Corneal cross-linking (CXL) with riboflavin and ultraviolet-A (UVA) light is considered an accepted treatment of various corneal conditions, including ectatic disorders such as keratoconus and post-LASIK ectasia.1–3 In ectatic disorders, CXL is used to halt progression by increasing the number of covalent bonds in the corneal stroma, thus increasing the tectonic stability of the tissue.4,5 Although the effectiveness of CXL in the treatment of keratoconus has been demonstrated in many studies, reports on adverse effects are scarce.6–13
It has been shown previously that the effect of CXL on a cellular level in the cornea continues for months or even years.14,15 Apoptosis of keratocytes takes place immediately after CXL.16 Over time, new keratocytes from the limbus repopulate the stroma and a flattening of corneal curvature is observed.2 Accordingly, nerve fibers in the anterior stroma are also affected.17 Similar nerve fiber damage after excimer laser ablation in most of the cases results in a clinically relevant neurotrophic dry eye syndrome for 3 to 6 months.18 The main factor is thought to be a reduced blinking rate due to the damage to the afferent corneal nerves.19 Clinical studies have shown that the regeneration of corneal sensitivity after laser ablation takes approximately 1 year.21,22 Moreover, it is conceivable that by intended cross-linking of the cornea, the conjunctiva, including limbal stem cells, may also be damaged, which may lead to the development of a whorl-like epitheliopathy disturbing the tear film.23,24
The aim of this study was to assess whether CXL induces or increases symptoms of dry eye.
Patients and Methods
We conducted a prospective single center cohort study including 30 consecutive eyes of 16 patients (median age: 31 years, mean age: 32 years, range: 23 to 51 years). Bilateral CXL was performed in the same session in 14 patients (4 female); 2 patients were treated in one eye only. The local ethics committee approved this study and patients gave their written informed consent. Eyes were eligible for inclusion in this study if the CXL was scheduled for the treatment of progressive keratoconus and the patient agreed to return to our center for follow-up after 3 and 6 months.
CXL was performed as originally described by Wollensack et al.4 Briefly, after instilling anesthetic eye drops and disinfecting the eyelids and ocular surface, the eye was draped in a sterile fashion. Then, the epithelium was completely scraped off except for a small limbal rim. A wet Chayet sponge was placed around the cornea to protect the ocular surface from desiccation. The cornea was soaked for 30 minutes with riboflavin 0.1% in dextran solution. Then, the sponge was removed and the cornea was illuminated with 365 nm UVA-light from 5 cm distance and a fluence of 3 mW/cm2 using a dedicated light source (UV-X 1000; IROC AG, Zürich, Switzerland). The cornea was kept moist during illumination by further applying riboflavin solution. Finally, in addition to the original protocol, a bandage contact lens was inserted at the end of the operation. Antibiotic eye drops were prescribed until epithelial closure was achieved a few days later and the contact lens was removed. No anti-inflammatory agents were administered.
Preoperatively, a patient questionnaire (as proposed by the mutual work group “Dry eye” of the German Ophthalmology Society and the German Ophthalmologists Association) was used to evaluate dry eye symptoms, environmental factors, contact lens use, systemic disorders, and medication (Table 1).
Table 1: Baseline Dry Eye Symptoms, Contact Lens Use, Systemic Disorders, and Medication
A slit-lamp examination of the ocular surface and the tear film was documented preoperatively and at the 3- and 6-month postoperative visits by the surgeon without prior access to the patient chart. Additionally, tear film break-up-time was noted. Vital staining of epithelial defects using fluorescein and staining of devitalized and keratinized cells with Rose bengal 1% was assessed.
Because great variability of the quality of the tear film and the ocular surface was anticipated, an intra-individual comparison of the following parameters was performed, which included (1) fluorescein staining (normal or pathologic; pathologic: more than 10 point-shaped or diffuse staining areas); (2) Rose bengal staining using the van Bijsterfeld index, calculated as the sum of the average intensity of staining of the ocular surface within the palpebral fissure (temporal and nasal conjunctiva and cornea; 0 to 3 points each, adding to maximum 9 points)24; (3) height of tear film meniscus at the lower eyelid margin (increased, normal, reduced); and (4) tear film break-up time (< 5 sec, 5 to 10 sec, > 10 sec).
In this study, we intra-individually compared eyes before and after intervention (dependent parameters). To test for significant differences with nominal data (fluorescein staining, tear film height, and tear film break-up time), we used the Cochran-Q-test. To test for significant differences with scaled data (Rose bengal staining), we used the Friedman test. Statistical analysis was performed using the SPSS for Mac software (version 20; SPSS, Inc., Chicago, IL).
Pathologic staining with fluorescein was evident in 1 eye before CXL, 1 eye of a different patient 3 months postoperatively, and 1 eye of a third patient 6 months postoperatively (Figure 1). Differences between visits were not statistically significant (P = .607)
Figure 1. Fluorescein staining of corneas (pathologic: more than 10 point-shaped or diffuse staining). Differences between visits were not statistically significant (P = .607).
Rose bengal staining at 3 and 6 months was comparable to preoperative staining (Figure 2). Differences between visits were not statistically significant (P = .590).
Figure 2. Rose bengal staining of ocular surface within the palpebral fissure using the van Bijsterfeld index (calculated as the sum of the average intensity of staining of the temporal and nasal conjunctiva and cornea; 0 to 3 points each, adding to maximum 9 points). Differences between visits were not statistically significant (P = .590).
Tear film height was reduced in more eyes 3 months postoperatively than before CXL. Tear film height was normal in all eyes 6 months postoperatively (Figure 3). Differences between visits were not statistically significant (P = .135).
Figure 3. Height of tear film meniscus on the lower eyelid margin. Differences between visits were not statistically significant (P = .135).
The number of eyes with reduced tear film break-up time (< 10 sec) was not significantly changed 3 and 6 months postoperatively (P = .247) (Figure 4).
Figure 4. Tear film break-up time. The number of eyes with reduced tear film break-up time (< 10 sec) was not significantly changed after 3 and 6 months (P = .247).
The use of artificial tears in two eyes before CXL was discontinued by the patient before the 3-month follow-up visit. Four other eyes that received no treatment before CXL were treated with artificial tears during the 6-month follow up period.
Examining the impact of CXL on the ocular surface by vital staining, our study did not reveal a statistically significant effect after 3 and 6 months.
Our findings suggest that potential limbal stem cell injury during the CXL procedure has no significant effect resulting in postoperative dry eye. Additionally, potential damage to corneal nerves during the CXL procedure does not appear to affect the surface lubrication after 3 months.
There are many other studies revealing an induction of ocular dryness after excimer laser vision correction.20–28 The biggest factor contributing to tear film irregularities after LASIK and photorefractive keratectomy is thought to be the corneal nerve damage caused by the laser ablation.28 This injury to the nerves reduces corneal sensitivity and blink rate for several months. Histologic recovery of corneal nerves after LASIK may last up to 1 year. In contrast, using confocal microscopy after CXL, Kymionis et al. found that the subepithelial nerve plexus was absent immediately after treatment but regeneration of nerves was evident after the third postoperative month.29 This may be the anatomical explanation of our results.
However, this study has some limitations. The first weeks after treatment are not within the scope of this study. In this period of time, there may be some effect due to the corneal epithelium abrasion and subsequent regeneration alone or in combination with the CXL that is no longer detectable after 3 months. Some imprecision may be associated with estimating the height of the tear film meniscus on the lower eyelid margin; therefore, only a qualitative score was used. Keratoconic eyes may have associated allergic conjunctivitis, which in turn may also cause dry eye symptoms. Because allergies follow a seasonal pattern, a significant change may have occurred within the follow-up period that affected the results. However, because recruitment was spread to more than 9 months, this effect may have been minimized. Additional tests including corneal sensitivity blink rate or tear film osmolarity and its composition may have provided additional information. However, the Luneau Cochet Bonnet Aesthesiometer was not used in this study because it is not sterilizable. A non-contact aesthesiometer was not available to us. Therefore, we cannot evaluate whether corneal sensitivity was affected by CXL. The assessment of the blink rate needs high-speed video imaging that is not part of our clinical routine. Finally, no tear film test or enzyme kit to date has proven its superiority over the clinical evaluation of dry eyes, so using these would have been associated with new imponderabilities.
We consider these findings significant when evaluating the safety of this time-proven procedure. However, our results may not be generalized to other treatment protocols because new UVA light sources with higher fluence rates and energy distribution within the light beam and new photosensitizer solutions and treatment protocols are becoming available.30–32 Even if the same amount of energy is delivered to the cornea with a higher intensity beam and a shorter duration, the effect on the ocular surface and stromal nerves may be different. In addition, the means of epithelium removal (ie, scraping or removal by laser photoablation) may play a role in the overall epithelium regeneration process, as well as in the impact on nerve fiber damage. The chemical interaction of varying photosensitizers may also play a role.33,34 A major variation from the technique described here is emerging treatments without prior epithelial removal.30,31 Further studies with these developments are desirable.
- Hafezi F, Kanellopoulos J, Wiltfang R, Seiler T. Corneal collagen crosslinking with riboflavin and ultraviolet A to treat induced keratectasia after laser in situ keratomileusis. J Cataract Refract Surg. 2007;33:2035–2040. doi:10.1016/j.jcrs.2007.07.028 [CrossRef]
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- Asri D, Touboul D, Fournie P, et al. Corneal collagen crosslinking in progressive keratoconus: multicenter results from the French National Reference Center for Keratoconus. J Cataract Refract Surg. 2011;37:2137–2143. doi:10.1016/j.jcrs.2011.08.026 [CrossRef]
- Saffarian L, Khakshoor H, Zarei-Ghanavati M, Esmaily H. Corneal crosslinking for keratoconus in Iranian patients: outcomes at 1 year following treatment. Middle East Afr J Ophthalmol. 2010;17:365–368. doi:10.4103/0974-9233.71600 [CrossRef]
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- Gokhale NS, Vemuganti GK. Diclofenac-induced acute corneal melt after collagen crosslinking for keratoconus. Cornea. 2010;29:117–119. doi:10.1097/ICO.0b013e3181a06c31 [CrossRef]
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Baseline Dry Eye Symptoms, Contact Lens Use, Systemic Disorders, and Medicationa
|Foreign body sensation||12%|
|Feeling of pressure||25%|
|Contact lens tolerance|
| Dry mucous membranes||12%|
| Oral contraceptives||6%|
| Artificial tears||7%|