Sinonasal Mucocele 8.4.1
Definition, Epidemiology, Pattern of Growth
Mucocele may be defined as an accumulation of secretion products, desquamation, and inflammation within a paranasal sinus with expansion of its bony walls. The lesion is limited by a wall ("sac") made by respiratory mucosa with a pseudostratified columnar epithelium. According to Lloyd et al. (2000a), who reported data on 70 mucoceles, 89% of the lesions were located in the fronto-ethmoid area, while maxillary, ethmoid and sphenoid mucoceles were rarely observed. Mucocele more frequently occurs in the fourth and fifth decades of life, with approximately the same distribution in men and women.
The development of the lesion is thought to occur as the result of sinus ostium blockage. The accumulation of mucus creates a positive pressure inside the cavity (Kass et al. 1999), which might explain resorption of the surrounding bony walls. Nevertheless, these factors cannot be considered fully responsible for the process, since sinus occlusion can also result in chronic sinus atelectasia (Kass et al. 1997). According to Wurster et al. (1986) and Lund et al. (1988), there is a dynamic process occurring at the interface between mucocele and bone, as suggested by high levels of bone re-absorbing factors such as PGE2, IL-1, and TNF, which have been demonstrated in the mucosa of the affected sinuses. Furthermore, histology more often demonstrates an active secretory columnar ciliated epithelium instead of a thin atrophic cuboidal epithelium, as would be expected from the positive pressure existing inside the lesion (Lund and Milroy 1991). As mucus continues to be produced, mucocele gradually expands, resulting in remodeling and/or erosion of the surrounding bone, with possible intraorbital and intracranial extension. Secondary infection (pyomucocele) can lead to a rapid expansion with significant risk of complications mostly involving the orbit.
Sinus ostia obstruction may be the consequence of chronic rhinosinusitis, allergic rhinitis, previous radiotherapy, trauma, scarring due to previous surgical procedures, or, more rarely, of sinonasal neoplasms. In recent years, some authors have alluded to a possible increase in the occurrence of mucoceles related to previous surgery (Rambaux et al. 2000); however, these data must be interpreted very cautiously.
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