A case of a mucocele in an Onodi cell.

Onodi cells have been defined as posterior ethmoid cells that have pneumatized laterally and superiorly to the sphenoid sinus. They are often close to the optic nerve or the internal carotid artery. A patient complained of a sudden decrease in left eye visual acuity and left eye pain due to formation of a primary mucocele in an Onodi cell. A coronal computed tomography scan and magnetic resonance imaging were useful for diagnosing the mucocele.