A nasal polyp is a clump of epithelial cells that forms a small bump or overgrowth of tissue along the lining the nasal cavity, the inside of the nose.
The nasal cavity is made up of three regions.
The first, is the nasal vestibule which is the area just inside the nostrils.
Beyond that, is the respiratory region which delivers air to the sinuses and lungs, and above it is the olfactory region, which is involved in smelling.
Lining the respiratory region are epithelial cells that create mucus to moisten the air and trap pathogens.
There are also air-filled spaces within the skull that are on either side of the the nose called paranasal sinuses which are lined by the same layer of epithelial cells as the respiratory region.
The paranasal sinuses are named for the bones that house the sinus: the sphenoid, located next to the eyes; the ethmoid, between the eyes; the frontal, above the eyes behind the forehead; and the maxillary, behind the cheeks and below the eyes.
Each of the sinuses normally produce mucus, which drain into the respiratory region.
Holes at the back of the respiratory region, called choanae act like funnels to direct the mucus into the throat to be swallowed.
Nasal polyps develop when epithelial cells that line the respiratory region simply overgrow - a process called hyperplasia.
Most of the time, one or more nasal polyps forms in the maxillary or ethmoid sinus.
Nasal polyps can get large - the size of a pea, but they are usually non-cancerous, meaning they don’t break through the basement membrane of the epithelium.
Unfortunately, they often obstruct air flow as well as mucus drainage which allows pathogens to linger in the sinuses and cause infections.
Recurrent infections causes mucosal swelling as immune cells infiltrate the tissue and create an inflammatory response.
The swelling makes airway obstruction and mucus drainage even worse.