What causes a sinus headache?

While a sinus infection (sinusitis) can be due to a viral infection much like a cold that causes swelling in the nose and decreases drainage of the sinuses, an allergic reaction (such as hay fever) may also begin the process of inflammation. As with any inflammation, there is swelling that in this case decreases the ability of the sinuses to drain and increased inflammatory fluid production (just like the weeping observed with a skin injury). The combination of more fluid and less drainage leads to the pain and pressure of a sinus headache.

Nature does not like stagnant fluid, and after a period of time bacteria and viruses can travel from the nasal cavities into the fluid and cause an infection. Less commonly, fungal infections may cause a sinus infection, and even more rarely, tumors can invade the sinus.

The maxillary sinus sits underneath the eye within the cheekbone. The upper teeth attach to the lower portion of this bone, and dental infections can travel up the root of the tooth and infect the sinus directly.