Keywords
Antrochoanal polyp - CT - osseous metaplasia
Introduction
Osseous metaplasia of inflammatory sinonasal polyp is an extremely rare entity. Preoperative imaging evaluation of sinonasal polyps is usually done by noncontrast CT scan of paranasal sinuses. Correct identification of this rare entity preoperatively and differentiation from radiological mimics is of utmost clinical importance.
Discussion
The term antrochoanal polyp refers to polyp that arises from antrum and extends out of maxillary sinus into nasal cavity through choana into nasopharynx.
Frosini et al.[[1]] in their landmark paper has proposed a mechanism in which antrochoanal polyp are formed. There is initial infective/allergic process that leads to obstruction to acinar mucous glands and retention cyst formation. Subsequently due to narrowing or complete blockage of maxillary ostium and accessory maxillary sinus ostium, there is development of high pressure within the maxillary cavity. This high pressure results in herniation of retention cyst through accessory maxillary ostium resulting in formation of antrochoanal polyp.
Histopathologically, antrochoanal polyps are inflammatory. The polyps are lined by ciliated respiratory epithelium. At the submucosal level, there are abundance of plasma cells with scanty eosinophils.[[2]] The overlying respiratory epithelium has been reported to undergo dysplastic or squamous metaplastic changes.[[3]]
Osseous metaplasia of nasal mucosal polyps is a known rare entity. Cases of mucosal polyps with osseous metaplasia have been reported in other parts of the body such as stomach,[[4]] rectum,[[5],[6],[7]] and colon.[[8]] Osseous metaplasia of mucosal polyps is a much rarer entity with cases being reported from external auditory canal, tongue, and nasal polyps.[[9]]
The exact mechanism of osseous metaplasia is still vastly unknown. Mesenchymal cells in mucosal polyps are believed to differentiate into osteoblasts with raised local alkaline phosphatase secretion.[[10]] Role of bone morphogenetic proteins[[11]] and upregulation of type-1 collagen and osteonectin[[12]] have been also implicated.
A literature survey reveals only few sporadic published cases[[9],[13],[14],[15],[16]] of osseous metaplasia of nasal polyp in last four decades and the largest case series was of five patients published by Kim et al.[[17]]
Osseous metaplasia is a rare entity and presence of bone density component within an endonasal lesion can simulate other common entities such as chronic fungal allergic polyps with hyperdense components, rhinoliths, and tumours such as osteosarcoma, chondrosarcoma, and fibro-osseous lesions.
In our case, NCCT showed hyperdense bony component which was centrally located. It was completely surrounded by soft tissue on all sides. There was mild widening of accessory maxillary ostium and mild remodelling of nasal cavity. No gross destruction of walls of maxillary sinus, nose, or nasopharynx was noted. These features made the alternate diagnosis of neoplastic lesion like osteosarcoma unlikely. The ring and arc pattern of chondroid calcification that is seen in chondrosarcoma was absent.
Our case could be easily distinguished from rhinoliths and fungal elements on NCCT because of large size, dumb-bell shape of bony component with linear well-defined trabeculae extending from the maxillary sinus through accessory ostium into nasal cavity, and nasopharynx.
Management of sinonasal polyps with osseous metaplasia is similar to other sinonasal polyps and endoscopic sinus surgery remains the current treatment of choice. However, due to hardness of the polyp, extraction of the mass can be challenging. Jacono et al.[[17]] has reported a case where the nasal polyp with osseous metaplasia was so hard that the mass had to be delivered through mouth.
Our case was managed through endoscopic sinus surgery uneventfully.
Conclusion
Though only few cases of osseous metaplasia have been published in literature till date and no cases have been published from India to our knowledge till date, radiologists need to be aware of the existence of this entity and be able to differentiate from other radiologic mimics. A large bony component within a polyp may lead to significant intraoperative difficulty in removal of the polyp and increase duration of operation thereby increasing the possibility of subsequent postoperative complications. Preoperative diagnosis of osseous metaplasia helps in improved surgical planning and may lead to eventual better patient outcome.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
