Ever had what seemed like a bad case of the sniffles - only for your face to suddenly descend into horrible pain, as intense pressure builds around your eye sockets and nose?
It might have been sinusitis - "an infection of inflammation of the air-filled sinus cavities surrounding your nose", says GP and Healthspan medical director Dr Sarah Brewer (healthspan.co.uk).
"You have four pairs of sinuses," she explains. "The frontal sinuses over your eyes; the ethmoids between your eyes; the sphenoids in the upper part of your nose behind your eyes, and the maxillary sinuses inside your cheekbones. These help to lighten the bones of the skull and improve the resonance of your voice. They also act as a safety 'crumple zone' to absorb blows to the face."
Some people suffer with chronic sinusitis, though this is somewhat different from having an acute infection, explains Mr David Howe, an ear, nose and throat consultant surgeon at Solihull's Spire Parkway Hospital (spirehealthcare.com).
"Sinus infections arise from a combination of blockage within the sinus and infection forming within it. This obstruction of the tiny ducts that drain the sinus into the nose leads to the symptoms of pain, pressure and congestion that people report with acute sinusitis," he says.
"Chronic sinusitis is a rather different condition and in the UK, is often associated with nasal polyps," says Howe. "This tends to cause patients to have a very blocked nose, loss of smell and recurrent discharge from the nose. It can affect men or women but is certainly more common in people who have asthma, cystic fibrosis or aspirin sensitivity."
So how come sinus infections can be so painful? "The lining of your sinuses secretes a thin, watery fluid that continually washes them clean. When a sinus is inflamed, it swells and produces larger amounts of mucus, so the fluid becomes thicker and more profuse. If the narrow drainage channel from a sinus becomes blocked by swollen tissue or mucus, the secretions remain trapped and pressure builds to cause a throbbing pain between the eyes, in the cheeks, headache and/or pain in the upper teeth (yep, you might feel like your teeth are all about to fall out!)," explains Brewer.
"You may develop tenderness over the sinus, with swelling and redness of overlying skin. If the infection's severe, pus will pool in the sinus and you will develop a fever. Some pus may drain out through the nose to cause increasing pain and a constant, nasal discharge that drops into the back of the nose. This triggers a cough and tastes foul."
Acute sinusitis, she explains, typically lasts up to three weeks - although the 'acute' very painful phase should hopefully ease long before that. "It is normally the aftermath of a cold or flu infection, and can occur after lots of flying," Brewer adds.
You don't always need antibiotics - however it's important to see a doctor if the acute symptoms don't improve, and seek medical attention if your symptoms become severe (for instance, a very high fever, redness around the eyes, a stiff neck, confusion and vision changes). Infections will normally clear up by themselves, and regular painkillers like paracetamol, plus over-the-counter decongestant tablets and nasal drops or sprays - ask your pharmacist which ones are suitable - can help ease symptoms.
Steaming can sometimes help too, and Alison Cullen, nutritional therapist for A. Vogel (avogel.co.uk), says: "Warm face packs placed over your sinuses can be comforting and reduce pain."
Howe explains: "Antibiotics are rarely needed, unless the patient becomes toxic or gets frontal headaches, suggesting a frontal sinusitis. Also If the acute episode lasts more than a few days, then antibiotics might be appropriate."
You can't completely guard against sinus infections, but doing your best to avoids colds and flu will help, along with keeping generally well - with a balanced diet, regular physical activity, good sleep and keeping stress levels in check - to help support your immune system.
Dr Brewer says Pelargonium (Pelargonium Cold Relief, £14.95 for 30 tablets, Healthspan) has been found to be effective in "reducing cough, sputum production, headaches and nasal discharge", while Cullen notes that hydration can also be an important factor. "It's important to ensure you drink plenty of fluid. This will help loosen and thin mucus which has accumulated in your sinuses or nasal passages, encouraging mucus to drain and help reduce blockages and pressure," she says.
As well as nasal polyps and pre-existing conditions like asthma, other factors in developing chronic sinusitis include a deviated (crooked) nasal septum, certain medications, and certain allergies and food intolerances may play a part for some people.
If you find you're prone to sinus infections, or are finding it harder and harder to shrug off symptoms, talk to your GP.
"Chronic sinusitis is more complex, and this often needs input from a specialist," says Howe.
"Confirmation of the diagnosis is key, with often the need for a nasoendoscopy (a thin flexible tube with a camera at the end is inserted up into the nose to get a good look at what's going on) to be certain. Then a combination of antibiotics and steroids may be used to treat the sinusitis. Approximately 50% of patients will respond to this, but a minority may require surgery eventually.
"Surgery is generally done for patients with chronic sinus disease, which doesn't respond to medication. Occasionally, patients with recurrent acute sinusitis may need treatment."
The procedure's now "almost always done endoscopically", Howe adds, meaning it's performed via the nostrils, with no need for incisions, and often as a daycare procedure under local anaesthetic.