What Are Refractive Errors?
Refractive errors occur when the eye cannot focus light precisely on the retina, resulting in blurred vision. They are the most common cause of correctable vision impairment worldwide and the primary reason most patients seek eye care. As a CPO, understanding the four main refractive errors helps you explain findings, assist with pre-testing, and anticipate the physician's correction goals.
Myopia (Nearsightedness)
Myopia occurs when the eye is too long relative to its focusing power, causing light to converge in front of the retina rather than on it. The result is clear near vision but blurred distance vision.
- Corrected with minus (concave) lenses
- Prevalence is increasing globally, particularly in East Asia
- High myopia (above -6.00 D) increases risks of retinal detachment, glaucoma, and myopic maculopathy
- Onset typically in childhood, often stabilizes in the mid-20s
Hyperopia (Farsightedness)
Hyperopia occurs when the eye is too short or the cornea is too flat, causing light to converge behind the retina. Young patients can compensate through accommodation (the lens flexing to add focusing power), but this effort causes eyestrain and headaches. Older patients lose this accommodative ability and experience blurred vision at all distances.
- Corrected with plus (convex) lenses
- Mild hyperopia may be fully compensated by a young patient's accommodation
- High hyperopia increases risk of angle-closure glaucoma due to a naturally smaller anterior segment
- Accommodative esotropia in children can result from uncorrected hyperopia
Astigmatism
Astigmatism occurs when the cornea or lens has an uneven curvature, like a football rather than a basketball. Different meridians of the eye have different focusing power, so no single focal point exists on the retina. Vision is blurred at all distances.
- Corrected with cylinder power in the spectacle prescription
- Specified by sphere, cylinder, and axis (e.g., -2.00 -1.50 x 090)
- With-the-rule astigmatism: steeper vertical meridian (axis near 90°), more common in children and young adults
- Against-the-rule astigmatism: steeper horizontal meridian (axis near 180°), more common in older adults
- Oblique astigmatism: steepest meridian at an oblique axis (not near 90° or 180°)
Presbyopia
Presbyopia is the age-related loss of accommodation (the ability to change focus for near objects) caused by progressive hardening of the crystalline lens. It becomes symptomatic in the mid-40s and affects virtually everyone eventually.
- Corrected with a reading addition (add power) to the distance prescription
- Delivered via reading glasses, bifocals, progressive lenses, or multifocal contact lenses
- The required add power increases with age: typically +1.00 at 45, progressing to approximately +2.50 by the mid-50s
Comparing the Four Refractive Errors
| Error | Problem | Blur Pattern | Correction |
|---|---|---|---|
| Myopia | Eye too long | Distance blurred, near clear | Minus lens |
| Hyperopia | Eye too short | Near blurred (or all distances in older patients) | Plus lens |
| Astigmatism | Uneven curvature | Blurred at all distances | Cylinder lens |
| Presbyopia | Stiff lens, no accommodation | Near blurred in middle-aged adults | Add power (bifocal/progressive) |
Key Takeaways
- Myopia: eye too long, distance blur, corrected with minus lenses
- Hyperopia: eye too short, near blur (or all distances), corrected with plus lenses
- Astigmatism: uneven corneal curve, blur at all distances, corrected with cylinder power
- Presbyopia: stiff lens from aging, near blur in 40s+, corrected with add power
- High myopia increases risk of retinal detachment and maculopathy
- High hyperopia increases angle-closure risk and can cause accommodative esotropia in children