**5. Application of intraoperative OCT for corneal pachymetry during collagen cross-linking**

#### **5.1 Advantages of OCT as a diagnostic tool for corneal imaging**

Anterior segment optical coherence tomography (AS-OCT) is a noninvasive imaging modality that can be used to obtain high-resolution, cross-sectional images of the anterior structures of the eye. In comparison with ultrasound pachymetry, AS-OCT has the advantage of providing a more detailed corneal anatomic profile during the procedure with a high degree of intra-observer repeatability and interobserver reproducibility of the pachymetric measurements.

OCT has been proven to be a useful diagnostic imaging modality for the diagnosis of keratoconus. OCT-derived corneal thickness distribution and asymmetry parameters have been shown to have a good correlation with established Scheimpflug-derived anterior surface irregularity indices and proven useful to detect suspect, early, and clinical keratoconus [13].

The high resolution of the spectral domain OCT which allows for reconstruction of epithelial and stromal thickness profiles is another advantage of OCT to study the altered epithelial thickness pattern in keratoconus due to corneal surface irregularity. OCT-derived epithelial thickness map parameters have shown promising results in early and advanced keratoconus detection [14, 15].

OCT has also been used for detection of demarcation lines after CXL which are considered as a measure of the treatment efficacy [16]. In addition, OCT has been proved to be useful to show changes after CXL including the epithelial remodeling after CXL, resulting in a thinner and more regular thickness profile [7].

#### **5.2 Application of intraoperative OCT in corneal and anterior segment surgery**

OCT as a real-time and high-resolution imaging modality provides additional information regarding the angle, corneal thickness, and other structures that are otherwise invisible or difficult to visualize using the normal operating microscope, making it a potential beneficial tool for anterior segment surgery. In addition, it could provide valuable information in the cases like corneal opacity which preclude visualization of the anterior segment structures of the eye.

Intraoperative OCT has also been used as a useful extension of the normal surgical microscope during anterior segment procedures for finding the plane of corneal dissection during anterior lamellar surgery, for assessment of graft-host relationship in penetrating keratoplasty, as a guide during the crucial aspects of posterior lamellar surgery, cataract surgery, and glaucoma procedures like canaloplasty or trabectome surgery [17–22].

*A Practical Guide to Clinical Application of OCT in Ophthalmology*

cm2

5.4 mJ/cm2

damage threshold [7].

the CXL procedure [8].

of the original procedure.

progression of keratoconus [5, 6].

stroma with 0.1% riboflavin-20% dextran solution (every 5 minutes until 30 minutes), and then application of UV-A light source (370 nm with irradiance of 3 mW/

) on the cornea for 30 minutes [2]. However, there have been other modifications such as preserving the corneal epithelium (epithelial-on method), increasing the intensity of the UV source and decreasing the irradiation time (accelerated method), and using a continuous versus a pulsed light source after the description

A significant increase up to 71.9 and 328.9% in corneal rigidity has been demonstrated in experimental studies in porcine and human corneas, respectively [4], and long-term studies have demonstrated the safety and efficacy of CXL in halting the

**2. Significance of corneal pachymetry as a safety criteria for CXL**

and the corresponding irradiance of 3 mW/cm<sup>2</sup>

**3. Pachymetric changes during CXL for KCN and importance of** 

irradiance at a depth of 400 μm is 0.18 mW/cm2

**intraoperative pachymetric monitoring**

Corneal thickness (measured by pachymetry) is one of the most important factors with respect to the safety of CXL; a minimum thickness of 400 μm is recommended to ensure the safety of the procedure and avoid the potential toxic effects of UV-A irradiation on the corneal endothelium [7]. Both an adequate corneal thickness and adequate riboflavin saturation of the cornea are necessary to ensure photochemical damage caused by the free radicals to the corneal endothelium. With the currently used irradiation doses in CXL (UV-A radiant exposure of

CXL in thin corneas with a minimum corneal thickness below 400 μm after epithelial removal has been reported to result in significant endothelial cell loss postoperatively, emphasizing the role of corneal pachymetry as a critical factor for

In addition to the initial pachymetric changes, significant changes in corneal pachymetry may occur during the different steps of the procedure [9–11]. Factors contributing to these changes include epithelial removal, dehydration due to corneal stroma due to exposure, osmotic effects of the riboflavin-dextran solution, and UV irradiation. The use of an eyelid speculum during instillation of riboflavin drops and the dextran containing riboflavin formulations (as compared to nondextran containing riboflavin iso-osmolar formulations) are especially among the potential factors associated with shrinkage of the corneal stroma during the

Kymionis et al. [10] reported a decrease of 75 μm and Muzzotta et al. [11] reported a 32.07% shrinkage of the corneal stromal thickness after riboflavin saturation of the stroma with the riboflavin-dextran solution. These findings indicate that despite an initial pachymetry value greater than 400 μm, a considerable proportion of patients will end up with a corneal pachymetry value below the critical thickness during the procedure, which may be a factor explaining complications like corneal edema despite an initial adequate pachymetry in some reports [12]. These pachymetric changes all could potentially jeopardize the safety of the procedure, further highlighting the importance of intraoperative pachymetric

), the estimated level of

which is by two factors below the

**40**

procedure.

monitoring during the operation.

As another application of intraoperative OCT, this modality has been used for anterior segment examination of patients under general anesthesia, diagnosis of complex wound apposition problems, with the advantage of providing additional information for understanding corneal wound-related problems [23].

With the adaptation of this technology, intraoperative online OCT provides additional information for anterior segment surgeons providing a real-time dynamic feedback of the various surgical steps during surgery. Nonetheless, shadowing produced by surgical instruments represents the main limitation of this technology [19].
