CPO FREE CPO Keratometry and Corneal Topography Questions and Answers 2 — Questions and Answers
Question 1: A patient with a history of myopic LASIK now requires cataract surgery. Standard keratometry is performed. Which of the following issues is MOST likely to cause an error in the IOL calculation?
- The keratometer will overestimate corneal power, leading to a myopic surprise.
- Traditional keratometry assumes a normal prolate shape and measures a flattened central zone, leading to an underestimation of corneal power. (Correct answer)
- The posterior corneal curvature, which is unchanged by LASIK, will be the primary source of error.
- The standard refractive index used by the keratometer is invalid for post-ablation corneas, causing a hyperopic surprise.
Correct answer: Traditional keratometry assumes a normal prolate shape and measures a flattened central zone, leading to an underestimation of corneal power.
After myopic LASIK, the central cornea is flattened. Traditional keratometers measure this central zone and assume a normal prolate corneal shape, which is no longer valid. This leads to an underestimation of the true corneal power, which can result in a postoperative hyperopic surprise (the patient is more farsighted than intended) if not accounted for in IOL calculations.
Question 2: A corneal topography map displays a classic 'crab claw' or 'butterfly' pattern of inferior steepening. This pattern is most characteristic of which ectatic condition?
- Keratoconus
- Corneal scarring
- Pellucid Marginal Degeneration (PMD) (Correct answer)
- Post-keratoplasty astigmatism
Correct answer: Pellucid Marginal Degeneration (PMD)
Pellucid Marginal Degeneration (PMD) is characterized by a peripheral band of thinning in the inferior cornea, causing the cornea above it to protrude. This results in a distinctive 'crab claw' or 'butterfly' pattern on topography maps, which is a key diagnostic feature distinguishing it from the more central, conical steepening seen in keratoconus.
Question 3: When reviewing a corneal topography map for a potential orthokeratology (Ortho-K) patient, you notice a 'smiley face' pattern on the differential map after the initial fitting. What does this indicate?
- A perfectly centered treatment zone.
- The lens is riding too low ('frowny face').
- The lens is decentered superiorly ('high rider'). (Correct answer)
- The treatment effect is too strong.
Correct answer: The lens is decentered superiorly ('high rider').
In Ortho-K troubleshooting, a 'smiley face' pattern on the differential topography map indicates that the lens is decentered superiorly, or is a 'high rider'. This is often caused by a lens that is too flat, having an insufficient sagittal height for the cornea.
Question 4: A patient presents with what appears to be irregular astigmatism on corneal topography. Before assuming a corneal dystrophy, what common and treatable condition should be ruled out first?
- Lens subluxation
- Severe ocular surface disease or dry eye (Correct answer)
- Early-stage glaucoma
- Retinal detachment
Correct answer: Severe ocular surface disease or dry eye
Severe ocular surface disease, particularly dry eye, can disrupt the tear film which is the first refractive surface imaged by a topographer. This can induce artifacts that mimic true corneal irregularity. Treating the underlying dry eye can often normalize the topography map, making it a critical first step in the differential diagnosis.
Question 5: During keratometry on a patient with advanced, irregular astigmatism from keratoconus, the mires appear distorted and difficult to align. Which of the following statements is the most accurate description of this situation?
- The keratometer is malfunctioning and requires recalibration.
- This indicates the patient is not a good candidate for any contact lenses.
- The findings are expected; keratometry is unreliable for quantifying highly irregular corneal surfaces. (Correct answer)
- Manual keratometry should be repeated with a larger mire pattern to obtain an accurate reading.
Correct answer: The findings are expected; keratometry is unreliable for quantifying highly irregular corneal surfaces.
Manual keratometers are designed to measure corneas with regular astigmatism where the principal meridians are 90 degrees apart. In cases of advanced irregular astigmatism, such as moderate to severe keratoconus, the corneal surface is too distorted for the mires to be properly aligned. This makes the keratometer an unreliable tool for measurement, and corneal topography is required for an accurate assessment.
Question 6: A patient who has undergone a penetrating keratoplasty (corneal transplant) has significant astigmatism. How is corneal topography most useful in the post-operative management of this patient?
- To determine the final spectacle prescription immediately.
- To decide if the corneal transplant needs to be repeated.
- To guide the selective removal of sutures to reduce astigmatism. (Correct answer)
- To measure the intraocular pressure without touching the graft.
Correct answer: To guide the selective removal of sutures to reduce astigmatism.
After a corneal transplant, sutures hold the donor cornea in place. These sutures can induce astigmatism. Corneal topography is used to identify the steepest meridian of the cornea. By selectively removing sutures along this steep axis, the surgeon or doctor can encourage the cornea to flatten in that meridian, thereby reducing the overall astigmatism.
CPO FREE CPO Keratometry and Corneal Topography Questions and Answers 2
A patient with a history of myopic LASIK now requires cataract surgery.
Standard keratometry is performed.
Which of the following issues is MOST likely to cause an error in the IOL calculation?