COT Contact Lens Principles 3 — Questions and Answers
Question 1: When converting a spectacle prescription to a contact lens prescription, which step accounts for vertex distance?
- Adding the cylinder axis to the sphere
- Adjusting lens power for the 12 mm distance from the eye (Correct answer)
- Increasing the base curve by 0.3 mm
- Multiplying the add power by 1.25
Correct answer: Adjusting lens power for the 12 mm distance from the eye
Vertex distance conversion is needed for prescriptions above ±4.00 D because the corneal plane is approximately 12 mm closer to the eye than spectacle lenses.
Question 2: Which term describes the zone of a rigid contact lens that creates the peripheral seal and allows tear exchange?
- Optical zone
- Intermediate curve
- Peripheral curve (Correct answer)
- Lenticular zone
Correct answer: Peripheral curve
The peripheral curve (or blend zone) is flatter than the base curve and creates the edge lift needed for tear circulation under the lens.
Question 3: A toric soft lens is rotating 20° counterclockwise from the intended axis. Which LARS rule applies?
- Add 20° to the cylinder axis on the order (Correct answer)
- Subtract 20° from the cylinder axis on the order
- Add 20° to the sphere power
- Order a steeper base curve
Correct answer: Add 20° to the cylinder axis on the order
LARS (Left Add, Right Subtract): if the lens rotates left (counterclockwise), add the degrees of rotation to the ordered cylinder axis.
Question 4: Which contact lens-related complication is characterized by upper tarsal conjunctival changes resembling cobblestones?
- Superior limbic keratoconjunctivitis
- Contact lens-induced acute red eye
- Giant papillary conjunctivitis (Correct answer)
- Corneal neovascularization
Correct answer: Giant papillary conjunctivitis
Giant papillary conjunctivitis (GPC) produces large papillae (>0.3 mm) on the upper tarsal conjunctiva, often from lens deposits or mechanical irritation.
Question 5: What is the recommended replacement schedule for a 'daily disposable' contact lens?
- Every 2 weeks with nightly cleaning
- Discarded after each single day of wear (Correct answer)
- Replaced monthly after daily cleaning
- Worn for up to 30 nights of continuous wear
Correct answer: Discarded after each single day of wear
Daily disposable lenses are designed to be worn once and discarded at the end of the day, eliminating the need for a lens care system.
Question 6: When assessing RGP lens centration, a lens that consistently rides superiorly is most likely caused by:
- Too steep a base curve fitting relationship
- Upper lid attachment due to lid anatomy or lens edge design (Correct answer)
- Insufficient optical zone diameter
- Excessive edge lift
Correct answer: Upper lid attachment due to lid anatomy or lens edge design
Superior decentration of RGP lenses is commonly caused by the upper eyelid capturing or attaching to the lens edge, especially with lid-attachment fitting philosophy.
Question 7: Which solution type uses a chemical preservative system to disinfect soft contact lenses in a single step?
- Hydrogen peroxide one-step system
- Saline rinse solution
- Multipurpose solution (MPS) (Correct answer)
- Enzymatic cleaner tablet
Correct answer: Multipurpose solution (MPS)
Multipurpose solutions combine cleaning, rinsing, and disinfecting in one bottle using preservatives such as PHMB or polyquaternium-1.
COT Contact Lens Principles 3
When converting a spectacle prescription to a contact lens prescription, which step accounts for vertex distance?