A tonometer is a medical device used to measure the intraocular pressure (IOP) within the eye. The intraocular pressure refers to the fluid pressure present inside the eyeball. Measuring the IOP is important because elevated pressure can be a sign of certain eye conditions, such as glaucoma.
There are different types of tonometers used in ophthalmology, including:
Applanation Tonometers: These tonometers directly measure the force required to flatten a specific area of the cornea (the clear front surface of the eye) using a small probe. The most common type of applanation tonometer is the Goldmann applanation tonometer.
Non-contact Tonometers: Also known as air-puff tonometers, these devices measure intraocular pressure without touching the eye. They work by emitting a puff of air onto the cornea and then measuring the response of the eye to the air puff.
Tono-Pen: The Tono-Pen tonometer is a handheld device that uses a small probe to gently touch the cornea and measure the IOP. It is commonly used in situations where portability and ease of use are important.
The measurement of intraocular pressure with a tonometer is a routine part of an eye examination, especially for individuals at risk of glaucoma or with known eye conditions. It is a relatively quick and pain
less procedure that helps eye care professionals assess and monitor eye health.
(i) Goldmann & Perkins applanation tonometry:
The Goldmann applanation tonometer measures the pressure inside the eye by flattening a small area of the cornea. It uses a special dye to highlight the tear film, and the image of the tear meniscus is divided into two parts. By aligning these parts, the intraocular pressure can be measured. The force required to flatten the cornea is converted to millimeters of mercury (mmHg) to determine the intraocular pressure.
(ii) Non-contact tonometry:
(a) Air puff tonometer: This type of tonometer uses a puff of air to measure intraocular pressure. The force of the air puff is increased until the cornea flattens, and the force is then translated into mmHg. Air puff tonometry may sometimes give inaccurate readings at high or low pressure levels, so multiple measurements are usually taken to establish the mean intraocular pressure.
(b) Ocular response analyzer: The ocular response analyzer also uses a column of air to measure intraocular pressure. It records the moment of corneal flattening and measures corneal elasticity. Mathematical calculations can be used to adjust the measured pressure based on corneal thickness. This type of tonometry is less dependent on corneal thickness compared to other methods.
(b) Indentation tonometry: Indentation tonometry measures intraocular pressure by indenting or sinking a weight or plunger into the eye. There are different types of indentation tonometry:
(i) Schiotz tonometer: A weighted plunger is placed on the cornea, and the depth of indentation is measured. The deeper the plunger sinks, the lower the intraocular pressure. A conversion table is used to determine the pressure in mmHg based on the scale reading.
(ii) Pneumotonometer: This tonometer combines applanation and indentation methods. It uses a silicone tip to indent the cornea, and the pressure exerted on the tip is equal to the intraocular pressure when the cornea is flat. The device measures this pressure and displays it in mmHg.
(iii) Tono-Pen: The Tono-Pen is a portable device that uses both applanation and indentation processes. It has a plunger that measures the resistance of the cornea and intraocular pressure. The point of corneal flattening is electronically recorded, and multiple readings are averaged to determine the intraocular pressure.
(c) Rebound tonometry: The rebound tonometer measures intraocular pressure by bouncing a small plastic ball against the cornea. The rate of deceleration of the ball determines the pressure. This method does not require anesthesia and is influenced by corneal thickness and other corneal characteristics.
(d) Pascal Dynamic Contour Tonometer: This tonometer measures dynamic pulsatile changes in intraocular pressure using a sensor. It is less affected by corneal thickness and curvature compared to the Goldmann tonometer. Disposable covers are used for each measurement, and the device provides a quality measurement indicator.
