Article

Tonometry types, tips, and tricks

Learn more about this method of measuring a patient’s intraocular pressure.

As the examination to determine pressure within the eye, according to Ophthalmic Medical Assisting, tonometry’s goal is to get reproducible and accurate intraocular pressure (IOP) readings.

Pressure is an important measurement to track in the management of multiple eye conditions and diseases. Beyond monitoring glaucoma patients, accurate IOPs are important in postsurgical care, injury/trauma cases, and certain pharmaceutical monitoring.

To care for our patients, we need to be familiar with the different tonometry modalities and how to use each type. If you have alternative ways to check IOP, you can choose which is most accurate for each patient and situation.

Here is a summary of the types of tonometers available as well as the tips and tricks to obtain an accurate IOP.

Types of tonometers

Applanation tonometry

  • Goldmann tonometer. Considered the Gold standard according to the AAO, a Goldmann is attached to a slit lamp and provides accurate and reproducible readings. Using a prism, it measures the force needed to flatten a 3.06mm diameter circle of the central cornea.
  • Perkins tonometer. This device is handheld version of the Goldmann that can be used without a slit lamp.

Contact tonometry

  • Tono-Pen (Reichert). This is a hand-held device with a plunger that protrudes from a footplate, when it meets the cornea — and the plunger and footplate share the force of resistance — it gives a reading. The readout will also provide a percentage of accuracy reading. These are portable, easy to use and helpful with patients who have decreased ability to sit behind a slit lamp.
  • Icare ic100 tonometer is a similar handheld device that uses rebound speed of a small magnetized probe. The patient does not need to have anesthetic drops due to the rapid measurement that makes taking pressure less stressful for patients who have aversion to drops, such as pediatric or developmentally disabled patients.
  • Pneumotonometer (Reichert). This uses a fenestrated silicone tip that floats on a stream of air that applies an increasing amount of air pressure until the force is equal to that of the pressure in the anterior chamber. The Pneumotonometer is the preferred method of testing for patients with an artificial cornea (kPro), wherein the probe is placed at the limbus. They also have little effect on the cornea, so useful after corneal surgeries or if needing to avoid an area secondary to a lesion or scar.

Non-contact tonometry

This device is widely known as the “air-puff” tonometer. It is great to use for screenings because it is accurate in normal ranges of pressure, eliminates the need for anesthetic drops, and requires less technical training to learn to operate.

Tips and tricks

Beyond the initial instruction you receive on how to perform tonometry, keep the following tips in mind to ensure an accurate and efficient assessment:

  • Have patients sit comfortably; they should not be straining to reach the head rest. Patient position and tight collars can affect IOP, according to the British Journal of Ophthalmology.
  • Have patients focus on a spot behind the technician. This prevents them from focusing on the tonometer tip, so they are less likely to flinch as the device nears their eye.
  • Ask patients to focus on keeping the opposite eye open wide — if they squeeze, it will be more difficult to get pressure on the primary eye. Ask them to open their eyes like they are “surprised” and to lift their eyebrows. By using their brow muscles to lift the eyebrows, it becomes much more difficult to squeeze their lids shut. If patients cannot open their eyes, you need to hold them open without placing pressure on the globe of the eye. You can achieve this by placing the pressure against the patient’s brow bone and cheekbone.
  • When using a handheld device, make sure the patient is looking ahead. Increase in IOP can be seen if the patient is looking superiorly, and a decrease in IOP can be seen with inferior gaze, according to Ophthalmology.
  • Always start at the same pressure on the dial. Do not set the number of the dial to last known IOP as this can make it easier to “cheat” by thinking you are close enough. After time, you will be able to assess how much you need to turn the dial just by seeing the mires at first contact.
  • With Goldmann tonometry, look at the mires. Are they fat and bright? This can be a sign of too much fluorescein or tearing and can cause a false high reading. Are they too thin and harder to see? This can mean the patient has very dry eyes, thin mires can cause a false low reading. Are the mires distorted? This can be a sign of a corneal lesion or high astigmatism (if this is the case, align the red mark on the tonometer base to the minus cylinder axis on the tonometer tip for the most accurate reading).
  • Check the cornea after taking an IOP. This will give you invaluable information, such as any staining left from taking the measurement, which can help you learn better technique. You may also note changes in corneal staining that may not have been present when you first instilled the drop.
  • Finally, calibrate frequently and keep all instrumentation protected, clean and in proper working order.

Conclusion

Methods to provide the most accurate and obtainable IOP readings for patients depend on factors including disease state, patient mobility and ease of transport. Becoming familiar and comfortable with the options will increase your ability to acquire accurate, consistent IOP readings, even in challenging cases. OP