The sinuses produce mucus that helps to clean the nose and to smell. It's moved out of the sinuses and into the nasal cavity by thousands of tiny hairs, called cilia, which operate in unison to form a sort of conveyor belt. Foreign particles and organisms entering the sinuses land in the mucus and are sent back to the nose. To get to the nose, the mucus has to pass through small holes in the bones that surround the sinuses.

Sinusitis usually begins during a bout of the common cold, influenza (flu), or some other viral infection. This causes the nasal mucous membrane (which is soft tissue inside the nose, not simply mucus) to swell. It can press against the hole through which mucus leaves a sinus.

Most cases of sinusitis are caused by viral infections. However, in a few people, a bacterial infection can develop. This is because when the sinus fills with mucus and empties of oxygen, it creates an ideal setting for bacteria to grow. The bacteria are often already in the nose, but don't cause an infection because they are held in check by the body's natural defences.

If the body's defences (e.g., the cilia, sinus drainage, or immune system) are not working properly, the bacteria can cause an infection. In rare cases, sinusitis can be caused by a fungal infection. People who get fungal sinus infections usually have other medical problems that affect their ability to fight infection (e.g., HIV, cystic fibrosis).

Other things that inflame the nose can also cause sinusitis. For example, hay fever and other allergies increase your chances of getting sinusitis.

Sinusitis has symptoms very different from a cold or the flu. The main symptoms are face pain or pressure, congestion, nasal discharge or post-nasal drip, and reduced ability to smell. The location of the pain depends on which sinus or sinuses are affected. Other symptoms include headache, bad breath, fatigue, and cough.

Infection of the lower (maxillary) sinuses causes toothache in the upper jaw and pain in the area under the eyes, while infection of the upper (frontal) sinuses causes pain in the temple. Infection of the small sinuses between the eyes (the ethmoid sinuses) causes pain between and behind the eyes.

Yellow or green pus may drain out of the nose, and there may be an unpleasant smell. Someone with sinusitis may feel generally unwell, and can have a low grade fever (less than 38°C).

A viral infection is different from bacterial sinusitis in that it generally resolves in less than 7 days. If symptoms persist for more than 7 days, the infection is likely bacterial in nature.

Acute sinusitis usually lasts about 2 weeks. In a few people, however, the infection can last longer. People who have more than 4 episodes per year are considered to have recurrent acute sinusitis and should be referred to a specialist. When symptoms last more than 12 weeks, the condition is called chronic sinusitis.

Serious complications of sinusitis can sometimes occur, including abscesses and meningitis (infection of the membranes surrounding the central nervous system). However, complications such as these are rare.

For acute sinusitis, usually your doctor will arrive at a diagnosis based on history (e.g., a recent cold), symptoms (when they appeared, the severity, and the duration), and an examination.

A diagnosis of acute bacterial sinusitis is usually made if you have:

• symptoms lasting more than 7 days, and
• yellow or green pus draining from the nose, and
• one of the following: facial pain/tenderness, changes to smell, or nasal obstruction.

Computed tomography (CT) scans can reveal the extent and location of infection in the sinuses. But these tests are not usually done unless a person has sinusitis complications, has chronic sinusitis that has not responded to treatment, or may need to have surgery.

When sinusitis is chronic or recurrent, your doctor may suggest allergy testing to see if allergies are causing sinusitis symptoms.

Seek medical attention immediately if you are experiencing any of the following symptoms: severe headache, altered mental status, change in your vision, or signs of a more severe infection (e.g., fever over 39°C).

Since most cases of sinusitis are caused by a virus, your doctor or pharmacist may only recommend oral (e.g., phenylephrine*, pseudoephedrine) or topical (e.g., oxymetazoline, xylometazoline) decongestants, saline nasal sprays or rinses, and pain medications such as acetaminophen or ibuprofen, to help relieve symptoms. If you have heart disease, high blood pressure, thyroid disease, diabetes, glaucoma, or prostate problems, or if you are pregnant or breast-feeding, talk to your doctor before taking these medications.

Decongestant sprays should not be used for more than 3 days in a row or they can cause "rebound congestion." Steam inhalation and cool mist humidifiers may also help. It may help to add eucalyptus and menthol-based decongestants to the hot water. Although they are safe for adults, decongestants are not recommended for young children.

If these measures do not improve symptoms after 10 days, or if your symptoms are severe, then your doctor may prescribe antibiotics. Antibiotics are not usually prescribed because acute sinusitis is usually viral in nature. The typical duration of antibiotic treatment is 5 to 7 days for adults and 10 to 14 days for children. If the infection becomes chronic (lasting more than 3 months) the antibiotic therapy may be used for longer, usually up to 6 weeks.

It's important that you take all of the antibiotic medication as prescribed, even after symptoms have disappeared. Otherwise the bacteria could return in a form that's resistant to the antibiotic.

For chronic sinusitis, your doctor may prescribe a corticosteroid (nasal spray or tablets) with or without antibiotics. Your doctor may also suggest saline rinses and decongestants to relieve your symptoms. Antihistamines may be suggested if allergies are contributing to chronic sinusitis. Surgery may be an option for people with chronic sinusitis that is not responding to treatment.

There's no guaranteed way to prevent all causes of sinusitis, but you can reduce your risk of viral infections by washing your hands frequently and properly, avoiding tobacco smoke, and reducing your exposure to things you are allergic to.