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Rhinitis can be broadly defined as inflammation of the nasal mucosa, and is generally subdivided into two groups: allergic and non-allergic. It has been estimated that allergic rhinitis has a high prevalence in the general population (10% to 40%), while non-allergic rhinitis alone is thought to affect more than 200 million people worldwide.
The prevalence of allergic rhinitis is increasing, and severe cases have been associated with impairments in quality of life, work performance and sleep patterns. Furthermore, non-allergic rhinitis is an underrecognized entity which affects many individuals and is often inadequately treated.
Allergic rhinitis represents the most common cause of allergic diseases. It is an IgE-mediated inflammatory chronic illness affecting nasal mucosa, characterized by the presence of rhinorrhea, itching, sneezing and congestion. A positive correlation between the clinical history and the allergen sensitization is usually enough to support the diagnosis of allergic rhinitis.
Upon exposure to an inciting allergen, a plethora of inflammatory cells (such as CD4-positive T cells, B cells, mast cells, macrophages and eosinophils) infiltrate the nasal lining. Cytokines that are then released promote the production of immunoglobulin E (IgE) by plasma cells, triggering in turn the release of mediators (such as histamine and leukotrienes) that are responsible for the development of characteristic symptoms.
There are two general types of allergic rhinitis: season and perennial. Seasonal allergies (usually due to cyclic airborne plant pollens) are more common and cause problems only at certain times of the year. Conversely, perennial allergies cause symptoms continuously throughout the year due to constant allergen exposure (such as house dust mites, mold spores, cockroaches, cigarette smoke and pet dander).
Furthermore, Allergic Rhinitis and its Impact on Asthma (ARIA) group classified allergic rhinitis to intermittent (less than 4 days per week and for less than 4 weeks) and persistent (more than 4 days per week or lasting more than 4 weeks regardless of the number of days per week) type.
According to the severity of the condition, allergic rhinitis is considered mild when patients generally have normal sleep pattern and have no trouble in performing everyday activities (including work or school). Such mild symptoms are usually intermittent. On the other hand, moderate or severe condition significantly affects sleep and activities of daily living.
Non-allergic rhinitis syndromes are heterogeneous and often unrelated to each other. The common feature of this group is chronicity and bothersome nasal conditions that do not involve allergic mechanisms. Some research points to neurosensory abnormalities as an important factor in non-allergic rhinopathy, but thus far they remain a murky group of overlapping syndromes.
There are at least nine subtypes that fulfill the criteria for non-allergic rhinitis: drug-induced rhinitis, gustatory rhinitis (rhinorrhea associated with eating hot and spicy foods), hormonally induced rhinitis, non-allergic rhinitis with eosinophilia syndrome (NARES), senile rhinitis, atrophic rhinitis, occupational rhinitis, infectious rhinitis and vasomotor rhinitis.
Among them, vasomotor (or sometimes called idiopathic) rhinitis represents the most commonly diagnosed form of non-allergic rhinitis, accounting for more than half of all the cases. Nasal symptoms can be sporadic or persistent, and are triggered by cold air, sudden changes in temperature, humidity, barometric pressure, alcohol intake, but also strong emotions.
Rhinitis medicamentosa or drug-induced rhinitis is another common variant of non-allergic rhinitis caused by the rebound nasal congestion that arises due to the chronic overuse of the topical nasal decongestants oxymetazoline or phenylephrine. Moreover, in aspirin-sensitive patients symptoms of nasal obstruction are quite common.
NARES is the acronym for non-allergic rhinitis with eosinophilia. On nasal smears between 5 and 20% of cells are eosinophils. This condition usually develops in adulthood with year round symptoms such as profuse rhinorrhea and nasal congestion. Serum eosinophil levels can also be elevated in certain instances.