Answer: Choanal polyp

Histology: The polypoid nature of the specimen is largely due to accumulation of submucosal edema fluid with microcyst formation. The inflammatory infiltrate is rather scant and composed mostly of lymphocytes and plasma cells. Eosinophils are notably absent. The overlying surface epithelium is comprised of pseudostratified respiratory epithelium. It is not thickened or permeated by intraepithelial neutrophils.

Discussion: By far the most common sinonasal polyp encountered by surgical pathologists is the inflammatory (i.e. allergic) polyp. These typically are encountered in adults as bilateral and multiple lesions in patients with allergic sinusitis. Outside of this typical setting, other types of sinonasal polyps should be considered. For example, bilateral multiple polyps in children should always prompt consideration of polyps of cystic fibrosis. Although the clinical history will often point to this diagnosis, in a small percentage of individuals with cystic fibrosis, polyp formation will be the initial manifestation of disease.

The presence of a sinonasal polyp that is solitary and unilateral raises the concern of a neoplastic process such as Schneiderian papilloma. Schneiderian papillomas may show the typical background features of an inflammatory polyp (e.g. stromal edema, chronic inflammation), but the epithelium is thickened, inverted and heavily permeated by infiltrating neutrophils.

The choanal polyp is an important exception to the generalized rule that nasal polyps are multiple, bilateral and arise in the setting of allergic sinusitis. These are typically solitary lesions that occur in adolescent males but can occur in females and adults as well. Although they usually present as nasal cavity masses, they originate in one of the sinuses and then involve the nasal cavity or nasopharynx secondarily as they extend through the ostium via a long pedicle.

By far the most common choanal polyp arise from the antrum of the maxillary sinus (antrochoanal polyps), but they can rarely arise from the sphenoid sinus as well (sphenochoanal polyp). Origin from this unusual and unexpected site can cause diagnostic confusion. The distinction from an inflammatory polyp is mostly clinical, but there are some subtle histologic differences. Compared to inflammatory polyps, sinonasal polyps tend to have a more fibrous stroma, they lack eosinophils, they contain fewer glandular elements, and they do not demonstrate thickening of the basement membrane.