THE SINUS AND PRESSURE
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The sinuses are hollow spaces inside the bones of the head. There is 1 over each eye, 1 in the cheek bone under each eye, and more inside the head adjoining the nasal cavity. Since they are hollow they contain gas and are subjected to the affects of Boyle's law. If you scuba dive and the pressure in the sinuses does not change as you descend, the crushing affect results in extremely sharp pain. The pain increases as descent is continued until bleeding occurs. The blood will enter the sinus and may relieve the pain to some extent. The diver may blow bloody strands of mucus from the nose for several days after the dive.
The Skull showing some of the sinuses
The tubes to the sinuses are larger and shorter than the eustachian tubes to the ears. Pressure is usually changed in the sinuses automatically as the diver descends and ascends. If the tubes are blocked by mucus due to a cold, allergy, or sinus condition, then a pressure differential may occur. The sinuses will not collapse before they bleeding because the bone is too strong for that to happen.
When you have a "head cold" it is the result of the sinuses being stuffed with mucus from forcefully blowing the nose. The dull pain in the cheeks and/or forehead is minute when compared to the pain caused by a sinus squeeze. A sinus squeeze pain is super-sharp and usually prevents the diver from descending further. Interestingly, if the squeeze is to the sinuses at the rear of the nasal cavity (Submaxiliary) the diver will be convinced there is something wrong with their teeth. The pain signal from those sinuses are connected to the same nerves coming from the teeth! Divers usually swear up and down they have to see their dentist following a dive when a submaxiliary sinus squeeze is encountered.
Reverse blocks in the sinuses are rare because the tubes are so large. If mucus started to block the exit of gas from a sinus upon ascent, the mucus plug would simply be blown into the nasal cavity. There may be slight discomfort as one ascends but the sinuses will usually clear by themselves.
The biggest problem this author has with the sinuses is when very cold water hits the the forehead. The brain freeze! Sometimes a gloved hand has to be placed on the forehead to relieve the pain. It happens if the wet suit hood does not meet the mask and there is a gap between the two.
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