To report a case of masked glaucomatous optic nerve head damage due to acute swelling in a primary open-angle glaucoma patient.

A healthy 21-year-old male visited our clinic complaining of blurred vision in the right eye for 1 week. The intraocular pressure (IOP) was 60 mmHg, as measured by a Goldmann applanation tonometer. No specific anterior segment finding other than severe corneal edema was found on slit lamp examination. Maximum tolerated medical therapy was performed, and a further examination was done 1 day after the IOP lowering. No glaucomatous change in the optic disc or retinal nerve fiber layer was observed on fundus examination and optical coherence tomography (OCT), but the optic disc of the right eye was more hyperemic than that of the left eye. A superonasal visual field defect was also observed using automated perimetry. After treatment, the IOP was kept within the normal range using IOP-lowering eye drops. However, an inferonasal retinal nerve fiber layer defect was observed on fundus examination and OCT, and a superonasal scotoma was detected by perimetry.