Urinary retention is the inability to empty the bladder completely, manifesting suddenly or developing gradually over time. This condition is distinct from urinary hesitancy, which is difficulty starting or maintaining the flow of urine. Retention means the bladder cannot be emptied at all, or a significant amount of urine remains after voiding. Acute urinary retention, characterized by a sudden, painful inability to pass urine, is a medical emergency requiring immediate attention to prevent damage to the bladder and kidneys.
Physical Blockages in the Urinary Tract
The most direct cause of urinary retention involves a physical obstruction blocking the flow of urine out of the bladder and through the urethra. These blockages prevent urine from exiting the body, even when the bladder muscle contracts correctly.
In men, the most common structural cause is Benign Prostatic Hyperplasia (BPH), the non-cancerous enlargement of the prostate gland. The prostate surrounds the urethra, and as the gland increases in size, it compresses this tube, creating resistance to the outflow of urine. This compression causes a weak stream and potentially leads to sudden, complete retention.
Beyond the prostate, narrowing of the urethra (urethral stricture) can impede flow. Strictures often result from scarring due to previous infections, trauma, or medical procedures. Solid masses, such as bladder or kidney stones, can migrate and become lodged in the urethra or at the bladder neck, creating an acute blockage.
External pressure from masses like tumors or cysts in the pelvic area can also press onto the urethra or bladder, physically impeding the pathway. In women, a condition called cystocele, where the bladder sags into the vagina, or a prolapsed uterus can cause a kink or pressure on the urinary tract, leading to retention. Constipation, where stool presses on the bladder and urethra, is another physical cause.
Nerve and Bladder Muscle Control Issues
In many cases, the urinary tract is physically clear, but communication between the brain and the bladder is compromised, a condition termed neurogenic bladder dysfunction. Urination relies on a coordinated signal: the detrusor muscle must contract to expel urine, and the sphincter muscles must relax to open the exit. If the nerves controlling this process are damaged, coordination fails, leading to retention.
Chronic conditions affecting the nervous system are frequent culprits, including diabetes, stroke, Multiple Sclerosis (MS), and Parkinson’s disease. Diabetes can cause peripheral neuropathy, damaging the sensory and motor nerves that signal bladder fullness and muscle contraction. A spinal cord injury can also sever the neural pathways that relay voiding commands, resulting in a loss of voluntary control.
The problem may also reside in the detrusor muscle itself, even if nerve signals are intact. If the bladder has been significantly overstretched, the muscle fibers can become weakened and unable to generate a strong enough contraction to fully empty the bladder. This detrusor muscle failure means the bladder is unable to push the urine out, even without an obstruction.
Medication Side Effects and Acute Factors
Certain medications can interfere with the nerve and muscle signaling required for urination, leading to retention as an unintended side effect. Drugs with anticholinergic properties are a major category, blocking the nerve impulses that signal the detrusor muscle to contract. These include medications used to treat overactive bladder, stomach cramps, and certain antidepressants.
Common over-the-counter cold and allergy remedies also pose a risk. Decongestants often contain alpha-adrenergic agonists, such as pseudoephedrine, which cause the internal urethral sphincter muscle to tighten. This tightening increases resistance at the bladder neck, making it difficult to initiate or maintain the urine stream, especially in men who already have an enlarged prostate.
Other drugs, including muscle relaxants, opioid pain medications, and certain psychiatric drugs, can also impair bladder function. Acute psychological factors, such as extreme anxiety or a phobia related to using public restrooms, can trigger a temporary, involuntary spasm of the sphincter muscle. This psychogenic retention is a temporary blockage caused by the body’s protective response to stress.
Inflammation and Infection-Related Causes
Inflammation or infection within the lower urinary tract can cause retention by creating a temporary obstruction due to swelling and muscle irritation. A Urinary Tract Infection (UTI) causes the lining of the urethra and bladder neck to become inflamed and swollen. This swelling narrows the passageway, making it difficult for urine to pass.
In men, prostatitis (inflammation or infection of the prostate gland) is a common acute cause of retention. Unlike BPH, acute prostatitis causes rapid and painful swelling of the gland, which quickly presses on the urethra and impedes flow. The pain and irritation from these infections can also trigger a protective reflex that causes the sphincter muscles to spasm, further inhibiting urination.