Tonometry is the measurement of the intraocular pressure (IOP; the pressure within the eye). Most of the instruments used in tonometry rely on deforming an area of the cornea with a small amount of force that is used to calculate the IOP. Tonometers can be divided into types that applanate, or flatten, the cornea and those that indent it. The accuracy of either type of tonometer assumes that all eyes have a similar ocular rigidity, corneal thickness, and ocular blood flow.

Applanation tonometry is based on the Imbert-Fick law that the IOP is equal to the amount of force needed to flatten a spherical surface divided by the applanated area. Goldmann applanation, the gold standard and the most commonly used form of tonometry, was introduced in 1954. This device can be used only in patients seated at a slit lamp. The cornea is viewed through a prismatic doubling device in the center of a cone-shaped head that is obliquely illuminated with a cobalt blue light (Fig. 3-1). Although the patient’s head is held steady, the applanation head is gently placed against a fluorescein-stained, anesthetized cornea (Fig. 3-2). The examiner sees a split image of the tear film meniscus around the tonometer head. These fluorescein rings just overlap when the pressure at the head equals the IOP. The graduated dial on the side measures the force in grams, which is converted into millimeters of mercury by multiplying by 10.

With a circular applanation surface 3.06 mm in diameter, the surface tension of the tear film counteracts the force needed to overcome the rigidity of the cornea, allowing the amount of force applied to equal the IOP. The tip flattens the cornea less than 0.2 mm, displaces 0.5 µL of aqueous, increases the IOP by 3%, and provides a reliable measurement of ±0.5 mm Hg. In corneas with high astigmatism (greater than 3 diopters), the flattest corneal meridian should be placed at 45 degrees to the axis of the cone. This can be done simply by placing the red line on the tonometer tip at the same axis of the minus (or flattest) cylinder of the eye.

Introduced in 1905, the Schiötz tonometer is the classic indentation tonometer and requires the patient to be supine (Fig. 3-3). As opposed to applanation tonometry, the amount of indentation of the cornea by the Schiötz tonometer is proportionate to the IOP. This deformation, however, creates an unpredictable and relatively large intraocular volume displacement. The 16.5-g Schiötz tonometer has a base weight of 5.5 g that is attached to the plunger. This weight may be increased to 7.5, 10, or 15 g for higher ocular pressures. The calibrated footplate of the tonometer is placed gently on the anesthetized cornea, and the free vertical movement of the attached plunger determines the scale reading. To estimate IOP, conversion tables based on empirical data from both human cadaver eyes and in vivo studies are available. The tables assume a standard ocular rigidity such that in eyes with altered scleral rigidity (e.g., after retinal detachment surgery), the Schiötz measurement may not be accurate.