COT Tonometry and Keratometry 4 — Questions and Answers
Question 1: The Schiotz tonometer measures IOP by the principle of:
- Applanation of a fixed area
- Indentation depth of the cornea under a known weight (Correct answer)
- Air-puff deflection of the corneal apex
- Optical coherence of the tear meniscus
Correct answer: Indentation depth of the cornea under a known weight
Schiotz tonometry measures how deeply a known weight indents the cornea; a softer eye allows greater indentation.
Question 2: What is the standard keratometry mire size that most manual keratometers measure?
- Central 1 mm of cornea
- Central 3 mm zone of cornea (Correct answer)
- Central 6 mm zone of cornea
- Central 9 mm zone of cornea
Correct answer: Central 3 mm zone of cornea
Standard manual keratometers sample approximately the central 3 mm zone of the anterior cornea.
Question 3: Which IOP range is considered statistically normal in most adult populations?
- 5–10 mmHg
- 10–21 mmHg (Correct answer)
- 22–30 mmHg
- 30–40 mmHg
Correct answer: 10–21 mmHg
The statistically normal IOP range for adults is approximately 10–21 mmHg, though glaucoma can occur at any pressure.
Question 4: During keratometry, the technician notices the mires cannot be brought into focus at the standard working distance. The MOST likely cause is:
- The patient has high myopia
- The corneal curvature is outside the instrument's measurable range (Correct answer)
- The room is too brightly lit
- The patient has a large pupil
Correct answer: The corneal curvature is outside the instrument's measurable range
Very flat (post-LASIK) or very steep (keratoconus) corneas may fall outside the diopter range measurable by a standard manual keratometer.
Question 5: After instilling fluorescein for Goldmann tonometry, the mire semicircles appear very wide and thick. The technician should:
- Increase the force dial immediately
- Ask the patient to blink to thin the tear film before retaking the reading (Correct answer)
- Switch to non-contact tonometry
- Record the reading as is since mire width does not matter
Correct answer: Ask the patient to blink to thin the tear film before retaking the reading
Excess fluorescein causes wide mires that make the endpoint harder to judge accurately; having the patient blink redistributes and thins the tear film.
Question 6: In keratometry, if the axis of astigmatism is at 45° or 135°, this is classified as:
- With-the-rule astigmatism
- Against-the-rule astigmatism
- Oblique astigmatism (Correct answer)
- Irregular astigmatism
Correct answer: Oblique astigmatism
Oblique astigmatism occurs when the principal meridians are oriented near 45° or 135° rather than the 90° or 180° axes.
Question 7: A patient with a thin cornea (480 µm) will have Goldmann applanation IOP readings that are:
- Falsely elevated
- Falsely depressed (Correct answer)
- Unaffected by thickness
- Elevated only if they have glaucoma
Correct answer: Falsely depressed
Thin corneas offer less resistance to applanation, so less force is needed to flatten them, resulting in a falsely low IOP reading.
COT Tonometry and Keratometry 4
The Schiotz tonometer measures IOP by the principle of: