Understanding and caring for a Schiotz tonometer

A Schiotz tonometer is an instrument for measuring the intraocular pressure (IOP). Although the Schiotz tonometer does not make as precise measurements as other types of tonometers, it is inexpensive, simple to use, durable, requires little maintenance, does not have electronics, does not require batteries, and can be stored for years between uses. These qualities make it well suited for screening and remote or mobile clinics. The Schiotz tonometer consists of a hollow barrel with a concave footplate and a holder (Figure 1). A free-fl oating, rod-like plunger with a 5.5 gram weight attached fi ts inside the barrel. When held vertically on top of the eye, the plunger will move downwards by gravity and indent the cornea. This very small up-and-down movement is magnifi ed by a lever arm to move a needle that gives a reading on a horizontal scale numbered arbitrarily 0–20. A fi rmer eye, due to higher IOP, will result in a lower indentation and a lower reading on the scale. Since the Schiotz tonometer does not measure pressure directly, a conversion table, supplied with the instrument, is used to translate scale readings into estimates of IOP in mmHg. To account for the range of pressure, other weights (typically 7.5 g and 10 g) are supplied that can be added to the plunger.