|Year : 2022 | Volume
| Issue : 3 | Page : 149-156
Evaluating corneal changes after corneal collagen cross-linking in keratoconus by optical coherence tomography
Abdallah Barakat, Ahmed Elmassry, Ihab Othman, Mohamed El-Kateb
Department of Ophthalmology, University of Alexandria, Alexandria, Egypt
|Date of Submission||05-Mar-2022|
|Date of Decision||02-Apr-2022|
|Date of Acceptance||26-Apr-2022|
|Date of Web Publication||30-Jul-2022|
Department of Ophthalmology, Faculty of Medicine, Alexandria University, Alexandria 21311
Source of Support: None, Conflict of Interest: None
Purpose The aim of this study was to evaluate the corneal changes after corneal collagen cross-linking (CXL) in progressive keratoconus using anterior segment optical coherence tomography.
Patients and methods This prospective interventional noncomparative case-series study was conducted on 30 eyes of 18 patients of both sexes with a mean age of 23.11±4.06 years diagnosed as progressive keratoconus patients with clear central cornea. All eyes underwent epithelium-off accelerated CXL. Anterior segment optical coherence tomography and Scheimpflug camera imaging were done to all cases preoperatively and at 1, 3, and 6 months, postoperatively.
Results The demarcation line (DL) was detected in all eyes 1 month postoperatively, and was still detected at 3 and 6 months, postoperatively. However, it was accurately measured until 3 months postoperatively. At 6 months, it was barely visible, so measuring its depth was difficult to be done. There was a statistically significant positive correlation between the thinnest corneal thickness and the central corneal DL depth (r=0.480, P=0.006). There was no statistically significant correlation between the maximum keratometric reading (K-max) and the depth of central corneal DL (r=−0.253, P=0.17). In addition, there was no statistically significant correlation between corneal densitometry and the depth of central corneal DL (r=−0.68, P=0.715).
Conclusions The DL is a direct clinical sign of corneal CXL and can be found within 6 months after the treatment, being most clearly visible at 1 month after CXL. However, the line starts to be less visible after 3 months and becomes barely visible after 6 months postoperatively, making it difficult to measure its depth at that time.
Keywords: anterior segment-OCT, corneal cross-linking, demarcation line
|How to cite this article:|
Barakat A, Elmassry A, Othman I, El-Kateb M. Evaluating corneal changes after corneal collagen cross-linking in keratoconus by optical coherence tomography. Delta J Ophthalmol 2022;23:149-56
|How to cite this URL:|
Barakat A, Elmassry A, Othman I, El-Kateb M. Evaluating corneal changes after corneal collagen cross-linking in keratoconus by optical coherence tomography. Delta J Ophthalmol [serial online] 2022 [cited 2022 Dec 3];23:149-56. Available from: http://www.djo.eg.net/text.asp?2022/23/3/149/353032
| Introduction|| |
Collagen cross-linking (CXL) is an established treatment for keratoconus (KC) and other ectatic corneal disorders, with proven efficacy in slowing or halting the disease progression. CXL treatment relies on a photochemical reaction between riboflavin (vitamin B2) and ultraviolet A (UVA). Riboflavin acts as a photosensitizer to induce cross-linking between the collagen fibrils and as a shield to protect the underlying tissues from the UVA damage .
A distinct line between the treated anterior corneal stroma and the posterior stroma [demarcation line (DL)] could be detected clearly by using anterior segment-optical coherence tomography (AS-OCT) and is considered by many authors as a sign of successful cross-linking procedure of the ectatic cornea ,,,. The exact cause of the DL appearance is still under investigation. Seiler and Hafezi  found that it may result from the difference in refractive indices or reflection properties of the cross-linked and the untreated corneal stroma. Mazzotta and colleagues using confocal microscopy, showed a transition area where an edematous zone with low cell density merged with a deeper zone with less edema and more keratocytes, at an average depth of 320 μm. The transition can be seen as an opaque/whitish line, when using an OCT for instance, because of the rapid change between the two zones .
Recently, the debate is focused on whether the depth of the corneal stromal DL is indeed, a true indicator of CXL efficacy or not. The main question is whether ‹the deeper, the better’ principle can be applied to CXL. Other confounding factors may interfere with the clinical interpretation of the depth of the corneal DL after CXL . Therefore, some authors believe that the ‹the deeper, the better’ principle is rather a simplistic approach for interpreting the clinical importance of the corneal stromal DL . Seiler and Hafezi  first reported the identification of a corneal stromal DL at a depth of 300 μm that was visible as early as 2 weeks after CXL using confocal microscopy.
The aim of this study was to evaluate the corneal changes after corneal CXL in progressive KC using AS-OCT.
| Patients and methods|| |
This prospective interventional noncomparative case-series study was conducted on 30 eyes of 18 patients of both sexes with progressive KC at Alexandria Faculty of Medicine, Alexandria, Egypt. The study was approved by the Ethics Committee of Alexandria Faculty of Medicine (IRB number: 00007555, approval date: 22/11/2018) and complied with the tenets of the Declaration of Helsinki. All patients signed a written informed consent to participate in the study and for publication of data before enrollment in the study. A written informed consent was also obtained from the patients before the procedure.
The study included patients with age range between 15 and 35 years of both sexes having progressive KC detected by one or more of the following changes over 12 months: an increase of 1.00 diopter (D) or more in the steepest keratometric (K) reading, an increase of 1.00 D or more in manifest cylinder, an increase of 0.50 D or more in manifest refraction spherical equivalent, or a decrease in thickness greater than 30 μm in the first, second, or third stage according to Amsler–Krumeich’s classification  with clear cornea. The exclusion criteria included the history of ocular trauma or intraocular surgery, corneal thickness less than 400 μu, prior herpetic ocular infection, post-Lasik ectasia, and other ectatic conditions.
Preoperatively, the patients were subjected to full history taking and detailed ophthalmological examination, including slit-lamp examination, manifest refraction, and uncorrected visual acuity and best-corrected visual acuity.
Preoperatively, Pentacam Scheimpflug system (HR Oculus; Optikgerate Gmbh, Wetzlar, Germany) was used to measure the corneal thickness, thinnest corneal location, and steepest K, and to measure objectively the corneal densitometry values. The Scheimpflug system quantifies the density of the cornea on a scale from 0 to 100. AS-OCT was done using the Spectralis HRA+OCT (Heidelberg Engineering, Heidelberg, Germany).
CXL was performed under aseptic conditions. Preoperatively, topical gatifloxacin eyedrops (Tymer, Jamjoom Pharma, Jeddah, Saudi Arabia) were used three times on the day of operation. One drop of benoxinate HCL 0.4% (Benox, EIPICO, 10th of Ramadan City, Egypt) was instilled every 5 min for 3 times. The central 7.0–9.0 mm of the corneal epithelium was removed by mechanical debridement using a hockey spatula. A photosensitizing solution, 0.1% Riboflavin (vitamin B2) with 1.0% hydroxypropyl methylcellulose solution (Peschke M – Isotonic Standard solution without dextran; Peschke Meditrade GmbH, Huenenberg, Switzerland), was then instilled every 5 min for 20 min. Then, the central cornea was exposed to a UVA source emanating from a solid-state device (CCL-365; Peschke Meditrade GmbH) with a working wavelength of 370 nm at surface irradiance of 9 mW/cm2 for 10 min at a working distance of 5 cm. During the treatment, riboflavin solution was applied every 3 min to ensure saturation.
Immediately postoperatively, a soft-bandage contact lens was placed until complete reepithelialization. The patients were instructed to use topical gatifloxacin 3% eyedrops four times/day for 1 week, topical tobramycin 0.3%+dexamethasone 0.1% eyedrops (Optidex-T; Jamjoom Pharma) four times/day for 1 week, and topical artificial tears sodium hyaluronate 2% (Hyfresh; Jamjoom Pharma) for 6 weeks. For pain relief, an oral analgesic was given, if needed, such as Ibuprofen (Brufen, Abbott Laboratories, Chicago, Illinois, USA) 400 mg three times per day.
The follow-up examinations were performed on days 1, 3, and 7, postoperatively. Subsequent examinations were performed at 1, 3, and 6 months, postoperatively. During the follow-up visits at 1, 3, and 6 months, uncorrected visual acuity and best-corrected visual acuity were assessed and slit-lamp examination was done by the same examiner (A.B.). The corneal densitometry was measured using the Scheimpflug image taken at the same axes as at the baseline visit. In addition, the keratometric values and minimal pachymetry were recorded and compared with the preoperative values.
AS-OCT was done during the follow-up visits at 1, 3, and 6 months using the Spectralis HRA+OCT, which is a spectral-domain OCT system that allows high-speed, high-resolution cross-sectional imaging. The CXL DL depth was measured from the corneal epithelium to the detectable hyper-reflective DL. Measurements were taken at the corneal center and at 1 and 2 mm from the corneal center in two locations: temporal and nasal.
Statistical analysis was performed with a commercially available software (StataCorp. Version 15, 2017; StataCorp LLC, College Station, Texas, USA; IBM SPSS Statistics for Windows, 2016, Version 24.0; IBM Corp., Armonk, NY, USA). A P value less than 0.05 was considered statistically significant.
| Results|| |
The study included 30 eyes of 18 patients with progressive KC. They included seven male patients (38.9%) and 11 female patients (61.1%). The mean age of the patients was 23.11±4.06 years with a range from 15 to 31 years.
From the refractive map of the Pentacam, the postoperative data at 1, 3, and 6 months were compared with the preoperative data. The mean preoperative flattest keratometric reading (K1) was 44.95±3.25 D, which minimally decreased to 44.24±3.03 D at the end of the follow-up, with no statistically significant difference (P=0.056). The mean preoperative steepest (K2) reading was 48.90±4.20 D and improved (flattening) to 47.94±4.45 D at the end of the follow-up period, which was a statistically significant difference (P=0.05). The mean preoperative K-max was 53.64±5.07 D and decreased to 52.69±5.37 D at the last follow-up period, a statistically significant difference (P=0.005). Progression of corneal steepening from the preoperative value was not observed in any of the treated eyes ([Table 1]).
|Table 1 Comparison between the studied periods according to the keratometric reading|
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[Table 2] shows that a significant corneal thinning was found between the pre-CXL and 1, 3, and 6 months post-CXL thinnest corneal thickness (P≤0.05). The pachymetric data did not reach the pre-CXL thickness by the end of 6 months.
|Table 2 Comparison between the studied periods according to the thinnest corneal thickness|
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There was a statistically significant increase in corneal densitometry at 1, 3, and 6 months postoperatively compared with the preoperative value (P<0.001, <0.001, and <0.001, respectively, [Table 3]).
|Table 3 Comparison between the studied periods according to corneal densitometry|
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Using AS-OCT, the corneal stromal DL was detected in all eyes at 1, 3, and 6 months, postoperatively, but it can only be properly measured at 1 and 3 months postoperatively, while at 6 months, it was barely visible and difficult to measure. [Table 4] and [Figure 1][Figure 2][Figure 3][Figure 4] illustrate the demarcation line depth (DLD) at the central, nasal, and temporal points of the cornea.
|Table 4 Comparison between the two studied periods according to the demarcation line depth|
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|Figure 1 Preoperative anterior segment optical coherence tomography showing no demarcation line.|
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|Figure 2 Anterior segment optical coherence tomography 1 month postcross-linking showing the demarcation line and its depth measurement at the center of the cornea and at 2 mm temporally and at 2 mm nasally, of the same patient.|
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|Figure 3 Anterior segment optical coherence tomography 3 months postcross-linking showing the demarcation line and its depth measurement at the center of the cornea and at 2 mm temporally and at 2 mm nasally, of the same patient.|
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|Figure 4 Anterior segment optical coherence tomography 6 months postcross-linking showing the demarcation line clearly at the temporal side. It fades out gradually to become barely visible at the center of the cornea and disappeared at the nasal side, of the same patient.|
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Using Pearson’s correlation, there was a statistically insignificant negative correlation between the DLD and K-max at 1 and 3 months post-CXL (r=−0.267 and −0.253, respectively, and P=0.147 and 0.170, respectively). On the other hand, there was a statistically significant positive correlation between the DLD and the thinnest location at 1 and 3 months post-CXL (r=0.496 and 0.480, respectively, and P=0.005 and 0.006, respectively). There was a statistically insignificant negative correlation between the DLD and corneal densitometry at 1 and 3 months post-CXL (r=−0.052 and −0.086, respectively, and P=0.781 and 0.715, respectively) ([Table 5] and [Figure 5] and [Figure 6]).
|Table 5 Correlation between the demarcation line depth at the corneal center and K-max, thinnest corneal thickness, and corneal densitometry|
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|Figure 5 Correlation between the center of demarcation line depth and the thinnest corneal thickness at 1 month postcross-linking.|
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|Figure 6 Correlation between the center of demarcation line depth and the thinnest corneal thickness at 3 months postcross-linking.|
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| Discussion|| |
From the AS-OCT results, the appearance of a stromal DL was observed in all eyes one month postoperatively and showed a mean DLD of 302.39±60.77 μm centrally, 281.01±55.22 μm temporally 2 mm from the corneal center, and 280.77±54.32 μm nasally 2 mm from the corneal center. Three months after CXL, the average DLD was 285.9±55.45, 273.0±55.32, and 280.77±54.32 μm, respectively, and was observed in all eyes but less clear than at the first month. At 6 months after CXL, the DL was observed in all the eyes but without the possibility of accurate measurement. Therefore, a reliable depth measurement can be obtained only if the line is visible clearly.
In the study of Doors et al. , the DL was clearly visible with reliable depth measurement after 1 month, and therefore, they suggested that AS-OCT should be performed within 1 month after CXL, as the DL was not detectable in most eyes by 3 months after CXL and was invisible in all included eyes at 6 months after CXL. Similarly, we found that the DL was barely visible but difficult to measure at 6 months post-CXL. Doors et al.  found that the mean stromal DLD was 313±61, 325±62, and 305±61 μm on the central, nasal, and temporal locations, respectively. However, in the study of Kymionis et al. , the mean corneal stromal DLD was 223±32 μm (range: 159–265 μm) after 1 month of CXL, which is shallower than the current study and Doors and colleagues results. The authors believed that the DLD after a very high-intensity 5-min CXL protocol seems to be shallower than the standard Dresden protocol.
Yam et al.  reported that at 6 months postoperatively, the mean DLD after epithelium-off CXL at surface irradiance of 9 mW/cm2 for 10 min was 281.4±53.3 μm, which is close to the present study results.
In the study by Lhuillier et al. , the mean depth of the stromal DL at 1 month AS-OCT evaluation was 331.2±62.7 μm, which is near to this study result. In addition, Mesen et al.  study showed DLD of 304.97±94.45 μm, which is also similar to our result.
Another study was conducted by Kymionis et al.  and aimed to compare the corneal stromal DLD after CXL by two treatment protocols. The corneal stromal DL was identified easily (i.e. scored as clearly visible) on AS-OCT in all eyes after 1 month of CXL.
Some studies showed a shallower DLD than this study. In Ng et al.  study, the mean DLD at the center was 203±45 μm. Bouheraoua et al.  examined 15 eyes of 15 patients who underwent epi-off A-CXL, in whom the corneal DL was visible in 87.5% (with a mean depth 184.2±38.9 μm). Similarly, Jiang et al.  as well as Moramarco et al.  had similar results. In the study of Abdel-Radi et al. , the mean DLD of A-CXL was 219.9±58.4 μm.
Regarding the keratometry readings, the preoperative mean K1 was 44.95±3.25 D and minimally decreased to 44.24±3.03 D at the end of follow-up, which is not statistically significant. The mean preoperative K2 was 48.90±4.20 D and decreased to 47.94±4.45 D at the end of the follow-up period, which was statistically significant. The mean K-max was 53.64±5.07 D preoperatively and decreased to 52.69±5.37 D at the last follow-up period, which was statistically significant. Many earlier studies like Wittig-Silva et al. , Jankov et al. , and Vinciguerra et al.  similarly demonstrated a decrease in K-max after CXL.
Regarding the thinnest location, the mean thickness decreased markedly at 1 month postoperatively by 21.8 μm. However, it increased gradually to reach near the baseline level at 6 months, postoperatively. These initial changes may be attributed to the corneal deepithelialization that was performed during the CXL procedure, postoperative keratocyte apoptosis, and structural changes in corneal collagen fibrils and extracellular matrix in the anterior stroma. Reduced corneal thickness may also be explained by the increase in endothelial pump activity or density induced by the treatment. The corneal thickness gradually increased after the first month of treatment. However, this increasing value did not reach the preoperative value by the end of 6 months of follow-up. These results were confirmed by the study of Helaly and Osman , who reported that the mean thinnest pachymetry decreased markedly at 1 month postoperatively by 51 μm. This is also in agreement with a previous report on transient thinning of the cornea with ultrasound pachymetry after CXL treatment by Holopainen and Krootila .
Regarding the corneal densitometry, there was a significant increase in the corneal stromal haze, peaking at 1 month postoperatively. Then, there was a gradual reduction of the corneal haze as measured objectively using Pentacam densitometry. However, at 6 months postoperatively, the values of corneal densitometry did not reach the baseline levels. The mean corneal densitometry remained higher than the baseline level, with an increase of 1.04. These findings were consistent with those found in the KC subgroup of the study conducted by Greenstein et al.  This was similarly reported by Gutiérrez et al.  even though they reported higher corneal densitometry values. This course also agreed with that reported by Helaly and Osman  and Badawi . The current study demonstrated that the corneal densitometry values were negatively correlated with the CXL DLD, but it was statistically insignificant.
| Conclusion|| |
The DL is a direct clinical sign of corneal CXL and can be found within 6 months after the treatment, being most clearly visible 1 month after CXL. However, the line starts to be less visible after 3 months and becomes barely visible after 6 months postoperatively, making it difficult to measure its depth at that time.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]