ABSTRACT
Acute visual loss caused by an infected mucocele in an Onodi cell is extremely rare. The Onodi cell is a pneumatized posterior ethmoid cell located laterally and superiorly to the sphenoid sinus and closely related to the optic nerve. Therefore, a mucocele affecting the Onodi cell that has encroached on the adjacent sphenoid bone forming the optic canal can rarely present with visual loss. We describe a rare case of retrobulbar optic neuritis caused by an infected mucocele in the Onodi cell. A 54-year-old male complained of headache and visual loss in his right eye. A computed tomography scan and magnetic resonance image demonstrated a mucocele occupying the Onodi cell on the right side. Surgical treatment with an endoscopic sinus approach was performed, resulting in improvement of visual acuity. A lesion in an Onodi cell may be associated with ocular symptoms even if the lesion is isolated or small. Imaging studies should be considered for the differential diagnosis because early diagnosis and prompt surgical treatment for mucocele are needed for recovery of visual function.