Teaching Case

A 57-year-old woman presented to the emergency department with three-week history of headache that worsened with intermittent left-orbital pain and diplopia for a week. The patient had no personal history of interest.

MRI shows a well-defined 2-cm ovoid lesion in a left posterior ethmoidal cell, hypointense on T2WI (Fig. 1a) and hyperintense on T1WI (Fig. 1b), associated with destruction of the lateral wall in NECT (Fig. 2). The lesion protrudes into the orbit at the level of the vertex, displacing the posterior portion of the medial rectus muscle, and courses a compression of the optic nerve both at the apex level and in the orbital canal. After contrast-administration, a diffuse enhancement of the sinus inflammatory component is observed, with no clear enhancement of the ethmoidal lesion in comparison with the precontrast T1WI sequence signal (Fig. 1c).

Paranasal mucoceles develop when chronic obstruction of a sinus ostium results in an accumulation and dilation of mucus within a paranasal sinus lined with respiratory epithelium. They arise most commonly in frontal sinus followed by the ethmoid, maxillary and sphenoid sinuses [1].
Clinical manifestation of paranasal mucoceles depends on the degree of the mass involvement and on its location. Typically, they may show rhinological, neurologic or ophthalmologic symptoms [2, 3]. The diagnosis of ethmoid sinus mucocele may not be straightforward initially, especially if clinical manifestation is subtle.
Fronto-ethmoidal sinus mucoceles may cause mass-effect upon the orbit and present with pain and ophthalmic symptoms such as proptosis, diplopia and periorbital swelling. As the thin walled lamina papyracea may be displaced into the optic canal by an expanding mucocele, visual compromise is more common with posterior ethmoid and sphemoid mucoceles. Optic neuropathy may occur due to direct mechanical compression, ischaemia or optic neuritis secondary to inflammation [3, 4, 5].
CT findings typically include an expansile, homogeneous mass that is not rim-enhancing. CT provides insight into positioning and potential bony erosion [1]. MRI is essential to differentiate mucocele from other lesions. Due to the protein concentration, the appearance of the mucocele varies [2]. Usually they show hypointensity in T1WI and hyperintensity in T2WI. Higher protein content may increase the T1-signal intensity and decrease the T2 signal intensity, and although normally they do not enhance after contrast administration, in some cases they may show peripheral enhancement [1, 6].
The treatment of mucoceles is based on surgical excision of the mass to prevent visual compromise or cases of piocele. In the case of visual problems, urgent operation is essential. The endoscopic approach has become preferred over the external approach because of minimal mucosal injury, short recovery time, and low recurrence rate [1, 3, 4].

Teaching points:
• Paranasal mucoceles develop when chronic obstruction of a sinus ostium results in an accumulation and dilation of mucus.
• Ethmoid sinus mucocele diagnosis may not be straightforward initially, especially if clinical manifestation is subtle.
• Posterior ethmoidal sinus mucocele may present with ophthalmic symptoms due to mass effect upon the orbit.
• CT provides insight into positioning and potential bony erosion of the lesion.
• They usually show hypointensity in T1WI and hyperintensity in T2WI, although higher protein content may increase the T1-signal intensity and decrease the T2-signal intensity. Normally they do not enhance after contrast administration.

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