Journal of the Egyptian Ophthalmological Society

: 2013  |  Volume : 106  |  Issue : 3  |  Page : 168--171

Possible changes in intraocular pressure measurements after corneal collagen cross-linking with riboflavin and ultraviolet A in eyes with keratoconus

Iman M Eissa1, Mostafa A El-Husseiny2, Ahmad Ismail2,  
1 Department of Ophthalmology, Lecturer of Ophthalmology, Cairo University Hospital, Cairo, Egypt
2 Researcher of Ophthalmology, Research Institute of Ophthalmology (RIO), Giza, Egypt

Correspondence Address:
Iman M Eissa
6 Othman-Diplomats-Towers, Othman Buildings, Korniche Al Maadi, Floor 6, Appt. 62, Maadi 11431, Cairo


Purpose The aim of the study was to determine the possible effect of corneal collagen cross-linking (CXL) with riboflavin and ultraviolet A on intraocular pressure (IOP) measurements using Goldmann applanation tonometry (GAT) in eyes with keratoconus. Design The study was designed as a prospective case series and conducted in Al Haram Eye Center and RCC Hospital, Giza, Egypt. Materials and methods This noncomparative study measured IOP using GAT before CXL and at 3, 6, and 12 months after CXL. Results The study evaluated 49 eyes (40 patients). There was a statistically significant increase in the measured IOP at 3, 6, and 12 months after CXL (P<0.001). The mean preoperative IOP was 11.2 mmHg (1.29 SD), whereas the mean postoperative IOP was 12.24 mmHg (1.13 SD) at 3 months, 12.37 mmHg (1.13 SD) at 6 months, and 12.55 mmHg (1.12 SD) at 12 months. We also found a direct correlation between preoperative central corneal thickness and IOP measurements at postoperative examinations. However, postoperative IOP measurements were not correlated with patient age, sex, or preoperative keratometric readings. Conclusion After riboflavin-ultraviolet A CXL in eyes with keratoconus, there was a significant increase in IOP measured using GAT, which was probably caused by an increase in corneal rigidity and not a true increase.

How to cite this article:
Eissa IM, El-Husseiny MA, Ismail A. Possible changes in intraocular pressure measurements after corneal collagen cross-linking with riboflavin and ultraviolet A in eyes with keratoconus.J Egypt Ophthalmol Soc 2013;106:168-171

How to cite this URL:
Eissa IM, El-Husseiny MA, Ismail A. Possible changes in intraocular pressure measurements after corneal collagen cross-linking with riboflavin and ultraviolet A in eyes with keratoconus. J Egypt Ophthalmol Soc [serial online] 2013 [cited 2023 Jan 31 ];106:168-171
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Full Text


Keratoconus is an asymmetric, bilateral, progressive ectasia of the cornea that affects approximately one in 2000 people [1],[2]. Compared with normal corneas, the mechanical stability of keratoconic corneas is decreased because of increased pepsin digestion and fewer collagen cross-links [3].

Current 'conventional' treatment options for keratoconus include both rigid gas permeable contact lenses and penetrating keratoplasty. Unfortunately, neither of these options treats the underlying cause of ectasia. Corneal collagen cross-linking (CXL), however, is a relatively recent procedure that was proven to slow down, stabilize, or even possibly reverse the progression of corneal ectasia in patients with keratoconus [4].

First studied in a series of time-response and dose-response assays on rabbit and porcupine eyes, researchers found a 70-300% increase in corneal rigidity after CXL [4],[5],[6],[7]. The first few CXL procedures on keratoconic human patients, however, dates only as far back as 1998. In fact, the first clinical application of CXL was used to treat corneal melt [8].

In 2003, research showed that CXL appeared to halt the progression of keratectasia [5]. Subsequent studies with limited follow-up data also supported these findings [9],[10]. In addition, a controlled, prospective study of the effect of CXL on keratectasia patients, which was completed in early 2009, showed that the corneal shape appears to undergo a process of regularization, evoluting toward a more 'normal' shape during the first year after treatment [11].

Results of several recent studies indicate that CXL results in corneal stiffening [12]. In a recent in-vitro study on human corneas, an overestimation of true intraocular pressure (IOP) was found as a result of CXL induced by riboflavin and ultraviolet A (UVA) [13]. To our knowledge, there is one clinical study in the literature [14] that directly evaluates the effect of CXL with riboflavin and UVA on IOP measurements by performing Goldmann applanation tonometry (GAT).

A standard CXL procedure begins with the administration of a topical anesthetic, followed by debridement of the central 7-9 mm of the cornea to allow a uniform diffusion of riboflavin into the stroma [5]. Next, 0.1% riboflavin, suspended in a 20% solution of dextran T500, is applied and allowed to permeate the cornea before UVA irradiation.

Riboflavin is reapplied every 5 min during a 30-min irradiation period. Following treatment, a topical antibiotic ointment is applied until corneal re-epithelialization is achieved [5]. Bandage soft contact lenses can be used for pain management and/or to enhance healing.

UVA irradiation has a toxic effect on cell viability and can cause keratocyte and corneal endothelial cell destruction or death, as well as possible lens and retinal damage [5],[15],[16].

In addition, it is suggested that CXL treatment be restricted to the anterior 250-350 μm of the stroma. Thus, CXL is not recommended in patients whose corneas are thinner than 400 μm [16]. Because 85-90% of UVA radiation is absorbed in the anterior 400 μm of the cornea, the procedure should spare the patient's deeper corneal structures, crystalline lens, and retina [15].

To protect the corneal endothelium and deeper ocular structures, the currently used treatment parameters are set; hence, the anterior 250-350 μm of the corneal stroma is treated. Accordingly, the current inclusion criteria require a minimum stromal thickness (without the corneal epithelium) of 400 μm, including a safety margin [16].

 Aim of study

The aim of this study was to evaluate the effect of riboflavin-UVA CXL on GAT readings in grade two keratoconus patients with a normal preoperative IOP and no history of glaucoma, diabetes, or other local eye disease. The correlation between preoperative and postoperative IOP measurements and preoperative corneal pachymetry was also evaluated.

 Patients and methods

Inclusion criteria and protocol

This prospective clinical study enrolled 49 eyes of 40 patients with keratoconus, who underwent corneal CXL induced by riboflavin and UVA in one or both eyes. All patients were examined using slit lamp biomicroscopy, and the IOP was measured by performing GAT. Fundus examination was performed in all patients using indirect ophthalmoscopy, and any patient suffering from coexisting retinal disease was excluded from the study. All patients were between 16 and 32 years of age. None of them was diabetic, nor did they have a local eye disease other than keratoconus. The clinical diagnosis of keratoconus was based mainly on corneal topography data (using Pentacam rotating Scheimpflug camera) and clinical signs such as Fleischer rings, Vogt striae, stromal thinning, and conical protrusion of the cornea.

After being informed about the nature of the study, before their participation all patients gave informed consent in accordance with the guidelines in the Declaration of Helsinki. Data obtained from the patient records included age and sex, preoperative and postoperative IOP measurements determined with GAT, keratometry (K) readings (Pentacam), central corneal thickness (CCT) using a Pentacam rotating Scheimpflug camera, and slit lamp biomicroscopy and fundus examination findings.

Surgical procedure

Corneal CXL was performed under sterile conditions. The patient's eye was topically anesthetized with benoxinate hydrochloride 0.4% eyedrops (Benox; Eipico). A 7.5-8.0-mm-diameter section of corneal epithelium (including the cone) was mechanically removed with a spatula, and sterile riboflavin 0.1% solution was instilled every 2 min for 30 min. UVA irradiation was performed using the commercially available UVA system (UV-X; IROC AG, Zurich, Switzerland). Irradiation was performed for 30 min, corresponding to a dose of 5.4 J/cm 2 . During treatment, riboflavin 0.1% solution was applied every 2-3 min to saturate the cornea. At the end of the procedure, a silicone-hydrogel bandage contact lens was applied until full re-epithelialization had occurred, which was typically after 4 days from surgery.


Tonometry measurements were recorded using GAT with sterile sodium fluorescein strips. The same tonometer (Haag- Streit AG) was used throughout the study. Tonometry measurements were taken at the center of the cornea (before and after CXL). Corneal topography, CCT, and K readings (Pentacam) were examined and recorded by a masked observer. The K measurements were actual K readings in the flat axis and steep axis. The GAT measurements, CCT, and K measurements before surgery as well as IOP measurements at 3, 6, and 12 months postoperatively were included in the statistical analysis.

Statistical analysis

The mean preoperative IOP was 11.2±1.29 mmHg, whereas the mean postoperative IOP values were 12.24 mmHg (1.13 SD) at 3 months, 12.37 mmHg (1.13 SD) at 6 months, and 12.55 mmHg (1.12 SD) at 12 months. An a-priori two-tailed paired t-test power calculation showed that a total sample size of at least 25 patients was required to achieve a power of 0.95. Results were presented as mean ± SD.

Analysis of variance was used to compare IOP measurements before and after CXL. Correlation analysis was used to test the influence of variables (e.g. patient age and sex, preoperative corneal pachymetry, preoperative K readings) on the postoperative changes in IOP measurements.

A P-value of less than 0.05 was considered statistically significant.

A one-way Kolmogorov-Smirnov test was used to examine the normality of the data.


The study enrolled 49 eyes of 40 patients with a mean age of 22.4 ± 3.1 years (range 16-32 years). The apices of the patients' cones were mostly inferior (38 eyes); the rest (11 eyes) were central. No intraoperative or postoperative complications occurred. There was a statistically significant increase in IOP measurements at 3, 6, and 12 months [Figure 1] after CXL when compared with the preoperative IOP (P = 0.000).{Figure 1}

The mean IOP measurement was 11.2±1.29 mmHg before CXL, whereas it was 12.24 mmHg at 3 months, 12.37 mmHg at 6 months, and 12.55 mmHg at 12 months after operation. There was no statistically significant change in IOP between the 3- and 6-month postoperative measurements (P = 0.341), nor between the 6- and 12-month postoperative measurements (P = 0.499). However, a statistically significant difference in IOP was noticed between the 3- and 12-month postoperative measurements (P = 0.011).

A strong positive correlation [Figure 2] was found between preoperative CCT and preoperative IOP readings (P = 0.000, r = 0.789) as well as IOP readings at 3, 6, and 12 months (P = 0.000, r = 0.652), which means that the higher the preoperative pachymetric reading the higher the preoperative and postoperative IOP readings. However, we found no statistically significant correlation between postoperative IOP measurements and patient age (P = 0.433), sex (P = 0.227), or preoperative K readings (P = 0.071).{Figure 2}


Collagen CXL with riboflavin and UVA is a rather new surgical technique used in the treatment of keratoconus. The treatment is based on the activation of photosensitizer riboflavin by UVA, which produces oxygen radicals that induce the formation of strong chemical bonds between the collagen fibrils and thus increases corneal stiffness [17]. Several studies [5,6] report corneal stiffening after riboflavin-UVA CXL to treat keratoconus. The corneal stiffening might be correlated with an increase in corneal and ocular rigidity. Ocular rigidity is a measurable physical parameter of the eye that expresses the elastic properties of the globe. In 1937, Friedenwald [18] described the coefficient of ocular rigidity as a 'measure of the resistance, which the eye exerts to distending forces'. Results in a recent in-vitro study [13] indicated that riboflavin-UVA CXL in human corneas results in overestimation of true IOP, in the range of 1.8-3.1 mmHg depending on the tonometer type (GAT, dynamic contour tonometry, Tono-Pen XL). Nevertheless, this overestimation was considerably smaller than the magnitude of overestimation expected from theoretic calculations [19], despite a reported increase in corneal rigidity after CXL in human corneas of up to 330% [6]. This result might be because of the maximum stiffening effect of CXL in the anterior corneal stroma [6],[15]. In another recent study [20], it was found that CXL with a dialdehyde agent in human corneas led to significant increase in wave velocity and transcorneal IOP measurements, despite a constant intravitreal pressure.

In our study, there was a statistically significant increase in measured IOP using GAT after CXL with riboflavin and UVA at 3, 6, and 12 months, postoperatively.

Biomechanical alterations and corneal rigidity increments are probably related to IOP changes after CXL. The change in IOP readings at 3, 6, and 12 months was positively correlated with preoperative CCT; yet, it was not correlated with patient age, sex, or preoperative K readings. Ehlers et al. [21] reported that applanation tonometry provided accurate IOP measurements only when the CCT was 520 mm, whereas thinner and thicker corneas gave false lower readings and false higher readings, respectively.

Nevertheless, in our study, neither CCT nor K readings changed after CXL. Although we believe that alterations in corneal rigidity and elasticity by CXL may induce an overestimation of IOP, we cannot exclude the possibility that the 'true' IOP increased after CXL. This possibility could be verified through a real-time measurement of the IOP using an invasive method. Thus, the preoperative IOP levels (used as baseline IOP) should be taken into account in patients treated with CXL, especially if glaucoma is suspected.

There are some limitations to this study. The change in IOP may have been a response to aqueous humor dynamics to the surgery (decrease in outflow from the effect on the trabecular meshwork), or the CXL procedure may have had an unknown effect on IOP readings. Moreover, this was a noncomparative study with no control group. Finally, the IOP measurements were performed using only GAT, without the use of other tonometer devices (e.g. dynamic contour).


In conclusion, after CXL with riboflavin and UVA in eyes with keratoconus, there was a significant increase in IOP measured using GAT. The increase was probably the result of an increase in corneal rigidity. The increase in corneal rigidity led to an increase in measurement and not to a true increase in IOP.


Conflicts of interest

There are no conflicts of interest.


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