Tonometry

Excerpt

Tonometry is a standard procedure employed by ophthalmologists to measure intraocular pressure (IOP) using a calibrated instrument. Devices used to measure IOP are based on the assumption that the eye is a closed globe with uniform pressure distributed throughout the anterior chamber and vitreous cavity. The normal range of intraocular pressure is 10 to 21 millimeters of mercury (mmHg), which is based on the average levels of IOP in normal individuals. Glaucoma is a progressive optic neuropathy that leads to irreversible visual field damage, retinal nerve fiber layer thinning, and possible central visual acuity loss at the end stages of the disease. There are various risk factors in the development and progression of glaucoma, the greatest being ocular hypertension (OHT), in which the IOP tends to be higher than 21 mmHg.

Treatment strategies, be it local IOP-lowering drops, laser, or surgery, all aim to lower IOP to a target IOP level to limit, prevent, or slow glaucomatous progression. Multiple methods of tonometry have been proposed since the 19th century to assess intraocular pressure. The true IOP in the eyeball can only be obtained by manometry, in which a probe is inserted into the eye, which is invasive, risky, and not practical in a clinical setting.

Numerous methods have been proposed since the development of the first tonometer in 1863 by von Graefe. These numerous methods are based on different principles of physics to obtain an estimate of the true IOP, which all aim to be accurate, specific, and representative of the actual IOP that is of utmost importance in the management of patients in a routine clinical setting. Instruments can be classified according to principles used in the device to measure IOP, which include applanation tonometry, indentation tonometry, rebound tonometry, Dynamic Contour tonometry, and continuous IOP monitoring.