Sinusitis can result from infection or allergy.

The sinuses are spaces in the head that are mainly full of air. Their exact function is unclear. They stop our heads being too heavy and may play a role in helping us hear more clearly.

There are four sinuses: the frontal, maxillary, ethmoid and sphenoid. The frontal sinuses are above our eyes; the maxillary sinuses are situated below the eyes on either side of the nose; the ethmoid sinuses are behind the eyes on either side of the nasal cavity; and  the sphenoidal sinuses are in the centre of the head, directly behind the ethmoid sinuses.  Each sinus has a narrow opening or tube connecting it to the back of the nose.

The lining of the sinuses produces a clear fluid - mucus - which constantly cleans them of unwanted material. This fluid passes through the drainage passages into the back of the nose and throat, from where it is swallowed. This happens continually, although we are usually unaware of it. When excess fluid is produced it is often known as phlegm, or catarrh. It can produce a chronic irritation in the throat known by the unglamorous name of post-nasal drip.

What are the causes?

There are two main causes of excess mucus production: infection and allergy.

Because the drainage holes from the sinuses are narrow they block up easily. When this happens, pressure builds up in the sinuses causing symptoms, which can vary from a mild feeling of congestion, as in a typical head cold, to severe pain.

A sensation of partial deafness or blockage in the ear may also occur. This is because the Eustachian tube, which connects the middle ear to the back of the nose, blocks up in just the same way as the nearby sinus openings.

Infections can vary from mild viral illnesses to more severe bacterial infections. In these cases the mucus becomes thick and is usually green or yellow in colour. Treatment with antibiotics is often necessary. Spreading into nearby structures, such as the eye or brain, is a serious but rare complication of untreated sinus infections.

Allergic reactions such as hay fever are most common in springtime when allergy-producing pollens are abundant, but can happen at any time in response to a variety of things, including house dust and animals.

For those with recurring sinus congestion and infections, various treatments are available. Sometimes surgery is necessary to drain a blocked sinus, or to enlarge the drainage passage. Those with allergies can be helped with nasal spays which suppress the allergic reactions. The use of antihistamine tablets, especially the newer ones with little or no sedating side effects, can be helpful.

If you get a blocked-up feeling in the head, simple remedies such as aspirin or paracetamol and steam inhalations may be all you need. But if you develop pain in the face or head, or find you are producing discoloured phlegm, seek medical advice.

Original material provided by myDr and Dr Michael Jones, February 2002. Edited by everybody, September 2005.