Urinary retention is a common post-operative complication, but what causes the condition, which patients are at risk for developing it and how is it diagnosed? Let's explore the answers to these and other questions about overfilled bladders.

1. Why does it occur?
An adult bladder has a capacity of 400ml to 600ml. The urge to void is first felt at 150ml. When the threshold to void is reached (approximately 300cc) the parasympathetic nerve fibers that innervate the bladder cause contraction of the detrusor muscle and relaxation of the organ's neck to permit urination (the emptying phase). In contrast, during the storage phase sympathetic nerve fibers relax the detrusor muscle and increase the tone of the bladder's neck. The spinal cord activity of these 2 systems is modulated by brainstem centers and by the cortex.

General anesthetics and analgesics such as inhalation agents and intravenous opioids interfere with the autonomic nervous system, which can decrease detrusor muscle contractions, thereby inhibiting the bladder's emptying phase.

Spinal anesthetics block activity along the nerve fibers that travel between the nerve centers of the brain and the bladder. Patients lose the sensation to void about 1 minute after being injected with spinal anesthesia, but will continue to feel dull pressure as the bladder reaches full capacity. In addition, the ability to contract the detrusor muscle is lost 2 to 5 minutes following the injection of local anesthetics and still persists even after bladder sensation is fully recovered. Spinal anesthesia with long-acting local anesthetic therefore contributes more to POUR than spinal anesthesia with short-acting local anesthetic, since the inhibitory effect of spinal blockade on bladder function lasts longer. Patients receiving spinal anesthesia with short-acting local anesthetic are often able to void shortly after outpatient surgery and are ready to leave the PACU quickly.

2. How is it diagnosed?
You can identify POUR through clinical exams, bladder catheterization or ultrasound scans.

• Clinical exams involve palpating the patient's suprapubic area in order to estimate the bladder's urinary volume (dullness up to the umbilicus felt upon palpation means the bladder contains roughly 500ml of urine). Pain or discomfort in the lower abdomen may indicate POUR, but the symptoms might be masked by regional anesthesia or other co-morbidities, and some patients might not be able to communicate their discomfort. As you can imagine, these methods of diagnosis are far from accurate and can result in overestimating a patient's bladder volume.
• Bladder catheterization can be used to assess and, if need be, treat POUR. While effective in diagnosing and treating the issue, catheterizing is an invasive procedure that's uncomfortable for patients and one that could damage the urethra and cause bladder infection. Treat potential underlying causes of POUR and consider other diagnostic techniques before resorting to catheterization.
• Ultrasound is being used in more and more facilities as an effective means for assessing POUR. It can also be used to monitor bladder volume to see if interventions are needed before volume gets too large. In recovery, ultrasound can determine if patients at high risk of POUR can be discharged without voiding. The advantages of the technology are many: It's a non-invasive way to measure bladder volume; it's easy to use; it has a short learning curve; and it provides accurate and specific measurements of a patient's bladder volume. (Clinical research suggests the technology underestimates volumes, but the approximate 25cc differences between ultrasound readings and actual measurements aren't clinically significant.)

3. What are the risk factors?
The elderly are at greater risk of developing POUR, with rates of incidence more than doubling after the age of 50 years. Men are more likely to experience POUR than women. Diabetics and alcoholics are also at increased risk. Medications typically administered during surgery, including anticholinergic agents, beta-blockers and sympathomimetics, can impair proper bladder function. Administering more than 250ml of intravenous fluid before surgery, which can lead to over-distention of the bladder, is a risk factor. Longer surgeries have also been linked to POUR, as have inguinal hernia repairs, urologic surgeries and lower-limb orthopedic procedures.

4. What complications can arise?
Patients experiencing pain from over-distended bladders may suffer from vomiting, heart arrhythmia, hypotension and hypertension, to name a few potentially serious adverse events. Patients who are catheterized due to POUR are at heightened risk of developing urinary tract infections. Experimental studies show that bladder over-distention caused by POUR can result in long-term bladder dysfunction that impacts the ability to void.

5. Discharge without voiding?
Bladder catheterization is viewed as the go-to option for treating POUR, but you may be able to limit catheterizations to only those patients who truly need it. You may even be able to send some high-risk patients home before they void.

Start by assessing a patient's risk factors for POUR (see "Risk Factors to Consider"). Following surgery, use ultrasound to measure the bladder volume in high-risk patients who've yet to void, but are ready to be discharged. You can send patients with bladder volume less than 600ml home without voiding. Catheterize (in and out) high-risk patients when post-op bladder volume exceeds 600ml over 2 hours. If high-risk patients void spontaneously, however, you can discharge them after you check leftover bladder volume. You can safely discharge patients presenting with no risk factors for POUR without voiding. All patients sent home without voiding should seek medical attention if they haven't yet urinated 8 hours after discharge.