism was found in 17.6%, more common than myopia (2.94%) in that age range.² The axis shifts with age: with-the-rule astigmatism predominates in youth, while against-the-rule and oblique astigmatism become relatively more common with advancing age.¹

Regular astigmatism results from a toric corneal or lenticular curvature producing two line foci (the conoid of Sturm) rather than a single point focus. Irregular astigmatism's most important cause, corneal ectasia, involves progressive thinning and anterior bulging of the cornea. In a Malawi refraction-clinic study, keratoconus accounted for 5.7% of patients, was most common at ages 18-35, and cylindrical refractive error was independently and strongly associated with a keratoconus diagnosis.⁴

Patients report blurred or distorted vision at distance and near, described as smearing, streaking, ghosting, or monocular diplopia that persists despite covering the fellow eye — distinguishing it from binocular diplopia due to ocular misalignment. Asthenopia (headache, eye strain) is common with uncorrected low-to-moderate astigmatism. Rapidly progressive, asymmetric, or high astigmatism, particularly in adolescence or young adulthood, should raise concern for keratoconus.

Key differentials: keratoconus or other corneal ectasia (must be actively excluded in progressive/asymmetric astigmatism), corneal scarring, pterygium encroaching on the visual axis, lenticular astigmatism from an asymmetric or subluxated lens, and post-surgical astigmatism.

Manifest refraction shows blurred acuity that improves substantially with cylindrical (toric) correction. Keratometry or corneal topography shows a regular, symmetric bowtie pattern (regular astigmatism) versus asymmetric, skewed, or irregular patterns (irregular astigmatism/ectasia). Retinoscopy shows a scissoring reflex in irregular astigmatism, distinct from the clean symmetric reflex of regular astigmatism. Suspected keratoconus warrants evaluation for inferior corneal steepening on topography, Vogt striae, Fleischer ring, or Munson's sign on slit-lamp exam.

Regular astigmatism is corrected optically with spectacle lenses, toric soft contact lenses, or, in the context of cataract surgery, toric intraocular lenses; refractive corneal surgery (LASIK/PRK) is an option for eligible candidates with stable, regular astigmatism. Irregular astigmatism requires a different approach since it cannot be corrected by simple spherocylindrical lenses. When irregular astigmatism from keratoconus coexists with cataract requiring surgery, corneal stabilizing procedures (crosslinking, intrastromal ring segments) may be used before biometry to obtain more predictable keratometry, and the choice between toric and monofocal IOL depends on the severity, regularity, and centration of the ectatic astigmatism — patients who will still need rigid gas-permeable or scleral lens correction for residual irregular astigmatism after surgery generally should not receive a toric IOL.³

• Monocular diplopia that persists despite covering the fellow eye should prompt evaluation for uncorrected astigmatism rather than being assumed neurologic
• With-the-rule astigmatism predominates in youth; against-the-rule/oblique become relatively more common with age, worth anticipating when planning toric IOL selection in older cataract patients¹
• Significant or asymmetric astigmatism, especially in a young patient, warrants corneal topography rather than reflexive spectacle correction⁴
• Common pitfall: prescribing toric correction without first confirming the astigmatism is regular
• Common pitfall: missing early or subclinical keratoconus by attributing progressive/asymmetric astigmatism to simple refractive change
• Common pitfall: assuming a single population-based prevalence figure applies universally, when rates vary by age, ethnicity, and diagnostic threshold^1,2
• Common pitfall: overlooking that residual irregular astigmatism after cataract surgery in a keratoconus patient may still require RGP/scleral lens correction³