(See also Control of Urination.)
People may retain urine because bladder muscle contractions are impaired, the opening of the bladder is blocked (bladder outlet obstruction), or there is a lack of coordination between bladder contraction and relaxation of the muscle that closes the opening of the bladder (urinary sphincter). Urinary retention is more common among men because prostate enlargement such as that due to benign prostatic hyperplasia may cause narrowing of the channel that carries urine out of the body (urethra).
Drugs, especially those with anticholinergic effects, such as antihistamines and some antidepressants, can cause urinary retention in both men and women. Other causes include a hard lump of stool that fills the rectum and presses on the urethra (fecal impaction) and neurogenic bladder in people with diabetes, multiple sclerosis, Parkinson disease, or prior surgery of the pelvis that damages bladder nerves.
Sometimes, people cannot urinate at all. In such cases, the bladder stretches very painfully over a few hours as it fills with urine and people have swelling in the lower abdomen.
More commonly, people are able to pass some urine but cannot completely empty their bladder. In such cases, the bladder slowly stretches without causing pain. However, people may have difficulty starting urination, a weak urine stream, or a sense that the bladder has not emptied completely. Because the bladder stays relatively full, people may sometimes have leakage of urine (overflow incontinence), urinating at night (nocturia), or frequent urination. Because the retained urine can be a breeding ground for bacteria, people may develop a urinary tract infection.
If a person is unable to pass any urine, the diagnosis is clear.
In other cases, doctors try to see how much urine remains in the bladder after the person has urinated as much as they can. Immediately after the person urinates, doctors either insert a catheter into the bladder to see how much urine comes out or do ultrasonography of the bladder to measure the amount of urine present. The amount of urine left after urinating is called postvoid residual volume. If this volume is more than about half a cup (slightly more in older people), urinary retention is diagnosed.
Doctors do a physical examination, usually including a rectal examination. In men, the rectal examination can indicate whether the prostate is enlarged. In men and women, the rectal examination helps identify a fecal impaction. Doctors may take a sample of urine to test for infection. Blood tests and imaging tests may be needed to determine the cause of urinary retention.
If people cannot urinate at all, doctors immediately insert a thin rubber tube into the bladder (urinary catheter) to remove the retained urine and provide relief.
The cause of urinary retention is treated. Drugs that can cause urinary retention are stopped whenever possible. Men who have an enlarged prostate may need prostate surgery or drugs to shrink the prostate (for example, finasteride or dutasteride) or drugs that relax the muscles at the neck of the bladder (for example, terazosin or tamsulosin). People who have nerve problems that interfere with bladder contractions or function may need to use a catheter themselves periodically or have a catheter permanently placed. Occasionally surgery is needed to direct urine from the bladder away from the urethra and out of the body.