What is the difference between with-the-rule and against-the-rule astigmatism and how are they corrected?

Astigmatism: With-the-Rule vs Against-the-Rule

With-the-rule (WTR) astigmatism has the steepest corneal meridian oriented vertically (approximately 90 degrees), while against-the-rule (ATR) astigmatism has the steepest meridian oriented horizontally (approximately 180 degrees). 1

Definitions and Axis Orientation

With-the-rule (WTR) astigmatism:

• The principal meridian of greatest refractive power is positioned between 75-105 degrees (near vertical) 2
• This is the most common type of astigmatism, occurring in approximately 73% of eyes with astigmatism 3
• More prevalent in younger individuals 4

Against-the-rule (ATR) astigmatism:

• The principal meridian of greatest power is positioned between 0-15 degrees and 165-180 degrees (near horizontal) 2
• Less common, occurring in approximately 16% of eyes with astigmatism 3
• Becomes more prevalent with aging as corneal curvature changes over time 4

Oblique astigmatism:

• Occurs when principal meridians fall between 16-74 degrees and 106-164 degrees 2
• Does not fit the WTR or ATR classification 2

Clinical Significance

ATR astigmatism provides superior uncorrected near vision compared to WTR astigmatism:

• In pseudophakic patients with 1.00-1.50 diopters of myopic astigmatism, those with ATR achieved significantly better uncorrected near visual acuity (p<0.001) 5
• Elderly patients (ages 60-80) with ATR myopic astigmatism after cataract surgery demonstrated significantly better uncorrected near vision (p<0.01) compared to those with WTR astigmatism 6
• This occurs because the horizontal meridian focus (in ATR) provides better near vision function 5

ATR astigmatism is associated with worse amblyopia treatment outcomes:

• Patients with hyperopic ATR astigmatism require longer duration of occlusion therapy and achieve fewer lines of visual acuity gain compared to hyperopic WTR patients (p=0.0143 and p=0.0000, respectively) 4
• Similar findings occur in myopic ATR versus myopic WTR patients (p=0.0392 and p=0.0192, respectively) 4
• Near vision impairment may be more amblyogenic than distance vision impairment during visual development 4

Correction Strategies

Low to moderate astigmatism (less than 3.00 diopters):

• Can be corrected with soft toric contact lenses or rigid gas-permeable contact lenses 1
• Full cylindrical correction may not be needed initially, especially in adults 1
• Substantial changes in axis or power are poorly tolerated in patients with previously uncorrected or partially corrected astigmatism 1

High astigmatism (3.00 diopters or more):

• Corrected effectively with rigid gas-permeable and hybrid contact lenses 1
• Bitoric or back-surface toric contact lens designs minimize corneal bearing and improve centration for greater amounts of corneal astigmatism 1
• Custom-designed soft toric contact lenses provide good centration, flexible wear schedules, and improved comfort 1
• Rigid gas-permeable scleral lenses (diameter >17mm) are an option for high and/or irregular astigmatism, particularly with anisometropia 1

Refractive surgery considerations:

• ATR astigmatism is a significant predictor of magnitude of error after small-incision lenticule extraction, contributing a constant 0.32 diopters of undercorrection (p<0.001) 3
• Target-induced astigmatism and ATR/WTR orientation together explain approximately 25% of variation in surgical outcomes 3
• Incorporating these parameters in preoperative planning may produce more consistent results, especially for high cylinder corrections 3

Common Pitfalls

Avoid overcorrection in first-time astigmatism correction:

• Adults with astigmatism may not accept full cylindrical correction in their first pair of eyeglasses 1
• Gradual correction may be better tolerated 1

Consider the functional advantage of ATR in presbyopic patients:

• When planning cataract surgery or refractive procedures in older patients, leaving mild ATR myopic astigmatism (1.00-1.50 diopters) may provide better uncorrected near vision 6, 5
• This can reduce dependence on reading glasses 5

Recognize that regular astigmatism has uniform curvature in each meridian with principal meridians 90 degrees apart, while irregular astigmatism (seen in keratoconus, corneal scarring, and post-surgical corneas) varies at different corneal points and cannot be fully corrected with spherocylindrical lenses 1