Tonometry and Keratometry Flashcards

The Schiotz tonometer measures IOP by the principle of:

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.

What is the standard keratometry mire size that most manual keratometers measure?

Answer: Central 3 mm zone of cornea

Standard manual keratometers sample approximately the central 3 mm zone of the anterior cornea.

Which IOP range is considered statistically normal in most adult populations?

Answer: 10–21 mmHg

The statistically normal IOP range for adults is approximately 10–21 mmHg, though glaucoma can occur at any pressure.

During keratometry, the technician notices the mires cannot be brought into focus at the standard working distance. The MOST likely cause is:

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.

After instilling fluorescein for Goldmann tonometry, the mire semicircles appear very wide and thick. The technician should:

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.

In keratometry, if the axis of astigmatism is at 45° or 135°, this is classified as:

Answer: Oblique astigmatism

Oblique astigmatism occurs when the principal meridians are oriented near 45° or 135° rather than the 90° or 180° axes.

A patient with a thin cornea (480 µm) will have Goldmann applanation IOP readings that are:

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.