Nasal polyps typically form after chronic inflammation of the nasal passages, according to Medical News Today. Conditions such as asthma, frequent sinus infections and allergies may lead to nasal polyps. Nasal polyps start out as irritation in the nasal mucosa, or the wet layer that protects the inside of the nose, notes Healthline. This layer swells during a nasal infection or an allergic reaction, and continued irritation may lead to nasal polyps.
A polyp forms after fluid accumulates in the cells of the nose and sinuses, says Medical News Today. Scientists believe gravity pulls these heavier, fluid-laden cells downward to form polyps. These fluid-filled sacs occur most often near the openings of the nasal passages, although they can form anywhere in the nasal cavity. Swelling causes polyps as opposed to an actual infection or a disease. Doctors may diagnose nasal polyps based on medical history and a visual examination of a patient's nasal passages, while a nasal endoscope may help a doctor view any nasal polyps hidden from an initial inspection.
As of 2015, doctors do not know the exact cause of nasal polyps, according to Mayo Clinic. Some evidence indicates people who develop nasal polyps have different immune system responses to nasal infections. These people may also have different chemical markers in mucous membranes versus people who do not develop nasal polyps.