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Tonometry
Last reviewed: 04.07.2025
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Tonometry is the measurement of intraocular pressure (the pressure inside the eye). The instruments used in tonometry deform the surface of the cornea with a small force, which is used to calculate the intraocular pressure.
Tonometers are either applanation or impression. Each type of tonometer operates on the assumption that all eyes have the same rigidity, corneal thickness, and blood flow.
Applanation tonometer
Applanation tonometry, introduced in 1954, is based on the Imbert-Fick law, which states that the intraocular pressure is equal to the force required to flatten a spherical surface by the contact surface of the tonometer. Goldmann applanation tonometry is the "gold standard" and the most commonly used tonometry method. The method is used only with the patient at a slit lamp. The cornea is examined through a double prismatic lens located at the top of the conical tip of the device, illuminated obliquely with cobalt blue light. With the patient's head still, the flattened tip is carefully placed against the fluorescein-stained, anesthetized cornea. The physician sees in the slit lamp the meniscus of the tear film around the tonometer tip. These fluorescent rings coincide when the tip pressure becomes equal to the intraocular pressure. The graduated scale on the device measures force in grams and converts them into millimeters of mercury by multiplying by ten.
With a flattening circle diameter of 3.06 mm, the surface tension of the tear film balances the force required to overcome its rigidity. Thus, the applied force corresponds to the intraocular pressure. The tip flattens the corneal surface by less than 0.2 mm, displaces 0.5 μl of moisture, increases the intraocular pressure by 3% and provides a reliable measurement result of ±0.5 mm Hg. With high astigmatism (more than 3 D), the flattest corneal meridian should be shifted by 45° relative to the cone axis. This can be easily achieved by aligning the red line on the top of the tonometer with the same axis of the minus cylinder of the eye.
Schiotz tonometer
The Schiotz tonometer, used since 1905, is a classic impression tonometer. For tonometry, the patient must lie on his back. Unlike the applanation tonometer, the degree of corneal indentation with the Schiotz tonometer is proportional to the intraocular pressure. This deformation creates unpredictable and relatively large shifts in intraocular volume. The Schiotz tonometer weighs 16.5 g, its main weight is attached to the plunger and weighs 5.5 g. At high intraocular pressure figures, this weight can be increased to 7.5; 10 or 15 g. The calibrated base of the tonometer is carefully placed on the cornea after its preliminary anaesthesia, and free downward vertical movement of the attached plunger determines the reading on the scale. Conversion tables based on empirical data from cadaveric eye studies and in vivo studies are used to assess intraocular pressure. These tables assume standard eye rigidity, so if the scleral rigidity is impaired (for example, after surgery for retinal detachment), the Schiotz tonometer may show distorted results.
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Perkins tonometer
This hand-held Goldmann-type applanation tonometer is particularly useful for examining infants and children. It has a battery-powered light source and can be used with the patient either upright or supine. The applanation force is varied by rotating a calibrated dial with the same measuring device as the Goldmann tonometer.
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Tono-pen
The manual tone-pen tonometer (Mentor Ophthalmix, Santa Barbara) can measure intraocular pressure in both a seated and supine patient. The method is especially suitable for children and patients with injured or edematous corneas, when it is impossible to use a slit lamp. In Mackay-Marg type tonometers, to which the tone-pen belongs, the effects of corneal rigidity are transmitted to the surrounding sleeve, so that the central plate measures only intraocular pressure. A microprocessor in the tone-pen, connected to a strain gauge, measures the force with which the central plate, 1.02 mm in diameter, flattens the corneal surface. With 4-10 measurements of the same eye, a final result is obtained with a variability between the lowest and highest acceptable results of less than 5, 10, 20% or more.
Pneumotonometer
The pneumotonometer is a hand-held device that can be used when a slit lamp is not available. The patient may be sitting or lying on his back during the examination, and the corneal surface of the eye being examined may be uneven. Like the Tono-Pen, this Mackay-Marg type tonometer has a sensitive surface in the center, and a surrounding roller transmits the force necessary to overcome corneal rigidity.
The central sensitive area is a silastic diaphragm covering the air plunger. When this elastic membrane is placed on the cornea, it obstructs the gas from escaping from the plunger, causing the air pressure to increase until it equals the intraocular pressure. An electronic sensor measures the air pressure in the chamber.