Understanding Refractive Errors
A refractive error occurs when the eye's optical system fails to focus incoming light precisely on the retina. In an ideal eye (emmetropia), the cornea and crystalline lens bend light rays so they converge exactly on the retinal surface, producing a clear image. When the eye's length, corneal curvature, or lens power does not match properly, the focal point falls in front of, behind, or at multiple locations relative to the retina, resulting in blurred vision.
Understanding each type of refractive error is essential for contact lens practitioners because the correction strategy differs for each.
Myopia (Nearsightedness)
Myopia occurs when light focuses in front of the retina rather than on it. This typically happens because the eye is too long (axial myopia) or because the cornea and/or lens have too much refractive power (refractive myopia).
Patients with myopia can see near objects clearly but distant objects appear blurry. The farther the focal point falls in front of the retina, the more blurred distance vision becomes.
Myopia is corrected with minus-powered (concave) lenses, which diverge light rays slightly before they enter the eye, pushing the focal point back onto the retina. In contact lens prescriptions, myopia is indicated by a negative sphere value (for example, -3.00 D).
Hyperopia (Farsightedness)
Hyperopia occurs when light would focus behind the retina if the eye's accommodative system were completely relaxed. This typically happens because the eye is too short (axial hyperopia) or because the cornea and/or lens have insufficient refractive power.
Young patients with mild hyperopia can often compensate through accommodation (the natural lens increases its power to bring the focal point forward onto the retina). However, this constant accommodation can cause eye strain, headaches, and fatigue, especially during near work. As accommodation decreases with age, hyperopia that was previously compensated becomes symptomatic.
Hyperopia is corrected with plus-powered (convex) lenses, which converge light rays slightly, bringing the focal point forward onto the retina. In prescriptions, hyperopia is indicated by a positive sphere value (for example, +2.50 D).
Astigmatism
Astigmatism occurs when the eye's refractive surfaces (primarily the cornea, sometimes the crystalline lens) have different curvatures in different meridians. Instead of being shaped like a basketball (spherical), the cornea is shaped more like a football, with one meridian being steeper than the other.
This uneven curvature means light focuses at two different points rather than a single point, creating blurred or distorted vision at all distances. Patients may describe images as smeared, doubled, or shadowed.
Astigmatism is corrected with toric contact lenses, which have different powers in different meridians to compensate for the eye's uneven curvature. The prescription includes a cylinder power and an axis that specifies the orientation of the correction.
Types of Astigmatism
- With-the-rule: The steepest meridian is vertical (near 90 degrees). More common in younger patients
- Against-the-rule: The steepest meridian is horizontal (near 180 degrees). More common in older patients
- Oblique: The steepest meridian falls between 30-60 degrees or 120-150 degrees
Presbyopia
Presbyopia is the age-related loss of accommodation, the eye's ability to increase its focusing power for near objects. The crystalline lens gradually becomes less flexible with age, and the ciliary muscle can no longer reshape it effectively. Presbyopia typically becomes noticeable around age 40 and progresses until about age 60.
Presbyopia affects everyone, regardless of whether they have other refractive errors. A myopic patient will still develop presbyopia; they may simply remove their distance glasses to read.
Contact lens options for presbyopia include:
- Multifocal contact lenses: Incorporate multiple focal zones (distance, intermediate, near) within a single lens using either concentric ring or aspheric designs
- Monovision: The dominant eye is corrected for distance and the non-dominant eye for near. This approach sacrifices some binocular vision but is simple and effective for many patients
- Modified monovision: Uses a single-vision lens on one eye and a multifocal on the other for a compromise approach
Refractive Error and Prescription Notation
A complete contact lens prescription for a patient with astigmatism might read: -3.00 -1.25 x 180. This means:
- -3.00: The spherical correction (myopia in this case)
- -1.25: The cylindrical correction (astigmatism amount)
- x 180: The axis orientation of the astigmatism correction
For spherical contact lenses, only the sphere power is needed. For toric lenses, all three values are required to properly correct the patient's vision.
Key Takeaways
- Myopia: light focuses in front of the retina; corrected with minus-powered lenses
- Hyperopia: light would focus behind the retina; corrected with plus-powered lenses
- Astigmatism: uneven corneal curvature creates two focal points; corrected with toric lenses
- Presbyopia: age-related loss of accommodation; managed with multifocal lenses or monovision
- With-the-rule astigmatism has a steep vertical meridian; against-the-rule has a steep horizontal meridian
- Small amounts of astigmatism may be managed with spherical lenses using spherical equivalent