Teaching Case

60-year-old male who was admitted for drainage of subdural hygroma, developed sudden painless loss of vision in left eye on postoperative day 5. Visual acuity (VA) in left eye was limited to perception of light, along with ipsilateral afferent pupillary defect. Fundoscopy was unremarkable.

Plain MRI orbit showed an expanded onodi cell on left side with homogeneous T2 hyperintensity and intermediate T1 signal. There is mass effect on left orbit posteriorly, causing effacement of medial extraconal space, laterally displacing medial rectus and compressing the optic nerve, mainly the intracanalicular segment. CECT orbit showed the left onodi cell mucocele with severe thinning of ipsilateral medial orbital wall and dehiscence of the optic canal. There was no proptosis.

Background

Onodi cell, also known as a sphenoethmoidal air cell, represents a variant anatomy of the posterior ethmoidal air cell, which pneumatizes with a superolateral extension in relation to the sphenoid sinus, thereby coming in close proximity with the optic canal and the nerve inside [1]. Mucoceles arising from paranasal sinuses are benign epithelial-lined cystic lesions that usually contain sterile mucus, which accumulated due to ostial obstruction. Once expansile, they can be locally aggressive with drastic ocular and intracranial complications [2].

Clinical perspective

Onodi cell mucocele is a rare entity but with a high risk of optic nerve involvement and subsequent visual impairment. Most common presentation is unilateral painless loss of vision, which can be sudden or progressive. It may be associated with retro-orbital pain [3]. The diagnosis is almost always made through imaging.

Imaging perspective

Both CT and MRI are complementary to each other for diagnosing this condition. On CT scan, an expansile soft tissue density lesion is noted in the posterior ethmoidal air cell that is extending superolateral to the sphenoid sinus. Bony changes such as thinning of lamina papyracea and dehiscence of optic canal are well appreciated on CT. MRI allows better visualisation of the optic nerve, especially the intracanalicular part. The signal intensity of the mucocele is variable depending on the protein concentration of its contents. Enhancement may be peripheral or completely absent [4–6].

Outcome

Prompt intervention by decompression of the nerve through endoscopic sinus surgery is the definitive treatment for mucocele-related compressive optic neuropathy. Prognosis depends on factors such as visual acuity at presentation, nature of onset—sudden or gradual, delay in surgery [4,7]. In this case, patient was started on methylprednisolone pulse therapy followed by endoscopic sinus surgery and optic nerve decompression. His vision improved dramatically, with a VA of 6/20 at the time of discharge.

Take home message

Onodi cell mucocele is a rare cause for compressive optic neuropathy, with imaging playing a crucial role in diagnosis. Prompt decompression of the nerve through endoscopic sinus surgery can potentially prevent permanent visual impairment.

Written informed patient consent for publication has been obtained.