Tonometry is a common procedure employed by ophthalmologists to measure intraocular pressure (IOP) using a calibrated instrument. Instruments measuring intraocular pressure assume 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. Multiple methods of tonometry currently exist to assess intraocular pressure [1][2].

Tonometry is used to measure intraocular pressure in open-angle glaucoma, acute closed-angle glaucoma, in the setting of ocular trauma without globe rupture, and before and after ophthalmic surgical procedures [1]. The procedure is most commonly employed as part of a clinic visit to an ophthalmologist to screen for and monitor IOP elevation in open-angle glaucoma [2].

These include:

• Ruptured globe: Avoid tonometry in the setting of a ruptured globe or suspicion of one. Additional pressure on the globe with tonometry methods can further damage the globe and cause extrusion of aqueous and vitreous humor. A ruptured globe is often apparent in an acute situation with the examination of the anterior ocular anatomy.
• Infection (relative): When an infection such as keratoconjunctivitis is suspected in one eye, thoroughly disinfect or use a new cover before measuring the contralateral eye and between patients to prevent disease transmission [2].
• Unhealed corneal abrasions or ulcers: Manipulation with tonometry can further aggravate the lesion [3]. Methods such as the Tono-Pen and pneumatonometer provide the ability to measure IOP at the peripheral cornea if the corneal lesion is located centrally.
• Inability to administer topical anesthesia can be a contraindication depending on the method, as most methods of tonometry require topical anesthesia before measurement of IOP. Methods that do not require topical anesthesia include rebound (I-Care) and non-contact "air puff" tonometry.

Methods  

Applanation Tonometry

Goldmann Applanation Tonometer

Goldmann applanation tonometry (GAT) is considered the standard method for measuring intraocular pressure and is most frequently used by ophthalmologists in the clinical setting. It utilizes applanation, which is the measurement of the force required to compress the cornea over a given area. The higher the intraocular pressure, the greater the force needed to compress the cornea [2]. Eye clinicians use Goldmann tonometry as a standard component of a slit-lamp eye exam [1]. To perform GAT, topical anesthetic drops and a fluorescein dye are first applied to the eyes. Filtered cobalt blue light is then used to visualize the fluorescein dye while the tonometer tip compresses the center of the cornea. The blue light and dye highlight the circular border of the tear film created by the applanator tip pressed against the cornea. A prism in the tip splits this circular film of fluorescein into two green semicircles when visualized from the slit lamp oculars. The clinician then alters the tonometer force against the cornea until the two semicircles overlap slightly (Figure 1). This slight overlap of the two semicircles indicates the cornea has been flattened by a calibrated amount to give an accurate intraocular pressure reading, measured in millimeters of mercury, that is read on the dial of the tonometer [3]. Disadvantages of the Goldmann method include a high level of skill to operate, inability to measure in supine patients, need for topical anesthesia, and decreased accuracy on an irregular or scarred cornea [1][2].

Perkins Applanation Tonometer

This hand-held tonometer uses the same mechanism of applanation as the Goldmann. However, due to its portability, it can be useful in settings where a slit lamp exam is not feasible, such as an emergency department or operating room. It can also be used in the upright or supine patient. Disadvantages of the Perkins method include a high level of skill to operate, decrease in stability with a handheld instrument, need for topical anesthesia, and decreased accuracy on an irregular or scarred cornea [1].

Non-contact Tonometers

Non-contact tonometers are also known as “air-puff” tonometers. They use a small puff of air directed at the cornea. The returning air from the surface of the cornea is measured by a membrane that records the force, which is converted to intraocular pressure. Advantages of this method include no requirement for topical anesthesia and its usefulness in screening, especially in children and adults who are unable to tolerate contact methods [2]. However, it is considered a less accurate method than GAT or pneumotonometer, although it has been shown to have comparable readings to GAT in some studies [4]. Non-contact tonometers are available in table-top and portable devices [1].

Ocular-Response Analyzer (ORA)

This is a recently developed type of non-contact tonometer that utilizes a column of air as the applanating force to deform the cornea. Based on the force of airflow and the rate of recovery from deformation, the optical sensors can evaluate the elasticity of the cornea to provide a corrected intraocular pressure and corneal hysteresis value [1][5]. Corneal hysteresis is the ability of the cornea to absorb and dissipate applied forces based on its viscoelastic properties. ORA is designed to improve IOP accuracy as it allows clinicians to account for the variability in corneal biomechanical properties seen among patients [5]. Corneal hysteresis has been demonstrated to contribute to glaucoma progression risk with lower values imparting a greater risk of glaucoma progression [6]. ORA cannot be used on supine patients.

Indentation Tonometry

Tono-Pen Indentation/Applanation Tonometer

The Tono-pen is an electronic hand-held device that uses a small plunger to record the force needed to applanate the cornea. The Tono-pen averages multiple readings of this small force that is converted to intraocular pressure [1]. It requires daily recalibration, topical anesthesia, and uses disposable covers. It can prove useful in portable screenings, emergency rooms, or operating rooms to measure intraocular pressure. Advantages of this method include the ability measure over soft contact lens, on an irregular corneal surface, averaging of multiple readings, and potential to measure at peripheral cornea if a central corneal scar or ulcer exists. It can also measure intraocular pressure independent of patient position [1].

Pneumatonometer

A pneumatonometer uses a stream of air to indent the cornea with a 5 mm diameter silicone tip. The force of air that indents the cornea is recorded and converted to intraocular pressure. This method correlates well with Goldmann tonometry in normal pressure ranges [1]. Similar to the Tono-pen, it has the advantages of measuring irregular cornea surfaces, over soft contact lens, at the peripheral cornea, and accuracy is independent of patient position, although it requires topical anesthesia [1]. The pneumatonometer is accessible in a hand-held probe attached to a table-mounted device. The device is also used for pneumatonography, which uses changes in IOP to measure the outflow resistance of aqueous humor from the anterior chamber. If increased outflow resistance is found, it can cause increased IOP and contribute to glaucoma. Algorithms are used to correlate the pneumatonography values to a Shiotz-based tonograph, but with shorter potential measurement times.

Schiotz Tonometer

This portable tonometer employs a rarely used, older technique. It consists of a weighted plunger attached to a footplate positioned on the cornea. Weights are stacked onto the probe to cause depression of the cornea.  The number of weights stacked onto the probe correlates to a calibrated intraocular pressure, using a conversion chart provided with the device [1]. The patient must be supine during measurement, and topical corneal anesthetic is required [2].

Rebound Tonometry

I-Care tonometer

This is a portable and simple-to-use hand-held device that uses a small probe that bounces of the cornea in the horizontal plane. The deceleration of the probe produces a level of voltage that is converted to intraocular pressure. The faster the rate of deceleration against the cornea, the higher the pressure. The slower the rate of deceleration, the lower the pressure [1]. Advantages of this method include no requirement for topical anesthetics and its usefulness in children and dementia patients who are not tolerant of a slit-lamp exam or more involved contact methods [5][6]. It cannot be used on supine patients, and accuracy decreases in the setting of corneal edema, making it less useful with acute IOP elevation associated with acute angle closure with corneal edema.

Dynamic Contour Tonometry (DCT)

Pascale DCT

This device utilizes a contour-matched, piezoelectric sensor to measure minuscule dynamic pulsations in intraocular pressure at the cornea [1]. This method allows measurement of IOP without deforming the cornea and therefore its readings are independent of corneal thickness, unlike other methods [5]. It is considered a more accurate method on regularly shaped corneas, but it’s more difficult to use properly. It also requires topical anesthesia and is less accurate on irregular corneas [6].