The inability to empty the bladder. Urinary retention can be caused by an obstruction in the urinary tract or by nerve problems that interfere with signals between the brain and the bladder. There are many causes, and the disorder can occur over a long period of time (chronic) or abruptly (acute). People with chronic retention are able to urinate but the stream is decreased and many times they cannot empty the bladder completely. Acute retention is an emergency since the kidneys can fail if the urine is not able to drain. In men, an enlarged prostate is the most common reason for urinary retention.