Challenges of the Intraocular Pressure Measurements in the Keratitis

Abstract

The maintenance of the aqueous humor circulation is vital for nourishing the anterior segment structures and maintaining the shape of the eyeball. Imbalances in the production and drainage of aqueous humor are well-known occurrences during inflammatory processes in the anterior chamber, with keratitis being a major contributor. Elevated intraocular pressure (IOP) is a common complication during active microbial keratitis. However, even under normal conditions, corneal biomechanical properties, thickness, and curvature can complicate the accuracy of IOP measurements. Ongoing research is exploring the relationship between corneal characteristics and IOP. Corneal conditions related to keratitis, such as band-keratopathy, corneal edema, astigmatism, and corneal ectatic disorders, pose significant challenges for managing high-pressure-related complications. Different IOP measurement techniques may be preferable in various corneal prominent conditions. Regular IOP checks are necessary to avoid possible optic nerve damage during keratitis treatment. It is crucial to select the appropriate measurement technique and consider potential over- and underestimations of IOP due to corneal disorders.

1. Introduction

Intraocular pressure (IOP), which refers to the internal pressure of the eye, is a measure of the fluid pressure inside the eye. However, it is not practically feasible to directly measure the pressure inside the eye in routine clinical practice. Therefore, all clinical methods of measuring IOP are based on estimating it through the external surface of the eye. None of these methods are precise enough to accurately measure the true IOP using invasive techniques. Even the Goldmann applanation tonometer (GAT), which is commonly accepted as the current gold standard tonometer, cannot consistently provide reliable measurements in all conditions. The fundamental principle of applanation tonometry is based on the Imbert–Fick law, which can be expressed by the equation: Intraocular pressure = Contact force/Area of contact. However, this formula would work accurately only if the cornea were infinitely thin, perfectly elastic, and flexible, which is not the case. The measurement of IOP using GAT can be influenced by factors such as corneal thickness, curvature, modulus of elasticity, rigidity, and tear film. In particular, if there are ocular surface pathologies present, these corneal parameters are affected, leading to compromised accuracy in IOP measurements.

IOP is a critical parameter in the diagnosis and management of ocular diseases, including keratitis. However, obtaining accurate IOP measurements in patients with keratitis can be challenging due to several factors. In this paper, we review the challenges associated with IOP measurements in keratitis, including the effects of corneal thinning and scarring, tear film instability, and the use of topical medications. We also discuss various methods for measuring IOP in keratitis patients, including GAT, Tono-Pen, dynamic contour tonometer, the rebound tonometer, the ocular response analyzer, and their limitations. Finally, we suggest strategies for overcoming these challenges and improving IOP measurements in keratitis patients.

2. Keratitis-related conditions affecting the IOP measurement

Keratitis is an inflammatory condition that affects the cornea, the transparent outer layer of the eye. It can be caused by infectious agents such as bacteria, fungi, or viruses, or by noninfectious factors such as trauma, contact lens wear, or autoimmune disorders. Keratitis can lead to a different range of corneal thickness differences from thinning–scarring to thickening, edema, and calcification which can affect the accuracy of intraocular pressure (IOP) measurements, a critical parameter in the diagnosis and management of ocular diseases, including keratitis.

Corneal thinning and scarring can affect the accuracy of IOP measurements as they can lead to a reduction in corneal rigidity, which can cause underestimation of IOP. Several studies have shown that IOP measurements in keratitis patients are lower than in normal subjects due to corneal thinning and scarring [1, 2]. In addition, corneal infiltrates, or subepithelial calcium hydroxyapatite deposition named band-keratopathy, can increase the thickness and rigidity of the cornea, leading to overestimation of IOP [3]. The Ocular Hypertension Treatment Study (OHTS) found that cornea thickness is a major determinant in the glaucomatous process. At first sight, all the thickening and thinning process of the cornea seems like the effect of central corneal thickness (CCT) on IOP measurements. But the effect of the keratitis not only affect the CCT but also affect corneal curvature, modulus of elasticity – rigidity and tear film. The range of cornea’s biomechanical properties, like energy absorption and resistance of the deformation, which influences its capacity to dampen fluctuations in IOP, may influence tonometry. For example, the edema may cause CCT thickening but also may bring resistance deficiency. So at some points, you may get under-estimated IOP values even in the thickened corneas. Consequently, throughout the progression of keratitis, these parameters are often interconnected and complex, emphasizing the need to consider them during clinical evaluation.

Besides the cornea-related parameters, tear film instability is another challenge in IOP measurements in keratitis patients. Tear film instability can lead to fluctuations in IOP measurements due to changes in tear volume and composition. Several studies have reported that tear film instability can affect the accuracy of IOP measurements obtained using GAT, the most commonly used method for measuring IOP [4, 5]. However, the effects of tear film can be avoided by dynamic contour tonometry, rebound tonometer, noncontact tonometry [6, 7].

The other independent factor from cornea is topical medications. Topical medications used for the treatment of keratitis can affect IOP measurements by altering the corneal properties. Although ophthalmic steroids can cause steroid-induced high intraocular pressure, several studies have shown that topical steroids can reduce corneal rigidity and lead to underestimation of IOP [8, 9]. Similarly, topical antibiotics can affect corneal thickness and rigidity, leading to inaccurate IOP measurements [10].

After taking account of these keratitis-related conditions, we will evaluate the different IOP measurement principles with their pros and cons.

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

Figure 1.

Goldmann applanation tonometry.

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.

Figure 2.

Air-puff tonometer and ocular response analyzer.

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 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, 16, 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.

	Advantages	Disadvantages
GAT	Widely accepted gold standard technique	7.35 mm2 contact area by truncated conic probe
	Designed by basic physic principle, Imbert–Fick law: P = F/S	Topical anesthesia and fluorescein drop
		contamination and cornea damage risk
		Upright position on a slit-lamp
		Influenced by the CCT, tear film, cornea biomechanics
		Subjective measurement
Air -Puff Tonometer	No need to touch the cornea	Influenced by corneal parameters (CCT-biomechanics-edema)
	Designed by the measurement of the air force of corneal flattening	Slit-lamp positioning
	Objective measurements (can be taken by nonmedical staff)	Ideal as a screening (less accurate than GAT)
	Reduced risk of infection and injury
Ocular Response Analyzer	No need to touch the cornea	Relatively expensive device
	Designed by the measurement of the air force inward and outward applanation of the cornea	Not extensive usage
	Capable of measuring corneal biomechanics (electro-optical system monitor the deformation of the cornea)	Slit-lamp positioning
	Corrected IOP calculation (less dependent measurements by corneal parameters-(CCT-biomechanics-edema))
	Objective measurements (can be taken by nonmedical staff)
	Reduced risk of infection and injury
Tono-Pen	Portable lightweight, handheld, battery-powered device	Contact method and cornea damage risk
	Principles of applanation and indentation-	Significantly affected by CCT
	Disposable latex cap – reduced risk of infection	Significant error for high IOP values
	Measurement in both supine and upright positions.	Intra-session repeated measurements’ variabilities are found high
	Better accuracy in edematous corneas	Significant variations from GAT
Rebound Tonometry	Widespread accepted and used device in a short exposed time	Influenced by the CCT, tear film, cornea biomechanics
	Designed by the rebound subtle probe’s velocity, which impacts the cornea	Influenced by peripheral corneal measurements
	Portable lightweight, handheld, battery-powered device	Underestimation IOP at a higher level
	Measurement in both supine and upright positions (last version)
	Small surface contact-less traumatic compared to GAT and provides measurement across the irregular cornea
	Excellent repeatability and good reliability (especially for normal range)
DCT	Designed by measurement of the dynamic pressure changes in a fluid-enclosed space	Need for a slit-lamp–topical anesthesia
	Detection of the dynamic pulsatile fluctuations and ocular pulse amplitude	Need for trained staff and highly cooperative patients
	Contour matching principle between cornea and tip – theoretically elimination of the corneal parameters	Long contact time (at least 8 s)-difficult to use
	Less influenced method by the properties of the cornea	Reduced accuracy in the irregular corneas
	Disposable sensor caps in order to avoid the risk of infection

Table 1.

Advantages and disadvantages of different IOP measurement techniques.

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.

4. Strategies for overcoming challenges in IOP measurements in keratitis patients

To overcome the challenges associated with IOP measurements in keratitis patients, several strategies can be employed. These include corneal pachymetry, tear film stabilization, and the management of topical medications.

Corneal pachymetry is a noninvasive method for measuring corneal thickness, which can help to correct IOP measurements in patients with corneal edema or thinning. Corneal pachymetry can be used to adjust IOP measurements obtained using GAT or as a baseline for measurements obtained using DCT.

Tear film stabilization can be achieved through artificial tears or punctual plugs, reducing fluctuations in IOP measurements due to tear film instability.

The management of topical medications in keratitis patients can help to reduce the effects of these medications on IOP measurements. This can include reducing the frequency or dose of medications that affect corneal properties or switching to alternative medications that have less impact on IOP measurements.

5. Conclusion

IOP measurements are critical in the diagnosis and management of ocular diseases, including keratitis. However, obtaining accurate IOP measurements in keratitis patients can be challenging due to several factors, including corneal thinning and scarring, tear film instability, and the use of topical medications. Various methods of tonometry, have been proposed as alternatives to GAT in keratitis patients, but their accuracy in this population has still limitations, and they are still under investigation. Strategies for overcoming these challenges, such as corneal pachymetry, tear film stabilization, and topical medication management, can help to improve the accuracy of IOP measurements in keratitis patients.

Acknowledgments

There is no funding to be declared.

Conflict of interest

The author declares no conflict of interest.