CPO FREE CPO Tonometry Principles and Procedures Questions and Answers 2 — Questions and Answers
Question 1: A patient who recently underwent a vitrectomy with an intravitreal gas bubble is scheduled for tonometry. Which of the following is the most critical consideration when measuring their IOP?
- The patient's head must be positioned upright to ensure accuracy.
- The presence of the gas bubble will artificially lower the IOP reading, requiring a correction factor.
- The IOP can fluctuate dramatically with changes in atmospheric pressure, such as changes in altitude. (Correct answer)
- Only a non-contact tonometer can be used to avoid disrupting the gas bubble.
Correct answer: The IOP can fluctuate dramatically with changes in atmospheric pressure, such as changes in altitude.
Intravitreal gas bubbles expand or contract with changes in atmospheric pressure. An ascent in altitude causes the bubble to expand, which can lead to a significant and dangerous increase in IOP. A descent can cause the bubble to contract. This volatility is a primary concern for patients with gas bubbles.
Question 2: When performing Goldmann applanation tonometry on a patient with significant corneal scarring, the paraoptometric is most likely to encounter which issue?
- A falsely low and inconsistent IOP reading due to the soft nature of scar tissue.
- Difficulty achieving clear, well-defined semicircular mires for an accurate measurement. (Correct answer)
- An increased risk of corneal abrasion, even with proper technique.
- Rapid dissipation of the fluorescein dye from the tear film.
Correct answer: Difficulty achieving clear, well-defined semicircular mires for an accurate measurement.
Corneal scarring creates an irregular surface, which distorts the applanated area and the resulting fluorescein mires. This makes it difficult to properly align the inner edges of the semicircles, leading to unreliable or unobtainable readings with Goldmann tonometry.
Question 3: A paraoptometric performs a calibration check on a Goldmann tonometer. At the '2' setting (20 mmHg), the feeler arm only begins to move when the dial is turned past 2.4. When turned back, it only moves again when the dial is below 1.6. What is the most likely immediate cause of this inaccuracy?
- A damaged or warped tonometer prism.
- An internal mechanism that requires lubrication. (Correct answer)
- Incorrect positioning of the calibration weight bar.
- A weak battery in the slit lamp illumination system.
Correct answer: An internal mechanism that requires lubrication.
This 'sticking' behavior, where the feeler arm does not move freely at the correct calibration points, is a classic sign that the internal mechanical components of the tonometer are not moving smoothly. The most common reason for this is a lack of lubrication, which can often be resolved by cleaning and lubricating the instrument.
Question 4: Which of the following tonometry methods is theoretically LEAST affected by corneal biomechanical properties like thickness, rigidity, and curvature?
- Goldmann Applanation Tonometry (GAT)
- Non-Contact (Air-Puff) Tonometry (NCT)
- Dynamic Contour Tonometry (DCT) (Correct answer)
- Tono-Pen
Correct answer: Dynamic Contour Tonometry (DCT)
Dynamic Contour Tonometry (DCT) is specifically designed to measure intraocular pressure largely independently of corneal properties. Unlike applanation tonometers that flatten the cornea, DCT uses a sensor tip with a contour-matched shape that rests on the cornea, minimizing its deformation and the influence of its biomechanical properties on the reading.
Question 5: A patient with nystagmus requires an IOP check. Which type of tonometer would be most challenging to use effectively on this patient?
- Rebound tonometer (e.g., Icare)
- Tono-Pen
- Non-contact tonometer (Correct answer)
- Pneumatonometer
Correct answer: Non-contact tonometer
Non-contact tonometry (NCT) requires the patient to remain very still and fixated on a target for the instrument to align properly and deliver the air puff to the central cornea. The constant, involuntary eye movements of nystagmus make achieving this precise alignment extremely difficult, often resulting in error readings or the inability to obtain a measurement at all.
Question 6: When measuring IOP on a patient who has undergone a penetrating keratoplasty (corneal transplant), what is a common finding when comparing Tono-Pen to Goldmann Applanation Tonometry (GAT)?
- Tono-Pen readings are generally identical to GAT readings.
- Tono-Pen tends to yield significantly lower IOP readings than GAT.
- Tono-Pen tends to yield significantly higher IOP readings than GAT, especially at lower pressures. (Correct answer)
- Neither instrument can be used on a post-keratoplasty eye.
Correct answer: Tono-Pen tends to yield significantly higher IOP readings than GAT, especially at lower pressures.
Studies comparing tonometry methods on post-keratoplasty eyes have shown that the Tono-Pen often overestimates IOP when compared to GAT. This discrepancy can be particularly pronounced in the lower IOP ranges. The altered corneal structure, including scarring and irregular curvature, affects the two instruments differently.
CPO FREE CPO Tonometry Principles and Procedures Questions and Answers 2
A patient who recently underwent a vitrectomy with an intravitreal gas bubble is scheduled for tonometry.
Which of the following is the most critical consideration when measuring their IOP?