PurposeTo investigate intraoperative anterior segment optical coherence tomography (AS-OCT) to compare pachymetry changes between two methods (group 1: speculum-on, group 2: speculum-off during riboflavin instillation stage) during corneal cross-linking /CXL) (9 mW/cm2 for 10 min) in keratoconic patients.MethodsIn this interventional case series, 11 eyes (11 patients) in group 1 and 14 eyes (14 patients) in group 2 were enrolled. Pachymetry measurements by spectral domain OCT was performed at baseline, after epithelial removal, after 30 min of imbibition with riboflavin, and after UV irradiation. Repeated measures analysis of variance was used to compare the trend of changes in different stages of the procedure within and between two groups.ResultsThe corneal thinnest pachymerty (CTP) at baseline, after epithelial removal, 30 min of riboflavin instillation, and irradiation was 469.8±26.5, 438.0±27.1, 354.3±28.4, 367.1±31.8 microns in group 1 and 463.2±25.1, 438.0±27.1, 421.6±54.0, 386.9±34.0 microns in group 2, respectively. Group 1 showed a significantly greater decrease in the corneal pachymetry (P<0.001) during riboflavin instillation, while in group 2 a greater decrease in CTP was observed during the UV irradiation phase (P=0.002).ConclusionsUsing the intraoperative OCT, the speculum-off group had a more stable corneal pachymetry during riboflavin instillation time. Despite this, the effect was not lasting throughout the procedure; significant changes were noted in the corneal pachymetry at the end of operation in both groups; further highlighting the limitations imposed by corneal stromal shrinkage using isotonic dextran containing riboflavin solution for CXL.