ABO Contact Lens Fitting 5 — Questions and Answers
Question 1: Corneal topography showing an asymmetric bow-tie pattern with skewed radial axes (SRAX) is most suggestive of:
- Pellucid marginal degeneration
- Keratoconus (Correct answer)
- Regular corneal astigmatism
- Post-refractive surgery ectasia
Correct answer: Keratoconus
SRAX on corneal topography, where the bow-tie axes are not aligned, is a hallmark early sign of keratoconus.
Question 2: A bifocal contact lens using an alternating (translating) design relies on which mechanism for near vision?
- Simultaneous vision through concentric zones
- Lens translation downward on downgaze to position the near segment in front of the pupil (Correct answer)
- Pupil-size-dependent optics
- Diffractive optical zones
Correct answer: Lens translation downward on downgaze to position the near segment in front of the pupil
Translating bifocal lenses shift downward in downgaze so the near segment aligns with the visual axis, similar to an executive bifocal spectacle lens.
Question 3: The minimum recommended Dk/t for daily wear soft lenses to avoid significant corneal hypoxia is approximately:
- 10 × 10⁻⁹
- 24 × 10⁻⁹ (Correct answer)
- 87 × 10⁻⁹
- 125 × 10⁻⁹
Correct answer: 24 × 10⁻⁹
Holden and Mertz established that a Dk/t of ≥24 × 10⁻⁹ is required for daily wear to limit chronic corneal edema to ≤4%.
Question 4: A patient wearing monthly replacement soft lenses is non-compliant with lens care. Which complication are they at GREATEST risk for?
- Spectacle blur
- Microbial keratitis, particularly Acanthamoeba (Correct answer)
- Corneal flattening
- Myopia progression
Correct answer: Microbial keratitis, particularly Acanthamoeba
Poor lens hygiene and improper care (especially tap water use) dramatically increase risk of microbial and Acanthamoeba keratitis.
Question 5: When fitting a toric soft lens, the practitioner should evaluate lens axis stability after how many blinks?
- Immediately upon insertion, no blinks needed
- After 1–2 blinks to allow settling
- After 10–15 minutes of adaptation and several blinks (Correct answer)
- After 8 hours of wear only
Correct answer: After 10–15 minutes of adaptation and several blinks
Toric soft lens rotation should be assessed after at least 10–15 minutes of adaptation so the lens has settled into its stable position.
Question 6: Which condition is an absolute contraindication to contact lens wear?
- Mild seasonal allergic conjunctivitis
- Aniridia with photophobia
- Active bacterial corneal ulcer (Correct answer)
- Low refractive error
Correct answer: Active bacterial corneal ulcer
An active corneal infection is an absolute contraindication because lens wear would exacerbate the infection and introduce additional pathogens.
Question 7: A steep scleral lens fit (excessive vault) over a keratoconic cornea will MOST likely result in:
- Corneal desiccation at 3 and 9 o'clock
- Fogging of vision from a thickened, stagnant fluid reservoir (Correct answer)
- Excessive lens movement and decentration
- Spectacle blur after removal
Correct answer: Fogging of vision from a thickened, stagnant fluid reservoir
Excessive vault creates a thick, stagnant fluid reservoir that scatters light and reduces visual acuity, often described as 'fogging.'
ABO Contact Lens Fitting 5
Corneal topography showing an asymmetric bow-tie pattern with skewed radial axes (SRAX) is most suggestive of: