According to WebMD, a distended gallbladder is most commonly caused by the blockage of the cystic duct, an outlet for the gallbladder fluids, by a gallstone. Swelling of the gallbladder can also occur without a gallstone, but this is rare and mostly occurs in people who are very sick from other conditions. If left untreated, chronic gallbladder swelling can lead to a thickening and hardening of the gallbladder walls.
WebMD states that the distension of the gallbladder is also known as cholecytitis. The buildup of bile in the gallbladder when it is blocked causes swelling of the tissues and pain, and it opens the organ to infection. The pain of cholecytitis tends to be local to the upper left side of the abdomen, but it can radiate to the back or right shoulder blade. That abdominal area is often also tender to the touch or to any other increase in pressure, such as from a deep breath. Nausea and vomiting are common side effects of this condition.
Treatment for a distended gallbladder depends on the severity of the condition, according to WebMD. Careful rest, intravenous antibiotics and pain medication are often sufficient for mild cases. More severe cases require surgery, often to remove the gallbladder entirely.