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Neuroradiology Case of the Week

Case 77

Ravinder Sidhu MD and P-L Westesson MD, PhD, DDS

Clinical Presentation: An 85-year-old male presented with right-sided head pain, more so on lying down on his right side.

Radiological Findings:  Non-contrast enhanced head CT showed a large, hypodense (CT value-7 HU), expansile mass of 6.3 x 4.8 x 4 cm size in the region of sphenoid sinus. The mass was seen to extend laterally into greater and lesser wings of sphenoid wings and expanding them. Posteriorly, the mass was extending till the clivus. Posterior ethmoid air cells were pushed anteriorly. Left temporal lobe and cavernous sinus were displaced upwards (Fig. 1). The left pterygoid plates were expanded and thinned out (Fig. 2A). Few hyperdense specks were seen at the periphery of the mass suggestive of thinning of the surrounding bone (Fig. 2B). There were small retension cysts in the maxillary antrum but these did not fill the entire cavity and did not expand and were therefore not mucoceles (Fig. 3).

Diagnosis: Sphenoid mucocele

Discussion:  Mucoceles are benign, expansile, cyst-like paranasal sinus lesions lined with a secretory respiratory mucosa of pseudostratified columnar epithelium. The etiology of mucoceles is not clear, however, prior sinus disease, allergic history and trauma have all been implicated. In all cases, the initial event leading a mucocele is a blockage of the draining ostium of the sinus. As the mucosa secretes, the mass slowly enlarges, expanding and eroding the adjacent bony structures [1]. The most commonly affected paranasal sinuses are the frontal and anterior ethmoid sinuses. Sphenoid sinus mucoceles are the least common. Patients with sphenoid mucocele may present with intractable headaches, ophthalmic disorders (decreased visual acuity, visual field defects, and exophthalmos), and endocrine disorder from sellar extension [2].
     Radiologically, plain radiographs usually reveal sinus enlargement with opacification. Sinus wall erosion may also be seen. Computed tomography is superior to plain films in delineating the limits of a mucocele and involvement of contiguous structures. The CT characteristics of mucocele include homogeneous, non-enhancing, expansile sinus masses that completely fill the potential sinus cavity expanding or remodeling surrounding bone margins. Mucoceles generally do not enhance with contrast, but acutely inflamed mucopyoceles may show rim-enhancement [3].
     MR imaging is superior for demonstrating the interface of the mucocele with surrounding cranial structures. On MR imaging, the signal intensity of mucoceles varies in accordance with fluid content, presence of a proteinaceous component or hemorrhage. They usually have low-signal intensity on T1-weighted images and a signal void on T2-weighted images due to inspissated debris. Nasal mucosa, due to its highly vascular nature, shows high-intensity on T2-weighted sequence similar to inflamed or edematous sinus epithelium. Discrimination between tumor and mucosa can be made possible with T2-weighted images. On T1-weighted MR images, signal intensities are quite similar with normal mucosa and tumor. However, inflamed mucosa, nasal polyp, retained secretions and mucoceles all exhibit strong MR signal on T2-weighted sequences. Tumors produce signals of only moderate strength on T2-weighted images, thereby helping in differentiating between tumor and hyperplastic mucosa/secretions [4, 5].
     The differential diagnosis of an expansile mass involving the sphenoid sinus should include, in addition to mucoceles, pituitary adenoma, craniopharyngioma, malignant sinus lesion, meningoencephalocele, nasopharyngeal tumor, sphenoid osteoma, and chordoma. These lesions can usually be differentiated from mucoceles clinically and radiologically by the presence of contrast enhancement and an invasive pattern of bone destruction.

References:

1. Atlas SW, Bilaniuk LT, Zimmerman RA. Orbit. In Stark, D.D. and Bradley, W. G., Eds. Magnetic Resonance Imaging. St. Louis: C.V. Mosby; 1988: pp 570-613.
2. Campbell RE, Barone CA, Makris AN et al. Image interpretation session: 1993. Fungal mucocele (aspergillosis) of the sphenoid sinus. RadioGraphics 1994; 14:197-199.
3. Som PM. CT of the paranasal sinuses. Neuroradiology 1985; 27:189- 201.
4. Flanders AE, Rao VM. Paranasal sinus mucocele: unusual MR manifestations at 1.5 T. Magn Reson Imaging. 1989; 7:333-337.
5. Tchoyoson Lim CC, Dillon WP, McDermott MW. Mucocele Involving the anterior clinoid process: MR and CT findings. AJNR Am J Neuroradiol 1999; 20:287-290.