Causes:

Bacteria are the most common direct cause of acute sinusitis. (Other organisms might be the infecting cause in less common cases.) The ability of bacteria or other organisms to infect the sinuses, however, must first be set up by conditions that create a favorable environment in the sinus cavities. Sinusitis is most often an acute condition, which is self-limiting and treatable. In some cases, however, the inflammation in the sinuses is lasting, or is chronic to begin with. The causes for such chronic sinusitis cases are sometimes unclear.

Upper Respiratory Infections

The typical process leading to acute sinusitis starts with a cold virus. Over 85% of people with colds have inflamed sinuses. These inflammations are typically brief and mild, however, and very few people with colds develop true sinusitis. Instead, colds and flu set the stage by causing inflammation and congestion in the nasal passages (called rhinitis), leading to obstruction in the sinuses. This creates a hospitable environment for bacterial growth, which is the direct cause of sinus infection. In fact, rhinitis is the precursor to sinusitis in so many cases that doctors now refer to most cases of sinusitis as rhinosinusitis.

Rhinosinusitis tends to involve the following sinuses:

• The maxillary sinuses (behind the cheekbones) are the most common sites.
• The ethmoid sinuses (between the eyes) are the second most common sites affected by colds.
• The frontal (behind the forehead) and sphenoid (behind the eyes) sinuses are involved in about a third of cold-related cases.

Conditions That Cause Chronic or Recurrent Sinusitis

Chronic or recurrent acute sinusitis typically results from one of the following conditions:

• Untreated acute sinusitis that results in damage to the mucus membranes
• Chronic medical disorders that cause inflammation in the airways or persistent thickened stagnant mucus (such as diabetes, AIDS, other disorders of the immune system, hypothyroidism, cystic fibrosis, Kartagener's syndrome, and Wegener's granulomatosis)
• Structural abnormalities
• Allergic reaction to fungi

Chronic or recurrent acute sinusitis can be a lifelong condition.

Inflammatory Response, Allergies, and Asthma

The absence of bacterial organisms as factor in many cases suggests that some instances of chronic sinusitis may be due to a continuing inflammatory condition. Many of the immune factors observed in people with chronic sinusitis resemble those that appear in allergic rhinitis, suggesting that sinusitis in some individuals is due to an allergic response.

Allergies, asthma, and sinusitis often overlap. Those with allergic rhinitis (so-called hay fever) often have symptoms of sinusitis, and true sinusitis can develop as a result of the mucus blockage it causes. A causal association, however, has not been proved, and many doctors believe allergies themselves rarely predispose to sinusitis. People with chronic sinusitis may also have an allergic reaction to fungal organisms.

Abnormalities of the Nasal Passage

Abnormalities in the nasal passage can cause blockage and thereby increase the risk for chronic sinusitis. Some abnormalities include:

• Polyps (small benign growths) in the nasal passage block mucus drainage and restrict airflow. Polyps themselves may be consequences of previous sinus infections that caused overgrowth of the nasal membrane.
• Enlarged adenoids can lead to sinusitis.

Adenoids are masses of tissue located high on the posterior wall of the pharynx. They are made up of lymphatic tissue, which trap and destroy pathogens in the air that enter the nasopharynx.

• Cleft palate
• Tumors
• Deviated septum (a common structural abnormality in which the septum, the center section of the nose, is shifted to one side, usually the left)

Bacteria

The Role of Bacteria. The role of bacteria or other infectious organisms is complicated in chronic sinusitis. They may have a direct, or an indirect, role. Bacteria may be living in the sinuses without causing an infection (called colonization) In some patients, infectious organisms play no role at all:

The bacteria most commonly implicated in sinusitis include:

• Streptococcus pneumoniae. This bacterium is found in up to 45% of adults and children with sinusitis.
• H. influenzae (a common bacterium associated with many upper respiratory infections). This bacterium causes about 25% of sinusitis cases in children. Studies have reported the presence of this bacterium in up to a third of adult sinusitis patients.
• Moraxella catarrhalis. Over 75% of all children harbor this bacterium, which causes about 25% of sinusitis cases.

Other possible bacterial culprits include:

• Other streptococcal strains
• Staphylococcus aureus
• P. aeruginosa, Klebsiella pneumoniae, Proteus mirabilis, Enterobacter species, and Escherichia coli
• Fusobacterium nucleatum and Prevotella intermedia)

Fungal Sinusitis

While fungi are an uncommon cause of sinusitis, the incidence of such infections is increasing. At least 5 - 10% of chronic rhinosinusitis patients may actually have allergic fungal sinusitis.

Many patients with chronic sinusitis may be colonized with fungi, but this does not necessarily mean the patient has a fungal infection causing their symptoms. Studies suggest that some people who suffer from chronic sinusitis have an immune and inflammatory response to fungi and may benefit from anti-fungal treatment.

Fungi involved in sinusitis include:

• Aspergillus is the most common cause of all forms of fungal sinusitis.
• Other fungi include Curvularia, Bipolaris, Alternaria, Dreschslera, Cryptococcus, Candida, Sporothrix, Exserohilum, and Mucormycosis.
• There have been a few reports of fungal sinusitis caused by Metarrhizium anisopliae, which is used in biological insect control.

There are four categories of fungal sinusitis:

• Acute or invasive fungal sinusitis. This infection is most likely to affect people with diabetes and compromised immune systems.
• Chronic or indolent fungal sinusitis. This form is generally found outside the U.S., most commonly in the Sudan and northern India.
• Fungus ball (mycetoma). This fungal sinusitis is noninvasive and occurs usually in one sinus, most often the maxillary sinus.
• Allergic fungal sinusitis. This form typically occurs because of an allergy to the fungus Aspergillus (rather than being caused by the fungus itself). In such cases, a peanut butter-like fungal growth occurs in the sinus cavities that may cause nasal passage obstruction and the erosion of the bones.

Fungal infections can be very serious, and both chronic and acute fungal sinusitis require immediate treatment. Fungal ball is not invasive and is nearly always treatable.

Fungal infections should be suspected in people with sinusitis who also have diabetes, leukemia, AIDS, or other conditions that impair the immune system. Fungal infections can also occur in patients with healthy immune systems, but they are far less common.