This is one of those yoga urban legends, just like many others, that are not only unsubstantiated by any kind of science, but are actually potentially a very dangerous advice. Here's why.
A sinus infection, and also severe types of cold congestion, fill up the sinuses which are normally full of air. Mucus takes over the air spaces, on the account of which we can't take a normal breath. This mucous expands and creates pressure all around it as bacteria grows in there and creates an infectious situation. This is why doctors prescribe antibiotics. They want to kill the bacteria and by doing so, to decrease the inflammation to the area. This, of course, is not the only way, but more on that later...
In a head stand, the blood pressure to the head and the brain increases dramatically, almost doubling. Which is why head stand is not a suggested pose for people with history of seizures, epilepsy, brain disorders, strokes, and even eye disorders such as glaucoma. The increased pressure can literally blow the weak links in the brain, causing at the very least, very bad headaches, and the very worse, a brain aneurysm. In the case of glaucoma, there is already strong pressure in the eye and increased pressure from the inversion can cause damage to the optic nerve and bleeding of the blood vessels in the eyes, even loss of vision.
When you have a sinus infection, you already have lots of pressure in your head. There are 4 pairs of sinuses. Frontal sinuses (behind the forehead), Maxillary sinuses (behind the nose), Ethmoid sinuses (behind the nose), and Sphenoid sinuses (behind the eyes.) Because of the bacteria present during inflamed sinuses, the increased blood pressure can force the bacteria into the bone, into the areas around the sinuses, creating the possibility for the following conditions to occur, according University of Maryland Medical Center:
Osteomyelitis - infection of the bones of the forehead and other facial bones. In such cases, one usually experiences headache, fever, and a soft swelling over the bone known as Pott's puffy tumor.
Infection of the Eye Socket, or orbital infection, which causes swelling and subsequent drooping of the eyelid, is a rare but serious complication of ethmoid sinusitis. In these cases, the unfortunate individual loses movement in the eye, and pressure on the optic nerve can lead to vision loss, which is sometimes permanent. Fever and severe illness are usually present.
Blood Clot, another danger, although usually rare increases when you are in a full inversion. The inversion and the pressure it builds can cause a small blood vessel to burst causing a blood clot to form. Usually in the area of the ethmoid or frontal sinusitis. If a blood clot forms in the sinus area around the front and top of the face, symptoms are similar to orbital infection. In addition, the pupil may be fixed and dilated. Although symptoms usually begin on one side of the head, the process usually spreads to both sides.
Brain Infection. The most dangerous complication of sinusitis, the likelihood of which increases when one is fully inverted, particularly frontal and sphenoid sinusitis, is the spread of infection by anaerobic bacteria to the brain, either through the bones or blood vessels. Abscesses, meningitis, and other life-threatening conditions may result. When you increase the blood pressure to the area you are forcing not just blood flow into the sinuses which most practitioner think is a good way to clear the sinuses, but you are also forcing an exchange of liquids with the sinuses. This means the bacteria in the sinuses via the blood, enters the brain. In such cases, you may experience mild personality changes, headache, altered consciousness, visual problems, and, finally, seizures, coma, and death.
Now, next time you hear a yoga teacher give you medical advice in a yoga class, preceded by "this does not feel good, but" think twice. Remember, most yoga teachers today started doing yoga yesterday and did their yoga teacher training from yoga teachers who started doing yoga last week.
Secondly, very few yoga teachers actually bother double checking the information they are given during training and just run with it like it's the Gospel.
Doubt your yoga teachers and protect yourself! Don't take anecdotal evidence as fact. Your brain is worth your extra consideration.
Now, what does help sinus infection and congestion... well, the good old neti pot with a saline solution keeps the bacteria count down and keeps the mucus membranes moist, which is a great preventative and a great way to clear the breathing passages when you are in trouble. Breathing most air/steam infused with eucalyptus oil also works wonders. Staying hydrated. Exercising the right side up is awesome. A yoga practice itself is a great preventative and it helps move things around, as long as you don't do full inversions. There are also tons of OTC medications and your doctor will be happy to give you antibiotics if you really think you need it.
Incidentally, I am teaching a weekend on inversion at Smiling Dog, March 11 - 13, so if you are into it, call Smiling Dog and sign up. Meanwhile, have a safe yoga practice.
See you on the mat.