What is Tonometry?
Tonometry or glaucoma test is the name of the test or procedure used to measure the intraocular pressure or pressure inside the eye. This is used to treat, monitor, and/or diagnose glaucoma which is one of the leading eye disorders that can cause permanent loss of vision. Most instruments measure the pressure in millimeters of mercury or mmHg. There are many different methods and instruments used to measure the intraocular pressure. Goldmann tonometry is the gold standard and most widely used by eye doctors to measure the pressure. What is an eye doctored called who can test for glaucoma? (Optometrist and Ophthalmologist). Many times the eye doctor will use an ophthalmoscope to look inside the eye to check for glaucoma damage to the optic nerve.
Types of Tonometers
- Applanation tonometers
- Pneumotonometer or air puff
- Non-transducer indentation tonometer
- Transducer-based indentation tonometer
- Rebound tonometer
- Dynamic contour tonometer
- Transpalpebral tonometer
- Applanation Tonometer
The Goldmann Applanation tonometer is a slit lamp mounted instrument. A numbing eye drop is instilled in the eye. The tonometer then pressed against the cornea. A prism in the tonometer is used to calibrate the pressure in the eye the cornea is flattened to the correct amount. Another example of a Applanation tonometer is the Perkins tonometer which is useful in children.
Pneumotonometer or Air Puff
A column of air is pulsed against the eye and the amount of force to flatten the cornea is measured. The instrument is made by Reichert which was American Optical in the past. Zeiss made an air puff tonometer as well.
Non-Transducer Indentation Tonometer
The Schiotz tonometer is the most common tonometer of this type. The tonometer has a curved footplate with a plunger through the footplate. Weights are placed on the tonometer and the amount of weight to indent the cornea to the proper amount is then converted to mmHg.
Transducer Indentation Tonometer
Examples of transducer indentation tonometers are Tono-pen, Accupen, and MackayMarg. The transducer has an Applanation surface and a central plunger. The tonometer contacts the eye and the plunger measures the resistance of the cornea. These instruments take multiple measurements and calculate an average of the readings.
Rebound tonometers work by a very small plastic covered fine wire quickly bounces against the cornea and recoils back into the instrument. The amount of deceleration pressure is then measured. Icare makes a rebound tonometer.
Dynamic Contour Tonometer
A dynamic contour tonometer uses a piezoelectric sensor which is embedded into the tip of the tonometer. The Pascal Dynamic Tonometer measures the pulsatile fluctuations in the eye pressure. It is less influenced by the corneal shape, rigidity of the cornea, and corneal thickness.
Transpalpebral tonometry measures the eye pressure through the eyelid. The Diaton tonometer and the Proview Eye Pressure Monitor measure the pressure through the tarsal plate. No topical anesthesia is needed but the reliability can come into question. Another form of transpalpebral measurement is palpation of the eye. The eye doctor gently applies pressure on the eye through the eyelids with their index and middle finger.