W5: Astigmatism & Anisometropia Correction Issues Notes

Astigmatism Correction Issues

  • Prevalence and risk

    • Low levels of astigmatism are common

    • high levels are uncommon but may lead to meridional amblyopia, particularly if the axis is oblique.

    • Meridional amblyopia can occur in young individuals with uncorrected high astigmatic errors.

  • Correction thresholds and pathology cues

    • need to correct >2D by age 2 years to maximise vision.

    • If there are large changes in astigmatism, suspect pathology such as:

      • Keratoconus

      • Cataract

      • Pterygia

  • Visual symptoms and axis effects

    • Asthenopia (eye strain) due to fluctuating accommodation can lead to headaches, tiredness, and watery eyes.

    • Magnitude effects: <0.75\,\mathrm{D} usually not problematic.

    • The effect of axis on vision/symptoms, from greatest to least: oblique axis, ATR (against-the-rule), WTR (with-the-rule).

    • Indications for correction include functional vision impairment, prevention of amblyopia, and perceptual concerns.

  • Clinical considerations for correction

    • Determine origin: lenticular (lens-based) vs corneal astigmatism.

    • Correction mode: spectacles vs contact lenses (CLs).

    • Axis and lens design: thicker meridian of the lens may influence slab or frame choice; frame selection can help accommodate thicker meridians.

    • Perceptual distortion: aim to provide the least cyl (cylinder) required for good vision to minimize distortion.

    • Large changes in prescription may require an adaptation period for the patient.

    • Distortion or distortion-induced postural changes due to cyl optics.

Anisometropia Correction Issues

  • Origins and natural history

    • Anisometropia is present at birth in many cases and is associated with large refractive errors.

    • A portion often disappears as part of passive emmetropization; lower levels (<2\,\mathrm{D}) are likely to disappear.

    • If anisometropia is ≥3\,\mathrm{D} and present at 1 year, there is a high risk that it will remain and amblyopia may develop.

  • Monitoring and prognosis

    • Monitor anisometropia every 4–6 months to determine if it is reducing.

    • If amblyopia is present, anisometropia requires correction.

    • The greater the degree of anisometropia, the more likely it is to be permanent.

  • Types and impact on visual function

    • Anisohyperopia (farsighted anisometropia) is more likely to cause amblyopia than anisomyopia (nearsighted anisometropia), but both can affect binocular vision (BV) and stereoacuity.

  • Indications for correction

    • Indications include risk of amblyopia, persistent anisometropia, and the presence of BV or stereoacuity issues.

    • If amblyopia is present or likely, correction is indicated to prevent or treat the condition.

  • Correction issues:

    • Px complains of distortion:

      • reduce intraocular difference in rx

      • increase thickness of thinner lens

      • contact lens correction

      • max tolerable rx for comfortable BV

      • check vertical lens centre - may have induced vertical prism.