Myopia, Hyperopia, Astigmatism, Presbyopia & Anisometropia

Refractive Errors: Myopia, Hyperopia, Astigmatism, Presbyopia & Anisometropia

Quick Answer: Ametropia is any refractive error where the eye cannot focus light precisely on the retina. The five core ametropias tested on the CLRE are myopia (eye too long), hyperopia (eye too short), astigmatism (cornea or lens not spherical), presbyopia (lost accommodation), and anisometropia (unequal refractive error between the two eyes).

Myopia (Nearsightedness)

In myopia, the eye's axial length is too long or the cornea is too steep, causing parallel light rays to focus in front of the retina. Distant objects appear blurry; near objects remain clear.

• Correction: minus (concave) lenses diverge light so it focuses on the retina
• Contact lens advantage: minimal image minification versus spectacles (spectacles minify roughly 1.5% per diopter at a 12 mm vertex)
• Onset: typically childhood or teenage years; progression often stabilizes in early adulthood
• Pathologic myopia: axial length >26 mm with associated retinal complications; co-manage with ophthalmology

A -5.00 D myope corrected with spectacles at 12 mm has an effective power at the corneal plane of about -4.72 D, but the contact lens must deliver close to the full -5.00 D (less vertex compensation is needed at moderate powers). For high myopes, this vertex difference becomes clinically significant — see Section 3.3.

Hyperopia (Farsightedness)

In hyperopia, the eye is too short or the cornea too flat; light focuses behind the retina. Young hyperopes can use accommodation to clear vision, but this fatigues the ciliary muscle and can cause asthenopia and near blur.

• Correction: plus (convex) lenses converge light onto the retina
• Spectacle vs. CL: hyperopes accommodate less in contact lenses than in spectacles (the base-out prism induced at near in plus spectacles drives extra vergence-accommodation; removing the spectacles removes that demand)
• Latent hyperopia: partially hidden by accommodation; cycloplegic refraction may be required for accurate measurement, especially in children
• High hyperope / aphakic: requires vertex compensation and high-plus CL designs

Astigmatism

Astigmatism occurs when the cornea or crystalline lens has unequal curvature across meridians, producing two focal points instead of one. Classify by steep-meridian orientation:

• With-the-rule (WTR): steep meridian is vertical (~90°); corrected with plus cylinder at 180°. Common in younger patients.
• Against-the-rule (ATR): steep meridian is horizontal (~180°); corrected with plus cylinder at 90°. Common in older patients and post-cataract surgery.
• Oblique: steep meridian between 30°–60° or 120°–150°.
• Irregular: non-orthogonal meridians from trauma, keratoconus, corneal scarring, or surgery; best corrected with an RGP or scleral lens that masks the irregular surface with a tear lens.

Correction options:

1. Toric soft CL: uses ballasting (prism or peri-ballast) and thin zones to stabilize the cylinder axis
2. RGP lens: the lacrimal (tear) lens between the rigid lens and cornea optically neutralizes corneal astigmatism
3. Scleral lenses: vault the cornea; the fluid reservoir masks both corneal and irregular astigmatism

A soft lens drapes over the cornea and does not mask corneal astigmatism — the corneal toricity is transmitted directly to the lens surface, so residual astigmatism appears and a toric soft lens (or RGP) is required.

Presbyopia

Presbyopia is the age-related loss of accommodative amplitude, beginning around age 40, caused by crystalline lens stiffening and reduced ciliary muscle effectiveness. The near point recedes; reading small print becomes difficult.

Contact lens options:

• Multifocal CLs (simultaneous vision): concentric or aspheric designs project distance and near images through the pupil simultaneously; the brain suppresses the unwanted image
• Alternating (translating) bifocal CLs: prism-ballasted lens that translates with down-gaze, positioning the near segment under the pupil
• Monovision: dominant eye fit for distance, non-dominant eye fit for near (typical add +1.50 to +2.50 D)
• Modified monovision: one eye distance, the other fit with a multifocal CL

Suppression, reduced stereoacuity, and nighttime glare are common adaptation issues with all CL presbyopic corrections. A trial period of 1–2 weeks is recommended.

Anisometropia & Aniseikonia

Anisometropia is a significant difference in refractive error between the two eyes (generally ≥1.00 D). With spectacles:

• Prismatic imbalance (Prentice's rule: P = c × F) causes eye strain and diplopia on off-axis gaze
• Aniseikonia (image size difference between eyes) — spectacle magnification differences of ~1.5% per diopter at 12 mm vertex exceed the brain's tolerance of about 3–5%

Contact lenses solve both problems because they move with the eye and sit at the corneal plane:

• Prismatic effect is negligible (no decentration)
• Magnification difference is minimal (near-zero vertex distance)

Worked Example: Anisometropia Calculation

A patient has the following spectacle Rx:

• OD: -1.00 DS
• OS: -6.00 DS

Inter-eye difference = 5.00 D.

Spectacle magnification difference between eyes at 12 mm vertex ≈ 5 D × 1.5%/D = 7.5% aniseikonia — well above the symptomatic 3–5% threshold. Patients frequently report headache, diplopia, and difficulty with depth perception.

With contact lenses, magnification difference drops to <1%, restoring binocular fusion and stereopsis.

Aphakia & Pseudophakia

• Aphakia: absence of the crystalline lens (post-cataract extraction or trauma). The eye loses +15 to +16 D of refractive power, requiring a high-plus contact lens (+10 to +12 D after vertex compensation). Pediatric aphakia is one of the most common indications for contact lenses in infants, because spectacles create unacceptable aniseikonia in unilateral aphakia.
• Pseudophakia: crystalline lens replaced with an intraocular lens (IOL). Residual refractive error is usually small and can often be corrected with a spherical or toric CL.

Amblyopia

Amblyopia ("lazy eye") is reduced best-corrected visual acuity in one eye due to abnormal visual development during the critical period (birth to ~7 years). Anisometropic amblyopia is a common form: the more ametropic eye receives a chronically blurred image during visual maturation, suppressing it in the visual cortex. Early contact lens correction of anisometropia in children is essential to prevent irreversible amblyopia — spectacles often fail because of aniseikonia, while CLs provide equal image size and clear retinal images to both eyes.