I have seen patients with problems like hives which wouldn't get better, they had no other symptoms. The hives got better immediately with treatment of sinusitis. I was amazed.
Sinus problems may also cause all kinds of allergies to worsen (it seems that sinusitis turns on the immune system) and can cause problems with asthma, intestinal symptoms, and a wide variety of other medical problems. I have had patients who have toothaches and have numerous dental procedures to no avail. "Migraines" are an especially common manifestation of sinusitis. One study found 50% of chronic migraine patients had chronic sinusitis.
Although rare, serious complications can occur as a result of sinus infections. This includes spread to the brain, eyes, and a blockage of vessels in the head. In addition, erosion of bone can occur as a result of the infection eating through the sinuses. Not a pleasant thought, and more incentive to get it properly treated.