The term “sinus” can mean any channel, hollow space, or cavity in a bone, or a dilated area in a blood vessel or soft tissue; most often sinus refers to the four, paired, mucus-lined air cavities in the facial bones of the head. The same kind of membrane lines the sinuses and nose, so nasal infections spread easily to the sinuses. In sinusitis, mucous membranes inflame and swell, closing sinus openings and preventing infected material from draining. If nasal inflammation, congestion, deformities, or masses block sinus openings, the sinus lining swells and inflames, absorbing pre-existing gas that forms negative pressure. When blockage occurs during descent, the relative vacuum in the sinus increases the risk of damage. Hemorrhage into the sinus and then into the divers mask may occur.
The frontal sinus is most commonly affected by barotrauma, followed by the maxillary sinus. With maxillary sinus involvement, the diver often experiences pain in the maxillary teeth due to compression of a branch of the fifth cranial nerve, which runs along the base of the maxillary sinus. Additionally, maxillary sinus barotrauma can cause compression of another branch of the fifth cranial nerve, causing tingling and numbness of the cheek and upper lip.
Do not dive if you have congested sinuses. Various over-the-counter and prescription medications open sinus passages. If a decongestant wears off during your dive, the sinuses become closed spaces (rebound effect) containing high pressure air. This can lead to a reverse block on ascent, where excess gas volume will not vent during ascent. Sinus barotrauma can also occur during ascent if blockage of a one-way valve of the sinus, by inflamed mucosa, cysts, or polyps, allows equalization on descent, but impairs it on ascent.
(Source: NOAA Diving Manual: Diving for Science and Technology, 5th Edition; 2013, Best Publishing Company)
NOAA Diving Manual, 5th Edition
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