What a Tonopen Monitors
A Tonopen (tonometer) measures intraocular pressure (IOP), which is the fluid pressure inside the eye. 1
Primary Function
- The Tonopen is a handheld, portable applanation tonometer that measures IOP by detecting the force required to flatten (applanate) a small area of the cornea 1
- It utilizes a free-floating 1-mm micro-strain gauge transducer surrounded by an outer ring that flattens the adjacent cornea to detect transmitted IOP 1
- The device measures 500 samples per second and averages 8-10 readings for each IOP determination within confidence limits 1
- Topical anesthesia is required for accurate measurement 1
Clinical Applications
- The Tonopen is particularly useful for measuring IOP in children where traditional slit-lamp mounted tonometry may be difficult, as it is a compact, portable instrument 1
- It serves as an alternative to Goldmann applanation tonometry in diseased, abnormal, or surgically altered corneas where standard measurements may be inaccurate 1
- The device is valuable when slit-lamp independent tonometry is needed, such as in bedridden patients or when examining patients who cannot be positioned at a slit lamp 2
Measurement Accuracy
- Tonopen measurements show good agreement with Goldmann applanation tonometry, with 95% limits of agreement of -6 to +8 mmHg 3
- The device demonstrates comparable accuracy to Goldmann tonometry when an average of 3 measurements is used, though individual measurement reproducibility is inferior by a factor of 2 2
- No reduction in IOP occurs after Tonopen measurement (unlike with Goldmann tonometry), which can be advantageous for serial measurements 2
Important Clinical Considerations
- A learning curve of approximately 10 measurements exists even for well-trained ophthalmologists when first using the Tonopen 2
- Consistency in using the same device for serial measurements is crucial, as switching between tonometry methods can introduce variability that masks true IOP changes 4
- There may be variability between different Tonopen units, indicating the importance of quality control during production 2