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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.
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:
Toric contact lenses have three key parameters beyond those of spherical lenses:
| Parameter | Description | Example |
|---|---|---|
| Sphere (Sph) | Same as spherical lenses — myopic (-) or hyperopic (+) | -2.50 |
| Cylinder (Cyl) | Corrective power for astigmatism; usually expressed in - cylinder in contact lenses | -1.75 |
| Axis | Orientation of the cylinder, 0–180° | 90° |
| Base Curve | Curvature of the back surface; affects fit | 8.6 mm |
| Diameter | Overall lens diameter | 14.5 mm |
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:
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.
Free CPO and CPOA exam prep questions on Opterio—including toric and multifocal lens topics.
Lens types, parameters, and fitting fundamentals for all contact lens types.
Understanding the refractive condition toric lenses correct.
Age-related accommodation loss—the driving condition for multifocal lenses.
Browse all CPO and CPOA study topics by category.
A toric lens has two different powers at perpendicular meridians—a spherical power for myopia or hyperopia, and a cylindrical power for astigmatism. Unlike spherical lenses that can rotate freely, toric lenses must maintain a specific orientation on the eye. They are stabilized using various mechanisms: prism ballast (thicker zone at the bottom weighs the lens down), peri-ballast (thin zones at top and bottom), or dual thin zones. The lens is designed to align the cylinder axis with the patient's astigmatic meridian.
Toric lens rotation means the stabilized axis does not align with the prescription axis. If a toric lens rotates clockwise by 10°, the cylinder correction is 10° off-axis, reducing its effectiveness. Paraoptometrics assess rotation by observing the orientation markings (typically at 6 o'clock when properly aligned) and documenting the rotation in degrees and direction. The prescribing doctor then applies the LARS rule (Left Add, Right Subtract) to adjust the ordered axis. For example, if the lens rotates 15° to the left (counterclockwise), add 15° to the ordered axis.
Simultaneous vision designs present both distance and near powers to the pupil simultaneously—the brain selects the appropriate image. These include concentric ring designs (center-distance or center-near) and aspheric designs with gradual power transitions. Alternating vision (translating) designs work like bifocal spectacles—the pupil shifts between zones during different gaze directions. Simultaneous designs are most common in soft lenses; translating designs are more common in rigid gas permeable multifocals.
Simultaneous vision multifocals present multiple focal images to the retina at once. The brain suppresses the out-of-focus image, but this neural adaptation is not perfect for everyone. Patients with larger pupils (more simultaneous zone overlap), higher prescriptions, or more demanding visual tasks (night driving, fine detail work) are most likely to experience ghosting, reduced contrast, or halos. These symptoms often improve over 1–2 weeks as neuroadaptation occurs. Patients who do not adapt within 4–6 weeks may need a different lens design or monovision.
Monovision is a strategy for correcting presbyopia with contact lenses by fitting one eye (typically the dominant eye) for distance and the other for near or intermediate vision. It is an alternative to multifocal lenses. The brain suppresses the out-of-focus image from each eye depending on the task. Advantages: often better distance and near clarity than multifocals; lower cost. Disadvantages: some loss of depth perception, may not be suitable for drivers or those with stereopsis demands. Modified monovision (one multifocal, one single vision) is a hybrid option.
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