Understanding a Sinus Infection: Sinus Anatomy & Function

The sinuses are air-filled around and connected to the nose. The maxillary sinuses are in the cheeks, the frontals in the forehead, the ethmoids between the eyes, and the sphenoids behind the nasal cavity. The sinuses serve several functions including lightening and cooling the skull and filtering & humidifying the air before it reaches the lungs.

The sinuses are lined with millions of cillia that secrete mucous which traps particles in teh air and has immunoglobulins that help fight infections. The cillia sweep the mucous with the trapped particles to the back of teh throat, and is then swallowed and the stomach acid destroys most dangerous particles. The sinuses typically secrete one quart of mucous a day.


Sinusitis, also known as rhinosinusitis, is any type of inflammation of the sinuses, but usually implies a sinus infection. The most common cause is viral, which typically resolves spontaneously within ten days. There are more than 20 million cases annually in the US.

Sinus infections are classified as:

  • Acute – symptoms lasting less than four weeks
  • Subacute – symptoms last between four and 12 weeks
  • Chronic – symptoms lasting more than 12 weeks. Chronic sinusitis is much harder to treat because after three months of inflammation, the sinus mucosa stops working and produces very thick mucous which leads to sinus blockage and more inflammation.
  • Recurrent acute – where four or more separate episodes of sinusitis occur within one year.
  • Acute Flare of Chronic Sinusitis – occurs when the symptoms of sinusitis worsen and require treatment to return to baseline.

Symptoms & Causes

Sinusitis can be one of the most difficult medical problems to diagnose as many of the symptoms overlap other common medical problems. While the etiology of sinusitis is varied, a strong link to allergies and asthma has been suggested.

Possible Signs of Sinusitis

  • A cold lasting more than two weeks accompanied by a fever
  • A thick, yellow or green nasal discharge
  • Post-nasal drip
  • Fatigue and irritability
  • Puffiness around the eyes

Diagnosis & Treatment

Diagnosing sinusitis is difficult because the symptoms are often vague and the exam is unremarkable. If symptoms persist greater than three months, particularly if medical therapy has failed, nasal endoscopy or a sinusitis CT may be required to make the diagnosis. This comprehensive assessment aids in the development of an appropriate treatment plan and a speedy recovery.

Treatment of sinusitis depends primarily on the length of symptoms. If less than 10 days, good hydration, saline irrigation, and over the counter medications are recommended for treatment. If symptoms persist greater than 10-14 days, this implies there is an underlying bacterial infection and antiobiotics and prescription nasal sprays are typically used. Chronis sinusitis is very difficult to treat and if medical therapy fails and the CT scan shows anatomic obstruction of the sinuses, surgery to improve the anatomy may be needed to get relief of symptoms.


Frontal & Lateral View of Sinuses

CT Scan of Normal Sinuses

Normal Sinuses

CT Scan of Paranasal Sinuses. Arrows point to areas of blockage.

CT Scan of Paranasal Sinuses. Arrows point to areas of blockage.


Avoiding steps to the common cold are helpful in preventing sinusitis. Washing your hands frequently and avoiding those who are ill can help to keep you healthy.