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Editor,—In the literature Onodi cells occur in 3.4–51% of people.1 2 The paranasal sinus “Anatomic terminology group” defines the Onodi cell as the most posterior ethmoid cell which pneumatises laterally and superiorly to the sphenoid and is intimately associated with the optic nerve. Using this definition the incidence of Onodi cells is 8–14%.3-5
The occurrence of optic neuropathy caused by a pathological process in an Onodi cell is explained by the close relation to the optic nerve, because it often runs within the small cavity of the Onodi cell.6
Orbital inflammation associated with paranasal sinusitis is a well known cause of optic neuropathy,7 but an isolated mucocele of an Onodi cell causing optic neuropathy is rare.8 9 We report a case of acute visual loss caused by an isolated mucocele of an Onodi cell.
A 41 year old man was referred to our outpatient department in February 1998 with acute visual deterioration in the right eye and a central scotoma. He complained about having visual impairment for the past 2 weeks and “black dots” in his central visual field. His medical and family history were unremarkable.
Examination disclosed a visual acuity of hand movements on the right and 20/20 on the left. A swinging flashlight test revealed an afferent pupillary defect on the right. Slit lamp biomicroscopy and direct and indirect ophthalmoscopy were normal on both sides. Goldmann perimetry showed a central scotoma of 20° on the right and no pathological findings on the left. The clinical otorhinolaryngological status was unremarkable. Magnetic resonance imaging (MRI) showed a kidney bean-shaped mass of 1 × 2 cm and high signal intensity in the T2 weighted images in the right orbital apex, which compressed the optic nerve superomedially (Figs 1 and2).
Optic neuropathy caused by an isolated mucocele in an Onodi cell was diagnosed. The patient underwent endoscopic microsurgical sinus surgery. Intraoperatively the diagnosis was confirmed. Nine days after surgery the patient's visual acuity recovered to 20/30 and the initial central scotoma was reduced to a small paracentral scotoma.
Three months later the patient presented again with a recurrence of the mucocele, but without changes in visual acuity and visual field in comparison with the last examination. Surgery was repeated and showed a blockage in the former surgical field causing the relapse of the mucocele. Three weeks after the second intervention visual acuity improved to 20/20 and only a very small paracentral scotoma was detected. One year after the second operation the visual acuity is stable and the paracentral scotoma has disappeared.
The importance of the most posterior localised ethmoidal cell and its close relation to the optic nerve where first described by Adolf Onodi (1857–1920), professor of laryngology, University of Budapest, Hungary in 1904.10
Most authors have found an incidence of 8–14%.3-6 Onodi cells are mainly pneumatised laterally. Its location is usually superior and lateral to the sphenoid sinus.
ENT surgeons who perform endonasal sinus surgery, especially, should know about the anatomical variation. There is the substantial risk of injury to the optic nerve and even more, to the internal carotid artery, which is in close anatomical relation to the Onodi cell.2
Optic neuropathy is a well known complication of paranasal sinusitis and mucoceles,7 but there are only few cases in literature which describe an isolated mucocele in an Onodi cell as the cause of optic neuropathy.8 9 The mechanism of retrobulbar optic neuropathy is a mechanical compression of the optic nerve, because it often runs within the small cavity of the Onodi cell.6
Axial and coronal MRIs are of great value in detecting those lesions in the orbital apex.8 9 Coronal computed tomography is helpful in the differential diagnosis of an osseous origin and preoperative evaluation.2 3 8 9
In our patient we saw that an immediate decompression of the optic nerve led to considerable improvement of visual acuity and field, even in a case of drastic functional impairment. A close interdisciplinary cooperation with other medical specialties such as ENT and neuroradiology is essential for adequate diagnosis and treatment.
In patients with retrobulbar optic neuropathy an isolated Onodi cell mucocele should be considered in the differential diagnosis.