Tonometry is a critical aspect of every ophthalmic evaluation. Intraocular pressure (IOP) is the only modifiable risk factor for the development and progression of glaucoma at all disease stages.^1-4 The relevance of IOP to ocular disease has been appreciated for centuries,⁵ although reliable instruments were not developed until the 20^th century. Following the introduction of Shiotz’s indentation technique, Goldmann’s applanation tonometer became and remains the gold standard. More recently, however, several techniques and devices have been developed in an attempt improve the accuracy and reliability of tonometry while optimizing convenience for both practitioners and patients. These include a variety of applanation, rebound, indentation, and hybrid tonometers. Rebound tonometry can be implemented both in the clinic and at home by patients, enabling 24-hour IOP monitoring. Moreover, several wearable or implantable telemetric tonometry devices are in various stages of development and positioned to revolutionize IOP monitoring in the near future.

Applanation Tonometers

Goldmann applanation tonometry (GAT) is the standard by which by IOP is measured in clinical trials, and therefore it is the benchmark by which IOP is interpreted in patients with glaucoma and ocular hypertension. Applanation tonometry is based on the Imbert–Fick law, which states (with several assumptions) that the pressure within a sphere is directly proportional to the force necessary to flatten its surface. Goldmann applanation tonometry provides accurate and consistent IOP measurements, but it does have limitations that can create challenges in a busy clinic. Applanation tonometry requires topical anesthetic and fluorescein application to the ocular surface. Corneal pathology affects GAT measurements (eg, edema, surface irregularity, band keratopathy). The presence of high astigmatism requires correct alignment of the prism with the steep axis or 2 measurements at orthogonal positions. Central corneal thickness affects IOP measurements with all devices to varying degrees, which is especially relevant after refractive surgery.⁶ Body habitus, breath holding, and patient straining to position at the slit lamp can impact GAT measurements. Moreover, measurements in eyes with small palpebral fissures can be challenging given the 3.06-mm tip diameter that must applanate the corneal surface unimpinged by the lids. These factors require the operator, often a technician, to be wary of potential pitfalls and skilled with the slit lamp. They also require the patient to be at least partially cooperative to ensure accurate readings, which can be difficult in young children.

Of note, the Correcting Applanation Tonometry Surface (CATS) tonometer prism may reduce the impact of corneal thickness on IOP measurements by applanation.⁷ The prism fits into existing GAT tonometers. In multiple studies, the correlation between CATS tonometry and standard GAT was good, and the difference was strongly associated with corneal thickness and hysteresis.^8,9 In eyes undergoing cataract surgery, CATS tonometry yielded an IOP that was closer to manometrically controlled IOP than Perkins tonometry.¹⁰

One important consideration, particularly in light of the COVID-19 pandemic, is that the GAT prism requires disinfection between uses. Methodologies for disinfection have been described elsewhere.¹¹ Disposable single-use GAT tips are available and produce IOP measurements that are comparable to traditional reusable Goldmann prisms,¹² but they are costly.

Despite these issues, GAT is mandatory in eye care practices and should be the default IOP measurement method. GAT measurements occasionally differ from IOP measurements obtained by other methods (see below). Sometimes, the difference is large enough to impact treatment decisions. Therefore, it is wise to measure IOP using GAT when other methods yield unexpected values or significant changes from baseline, or when making surgical decisions, such as the use of minimally invasive glaucoma surgery (MIGS) alongside cataract extraction.

The Perkins tonometer is a handheld applanation device that is infrequently used. It is helpful in situations where applanation IOP measurements are needed but slit lamp positioning is challenging or impossible. This includes patients with difficulty positioning at the slit lamp due to body habitus or musculoskeletal problems, for hospitalized patients who cannot be transported to the clinic, and for examinations under anesthesia. Although other handheld tonometers can be easier to use (see below), Perkins tonometry may be useful in specific situations as above when applanation tonometry is preferred or when handheld options are known to produce aberrant results in particular patients.

Pneumatonometry

The pneumatonometer (Reichert) utilizes a relatively large, 5-mm pneumatic sensor that contacts the corneal surface and, as such, requires application of a topical anesthetic and good exposure. The underlying principle is similar to GAT. Pneumatonometry uses air flow to increase the force of the probe on the cornea until of the cornea is flattened. Pneumatonometry may perform better than GAT in eyes with corneal abnormalities, such as central or paracentral scars. Some have proposed using measurements at the limbus or sclera to estimate IOP in patients with severe corneal pathology,¹³ although this may be too unreliable for routine clinical implementation. Pneumatonometry generally correlates well with GAT, but it remains affected by corneal thickness and may underestimate IOP at lower pressures.^6,14 Unlike GAT, the pneumatonometer provides a printout of measured IOP over the few seconds of the measurement, which is useful for judging the quality of the reading and visualizing the ocular pulse amplitude. Although rarely used in clinical practice, the pneumatonometer can also be used to perform pneumatonography.

Dynamic Contour Tonometry

The Dynamic Contour Tonometer (DCT, Pascal; Ziemer Ophthalmic Systems) has an appearance that is similar to a Goldmann tonometer, but it contains a pressure transducer at the tip that resides inside of a concave surface that conforms to the cornea to measure IOP. Theoretically, corneal biomechanics (eg, thickness, hysteresis, changes induced by prior refractive surgery) impact DCT less than GAT.¹⁵ In general, agreement with GAT is quite good.¹⁶ The DCT also measures ocular pulse amplitude.

Handheld Tonometers

The 2 most widely used portable tonometers are the Tono-Pen (indentation tonometry, Reichert) and iCare (rebound tonometry). These tonometers contact a smaller portion of the cornea than GAT and thus are minimally affected by corneal astigmatism. The Tono-Pen is lightweight and durable, fits into coat pockets, and can be used with any patient position, including supine. Like all tonometry methods, it is possible to artifactually elevate the IOP with inadvertent pressure on the globe if the operator is not careful. It can seem faster to implement than applanation tonometry, but the Tono-Pen requires calibration before each use, addition of a sterile cover, and application of topical anesthesia. Of note, Latex-based disposable tips can cause significant allergic responses for patients with Latex allergies.¹⁷ There is a reasonable correlation between Tono-Pen and GAT, but Tono-Pen may underestimate at higher IOPs and overestimate at lower IOPs without a clear numerical cutoff for accuracy.^15,18,19

The iCare is a newer device that uses a 1.7-mm, disposable, lightweight probe mounted on a thin metal shaft to measure IOP by determining the kinetics by which the probe rebounds off the cornea. Unlike GAT or Tono-Pen, rebound tonometry does not require a topical anesthetic. It is an excellent choice for pediatric patients or adult patients who are minimally cooperative because the device can quickly obtain measurements through small palpebral fissures. It is somewhat less portable than the Tono-Pen. Some versions (IC100, TAO1i) require the device to be held upright, but newer models (IC200) can measure IOP with supine patient positioning. Importantly, it is gentle to the ocular surface because it does not require topical anesthetic or fluorescein, and the rebound technology places minimal force on the cornea, making it an appealing option for anterior-segment clinics in particular.²⁰ There is good correlation between iCare and GAT for most patients, but iCare may overestimate IOP in children and underestimate IOP in older patients.^21-23 Rebound tonometry is particularly useful postoperatively because it requires less manipulation of the eyelids with potential for pressure on the globe.

Noncontact Tonometry

“Air puff” tonometers and the Ocular Response Analyzer (ORA; Reichert) use forces of air to measure the IOP. Noncontact tonometers do not require topical anesthesia or significant operator experience, and thus may be useful for screening by untrained users. Historically, air puff tonometry has been considered unreliable. Newer models maintain reasonable agreement with GAT, but may have a tendency to overestimate IOP.^24,25 The ORA is unique in that it can measure both IOP and corneal hysteresis. It ejects a puff of air toward the cornea until the cornea becomes concave. Infrared lasers and optical sensors are used to measure the geometric conformation of the cornea, and relevant metrics are recorded twice during that process (once when the cornea flattens on the way to becoming concave, and once when while rebounding back to convexity). The ORA reports a standard IOP measurement, an IOP measurement that is corrected for corneal biomechanical properties (IOPcc), and 2 biomechanical factors: corneal hysteresis and corneal resistance factor.²⁶ In a prospective, observational cohort study of glaucoma patients, IOPcc better correlated with future glaucoma progression than GAT.²⁷ The prospective Diagnostic Innovations in Glaucoma Study demonstrated that corneal hysteresis significantly predicted the rate of visual field progression over time, independent of IOP.²⁸ Therefore, evidence suggests that measurements obtained with the ORA are useful for glaucoma practices by providing data relevant to risk stratification.

Many Options

Taking this all into consideration, which tonometer is right for your practice? Goldmann applanation tonometry remains the reference standard given its proven accuracy and reliability, and as such is obligatory in every eye care practice. In our opinion, the best portable supplement to the GAT is the iCare rebound tonometer, given its simplicity and speed, its accuracy over a range of IOPs, its ability to measure IOP without topical anesthesia, its preservation of the ocular surface, its ease of use in general and with pediatric and supine patients specifically, and its relative portability. The ORA provides information relevant to glaucoma specialty evaluation. Other tonometers are likely useful only in limited settings. The Perkins may be preferred when applanation is essential but cannot be performed due to positioning. The Tono-Pen is durable and portable, and it is handy for inpatient hospital consults.

The Future of IOP Monitoring

Diurnal curves and 24-hour monitoring have been proposed to aid in the evaluation of glaucoma patients but are time consuming and resource intensive.²⁹ Emerging technologies are likely to change the way in which clinicians obtain IOP data in glaucoma patients in the near future. A version of the iCare rebound tonometer, the iCare Home, is available for self-use by patients outside of the ophthalmology clinic. Following a brief training period, many patients can obtain reliable measurements and can do so at any time of day during the course of their routine activities.³⁰ Tonometric data are wirelessly uploaded to a secure cloud database for review by the clinician and may be useful in identifying patients with occult IOP elevation.³¹ Perhaps equally exciting is the prospect of automatic telemetric IOP monitoring through wearable or implantable sensors. The Sensimed Triggerfish contact lens sensor measures IOP-related changes in corneal curvature and has been used to discern trends in IOP over extended periods of time.³² Eyemate,³³ Injectsense, and Qura are companies that are developing implantable telemetric IOP sensors for continuous IOP monitoring.³⁴ In appropriate patients where the benefits of continuous real-time IOP monitoring outweigh the risks of implantation, continuous monitoring will provide a much deeper understanding of IOP variation, the effects of medications and surgery on average IOP and fluctuation, and how these relate to the development and progression of glaucoma. Implantable IOP sensors also offer the opportunity to reliably measure IOP in patients with corneal pathology that presently prohibits accurate tonometry, such as those with keratoprostheses.

Summary

Many variables factor into the decision of a tonometer to use for an individual patient (Table 1). The GAT and iCare tonometer are currently the most universal, and the ORA is especially relevant to glaucoma care. Other tonometers may be useful in specific situations. Future tonometric methods will need to address the universal impact of corneal thickness and biomechanics, as well as occasional instances of severe corneal pathology on IOP measurements. Moving forward, improved understanding of how corneal biomechanics and continuous IOP fluctuation affect glaucoma onset and progress will undoubtedly improve clinical care. GP

 

 

Pujan Dave, MD, and Thomas V. Johnson, MD, PhD, are with the Glaucoma Center of Excellence, Wilmer Eye Institute, Johns Hopkins University School of Medicine in Baltimore, Maryland. The authors report no relevant disclosures. Reach Dr. Johnson at johnson@jhmi.edu.

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