What is a Tonometer?

A tonometer is an instrument that is used to check eye pressure. The original type of tonometer is still being used most often today; however, since some patients are examined in an emergency room or in a hospital bed, portable tonometers have been developed. Other tonometer technologies have been created as well to determine if there is a way to improve accuracy of the pressure measurement.

Use

• At eye exams, the eye doctor checks eye pressure using a tonometer. This process is especially important for patients who already have a diagnosis of glaucoma. The eye pressure is measured by the resistance to indentation of the cornea by the tonometer. Some tonometers touch the eye, while others use air. When using a tonometer, doctors must take other aspects into account such as astigmatism (shape of the cornea) and thickness of the cornea when reading the measurement. These factors have spurred development of new kinds of tonometers.

Applantation

• Applanation tonometry is considered to be the gold standard of tonometry and the Goldmann applanation tonometer is the one most commonly used by eye doctors. To perform this procedure, the eye doctor will place an anesthetic eye drop containing the dye fluoroscein into the patient's eye. The patient will then sit at a slit lamp and the doctor will gently place the Goldmann tonometer against the numbed cornea to measure the pressure. The applanation piece needs to be cleaned after each examination. This form of tonometry is considered by many to be the most accurate way to measure eye pressure.

Non-contact tonometers

• Some eye doctors use non-contact tonometers which means that the instrument does not touch the cornea and eliminates the need for anesthetic drops. Non-contact tonometers work by ejecting a puff of air against the cornea. This method may not always be very accurate. For example, a patient may squint which may increase the pressure reading. A newer type of non-contact tonometer is the Ocular Response Analyzer, which works a little differently. It measures the displacement of the cornea twice as it moves away with the puff of air and as it reshapes itself. The accuracy of the Ocular Response Analyzer is equivalent to Goldmann applanation.

Portable tonometers

• While a person with an eye injury is being examined in the emergency room, it may be difficult to sit the injured patient at a slit lamp to measure the eye pressure. There are several tonometers that are portable and smaller for use. One is the tonopen which looks like a pen and is gently tapped against the anesthetized cornea to obtain a pressure reading. The patient can be lying down for this test without much alteration to the eye pressure. Another portable tonometer is the pneumotonometer contact device but this is significantly larger and has been replaced by the tonopen in most cases.

Dynamic Contour Tonometer

• Another tonometer received approval from the FDA in 2003. The Dynamic Contour Tonometer (DCT) is attached to the slit lamp like the Goldmann tonometer but it works in a different way. The DCT is placed against the numbed cornea as well but it uses a different scinetific principle than the Goldmann causing less distortion of the cornea. A study by Kniestedt, Nee and Stamper demonstrated the accuracy of the readings and further studies suggested that DCT may be used in different types of corneas.

Other tonometers

• Newer innovations in tonometers are underway. One is a tonometer that can measure pressure through the eyelid. This type of tonometer would mean that there is would be no need for anesthetic drops or sterilization and could make it easier for physicians to use and more pleasant for patients. Some of these newer types of tonometers are very expensive right now; it is unknown at this time if these newer forms will take the place of the Goldmann.