Nasal physiology is greatly dependent on the physical structure of the nose. Seemingly individual aspects of the nasal cavity collectively affect nasal function.

Rhinitis is defined as inflammation of the nasal mucosa and is characterized by a symptom complex that consists of any combination of the following:

• Sneezing,
• Nasal congestion,
• Nasal itching, and
• Rhinorrhea.

The eyes, ears, sinuses, and throat can also be involved. Rhinitis of the nose can be infective (as infective rhinitis, e.g. common cold, upper respiratory tract infection), or non-infective (inflammatory) e.g. allergic rhinitis.

Allergic rhinitis is the most common cause of rhinitis. It is an extremely common condition, affecting over 20% of the population. Allergic rhinitis involves inflammation of the mucous membranes of the nose, eyes, eustachian tubes, middle ear, sinuses, and pharynx.

Immunologically, the inflammation of the mucous membranes is characterized by a complex interaction of inflammatory mediators that is triggered by an immunoglobulin E (IgE)–mediated response to an extrinsic protein (allergen). The tendency to develop this allergic (IgE-mediated) reaction to extrinsic allergens (proteins capable of causing an allergic reaction) has a genetic component. In susceptible individuals, exposure to certain foreign proteins (allergen) leads to allergic sensitization, which is characterized by the production of specific IgE directed against these proteins.

These specific IgE coat the surface of mast cells (inflammatory cells), which are present in the nasal mucosa. When the specific protein (eg, a specific pollen grain, or dust mite) is inhaled into the nose, it can bind to the IgE on the mast cells, leading to immediate and delayed release of a number of mediators. These mediators lead to the symptoms of rhinorrhea, nasal congestion, sneezing, nose itching, redness, tearing, swelling, ear pressure, and postnasal drip. Mucous glands are stimulated, leading to increased secretions. Vascular permeability is increased, leading to plasma exudation.

There are 2 main phases;

• The immediate phase and
• The delayed phase.

The immediate reactions (usually within 20 to 30 mins) in the nasal mucosa induce acute allergy symptoms (eg, nasal itch, clear nasal discharge, sneezing, and nasal congestion).

The late-phase reaction occurs hours later, secondary to the recruitment of inflammatory cells into the tissue by the action of mediators released by the mast cell.

Allergic rhinitis can be associated with a number of co-morbid conditions, including asthma, atopic dermatitis, and nasal polyps. Evidence now suggests that uncontrolled allergic rhinitis can actually worsen the inflammation associated with asthma or atopic dermatitis.

According to the Allergic Rhinitis and its Impact on Asthma (ARIA) guidelines, the relationship between the development of allergic rhinitis and the development of asthma has been shown to be related and are manifestations of 1 syndrome in 2 parts of the respiratory tract. Within the past 5 years, increasing numbers of studies in the asthma/allergy literature support the concept of "One airway, one disease" (the unifying theory).

Rhinitis has been shown to coexist in more than 75% of patients with allergic asthma. Results also have revealed that an adult with a family history of asthma or rhinitis has a 3- to 4-fold higher risk of developing asthma and a 2- to 6-fold increased risk of developing rhinitis over an adult with no relevant family history.

Allergic rhinitis has been identified as a significant risk factor for the development of bronchial asthma, it is well accepted that allergic rhinitis may precede the onset of asthma symptoms and that asthma will not be optimally controlled unless the allergic rhinitis is also effectively managed.

Based on the ARIA guidelines, allergic rhinitis is divided into 2 main types:

• Intermittent disease and
• Persistent disease.

Patients with intermittent disease have symptoms for less than 4 days/week and less than 4 weeks/year, whereas patients with persistent disease have symptoms for more than 4 days/week for more than 4 weeks/year.

The complications of allergic rhinitis include

• Otitis media (acute otitis media and/or otitis media with effusion),
• Eustachian tube dysfunction (blocked swollen eustachian tube),
• Acute sinusitis, •Chronic sinusitis and
• Significant impairment of quality of life (such as fatigue and drowsiness)

The reduction in quality of life may be due to the disease or to medications; this malaise can lead to impaired work and school performance, missed school or work days, and traffic accidents. The overall cost (direct and indirect) of allergic rhinitis was recently estimated (in United States) to be $5.3 billion per year.

Symptoms and signs of allergic rhinitis include rhinorrhea (runny nose), nasal congestion, sneezing, nose itching, redness, tearing, swelling, ear pressure, and postnasal drip. Most patients also have watery eyes and itchy eyes. Throat symptoms include persistent phlegm, recurrent sore throat and chronic cough. "Allergic shiners" are dark circles around the eyes and are related to vasodilation or nasal congestion. "Nasal crease" is a horizontal crease across the lower half of the bridge of the nose that is caused by repeated upward rubbing of the tip of the nose by the palm of the hand (ie, the "allergic salute"). The mucosa of the nasal turbinates may be swollen (boggy) and have a pale, bluish-gray color. With the swollen turbinates, the nasal passage is narrowed, resulting in difficulty breathing and nasal congestion.