Pneumoperitoneum: the misleading double wall sign.

Both sides of the gut wall seen on a plain film of the abdomen is regarded as evidence of free air in the peritoneal cavity. However, this appearance may be precisely simulated when two loops of distended intestine lie in contact with each other. In vitro studies and clinical examples emphasise the necessity of exercising caution when making the diagnosis of pneumoperitoneum on this sign alone.