Evaluation of Higher Order Aberrations after Cross-linking in patients with early Keratoconus

INTRODUCTION

Non-inflammatory corneal thinning and steepening are hallmarks of keratoconus (KC), which affects both eyes.¹

The causes of KC are several. ² If left unchecked, keratoconus typically worsens until the third or fourth decade of life, when it stops growing. Although it may begin later in life and progress or stop at any age, it is possible that it is hereditary in some cases. ³

Most of the time, this disease is limited to the cornea's inferior central two-thirds. There have been reports of corneal steepening that is specific to each keratoconic eye having central, inferior nasal, and inferiorly located cones. There have also been reports of superior cones. More than half of keratoconus patients have cones that are most typically shifted inferior temporally, according to numerous studies. ⁴

 A wide range of visual issues can be caused by keratoconus, including significant amounts of myopic astigmatism, with blurring and distortion, to ghosting and poor low-contrast acuity. In the severest form of the disease, corneal scarring is common. In keratoconus it is believed that corneal thinning,

protrusion, and scarring causing the loss in visual performance. ⁵

In severe stages, KC is rather easy to detect, but in the early stages, where the cornea appears to be healthy, it is more difficult. Astigmatism with an oblique axis, increasing degrees of astigmatism, and advancement in astigmatism and/or spherical refraction are all indications that KC should be considered in patients. ⁶

To improve corneal tissue's mechanical and chemical stability, corneal collagen cross-linking (CXL) was developed. The major goal of this treatment is to establish new chemical connections inside the corneal stroma by highly targeted photo-polymerization while minimizing exposure to the surrounding eye structure. Free radicals drive the mechanism, which increases covalent bonding within and between collagen fibrils and between collagen molecules. Due to the cornea's structural alterations as well as its biochemical properties and biomechanical, the cornea's collagen fibrils have grown in diameter, making it more resistant to enzyme digestion and hydrothermal shrinking, which indicates better stability. ⁷

Preserving current vision and avoiding corneal transplantation are two of the key goals of treatment in terms of clinical outcomes. ⁸

PATIENTS AND METHODS

In this study, 40 eyes evaluated for CXL and meeting the inclusion criteria were compared prospectively.

The study conducted at Nour El Hayah Eye Hospital Cairo Egypt.

All patients provided written informed consent after the nature of the procedure was explained in addition to the benefits and possible risks explained.

Full medical history has been taken and ocular examination done to exclude any other pathology.

Medical /Ophthalmic history was taken including symptoms of HOAs like double vision, halos, glare, night blindness, starburst patterns and blurring.

Ophthalmologic examination including: (1) Uncorrected visual acuity (UCVA), Best corrected visual acuity (BCVA). (2)            Ocular examination using slit lamp biomicroscopy.

Assessments of corneal topographic changes before and 6 months after CXL.

Evaluation of HOAs by Shack-Hartmann aberrometry ( Bausch & Lomb Zywave III) before and 6 months after CXL.

The Technique of CXL:

The technique was performed under topical anaesthesia under sterile condition. Riboflavin is injected into the corneal stroma after the corneal epithelium is removed using a blunt spatula.

Before and after the treatment, 0.1 percent riboflavin eye drops are used(Riboflavin eye drops 0.1% are applied to exposed stromal surface five minutes prior to the CXL procedure and then every five minutes during the treatment) exposure to UVA radiation (370nm) at 3mW/ cm2 for 30 minutes on the cornea's surface. If the cornea is thicker than 400 micrometers, the method is safe and does not cause endothelium toxicity. When riboflavin is used at this low riboflavin content at this UV wavelength and energy level.

Postoperatively, antibiotic drops in the form of moxifloxacin and corticosteroid drops in the  form of prednisolone acetate 1% were administered four times per day for one week. A bandage lens is placed over the cornea. The patients were also prescribed lubricant gel that was administered four times per day for one week.

Slit-lamp examinations were performed on all patients on the day of surgery, as well as one week and one month thereafter. All patients have had corneal topography and Zy wave aberrometers done six months after CXL to examine changes in the HOAs before and after the procedure.

SPSS application was used to enter, validate, and analyze the acquired data (Statistical Package for Social Science).

Tables and graphs of collected data are examined using proper statistical procedures by computer software.

Statistical methods:

SPSS version 18 was used to analyze the collected data (SPSS Inc., Chicago, IL, USA). Statistics such as mean and standard deviation (SD) were used for quantitative information, and percentages were used for qualitative information. On the other hand, the Chi square or Fisher's exact test was employed to establish whether or not the difference was statistically significant for quantitative variables. It was considered statistically significant when the P-value was 0.05 or lower.

RESULTS

Study population:

The study included 40 eyes of 40 patients.

All patients completed their six months follow-up period.

Table 1: All patients studied had their age and sex documented.

Table 2: Comparison between pre-operative and post-operative Root mean square (RMS) Higher Order Aberration (HOA) in studied patients.

There was statistically significant (p-value = 0.002) decreased RMS HOA (5mm) post-operatively (1.11 ± 0.36). There was highly statistical significant (p-value < 0.001) decreased RMS HOA (6mm) post-operatively (1.29 ± 0.41) .

Table 3: Comparison of pre- and post-operative Higher Order Aberration (HOA) levels in the patients being studied.

This table shows:

Neither horizontal nor vertical trefoil had statistical significance (p-value 0.05) before or after surgery.

Highly statistical significant (p-value < 0.001) decreased post-operative Horizontal coma (0.494 ± 0.052) when compared with pre-operative Horizontal coma (0.601 ± 0.053).

Highly statistical significant (p-value < 0.001) decreased post-operative Vertical coma (1.42 ± 0.07) when compared with pre-operative Vertical coma (1.64 ± 0.07).

Highly statistical significant (p-value < 0.001) decreased post-operative spherical aberrations (0.406 ± 0.051) when compared with pre-operative spherical aberrations (0.520 ± 0.061).

Statistically significant (p-value = 0.011) decreased post-operative 2^nd horizontal trefoil (0.039 ± 0.008) when compared with pre-operative 2^nd horizontal trefoil (0.042 ± 0.006).

Statistically significant (p-value = 0.044) decreased post-operative 2^nd vertical trefoil (0.074 ± 0.008) when compared with pre-operative 2^nd vertical trefoil (0.078 ± 0.008).

Highly statistical significant (p-value < 0.001) decreased post-operative 2^nd horizontal coma (0.094 ± 0.032) when compared with pre-operative 2^nd horizontal Coma (0.126 ± 0.025).

Highly statistical significant (p-value < 0.001) decreased post-operative 2^nd vertical coma (0.286 ± 0.047) when compared with pre-operative 2^nd vertical Coma (0.378 ± 0.059).

Table (4): Pre and postoperative K values in the patients investigated are compared.