ABO Contact Lenses 2 — Questions and Answers
Question 1: A patient wearing soft contact lenses complains of reduced vision late in the day. Slit-lamp examination reveals corneal neovascularization at the superior limbus. What is the most likely cause?
- Dry eye syndrome
- Chronic hypoxia from low-Dk lens (Correct answer)
- Lens protein deposits
- Giant papillary conjunctivitis
Correct answer: Chronic hypoxia from low-Dk lens
Chronic hypoxia from wearing low-oxygen-transmissible (low-Dk) lenses causes limbal vessel ingrowth to compensate for oxygen deprivation.
Question 2: Which base curve selection guideline applies when fitting rigid gas-permeable (RGP) contact lenses?
- Fit 0.50 D steeper than flat K
- Align the lens base curve with the flattest K (Correct answer)
- Fit 1.00 D flatter than steep K
- Match base curve to the average of both K readings
Correct answer: Align the lens base curve with the flattest K
The standard RGP fitting philosophy is to align the lens base curve with the flattest keratometry reading to achieve an 'on K' or alignment fit.
Question 3: A toric soft lens rotates 15° nasally on a patient's left eye. Using the LARS rule, how should the lens axis be adjusted?
- Add 15° to the prescribed axis (Correct answer)
- Subtract 15° from the prescribed axis
- Rotate the lens 15° temporally by adjusting truncation
- Reorder a lens with 15° less base curve
Correct answer: Add 15° to the prescribed axis
LARS (Left Add, Right Subtract) means if the lens rotates left (nasally on left eye), add that amount of degrees to the prescribed axis.
Question 4: What is the primary advantage of silicone hydrogel contact lenses over conventional hydrogel lenses?
- Greater water content
- Superior protein deposit resistance
- Significantly higher oxygen transmissibility (Dk/t) (Correct answer)
- Better UV protection
Correct answer: Significantly higher oxygen transmissibility (Dk/t)
Silicone hydrogel lenses have dramatically higher Dk/t values due to silicone's high oxygen permeability, reducing hypoxia-related complications.
Question 5: A patient presents with acute red eye after sleeping in extended-wear contact lenses. Slit-lamp shows a stromal infiltrate with an overlying epithelial defect and mucopurulent discharge. What condition is most suspected?
- Contact lens-associated red eye (CLARE)
- Microbial keratitis (Correct answer)
- Superior limbic keratoconjunctivitis
- Sterile corneal infiltrate
Correct answer: Microbial keratitis
An epithelial defect with stromal infiltrate and mucopurulent discharge is the hallmark presentation of microbial keratitis, a vision-threatening emergency.
Question 6: Which contact lens parameter describes the amount of lens material per unit area of lens surface?
- Center thickness
- Dk value
- Water content
- Dk/t (oxygen transmissibility) (Correct answer)
Correct answer: Dk/t (oxygen transmissibility)
Dk/t (oxygen transmissibility) accounts for the lens Dk divided by its thickness (t), reflecting actual oxygen passage through the specific lens as worn.
Question 7: A bitoric RGP lens is indicated when the difference between corneal toricity and residual astigmatism exceeds what threshold?
- 0.50 D
- 0.75 D
- 1.00 D (Correct answer)
- 1.50 D
Correct answer: 1.00 D
When residual astigmatism (internal astigmatism not corrected by the lens-tear interface) exceeds approximately 1.00 D, a bitoric lens design is indicated.
ABO Contact Lenses 2
A patient wearing soft contact lenses complains of reduced vision late in the day.
Slit-lamp examination reveals corneal neovascularization at the superior limbus.
What is the most likely cause?