Astigmatism

Overview

Astigmatism is a refractive error in which the cornea (most commonly) or, less often, the crystalline lens has unequal curvature across different meridians, so that incoming light is focused at two separate focal points rather than one. This produces blurred or distorted vision at both distance and near, often described by patients as "smearing," "ghosting," or "shadowing" rather than the more uniform blur typical of pure myopia or hyperopia. Astigmatism very commonly coexists with myopia or hyperopia rather than occurring in isolation.

Epidemiology & Risk Factors

Astigmatism is common across the lifespan and across populations, though reported prevalence varies by age group, ethnicity, and the diagnostic cylinder-power threshold used to define it. In a population-based study of underserved rural areas in Iran using a cylinder-error cutoff of 0.50 D, overall prevalence was 35.6%, ranging from a low of 13.6% in the 6-20 year age group to 82.5% in adults over age 70 — with age and education level significantly associated with astigmatism in multivariable analysis.¹ In a large population-based study of Chinese preschool children aged 1-6 years (the Changsha Children Eye Study), astigmatism (defined as cylinder ≥1.50 D) was found in 17.6% of children, more common than either myopia (2.94%) or was comparable to hyperopia (13.8%) in that same age range.²

The axis of astigmatism also shifts with age. With-the-rule (WTR) astigmatism — where the steepest corneal curvature is vertical — was the most prevalent pattern in both the pediatric Chinese cohort and the Iranian population overall, but its relative prevalence declined with increasing age, while against-the-rule (ATR) and, to a lesser extent, oblique astigmatism became relatively more common in older adults.¹

Irregular astigmatism — as opposed to the regular, symmetric astigmatism described above — is a distinct entity with a different set of causes, most importantly corneal ectatic disease. In a facility-based study of patients attending referral refraction clinics in Malawi, keratoconus accounted for 5.7% of patients presenting to those clinics, was most common among individuals aged 18-35, and cylindrical (astigmatic) refractive error was independently and strongly associated with a keratoconus diagnosis in multivariable analysis.⁴

Pathophysiology

Regular astigmatism results from a toric (football- or rugby-ball-shaped, rather than spherical) curvature of the cornea or, less commonly, the crystalline lens, such that one meridian has greater refractive power than the meridian perpendicular to it. This produces two separate line foci (rather than a single point focus) at different distances from the retina — the basis of the classic conoid of Sturm used to explain astigmatic optics.

Irregular astigmatism arises when the corneal surface is asymmetric or non-uniform in a way that cannot be corrected by a simple spherocylindrical lens. The most clinically important cause is corneal ectasia (keratoconus and related disorders), in which progressive thinning and anterior bulging of the cornea produces asymmetric, often rapidly evolving, high astigmatism that is a specific red flag rather than a variant of ordinary refractive astigmatism.⁴

Clinical Presentation

Patients typically report blurred or distorted vision at both distance and near, sometimes described as smearing, streaking, ghosting, or double vision in one eye (monocular diplopia) that does not resolve when the fellow eye is covered — a useful distinguishing feature from the binocular diplopia of ocular misalignment. Headaches or eye strain with prolonged visual tasks (asthenopia) are also common, particularly with uncorrected low-to-moderate astigmatism. Rapidly progressive, asymmetric, or high astigmatism — particularly presenting in adolescence or young adulthood — should raise concern for keratoconus rather than being treated as routine refractive astigmatism.

Exam Findings

• Blurred visual acuity that improves substantially with cylindrical (toric) lens correction on manifest refraction
• Keratometry or corneal topography demonstrating regular, symmetric, bowtie-pattern astigmatism (regular astigmatism) versus asymmetric, skewed, or irregular patterns (irregular astigmatism/ectasia)
• Retinoscopic reflex showing a scissoring pattern in irregular astigmatism, distinct from the clean, symmetric reflex of regular astigmatism
• In suspected keratoconus: inferior corneal steepening on topography, Vogt striae, Fleischer ring, or Munson's sign on slit-lamp exam

Classification

• Regular astigmatism: the two principal meridians of maximal and minimal curvature are perpendicular to each other; correctable with a spherocylindrical lens
  • With-the-rule (WTR): steepest curvature near the vertical (90°) meridian
  • Against-the-rule (ATR): steepest curvature near the horizontal (180°) meridian
  • Oblique: steepest meridian oriented away from both the vertical and horizontal axes
• Irregular astigmatism: the principal meridians are not perpendicular, or curvature varies unpredictably across the corneal surface; not fully correctable with a simple spherocylindrical lens and often requiring rigid gas-permeable or scleral contact lenses for optimal correction. Most commonly caused by corneal ectasia (keratoconus, pellucid marginal degeneration), corneal scarring, or irregular healing after corneal surgery or trauma

Differential Diagnosis

• Keratoconus or other corneal ectasia (must be actively excluded, particularly in progressive or asymmetric astigmatism)
• Corneal scarring from prior infection, trauma, or surgery
• Pterygium encroaching on the visual axis, inducing corneal flattening/astigmatism
• Lenticular astigmatism from an asymmetric or subluxated crystalline lens
• Post-surgical astigmatism (following cataract, corneal, or refractive surgery)

Management

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 (e.g., LASIK/PRK) is also an option for eligible candidates with stable, regular astigmatism.

Irregular astigmatism requires a fundamentally different approach because it cannot be corrected by simple spherocylindrical lenses. When irregular astigmatism coexists with cataract requiring surgery — as in keratoconus — management requires careful preoperative planning: corneal stabilizing procedures (such as corneal crosslinking or intrastromal corneal ring segments) may be used before biometry to obtain more predictable keratometry values for intraocular lens power calculation, and the choice between a toric and a 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, since a toric IOL's benefit depends on the astigmatism being regular enough to correct with a fixed cylindrical power.³

Complications

• Uncorrected astigmatism can contribute to amblyopia in children if significant and asymmetric between the two eyes (meridional or anisometropic amblyopia)
• Undiagnosed progressive keratoconus can lead to significant, sometimes irreversible, loss of best-corrected visual acuity if not identified and treated (e.g., with corneal crosslinking) before advanced thinning and scarring occur
• Inappropriate toric IOL selection in an eye with unrecognized irregular astigmatism can leave a patient with a poor refractive outcome despite technically successful cataract surgery

Clinical Pearls

• Monocular diplopia that persists despite covering the fellow eye should prompt evaluation for uncorrected astigmatism (or another anterior segment cause) rather than being assumed to be neurologic.
• With-the-rule astigmatism predominates in children and young adults, while against-the-rule and oblique astigmatism become relatively more common with advancing age — a pattern worth anticipating when planning toric IOL selection in older cataract patients.¹
• Cylindrical refractive error was independently and strongly associated with a keratoconus diagnosis in a facility-based study of refraction-clinic patients — significant or asymmetric astigmatism, especially in a young patient, warrants corneal topography rather than reflexive spectacle correction.⁴
• In patients with keratoconus needing cataract surgery, corneal stabilization before biometry improves the predictability of lens calculations, and the decision between toric and monofocal IOL should be individualized to the severity and regularity of the ectasia.³

Common Pitfalls

• Prescribing a toric contact lens or IOL for astigmatism without first confirming the astigmatism is regular — toric correction does not adequately address irregular, ectatic astigmatism.
• Missing early or subclinical keratoconus by attributing progressive or asymmetric astigmatism to simple refractive change rather than obtaining corneal topography.
• Assuming a single population-based prevalence figure for astigmatism applies universally, when reported rates vary meaningfully by age group, ethnicity, and diagnostic cylinder threshold across different studies.¹²
• Overlooking that residual irregular astigmatism after cataract surgery in a keratoconus patient may still require rigid gas-permeable or scleral lens correction even with a well-selected IOL.³