Specialty contact lenses—including toric lenses for astigmatism and multifocal lenses for presbyopia—represent a growing segment of contact lens practice. Understanding how these lenses work, how to assess fitting issues, and how to educate patients are important competencies for CPO and CPOA candidates.
Toric Contact Lenses: Correcting Astigmatism
Patients with more than approximately 0.75–1.00 D of astigmatism often experience residual blur with spherical contact lenses. Toric lenses provide cylinder correction in addition to the spherical power. Understanding the key concepts:
Lens Parameters
Toric contact lenses have three key parameters beyond those of spherical lenses:
| Parameter | Description | Example |
|---|
Stabilization Methods
Prism Ballast
Thicker inferior zone weighs the lens, gravity anchors the bottom. Most common method.
Peri-Ballast
Thin zones at 12 and 6 o'clock positions; blink mechanics orient the lens.
Accelerated Stabilization
Multiple thin zones interact with lid dynamics during blinking (e.g., Acuvue's 4-petal design).
The LARS Rule for Rotation
Left Add, Right Subtract
When a toric lens rotates away from correct orientation, the ordered axis must be adjusted to compensate. The LARS rule tells you which direction to adjust:
- If the lens rotates to the LEFT (counterclockwise), ADD that many degrees to the ordered axis
- If the lens rotates to the RIGHT (clockwise), SUBTRACT that many degrees from the ordered axis
- Example: Ordered axis 90°, lens rotates 15° left → New order axis = 90 + 15 = 105°
Multifocal Contact Lenses: Correcting Presbyopia
Presbyopia begins to affect most patients in their early-to-mid 40s as the crystalline lens loses its ability to accommodate. Multifocal contact lenses provide simultaneous correction for distance, intermediate, and near vision.
Design Types
Center-Distance Design
Central zone provides distance correction; peripheral zone(s) provide near correction. Better for outdoor activities; dominant eye often preferred.
Best: Patients who prioritize distance vision
Center-Near Design
Central zone provides near correction; peripheral zone provides distance. Better for reading-heavy tasks; often used for non-dominant eye in modified monovision.
Best: Patients who prioritize near tasks
Aspheric Design
Gradual power change from center to periphery without discrete zones. More natural optics but can compromise contrast.
Best: Patients with moderate presbyopia and smaller pupils
Monovision
One eye set for distance (dominant), other for near. Not technically multifocal but common presbyopia strategy.
Best: Patients who cannot tolerate simultaneous vision design
Common Patient Complaints & Responses
- Distance is blurry — Verify the dominant eye has the distance correction. If using center-near in both eyes, switch dominant eye to center-distance. Consider reducing the Add power slightly.
- Near is blurry — Consider a higher-Add lens or switch the non-dominant eye to center-near design. Ensure adequate near working distance is used during trial.
- Ghosting / halos at night — Common in early adaptation period. If persisting beyond 2 weeks, consider a different design or monovision alternative.
- Fluctuating vision with blink — Check for lens rotation (toric component), poor centering, or dry eye reducing tear film stability over the lens.