**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.

**Keywords:** keratitis, intraocular pressure, pressure measurement, corneal disorders, cornea

## **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].

*Challenges of the Intraocular Pressure Measurements in the Keratitis DOI: http://dx.doi.org/10.5772/intechopen.112417*

 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.
