The three primary functions of the nose are to aid in respiration, to filter and defend against external particles and allergens, and to enable olfaction. The nasal cycle is a physiologic alternation of resistance between the two nasal airways, created by changes in congestion and decongestion, and may aid in respiratory defense.[4][5]

Respiration

As air is inhaled, the nasal cavity assists in respiration by preparing air for oxygen exchange. Due to the narrow nature of the cavity, inhaled air is rapidly introduced to a large mucosal surface area with a rich supply of blood at body temperature. This process facilitates rapid acclimation of the inhaled air to temperature better suited for the lungs.[6] Humidification functions to protect the fragile respiratory and olfactory epithelia.[7]

Defense

The nasal cavity also aids in defense of respiratory tissues. Mucus secretions trap particles and antigens carried into the respiratory system during inhalation. As pathogens become trapped in these secretions, they are bound by secretory IgA dimers (a component of the adaptive immune response), which prevents attachment of pathogens to host epithelium, thus hindering invasion. Mucus can also contain IgE, which is involved in the allergic response and can cause a pathologic type 1 hypersensitivity reaction.[8] Cilia within the nasal cavity also function to propel mucus away from the lungs in an attempt to expel trapped pathogens from the body.[9] The normal bacterial flora in the nasal mucosa also protects from invasion by competing with invading bacteria for space and nutrients.[10]

Olfaction

Additionally, the nasal cavity enables olfaction. Olfaction helps to identify sources of nearby danger or nutrition, as well as influencing mood and sexuality.[11] As air enters the nasal cavity, the turbinates function to direct a portion of the airflow to the higher regions of the cavity. The olfactory cleft is at the roof of the nasal cavity near the cribriform plate. Olfactory receptors located here bind to odorants carried into the nose during inhalation and send signals to the olfactory cortex and other brain regions.[12]

Nasal Endoscopy

Nasal endoscopy utilizes fiber optic cables and a light source to visualize the nasal and sinus cavities that otherwise would be impossible to see. Visualization is minimally invasive and the preferred method for evaluating a wide variety of medical problems affecting the nose and sinuses.[13] Many common surgical procedures within the nasal cavity are completed under visualization by a nasal endoscope.[14]

Rhinomanometry

Rhinomanometry is used to evaluate the respiratory function of the nose by measuring pressure and flow during normal respiration. Obstruction will lead to higher than expected pressures and decreased flow. Two primary methods are used:  anterior or posterior. Anterior rhinomanometry takes unilateral measurements of one nostril at a time, whereas posterior rhinomanometry evaluates both simultaneously. Anterior rhinomanometry is more commonly used, and primarily evaluates on which side of the septum the obstruction arises.

Acoustic Rhinometry

Acoustic rhinometry is primarily to measure the cross-sectional area and length of the nose and nasal cavity. It uses sound waves and measures reflection in the nasal cavity. Evaluation of nasal cavity geometry and abnormalities is possible.[15]

CT/MRI

CT is the modality of choice for evaluating congenital, inflammatory, benign, and malignant pathologies in the sinonasal region.[16] CT and MRI have the advantage of showing anatomic structures that would otherwise not be visible, and they display fine anatomic detail.[17]

Saccharin Test

The saccharin test is primarily for measuring the nasal mucociliary clearance time.[18] A small drop of saccharin is placed below the inferior turbinate, where it can be swept back toward the nasopharynx. The patient should perceive a sweet taste as the cilia sweep the saccharin posteriorly.  Delayed taste recognition indicates poor ciliary performance.

UPSIT

Many smell identification tests are available, but the University of Pennsylvania Smell Identification Test (UPSIT) is the gold standard due to its reliability. The test consists of 40 questions, each with a different odor released upon scratching. The examiner asks the patient to identify the odor released. This test can assist in the diagnosis of many conditions, such as Alzheimer's disease, Parkinson's disease, Huntington disease, and others.[19]

Allergic Rhinitis

Allergic rhinitis is an IgE-mediated reaction that leads to irritation, pruritus, sneezing, and congestion. As allergens are introduced to the host, the immune system is sensitized. Repeat exposure causes cross-linking of IgE bound to mast cells, which release histamine, bradykinin, and leukotrienes, ultimately resulting in increased mucus secretion and blood vessel dilation within the nasal cavity.[20] These same chemical mediators also activate sensory receptors and lead to activation of CNS reflexes such as sneezing and nasal irritation.[21] Nasal symptoms can last for several hours, and the airway can become more reactive to the initial insult and even nonallergenic stimuli, including strong odors and environmental irritants.[22]

Nasal Polyposis

Nasal polyps are localized expansions of edematous mucosa within the nasal cavity, which can reach 3 to 4 centimeters in length. Chronic inflammation of the nasal mucosa can ultimately lead to the formation of nasal polyps. Inflammation leads to disruptions in the tight junctions between epithelial cells within the nasal [23] cavity and a decrease in the effectiveness of cilia to clear mucus and antigens within the cavity, which may contribute to the formation of nasal polyps.[24]  The actual pathogenesis of nasal polyps has not been determined and is likely multifaceted.[25]

Epistaxis

Epistaxis, or nosebleed, occurs when a vessel within the nasal cavity is ruptured. The nasal mucosa is home to an abundant blood supply, and rupture can occur spontaneously or due to trauma. Epistaxis in the pediatric population is most often due to digital trauma.[26] Epistaxis rates increase in frequency during the winter months due to the decrease in humidity and temperature, which causes a reduction in nasal humidification, making the cavity more susceptible to bleeding.[27]    

Nasopharyngeal Carcinoma

Nasopharyngeal carcinoma arises from the squamous epithelium within the nasal cavity. Epstein-Barr virus, ethnicity (particularly Southeast Asians), and environmental exposures have all shown to play a role in the development of nasopharyngeal carcinoma.[28]

Diseases of the nasal cavity can be due to a wide variety of etiologies. Viral, bacterial, and fungal pathogens can cause an infection within the nasal cavity or the surrounding sinuses. Particular consideration is necessary for those infections that can spread from the nasal cavity to surrounding or connected structures. Because the Eustachian tube enters into the nasal cavity, an infection can spread through the tube to the middle ear. For patients with persistent otitis media, often children, tympanostomy tubes should be considered. Diabetes patients with poorly controlled glucose levels are also at risk for serious but rare infection mucormycosis. If untreated, this infection can quickly spread from the nasal cavity to the CNS. Many viral and bacterial causes of pneumonia are spread through nasal inhalation and initially infect the nasopharynx.

Anatomic abnormalities can also cause complications requiring evaluation. Concerning symptoms include repeated sinus infections, sleep apnea, difficulty breathing, and loss of olfaction, among others.

Nasal cavity tumors are more often malignant than benign, and the development of any mass within the nasal cavity warrants further evaluation.[29]