2.2.1. Tono-Pen® (Reichert; Depew, New York, NY, USA)

This is a hand-held electronic device that measures the force needed to applanate the cornea via a plunger. Prior to measurement, a topical anesthetic is applied to the eye, and a sanitized disposable cover is placed over the device tip. The operator then lightly taps the central cornea with the device tip multiple times until 10 measurements are recorded. The average IOP of the 10 measurements, along with a statistical confidence indicator, are displayed. The Tono-Pen® is easy to use and has reasonable accuracy when compared with Goldmann applanation, the gold standard for IOP measurement. A masked, randomized study on 270 eyes showed that Tono-Pen® measurements were 1.7 mm Hg higher than Goldmann applanation for IOPs from 6 to 24 mm Hg [5]. Another study looked at 197 eyes with glaucoma or ocular hypertension and found that Tono-Pen® measurements had a high correlation (*r* ≥ 0.86) with Goldmann applanation. However, at high IOPs (≥30 mm Hg), Tono-Pen® tended to underestimate Goldmann; and at low IOPs (≤9 mm Hg), Tono-Pen® tended to overestimate [6]. Another study [7] of 142 eyes reported a correlation of coefficient of 0.84 between Tono-Pen® and Goldmann measurements. This study likewise subdivided the eyes into IOP ranges. For eyes with low IOPs (4–10 mm Hg), the Tono-Pen® measured an average 1.78 mm Hg higher than Goldmann applanation. For eyes with IOP in the normal range (11–20 mm Hg), the Tono-Pen® measured an average 0.07 mm Hg lower than Goldmann applanation. For eyes with elevated IOPs (21–30 mm

Hg), the Tono-Pen® measured an average 1.27 mm Hg lower than Goldman applanation. Additionally, for eyes with very elevated IOPs (31–45 mm Hg), the Tono-Pen® measured an average 4.15 mm Hg lower than Goldmann applanation. The higher the IOP, the more the Tono-Pen® underestimates. Tono-Pen IOP measurements have also been shown to be increased by a greater central corneal thickness (CCT) and a greater corneal resistance factor (CRF) [8–13]. However, for most eyes, the Tono-Pen® has reasonable accuracy. The Tono-Pen® is particularly useful for patients with a corneal edema or scar, as Goldmann applanation can underestimate the IOP in the presence of a spongy, edematous cornea and overestimate the IOP in the presence of a calcified scar; the Tono-Pen® is less affected by corneal edema and the device tip can be easily directed away from the scar when measuring. Another distinct of advantage of the Tono-Pen® is that the patient does not need to be upright. If the patient can only remain supine due to a medical condition or cannot position one's head vertically due to neck or spinal disease, the Tono-Pen® can still be used, as long as the operator ensures that the device tip taps the cornea perpendicularly. Other devices require the head to be upright for accurate measurement.

#### 2.2.2. Air Puff Non-Contact Tonometer

This is a non-contact way to measure IOP. An electric device delivers a puff of air, and the force required to applanate the cornea is recorded. Multiple measurements are taken and the average IOP measured is calculated. Because there is no physical contact to the eye, a topical anesthetic is not required and there is no risk of corneal abrasion or infection from the equipment. The device is automated and easy to use. The noncontact tonometers available on the market include the Pulsair Desktop Tonometer (Keeler; Malvern, PA, USA), the CT-80 (Topcon; Tokyo, Japan), the NT-530/510 (NIDEK; Gamagori, Japan), and the TX-20 (Canon; Tokyo, Japan). Many studies found that noncontact tonometer IOP measurements are in moderate agreement with that of Goldmann applanation; as a result, the authors concluded that air puff tonometers can serve as good screening tools but are not accurate enough to substitute for Goldmann applanation [14–18]. Of note, there is an air puff tonometer that is compact and weighs approximately 2.5 kg called the Pulsair IntelliPuff (Keeler; Malvern, PA, USA). It is portable and can be easily brought to a glaucoma screening venue. Hubanova et al. [19] compared IOP measurements between the IntelliPuff tonometer and Goldmann applanation on 137 eyes and found that there was good agreement with an intraclass correlation coefficient of 0.86. The IntelliPuff tonometer overestimated the IOP by 1.5 ± 1.8 mm Hg in normotensive eyes and 2.3 ± 4.8 mm Hg in hypertensive eyes. Air puff is particularly useful in young children who do not tolerate eye drops well or are particularly anxious, as anesthetic eye drops are not required and there is no tip or probe that contacts the eye. However, some patients do not find the air puffs comfortable and would prefer other methods of measurement.

#### 2.2.3. iCare (iCare Finland Oy; Helsinki, Finland)

This is a hand-held device that bounces a light-weight probe off the cornea. The contact is gentle enough that no topical anesthetic is needed. The IOP displayed is a function of the probe's deceleration at contact and the contact time, as measured by an induction-based coil system. The first generation TA01i model and the second generation ic100 model require the patient to be upright for measurement. Nakakura et al. [20] compared measurements of the iCare TA01i, iCare ic100, and Goldmann applanation on 106 eyes, and found that both iCare models measured significantly lower IOPs than Goldmann applanation (12.2 ± 2.9, 11.7 ± 3.0, and 16.9 ± 3.2 mm Hg, respectively). Furthermore, both iCare models' IOP measurements were correlated with central corneal thickness (*r* = 0.50). In contrast, Gao et al. [21] compared TA01i iCare measurements to that of Goldmann applanation on 672 eyes and found no significant differences between the two (18.30 ± 5.10 and 18.52 ± 4.46 mm Hg, respectively; *p* = 0.19), with a correlation coefficient *r* = 0.806. However, for eyes with IOP ≥ 23 mm Hg by Goldmann applanation, the iCare measurements were significantly lower (1.66 mm Hg, *p* = 0.007) than that of Goldmann applanation. Central

corneal thickness had a stronger correlation with iCare measurements (*r* = 0.39) than with Goldmann applanation (*r* = 0.19). Subramaniam et al. [22] compared IOP measurements of iCare ic100 with Goldmann applanation in 1000 eyes and reported an intraclass correlation coefficient of 0.73. The ic100 measurements were significantly lower than Goldmann applanation measurements (12.1 vs. 16.2 mm Hg), even when the data were subdivided into different ranges of IOP. In January 2020, the iCare ic200 model was granted marketing authorization in the United States; it allows for IOP measurement even when the patient is reclined or supine. Badakere et al. [23] compared the ic200 with Goldmann applanation in 156 eyes and found that the ic200 readings were on average 1.27 mm Hg higher, but with no statistically significant difference.

A unique benefit of a portable tonometer that does not require a topical anesthetic is that home measurements can be performed. The iCare HOME is a device that allows for easy self-measurement. After loading a single-use probe, the device is placed in front of the eye at an appropriate distance (adjustable with the device's forehead and cheek supports). A hold of a button allows for six consecutive measurements, and the average measurement, along with the time of measurement, are saved in the device. When the patient returns the device to the clinic, these measurement data can be extracted and a diurnal IOP graph can be generated. Being able to take multiple measurements throughout the day at home is particularly useful in glaucoma patients with disease progression than normal IOPs measured in clinic. In this scenario, glaucoma specialists must determine whether the disease progression is a result of the "normal IOP" measured in clinic being above the target IOP to halt progression, or whether there are IOP elevations not detected because they occurred outside of clinic hours. The iCare HOME is a useful device that can answer this question. A comparison [24] of the iCare HOME measurement by the patient versus Goldmann applanation reported a high correlation (*r* = 0.846); the iCare HOME on average measured 0.70 mm Hg greater than Goldmann applanation (*p* < 0.001), and this difference increased by 1.2% for every 10% increase in central corneal thickness. Importantly, 98% of the 128 participants were able to use the iCare HOME.

#### 2.2.4. Ocular Response Analyzer (Reichert; Depew, NY, USA)

This is a desktop device that uses a stream of air to applanate the cornea. No topical anesthetic is needed. Infrared light is emitted onto the cornea, and an infrared light detector measures a peak in light intensity when the cornea is flat. At this state, the inward applanation pressure is measured. The force of air then increases so that the cornea becomes concave, and then decreases until the cornea is flat again. At this state, the outward applanation pressure is measured. The entire measurement process takes about 20 milliseconds. The inward applanation pressure is greater than the outward applanation pressure, and this difference is the biomechanical property of the cornea termed corneal hysteresis. The device displays the inward intraocular pressure measurement (which should be identical to Goldmann tonometry) and the intraocular pressure measurement corrected by corneal hysteresis. Ehrlich et al. [25] and Ogbuehi et al. [26] compared ocular response analyzer (ORA) IOP measurements with those of Goldmann tonometry and found no statistically significant difference between them. However, a number of studies [27–31] found that the ORA significantly overestimated IOP when compared with Goldmann applanation. The importance of the ORA lies in its ability to measure corneal hysteresis, which is a known risk factor for glaucoma progression [32]. Eyes with a corneal hysteresis < 10 are 2.9 times more likely to have moderate to severe glaucoma than eyes with a corneal hysteresis ≥ 10; thus, corneal hysteresis can serve as a screening tool for glaucoma [33].
