Continuing Education Activity

Tonometry is a common procedure employed by healthcare professionals to measure intraocular pressure (IOP) using a calibrated instrument. IOP is important in diagnosing, screening, and managing ocular hypertension and glaucoma. 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. This activity discusses the numerous past and present methods of tonometry available to assess intraocular pressure and identify potential diseases causing abnormalities in eye pressure.

Objectives:

• Identify the indications and contraindications of different types of tonometry.
• Describe the equipment, personnel, preparation, and technique in regard to performing tonometry.
• Summarize the appropriate evaluation of the potential complications of tonometry.
• Explain interprofessional team strategies for improving care coordination and communication to advance the proper use of tonometry and optimize patient outcomes.

Introduction

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.[1] 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,[2][3] the greatest being ocular hypertension (OHT), in which the IOP tends to be higher than 21 mmHg.[3] 

Treatment strategies, be it local IOP-lowering drops, laser, or surgery,[4] all aim to lower IOP to a target IOP level to limit, prevent, or slow glaucomatous progression.[5][6] Multiple methods of tonometry have been proposed since the 19th century to assess intraocular pressure.[7] 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.[8] 

Numerous methods have been proposed since the development of the first tonometer in 1863 by von Graefe.[9] 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.[10]

Anatomy and Physiology

IOP is determined by a delicate equilibrium between aqueous humor production and outflow. Elevated IOP can arise from increased production and/or decreased outflow mechanisms. The most common cause of elevated IOP in glaucoma is reduced aqueous humor outflow. Many forms of glaucoma are defined by elevated IOP, including primary open-angle glaucoma (the most frequent), closed-angle glaucoma, and secondary glaucoma (i.e., steroid-induced, pseudoexfoliation, traumatic, pigment dispersion glaucoma, etc.), and congenital glaucoma.[11][12][13][14][15][14][13][12][11][16][17] 

Most forms of open-angle glaucoma are silent sight-threatening diseases considering that central visual acuity tends not to be influenced, there is no associated pain or irritation, and peripheral visual field loss normally occurs gradually over numerous years in eyes with high IOP. Asymptomatic chronic elevated IOP leads to irreversible glaucomatous optic neuropathy, retinal nerve fiber layer thinning, and visual field damage. Tonometry is of great importance in the screening, diagnosis, and management of all patients, especially those with risk factors for developing glaucoma.[10] Tonometers must be properly calibrated, easy to use in routine clinics, and provide precise, accurate measurements with low inter-and intra- variability, preferably not influenced by external and ocular factors.[18]

Indications

Tonometry is utilized to measure IOP, which is essential in all forms of glaucoma. Tonometers are most commonly employed as part of a routine clinic visit by the clinician to screen for and monitor IOP elevation in glaucoma.[19] It is also used in the setting of ocular trauma without globe rupture, in cases of ocular inflammation and infection, and before and after ophthalmic surgical procedures.[7] It is crucial in managing patients at risk of developing the disease or those in therapy for glaucoma to prevent or slow down glaucomatous morphologic and functional defect progression. IOP is of clinical importance in numerous clinical situations, which include:

• Screening for OHT and glaucoma in all adult patients, especially those with known risk factors.[20][21]
• Diagnosis and management of patients with OHT or in therapy for glaucoma.[22][23]
• Differential diagnosis and management of various ophthalmologic conditions can give rise to high levels of IOP, including pigment dispersion syndrome,[16] ocular infections, [24] pseudoexfoliation syndrome,[13] eye inflammation, Posner-Schlossman syndrome,[25] use of steroids in cortisone responders,[13] uveitis,[26] surgery,[27][28] trauma,[15] etc.

Contraindications

Information regarding IOP is of clinical importance during the ophthalmologic examination. There are situations, however, in which tonometry is challenging to obtain with certain instruments or can worsen the clinical conditions of the patient in specific conditions, which include:

Ruptured globe: Contact tonometry should be avoided or limited in the setting of a ruptured globe or suspicion of one.[15] Additional pressure on the globe with tonometry methods can further damage the globe and cause extrusion of the aqueous and vitreous humor. A ruptured globe is often apparent in an acute situation with the examination of the anterior ocular anatomy.

Infection: When an infection such as keratoconjunctivitis is suspected in one eye, the clinician should thoroughly disinfect or use a new cover before measuring the contralateral eye. This is also important when using the same tonometer between patients to prevent disease transmission.[19]

Unhealed corneal abrasions or ulcers, corneal scars, and elevated eyes with astigmatism: Manipulation with tonometry can further aggravate active corneal lesions.[29] The precision and accuracy of several tonometers that have direct contact with the cornea can be limited in the presence of scarring or high astigmatism; thus, proper instrument use in these select patients is of clinical importance in measuring IOP.[30]

Inability to administer or intolerance/allergies to topical anesthesia and/or fluorescein: This contraindication is applicable to methods of tonometry requiring topical anesthesia before measurement of IOP.[31] Alternative methods that do not require topical anesthesia can be considered in these patients, which include rebound (iCare) and non-contact "air puff" tonometry.[10]

Difficulties or inability to use the tonometer: Certain tonometry, like the gold standard Goldmann applanation, can only be used with a collaborative individual in an upright position. Other portable manual instruments, like iCare rebound tonometers, can be used in a supine position and require little patient collaboration, thus can be considered in bedridden and non-collaborative individuals and children.[10]

Equipment

Methods  

Applanation Tonometry

Goldmann Applanation Tonometer (GAT)

In 1948, Hans Goldmann proposed GAT to measure IOP.[32] To this date, GAT remains the gold standard method for measuring IOP and is most frequently used by ophthalmologists in a routine clinical setting.[33] It utilizes applanation, which measures the force required to compress the cornea over a given area. Applanation tonometry is based on the Imbert-Fick law from the 1800s, which states that the pressure inside the eye (P) can be approximated by the force (F) needed to flatten a fixed area of the cornea divided by the surface area (S) of the flattened region of the surface.[34][35] The higher the intraocular pressure, the greater the force needed to compress the cornea.[19] 

Eye clinicians use GAT as a standard component of a slit-lamp (See slit-lamp and GAT tonometer images) eye exam.[7] 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 with a diameter of 3.06 mm (See GAT tip images). The blue light and dye highlight the circular border of the tear film created by the applanator tip pressed against the cornea (See GAT tip images). A prism in the tip splits this circular film of fluorescein into two green semicircles when visualized from the slit lamp oculars (See images with arcs). The clinician then alters the tonometer force against the cornea by turning the dial on the side of the instrument (See tonometer with dial images) until the two semicircles overlap slightly (See images with arcs). This slight overlap of the two semicircles indicates that the cornea has been flattened by a calibrated amount to give an accurate IOP measurement in mmHg, which is read on the dial (see tonometer with dial images) of the tonometer.[29]

Several factors can influence the accuracy and precision of IOP measurements, which include central corneal thickness (CCT), elevated astigmatism, ocular rigidity, corneal hysteresis, tear film, corneal edema, the amount of fluorescein, previous refractive surgery, etc.[36][37][32][34][32][37][36] Although GAT is considered the gold standard method to measure IOP, several limits have enticed the proposal of new tonometers in the past 70 years.[7] The disadvantages of the Goldmann method include a high level of skill to operate, inability to measure in supine and non-collaborative patients, need for topical anesthesia and fluorescein, and decreased accuracy on an irregular or scarred cornea.[7][19]

Perkins and Draeger Applanation Tonometer

The Perkins and Drager handheld tonometers use the same mechanism of applanation as the Goldmann.[7] Considering the portability of these devices, these tonometers can be useful in settings where a slit lamp exam is not feasible, such as an emergency department or operating room, and can also be used in upright or supine positions. The tonometers have a built-in forehead rest, illuminating blue light source, a prism holder, a thumb-wheel dial with a scale to measure IOP, and a power source. The limits of these tonometers are similar to those in GAT, considering that these tonometers use the applanation principle to measure IOP. Other disadvantages of these portable devices include a high level of skill to operate and a decrease in stability with the handheld versions.[38]

Non-contact Tonometers

Non-contact tonometers (NCT),[7] which first came available in the 1970s, are also known as "air-puff" tonometers. Several desktop and portable handheld models are available from various manufacturers.[39][40] Numerous modern desktop instruments used as pachymeters and refractometers also have a built-in NCT to provide versatility and convenience for clinicians (See refractometer with pachymeter and NCT images). These instruments use a small puff of air to applanate the central cornea.[41] The returning air from the cornea's surface is measured by a membrane that records the force using sensors, detectors, and beams of light, which are then converted by an internal algorithm to give IOP measurements. The advantages of NCT include no requirement for topical anesthesia and its usefulness in screening, especially in children and adults who cannot tolerate contact methods.[19] NCT can be helpful in a clinical screening setting; however, it is considered a less accurate, precise, and sensitive method than GAT, in addition to being instrument-dependent and highly influenced by CCT, thus not advisable in the management of patients with glaucoma or individuals with OHT or other risk factors.[42][43][42][44][45]

Ocular-Response Analyzer (ORA) and Corvis ST Tonometers

ORA was developed in 2005 and represented a newer version of NCT that utilizes a column of air as the applanating force.[7] The flow of air is used to move the cornea inwardly, which slowly decreases when the cornea indents outwards and then moves to the flattened state.[46] 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 corneal hysteresis (CC) value, a corrected intraocular pressure (IOPg), and a corneal-compensated measurement (IOPcc).[47] 

Corneal hysteresis is the ability of the cornea to absorb and dissipate applied forces based on its viscoelastic and elasticity properties. ORA was designed to theoretically improve IOP accuracy, allowing clinicians to account for the variability in corneal biomechanical properties seen among patients.[47][48] Corneal hysteresis has been demonstrated to contribute to glaucoma progression risk, with lower values imparting a greater risk of glaucoma progression.[49][50] Like GAT, ORA is a desktop instrument and thus cannot be used on supine patients. Although the corneal compensation parameters have been developed to provide IOP measurements less affected by corneal factors, studies have shown overestimated ORA measurements in eyes with high levels of IOP.[51] Further clinical studies are needed to assess the clinical use of ORA in glaucoma management. 

Corvis ST is an NCT based on the indentation of the cornea by a jet of air, which was proposed in 2011 by Oculus in Germany.[52] A built-in Scheimpflug camera is used to provide precise measurements of the deformation of the cornea during the air-induced indentation process.[53] The instrument also has a pachymeter and provides several other corneal parameters.[54] Although the precision of the Corvis ST readings has been reported to be very good, and measurements appear to be less influenced by corneal factors, studies have shown that the Corvis ST tonometer tends to underestimate IOP when compared to GAT.[55][56][57][58]

Indentation Tonometry

Schiotz Tonometer

The principle of indentation is based on the fact that weight or force will sink more into a softer object than a harder one. The Norwegian physician, Hjalmar Schiotz, invented the first clinically useful indentation tonometer in 1905.[59] This portable tonometer (See Schiotz images) 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.[7] The patient must be supine during measurement, and a topical corneal anesthetic is required.[19] This tonometer is relatively inexpensive and easy to use and is sometimes still used in children under general anesthesia and developing countries.[60] However, this tonometer is not routinely used in a clinical setting, considering that IOP measurements are subject to several errors, including defective instruments or calibration, high variability, bad positioning on the eyeball, and influences due to ocular rigidity and other ocular factors.[61]

Tono-Pen Indentation/Applanation Tonometer

The Tono-pen is a battery-powered, lightweight, portable handheld device that uses a small plunger to record the force needed to applanate the cornea. This tonometer is based on both the indentation and applanation principles. An early version of this instrument was introduced in 1959 by McKay and Marg.[62] The Tono-pen averages multiple readings of this small force converted to IOP.[7] The newer versions of the Tono-Pen include XL and Avia, which give IOP measurements as an average of  4 and 10 measurements, respectively.[9] 

It requires daily recalibration and topical anesthesia and uses disposable covers (See Tono-pen image). It can prove useful in portable screenings, emergency rooms, bedridden patients, or operating rooms to measure intraocular pressure. Advantages of this method include the ability to measure over a soft contact lens, on an irregular corneal surface, averaging multiple readings, and the potential to measure at the peripheral cornea if a central corneal scar or ulcer exists. It can also measure intraocular pressure independent of patient position.[7] Studies have shown Tono-Pen to be useful in certain clinical situations; however, the intrasession variability has been shown to be rather high, and measurements with Tono-Pen appear to be significantly affected by CCT.[63][64]

Pneumatonometer

The pneumatonometer uses a stream of air to indent the cornea with a 5 mm diameter silicone tip on a piston that rides on the air.[7] This tonometry is based on both indentation and applanation principles.[65] The force of air that indents the cornea is recorded and converted to intraocular pressure. A built-in transducer is used to convert the force generated during corneal applanation into IOP in mmHg. Studies have shown that this method correlates well with GAT in normal pressure ranges.[66] 

The Pulsatile Ocular Blood Flow pneumotonometer is the newer version that provides pulse fluctuation and information regarding ocular blood pulse.[67] Similar to the Tono-pen, it has the advantage of measuring irregular cornea surfaces, over a soft contact lens, at the peripheral cornea. Accuracy is independent of patient position, although it requires topical anesthesia.[7] The pneumatonometer is accessible in a handheld probe attached to a table-mounted device. The device is also utilized for pneumatonography, which uses changes in IOP to measure the outflow resistance of aqueous humor from the anterior chamber.[68] If increased outflow resistance is found, it can cause increased IOP and contribute to glaucoma. However, the clinical usefulness of this tonometer remains debatable, considering that studies have shown IOP values with pneumatonometry tend to be overestimated and more influenced by corneal parameters compared to gold standard GAT.[69]

Rebound Tonometry

iCare tonometer

The iCare tonometer, based on the principles of rebound, is a portable battery-powered handheld device that is easy to use.[70] Several versions of this tonometer include the iCare TA01i, IC100, IC200, PRO, and HOME (See images with different iCare models). It utilizes a small disposable probe (composed of a tiny plastic round tip on a thin steel wire) that bounces off the cornea in the horizontal plane. The deceleration of the probe produces a level of voltage that is converted to IOP.[70] The faster the rate of deceleration against the cornea, the higher the pressure. The slower the rate of deceleration, the lower the pressure.[71] The advantages of this method include ease of use,[33]  no requirement for topical anesthetics, and its usefulness in children, non-collaborative individuals, those allergic to fluorescein and anesthetic drops, and dementia patients who are not tolerant of a slit-lamp exam or more involved contact methods.[47]

The HOME version can be used autonomously for at-home use in managing glaucoma patients, especially in suspect normal-tension glaucoma patients that tend to have normal IOP during clinical examinations.[72][73][74] Most versions need to be used while the patients are in an upright position. However, the iCare PRO, which became available in 2011, was designed with a shorter probe and can be used in a supine position.[75] The iCare tonometer can be of important clinical use in screening, routine examination of normal adults and children, and in patients with recent ocular surgeries, considering the facility, rapidness, limited pressure applied on the cornea, and no need for fluorescein or anesthetic drops.[70][27]

Several studies have shown that when compared to GAT and other tonometers, iCare showed good agreement, specificity, reliability, and reproducibility, especially for low-to-moderate IOP measurements.[76][77][78][77][76][79] Considering the facility, low cost, and good agreement with GAT, numerous clinicians currently find the iCare tonometer useful in a routine clinical setting, of course, keeping in mind that elevated IOP readings with rebound tonometry need to be confirmed with GAT. The gold standard in tonometry remains GAT, which to date cannot be replaced, especially when managing patients with glaucoma or individuals at risk of developing OHT.

Dynamic Contour Tonometry (DCT)

DCT was developed in 2003 and calculates IOP based on the Pascal principle, which deals with the pressure of a fluid within an enclosed space.[80][81] Similar to GAT, this tonometer needs to be mounted on a slit-lamp (See DCT Pascal image), with the patient in an upright position, and the use of anesthesia drops (but not fluorescein) considering the tip of the tonometer needs to be in contact with the cornea. This device utilizes a contour-matched, piezoelectric sensor to measure minuscule dynamic pulsations in intraocular pressure at the cornea.[82] Disposable plastic caps are used on the sensor tip to prevent cross-infection. This method allows the measurement of IOP without deforming the cornea; therefore, its readings are independent of corneal thickness, unlike other methods.[47][18][83] 

Studies have shown DCT to have good precision, reproducibility, and correlatability compared to GAT and other tonometers.[84][85][86][85][84][87] When using DCT, the tip needs to be in contact with the cornea for about 10 seconds to provide an accurate measurement of IOP; thus rather difficult to use and not ideal for all patients.[33] The other disadvantages include the high learning curve, the need for a slit-lamp and anesthetic drops, difficulties to use in a routine clinical setting, and IOP measurements influenced by several artifacts, measurement errors, CCT, and other corneal parameters.[88][89][90][91][92]

Continuous IOP Monitoring

The routine clinical setting normally provides the ophthalmologist with only a few spot readings of IOP for each patient. Patients can have ophthalmologic examinations up to 4 times yearly; however, in normal cases, this tends to be biannual or annual for the most part. In the ideal world of glaucoma management, IOP would be continuously measured 24 hours a day to provide the true fluctuations of IOP and IOP spikes during the waking and sleeping hours, which could have a great impact on understanding the true target pressure, progression rates, and the best individually tailored treatment strategies for each patient.[93][94][95] Unfortunately, current methods are not available for routine clinical use. However, several options for continuous IOP monitoring have been proposed using an externally applied contact lens or an intraocular surgically inserted lens or sensor.[96]

Regarding sensors applied to contact lenses on the cornea surface, a company from Switzerland launched the Sensimed Triggerfish system.[97] This 24-hour IOP-monitoring device is composed of a disposable silicone contact lens with an embedded electromechanical microprocessor that collects more than 280 repeated IOP measurements during the 24 hours of application.[98] The data is sent with a wireless antenna on the lens to a portable recorder that the patient carries in a handy shoulder purse. Studies have shown that the Triggerfish contact lens is well tolerated, provides reproducible readings of IOP, and can help detect IOP peaks throughout the day.[99][100][101] 

The main limits of this system, however, include no direct correlation between corneal changes and IOP readings and poor correlation with GAT readings regarding measurements taken several hours after the device has been activated.[102][103] The clinical use of Triggerfish and its application in routine use remains debatable. 

Implantable IOP-sensor devices, which can be positioned during cataract surgery in the anterior chamber, suprachoroidal space, or embedded in a foldable artificial intraocular lens, have been proposed; however, they remain experimental and not of widespread use yet.[104][105][106][105][107]   

Technique or Treatment

Technique Considerations

• Care must be taken to not artificially raise the intraocular pressure by pressing on the eyelid during IOP measurement. Ensure the patient is comfortable and not attempting to squint, as this can raise IOP.
• Ensure that the patient is appropriately positioned for the specific tonometry method (seated versus supine). Table-top methods such as GAT can only be used in upright and seated patients. Rebound (I-Care) tonometry can only be used in the horizontal plane.[7]
• Clean and disinfect all contact tonometers between patients unless they utilize disposable covers. This assists in preventing disease transmission and possible corneal abrasions from residual debris.
• Methods that rely on applanation are affected by the central corneal thickness and other biomechanical properties such as elasticity and hysteresis.[44] The calibration of these methods assumes uniform corneal thickness and biomechanical properties. Thinner and more elastic corneas will result in lower readings, while thicker and more rigid corneas will result in higher readings due to the variability in forces needed to deform the cornea. Additional factors that may affect tonometer accuracy are astigmatisms, irregular corneas, Valsalva maneuver, breath-holding, and natural variation of IOP during the cardiac cycle.[7]
• GAT currently remains the gold standard technique in measuring IOP, especially in managing patients with OHT, risk factors, and in therapy for glaucoma. Elevated IOP measurements taken with alternative tonometers should be confirmed with GAT.

Complications

Corneal abrasion, aggravation of globe rupture, the transmission of infection, and reaction to ocular drugs are potential complications of tonometry. The risk of these complications is considerably low (less than 1%). Non-contact methods and rebound tonometry can be considered in special cases, even if GAT remains the gold standard approach to tonometry.

Clinical Significance

Measurement of intraocular pressure is important in the screening and monitoring of glaucoma, a progressive optic neuropathy that can be slowed with intraocular pressure reduction. Intraocular pressure is the only modifiable risk factor for glaucoma progression at this time.[7] Prescribing medications to reduce intraocular pressure and monitoring pressure over time using tonometry has been demonstrated to slow the progression of peripheral and central vision loss related to glaucoma. Tonometry is also used to evaluate acutely elevated intraocular pressure, as seen in acute-angle closure glaucoma and after ocular trauma. Acute angle-closure glaucoma is an ophthalmic emergency requiring immediate intervention to lower IOP and avoid vision loss.[19]

Enhancing Healthcare Team Outcomes

In addition to an ophthalmologist, an optometrist, ophthalmic technician, nurse, or emergency medicine physician may use tonometry during an ophthalmic office visit, emergency room visit, or ambulatory settings as needed. Communication among these professionals, operating as an interprofessional team, regarding the particular tonometry method is important so eye pressure measurements can be accurately compared throughout time.  

Portable techniques such as the Tono-pen and rebound tonometer are simple to use, while stationary methods such as GAT require more specialized training. Each medical professional mentioned above may use various tonometry methods based on comfort level and training. A point of emphasis among all professionals performing tonometry must be the proper use of topical anesthesia to minimize patient discomfort for methods including Tono-pen and GAT. Avoid any tangential movement across the cornea during tonometric measurement to prevent corneal abrasions. Additionally, ensure any disinfecting solution is rinsed from the tonometry surface that contacts the cornea to avoid potential toxic keratitis.  

Tonometry readings may need to be re-measured among professionals to ensure consistency of readings. If there is a question about the accuracy of a reading or device, an alternative tonometry method may be used to help confirm or refute the reading. In an acute setting where acute-angle closure glaucoma is suspected, an emergent intraocular pressure reading must be acquired. This is typically initially done in the emergency room by a physician or nurse, with a consulting ophthalmologist confirming the pressure on arrival and administering pressure-lowering medication or performing additional interventions to save vision.[108] [Level 1]