It is aimed to investigate the role of supplemental systemic oxygen therapy during accelerated corneal collagen crosslinking (CXL).

In this prospective randomized clinical trial, patients with progressive keratoconus who are candidates for receiving CXL will be included. The patients will be randomized and allocated to three different CXL protocol. In the first group (OA-CXL), CXL is performed using an accelerated protocol (9 mW/ CM2 for 10 minutes) in addition to the delivery of systemic oxygen with a rate of 5 liters/min through a nasal mask for 10 minutes during UV-A ablation. Patients in the second group (A-CXL) will receive CXL with the same accelerated protocol without additional oxygen therapy. Conventional CXL using 30 mW/CM2 UV-A ablation for 30 minutes is perfomed in the patients of the third goup (C-CXL). Maximum keratometry in Sirius corneal tomography as the primary outcome and uncorrected and corrected distance visual acuity (UDVA, CDVA) and corneal biomechanical properties including corneal resistance factor (CRF) and corneal hysteresis (CH) as secondary outcome measures are followed for a six- and 12-months period. A P-value of less than 0.05 is considered as statistically significance level.

Inclusion Criteria:

• definitive diagnosis of progressive keratoconus for whom CXL is indicated were enrolled in this study
• The indication is identified by a cornea specialist and was considered as follows.
• Patients with the definitive diagnosis of KC who aged 18 or more were considered for the procedure if they had an increase of 1 D in Kmax or an increase of 1D in the manifest cylinder over the past 12 months.

Exclusion Criteria:

• patients with the previous history of corneal surgery
• corneal scar or pathologies like herpetic eye disease or severe dry eye
• corneal thickness less than 400 micrometers
• pregnant or nursing women
• corneal Kmax more than 60
• age over 35 years
• severe ocular surface disease