Your head contains four sets of sinuses located throughout your face. They are located above your eyes, behind your nose, and behind the cheekbones.
Your sinuses are air-filled cavities surrounded by bone and lined with a mucous membrane. Air travels to and from your sinuses through small airways. Unlike your middle ear, which requires special attention to equalize, normal and healthy sinuses equalize automatically as you breathe, and methods such as the Valsalva maneuver have no effect on sinus pressure.
When your sinuses are congested, air cannot freely pass in and out of the sinus cavity. This prevents equalization of the sinus, and causes a painful squeeze during descent. If you feel pressure anywhere behind your face during descent, your sinuses aren't equalizing, and you should abort the dive.
As a sinus squeezes, the soft tissues that line the sinus are stretched inward. This causes the blood vessels to rupture and fill the sinus with blood. Then, as you ascend, the remaining air inside the sinus expands. Initially, the blood inside the sinus prevents the air from returning to its original volume. This causes painful pressure against the lining of the sinus. Eventually, the pressure frees the congestion and forces the blood and mucous out of the sinus and into your nasal passage.
A sinus squeeze results in a bloody nose, but this does not mean you must suspect a sinus squeeze whenever you have a bloody nose. Many people suffer from bloody noses as a result of pressure changes, so don't be alarmed unless it's accompanied by pain in the sinus area.
Sinus Reverse Blocks
If your sinuses become congested during a dive, a reverse block can occur during ascent. This is a painful situation that causes air to expand against the inside of the sinus.
Most sinus reverse blocks occur when decongestant drugs wear off during a dive. For this reason, you should avoid using decongestant medications prior to a dive.
If a reverse block does occur, you should ascend as slowly as possible to allow time for the air to escape the congested sinus. If your air supply is low and you have to ascend, you might be able to equalize the sinus by closing your mouth, pinching your nose, and gently inhaling.