Patient had left eye proptosis.
There is evidence of an expansile lesion within the left frontal sinus, bulging into the superior aspect of the orbit with indentation on the globe. This lesion is hypointense on the T1W images and hyperintense on the T2W and STIR image.
An expansile lesion of a paranasal sinus is called a mucocele. They are
sterile lesions. They may present with facial deformity, proptosis or
enophthalmos, loss of vision or diplopia, facial pain, headache or nasal
obstruction. If it gets infected, it is referred to as a mucopyocele. Two thirds
occur in the frontal sinuses, 20% to 25% in the ethmoid sinuses, about 10% in
the maxillary sinuses and only a few percent in the sphenoid sinus.
Frontal sinus mucoceles typically extend into the ipsilateral orbit at the
superomedial orbital margin. They tend to cause pressure deossification of the
posterior sinus table and bulge intracranially. Ethmoid sinus mucoceles extend
primarily into the orbit. However, intracranial growth can occur and can spread
dorsally into the sphenoid sinus. Sphenoid sinus mucoceles may compress the
optic nerve. Maxillary sinus mucoceles can erode back towards the
pterygopalatine fossa and central skull base or towards the orbit.
Pathology:
They develop when a sinus ostium or a compartment of a sinus becomes obstructed and the sinus fills with mucoid secretions. The cavity slowly expands as the surrounding sinus walls are remodeled. Histologically, a mucocele is typically described as a cyst lined by flattened, pseudostratified, ciliated columnar epithelium. There is reactive bone formation lying in proximity to the cyst epithelium. Reactive changes, such as fibrosis, granulation tissue, hemorrhage and cholesterol granuloma may be seen, and in long-standing cases squamous metaplasia may even be seen. If a completely filled sinus has normal bony sinus walls without sinus cavity expansion, it is an obstructed sinus and not a mucocele.
Imaging:
On plain Radiographs, the mucocele may appear as a clouding of the involved
sinus. The normal thin mucoperiosteal white line becomes attenuated. Dense
reactive bone may surround an involved sinus. The sinus would be expanded.
On CT, they appear as airless sinuses that have an expanded sinus cavity
surrounded by remodeled bone. This bone may be focally eroded but is usually
intact. The HU value of the sinus secretions is usually between 10 to 20 HU.
Occasionally, the attenuation of the secretions may be higher, probably due to
an increased protein concentration.
On MRI, mucoceles can have varied appearances that reflect the changing protein
concentration, amount of free water and viscosity. The sinus is airless and the
cavity is expanded. Mucoceles, when large, may coexist with polyps. Focal areas
within may be due to desiccation or may represent a site of local hemorrhage.
This leads to inhomogeneity of signal intensity within the involved sinus. They
may be hypointense on the T1W images and hyperintense on the T2W images,
hypointense on the T1W and T2W images, hyperintense on the T1W images and
hypointense on the T2W images or hyperintense on the T1W and T2Wmages.
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