In music, a tonometer is an instrument used to determine the pitch or vibration rate of tones, such as a tuning fork.
- Tonometry is the measurement of tension or pressure.
- A tonometer is an instrument for measuring tension or pressure.
In ophthalmology, "Tonometry" is the procedure eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. (actually the measuring of pressure in your eyeball). High pressure inside the eye is caused by a disease called "glaucoma," which can damage your vision if it is not treated. It is recommended that all adults over age 40 get checked for glaucoma every 3 to 5 years by having their eye pressures measured. Most tonometers are calibrated to measure pressure in mmHg.
Methods of Ophthalmotonometry
Semicircles seen during Goldmann tonometry through slit lamp
Applanation tonometry measures intraocular pressure either by the force required to flatten a constant area of the cornea (e.g. Goldmann tonometry) or by the area flattened by a constant force. Goldmann tonometry is considered to be the gold standard in tonometry as it is the most widely accepted method of determining intraocular pressure. However Goldmann tonometry is also an inherently imprecise measurement.
In Applanation tonometry, a special calibrated sterile probe attached to a slit lamp biomicroscope is used to flatten part of the cornea. Because the probe makes contact with the cornea, a topical anesthetic, such as oxybuprocaine, tetracaine, alcaine, proxymetacaine or proparacaine is introduced onto the surface of the eye in the form of one or a few eye drops. A yellow fluorescein dye is used in conjunction with a cobalt blue filter to aid the examiner in determining the IOP. Perkins tonometer is a special type of portable applanation tonometer, which allows measurement of IOP in children, patients unable to cooperate for slit lamp exam, and in anesthetised patients.
Pneumotonometry, also known as non-contact tonometry or air-puff tonometry, uses an instrument that senses deflections of the cornea in reaction to a puff of pressurized air. Although not considered to be the most accurate way to measure IOP, it is often used as a simple way to screen for high IOP. It is an easy way to test children. Because non-contact tonometry is accomplished without the instrument contacting the cornea the potential for disease transmission is reduced.
What happens when the test is performed?
- The pressure inside your eye is always measured from the outside. In most cases, if you are at an eye clinic, the pressure can even be measured without anything actually touching your eye. The eye doctor will have you look up close at an instrument that blows a small puff of air into your eye. It then uses a special kind of sensor (like a tiny radar detector) to detect the amount of indentation that the air puff causes on the surface of the eye. This indentation is normal and only lasts for a fraction of a second.
- Sometimes patients need to have their eye pressure measured but they are not in an eye clinic with this type of machine (for example, some patients need to be checked for glaucoma in an emergency room). In this case, the pressure can be measured with an instrument resembling a pen. One end of the instrument is placed on the surface of the eyeball. This feels like having a contact lens put in your eye.
- To help diagnose glaucoma or high eye pressure.
- The test is often part of a routine eye exam.
How it works
The pressure of the aqueous fluid inside the eyes is measured.
A topical anaesthetic is applied to the eyes.
- The tonometer in the form of a contact lens is applied directly to your eye's surface.
- During indentation tonometry, you lie on a table while the tonometer is placed on your eye, and a pressure reading is taken.
- During applanation tonometry, you sit with your head in a brace that has a chin rest and a padded bar for your forehead. The tonometer is then placed against your anaesthetised cornea.
- In non-contact tonometry, the tonometer does not touch your eye. Instead, it blows a puff of pressurised air into your eye and records the amount of pressure.
After the test
- You are free to leave and resume normal activities.
- You should avoid rubbing your eyes for at least 30 minutes after the test.
Factors affecting results
An irregularly shaped cornea requires a special tonometer.
Pressure readings are in millimetres (mm) of mercury (Hg). A normal reading is about 20 mm Hg or lower. Higher readings may indicate either glaucoma or ocular hypertension.
- It's a quick, easy test for glaucoma.
- It's non-invasive.
- Other tests may be necessary to diagnose glaucoma; these include visual field tests and ophathalmoscopy to evaluate the optic nerve.
Other types of testing include:
Electronic indentation tonometry. The Tono-Pen is a portable electronic, digital pen-like instrument that determines IOP by making contact with the cornea, afer use of topical anesthetic eye drops. This is especially useful for very young children, patients unable to reach a slit lamp due to disability, or patients who are uncooperative during applanation tonometry.
Impression tonometry, also known as indentation tonometry, measures the depth of the impression produced by a small plunger carrying a known weight. Intraocular pressure is determined by assessing the movement of the plunger to a calibrated scale.
Transpalpebral tonometry measures intraocular pressure through the eyelid. It is regarded as a simple and safe method of ophthalmotonometry. Transpalpebral tonometry requires no contact with the cornea, therefore sterilization of the device and topical anesthetic drops are not required and there is very little risk of infection.