tonometers

Tonometers are instruments used to measure intraocular pressure (IOP), the fluid pressure inside the eye. IOP is a key factor in glaucoma risk and is typically reported in millimeters of mercury (mmHg).

Tonometers can be broadly categorized into contact and non-contact devices. Contact tonometers require a probe or prism to touch the cornea and are typically used with topical anesthesia; examples include the Goldmann applanation tonometer and other prism-based devices, as well as indentation tonometers like the Schiötz. Non-contact tonometry uses a puff of air to deform the cornea and infer IOP without contact.

The most widely used method is applanation tonometry, which estimates IOP by measuring the force necessary to flatten a mapping area of the cornea. Rebound tonometry uses a lightweight probe that briefly contacts the cornea and measures deceleration; dynamic contour tonometry uses a sensor that matches the corneal contour.

Readings can be affected by corneal thickness, rigidity, curvature, edema, medications, and diurnal variation. Central corneal thickness, for example, can cause over- or underestimation. Accurate interpretation often requires consideration of these factors and sometimes multiple measurements.

In practice, IOP readings guide risk assessment and treatment decisions, but IOP alone does not diagnose glaucoma. Trends over time and other assessments of optic nerve health are important.

Historical note: The applanation principle underpins several devices and Goldmann applanation tonometry is considered the reference standard; newer devices offer non-contact or rebound options with varying ease of use and accuracy.