While sinus anatomy can be likened to a house, sinus function runs more like a car. With an automobile, you know there’s a lot of firing and whirring going on under the hood, but as long as the car runs OK, you tend not to think about it.

Then, after months or years without any problems, you start to hear a strange knocking sound. You hope it will go away, but instead the engine conks out on the highway, stranding you on your way to work.

Similarly, when your sinuses are healthy, you breathe in and out easily, and you never contemplate all the mucus production and air-conditioning occurring inside your nasal cavity.

Then you notice you’re congested, and after several days or weeks, you start to feel sinus pressure in your face or forehead. You hope it will go away, but instead you eventually find yourself so stuffed up or in so much pain you need to call your doctor.

I can’t tell you how to keep your car on the road, but I can help you understand the chain of events that leads to sinusitis. We’ll look at the causes of sinusitis and the resulting symptoms.

Direct Cause

Although there are many root causes of sinusitis, they all fall into one of three broad categories: anatomical, genetic, and environmental. We’ll cover each of these causes. For now, the critical fact is that regardless of the specific cause, in most cases, it will lead to a common triggering event: the ostiomeatal complex becomes obstructed.

Remember the OMC? That’s the “turnstile”— the narrow area in the middle meatus through which mucus drains from the sinuses into the nose. When the OMC gets blocked, in short order mucus backs up and the doors from the sinuses (the ostia) become blocked as well, as shown in Figure below.

Normal flow of mucus from inside the sinuses through the ostia and into the nasal cavity is shown on the left side of the diagram. Note that in the maxillary sinus, the mucus flows upward to reach the natural drainage ostium before turning downward to flow into the nose. The right side shows what happens when mucosa lining the sinuses becomes inflamed and blocks the OMC. Mucus trapped within the maxillary and frontal sinuses leads to bacterial overgrowth and sinusitis.

The cilia stop beating effectively, and drainage from the sinuses stops or is severely curtailed. That’s when the real trouble begins. Glands within the sinuses continue to produce mucus, which cannot drain.

Soon the blocked sinus fills up with mucus. The warm, moist environment of the clogged sinus serves as what biologists refer to as the perfect culture medium. Even when you’re healthy, small quantities of bacteria live in your nose and sinuses.

But bacteria in the stagnant mucus rapidly multiply, so instead of a few hundred stray bacteria in your sinuses, you soon have several thousand, several hundred thousand, and eventually millions. Large numbers of bacteria constitute an infection.

Once your sinuses are infected, things quickly go from bad to worse. Your body kicks into action to fight the infection with what’s known as the immune response. Unfortunately, this normally beneficial process is what ultimately makes you miserable.

To fight the bacteria, your mucus-secreting glands shift into overdrive, cranking out even more mucus than normal. Equally important, the mucous membrane that lines your sinus swells as blood vessels dilate in preparation for battle against the bacteria.

White blood cells in the blood circulating through this membrane are then recruited to attack, engulf, and destroy the bacteria. As this conflict rages, the white blood cells and the bacteria release a variety of substances that further stimulate the inflammatory process.

These activators include interleukin, major basic protein, and leukotrienes. The addition of white blood cells and bacteria to mucus makes it much thicker than usual, and often it becomes yellow or green.

This thick substance is referred to as pus or purulent mucus. Some of the pus in the sinuses manages to squeeze out the ostia, through the OMC, and into the back of the nasal cavity.

It tends to pool in the throat, causing irritation, or drain downward as troublesome postnasal drip. A full-blown case of sinusitis is likely to worsen before it gets better.

To fight the infection, the sinuses become more inflamed, which causes more swelling, which leads to more blockage, which allows for more bacterial overgrowth. You’ve now developed the “sinusitis cycle,” as shown in Figure below.

This vicious cycle is eventually broken when the bacteria are eliminated by the body’s immune system, medications, or, occasionally, surgery. Then inflammation and blockage are finally reduced, enabling the sinuses to drain normally again.

Sinusitis Symptoms

A number of symptoms can occur during a sinus infection. Let’s start by looking at the Big Three that many people experience: pain and pressure, difficulty breathing and congestion, and postnasal drip.

The dull, throbbing pain, ache, or pressure that is a hallmark of sinusitis results from inflamed tissue putting pressure on nerve endings in the lining of your sinuses.

This pain’s location is often specific to the infected sinus:

• Frontal sinusitis causes forehead pain or headache.
• Maxillary sinusitis causes cheek pain, which may radiate to the teeth in the upper jaw.
• Ethmoid sinusitis causes pain between the eyes or across the bridge of the nose.
• Sphenoid sinusitis causes pain behind the eyes, on the top of the scalp, or along the back of the head.

Depending on which part of the OMC is blocked, any one or all of your sinuses can become involved during an infection. If just one maxillary or frontal sinus is blocked, for instance, your symptoms may be localized to one cheek or one side of the head.

If all four pairs of sinuses are infected (what’s known as pansinusitis), your pain and pressure are usually more diffuse and may encompass the entire face or head.

The combination of swollen nasal membranes and excess mucus makes it difficult or impossible to breathe through your nose. This blockage can affect one or both sides of the nose. Because of the nasal cycle, it is not uncommon for nasal obstruction to alternate between sides.

For some sinus sufferers, the term congestion refers not to blocked nasal breathing but to a sense of fullness or blockage of the face, particularly in the cheeks. This sensation is caused by blockage of the sinuses themselves.

When ostia swell shut, the mucous membranes in the sinuses absorb oxygen, creating negative pressure (or a vacuum), which can produce this sense of facial congestion or even pain.

As we’ve discussed, mucus from the sinuses normally drains in minute quantities into the nose and down the back of the throat before being swallowed. During an infection, increased secretion by the nose and sinuses results in a larger volume of mucus, which is often thick and yellow or green.

This mucus, which may contain large quantities of bacteria and white blood cells, flows down the back of the nose and throat. Some people blow a large amount of mucus out the front of the nose as well.

Other Sinusitis Symptoms

Along with the Big Three, other sinusitis symptoms can include any of the following:

• Loss of smell. The roof of the nasal cavity is lined with specialized tissue known as olfactory epithelium. This tissue contains specific smell receptors that are triggered by the odor molecules you breathe in.

Swelling of the membranes in the nose can block these molecules from reaching the smell receptors, leaving you with a reduced sense of smell.

• Loss of taste. A normal sense of taste, particularly the ability to perceive different flavors, depends on an intact sense of smell. Consequently, many people who lose all or part of their sense of smell from sinusitis also describe a corresponding decrease in their sense of taste.

• Bad breath. The discolored mucus that drains from infected sinuses contains bacteria and debris that emit foulsmelling odors. As a result, thick mucus that runs down your throat may produce bad breath (halitosis) when these odors are exhaled.

The bad breath you experience during sinusitis can be more powerful than conventional bad breath, which results from bacteria in the mouth. Mouthwash is likely to offer only a temporary reprieve.

• Cough. When mucus runs down the back of the throat, it may touch the vocal cords and trigger an involuntary cough response. Coughing is often worst upon awakening because mucus from the nose and sinuses pools in the throat overnight.

If this mucus seeps between the vocal cords and into the trachea or windpipe, vigorous coughing may be needed to clear these secretions and protect the lungs.

• Sore throat. The thick mucus that drains during a sinus infection is more acidic than normal watery mucus, so it can irritate the membranes that line your throat.

• Fatigue. Your body uses extra energy to mount an immune response. This shifting of calorie reserves from normal daily activity to infection fighting can leave you feeling tired.

In addition, poor nasal breathing and frequent coughing during the night can cause poor quality sleep, resulting in less energy during the day.

• Ear fullness. Mucus drainage and inflammation from sinusitis may block the Eustachian tubes, passageways that connect your ears to the back of your nose (see Figure 2). When these tubes are open and function properly, they equalize pressure between the inside of the ears (known as the middle ear space) and the outside atmosphere.

If they become blocked, you can develop an uncomfortable feeling of fullness or pressure in the ears, similar to what you may experience when you’re on a plane descending from a high altitude.

• Fever. Occasionally, the body may mount a fever in response to the marked inflammation and the presence of a large quantity of bacteria during a sinus infection.

Just as a broken-down car forces you to visit an auto shop, malfunctioning nasal passages mean it’s time to go to the doctor. Like a mechanic examining an engine, physicians can use their tools of the trade to pinpoint the problem and recommend treatment.

Next, we’ll look at what distinguishes sinusitis from related disorders and what you can expect when you visit your doctor.