**3. Methods for measuring IOP in keratitis patients**

#### **3.1 Applanation tonometries**

*Goldmann applanation tonometry (GAT)* (see **Figure 1** ) is the leading method of the applanation tonometries and is widely accepted as the gold standard for IOP measurement. However, it has several limitations in keratitis patients. It requires a clear cornea for accurate measurements, which is not always possible in keratitis patients with corneal edema, scarring, or astigmatism. In addition, GAT measurements can be affected by tear film instability, as mentioned earlier. Also, if there are coexisting eye lid scarring-retraction pathologies, this will lead to overestimation of the IOP measurement. The CCT is considered as the most important parameter in the GAT measurements, and many studies address this problem by proposing a number of correction formulas. Although CCT may give information about the estimation of the real IOP, the CCT-based correction formula is not advised to be

 **Figure 1.** *Goldmann applanation tonometry.* 

applied to individuals [11, 12]. The other limitations of this technique are the risk of contamination and the pulsatile changes in the measurements.

There are also noncontact types of applanation tonometers that eliminate contamination risks. One of them is *air-puff tonometer* (**Figure 2a)**. Air-puff tonometers employ a rapid and controlled puff of air to applanate the cornea and measure IOP. They offer several advantages over traditional methods, including noninvasiveness, patient comfort, and rapid measurements. As for the first property, air-puff tonometers do not require any physical contact with the cornea, reducing the risk of infection and injury. For patient comfort, these tonometers eliminate the need for topical anesthetic eye drops and the discomfort associated with corneal contact. Finally, for rapid measurements, air-puff tonometers provide quick IOP readings, making them suitable for large-scale screenings and busy clinical settings. However, limitations exist with air-puff tonometry, including potential variability in measurements due to factors such as corneal thickness, ocular surface irregularities, and patient cooperation. Also, its measurements are brand-dependent and less accurate than the GAT's. Mostly, it underestimates IOP at high ranges and overestimates IOP at low ranges as compared to the GAT.

In this device, air-puff applanating force, flattens the cornea, and this force is covered to the mmHg. As it is expected, corneal infiltrations, deformations, and irregularities may lead to resistance or softening on the applanation force, and in consequence, this might lead to under or over-estimation of the IOP.

The other noncontact type of applanation tonometer is *ocular response analyzer* (**Figure 2b**). It also applanates the cornea by air-puff, but the air column continues to emit with increasing intensity until the cornea is indented. Then the force of the air column decreases until the cornea is once again at a point of applanation. The difference in the pressures at the two applanation points is a measure of the corneal biomechanic properties (rigidity or floppiness). When confronting the unusual cornea, it helps us to think about corneal biomechanics to accurately assess IOP and glaucoma risk. Mathematical equations are used to "correct" the applanation point for high or

**Figure 2.** *Air-puff tonometer and ocular response analyzer.*

low elasticity. This "corrected" IOP is thought to be less dependent on corneal thickness than other forms of applanated pressures [13].

## **3.2 Indentation tonometers**

The principle of indentation tonometry is that a force or a weight will indent an eye surface by way of the transducer to detect the transmitted pressure. The prototype of the indentation tonometers is the Schiøtz tonometer which was introduced many years ago and is no longer currently used.

The *Tono-Pen* (**Figure 3**) is the current form of the indentation tonometers. Indeed, the Tono-Pen involves both applanation and indentation processes, and it works both process calculations in multiple measurements. It is a small, handheld, battery-powered portable device and brings many advantages at some points. The main advantages of the Tono-Pen are its portability, not requiring a slit-lamp, or electricity, and its measurement ability in both supine and upright positions. A disposable latex cap is used for each patient, which helps to reduce the risk of infection between patients. More than that, Tono-Pen comes into prominence, especially in patients with eye scarring or irregular corneas like keratitis. Its measurements are well-correlated with Goldmann tonometry within normal IOP ranges. Moreover, it provides better accuracy in edematous corneas than GAT and dynamic contour tonometry [14]. But, Tono-pen was found to consistently underestimate IOP, with a significant error for IOP values >30 mmHg; also Tono-Pen can be significantly affected by CCT.

**Figure 3.** *Tono-Pen.*

## **3.3 Rebound tonometry**

Rebound tonometry has emerged as a noninvasive and reliable method for assessing IOP. They are the last generated tonometer models but are well-accepted and widely used devices worldwide (**Figure 4**). Like Tono-pen, it is portable, fast, and easy to use and does not need a slit-lamp or electricity. Its 1.8 mm diameter subtle probe impacts onto the cornea and then rebounds from the eye with a different velocity, which varies according to the IOP. Its small surface contact makes it suitable for measuring damaged corneas. Also, the subtle probe may be less traumatic on the cornea than GAT, and it could offer a better alternative in keratitis patients to provide information regarding IOP. Subtle probe contact leads to minimal discomfort during the procedure, making it suitable for individuals who may be sensitive or anxious about eye examinations. Rebound tonometers are portable and easy to operate, making them suitable for use in various clinical settings. The simplicity of the technique allows healthcare professionals to quickly and accurately measure IOP, facilitating screening programs and enabling frequent monitoring of patients with glaucoma. Moreover, it also has a high degree of versatility and reliability. Rebound tonometry provides accurate IOP measurements across a wide range of corneal conditions and shapes. It is less influenced by corneal thickness and biomechanical properties, factors that can affect other tonometry methods. This versatility ensures more reliable and consistent IOP readings, enhancing the diagnosis, treatment, and management of ocular conditions. IOP measurements obtained with this device have also been shown to be influenced by CCT with higher IOP readings with thicker corneas. Also, it is affected by other biomechanical properties of the cornea, including corneal hysteresis and corneal resistance factor.

**Figure 4.** *Rebound tonometry.*

## **3.4 Dynamic contour tonometry (DCT)**

DCT is a method of tonometry that measures IOP by detecting changes in the contour of the cornea. It works according to the Pascal principle which the pressure changes applied to the wall surface of a fluid in a contained enclosed place. DCT utilizes a sensor tip to detect changes in the ocular pulse waveform, enabling accurate IOP assessment. It takes about 8–10 sec corneal contact in order to provide IOP measurement. The advantages of DCT include accuracy and reproducibility as it accounts for corneal biomechanical properties and ocular pulsations, leading to more precise IOP measurements compared to traditional methods. Ocular pulse amplitude (OPA) measurement provides indirect information about choroid perfusion and also the eradication of the pulsatile changes on IOP. Several studies have shown that DCT is a reliable method for measuring IOP in keratitis patients [15–17]. Although reduced accuracy in the presence of irregular corneas, DCT is also applicable to various corneal conditions and shapes, making it suitable for a diverse range of patients. Nevertheless, DCT also has limitations, including difficulties, such as the need for a slit-lamp, topical anesthetic, longer corneal contact in a good head and eye position, trained staff, and highly cooperative patients.

**Table 1** summarizes frequently-used IOP measurement tonometers and presents their advantages and disadvantages.

#### **3.5 Digital palpation**

After familiarizing with various instruments produced through different principles of physics, which possess numerous advantages and weaknesses, it is possible that none of these devices may be effective in certain exceptional circumstances. In cases of severe eye pain and sensitivity, suspicion of globe rupture, indications of severe infection, and specific situations where sufficient lid aperture cannot be achieved, it may be necessary to perform intraocular pressure estimation using fingertip. Making a comparison with the patient's unaffected eye in these situations can facilitate the estimation process.
