“You need to have a rectal exam” you explain to one of your patients. He’s a 47-year-old with chronic back pain who is in the ED for an exacerbation of his pain accompanied by many of the associated accoutrements, such as sciatica, numbness and trouble urinating. You wonder if some time later during his ED course he will also inform you that he is out of his pain medication. “I’m not a fan of doing rectal exams,” you explain, “but sometimes it’s very important. If you are having trouble urinating, it might mean that you have a herniated disc pressing on nerves at the tail of your spinal cord and that could cause permanent neurological damage. I need to check your muscle strength down there and also see if you have something else that could be causing the trouble urinating such as an enlarged prostate gland.”
“No way, Doc! No way anyone’s going to stick their finger in my bum, not even you! There’s gotta be some other test you can do. How about an MRI?” You try a half dozen ways to convince him that the rectal exam is the best way to figure things out rapidly, and that the radiologist won’t approve an MRI unless the physical exam, including the rectal exam, has been completed. You’ve already done a chart biopsy, so you know that this particular patient is in the ED frequently for back pain and opiate refills and has had prior negative MRIs for the same exact presentation, including urinary retention. You also know that BPH and opiate-induced urinary retention are both far more common than cauda equina syndrome. You don’t want to give this guy more narcotics and you definitely don’t want him lingering in your ED for the 4+ hours it will likely take to get an MRI done and read by the radiologist. However, what you don’t want even more is to miss cauda equina syndrome just because your patient is a frequent flier. A rectal exam for tone, sensation and volition, plus a check of the bulbocavernosus reflex for completeness sake is the best and quickest way to confirm your suspicions and send him packing. Unfortunately, you can’t do the rectal without his consent.
Your next proposed solution, a Foley catheter to check for a post-void residual, is also met with the same excited refusal. So far, all of your objective examination and reflex testing has been normal, but you really need the rectal exam to make a bullet-proof chart. What other options do you have? You decide that the best plan B is to carefully document in the chart your attempts to convince the patient, along with his reasons for refusal, and then use your ED’s bedside ultrasound machine to do a post-void residual estimate of his bladder volume. While you are there you can try to take a look at his prostate size as well. Fortunately he agrees to let you scan him. With the machine plugged in and powered up, you take the following images.
How big is his prostate gland? How large is his bladder volume? Are these values normal or pathologic?
Conclusion on next page
Dx: No Cauda Equina, OK for Discharge
The ultrasound demonstrates a mildly enlarged prostate gland and a likely insignificant amount of post-void residual urine (see instructions of volume estimate calculations in the section below). The prostate is the gray apple-shaped structure that is measured with electric calipers (A----A and B----B) on the cross sectional image. The bladder is black and though usually shaped like a garden pot or rounded off triangle, is bat-shaped on this cross sectional image due to the intruding prostate gland. On the longitudinal image the bladder is shaped more like a bicycle seat. Not the hyperechoic posterior enhancement far field to the bladder and to a lesser extent posterior to the prostate gland.
Given the relatively small amount of urine in the bladder along with multiple alternative causes for urinary retention, namely the opiates he has been taking and the enlarged prostate gland, your suspicion for cord compression drops.
Moreover, there is now no objective evidence of cauda equina syndrome, and this patient has had multiple prior visits for back pain, including some coupled with a complaint of trouble urinating. Based on all of this you feel comfortable that this particular repeat customer is safe for discharge home without any further testing. You give him appropriate aftercare instructions, return precautions and a referral to a pain specialist and a urologist for further evaluation. He, of course, requests a med refill because he is out of his Vicodin and his Percocet. You tell him you don’t feel comfortable with him getting his prescriptions from the ED for this and that is why you have referred him to a pain specialist. Furthermore, you recommend he avoid using opiates for the next 24 hours to make sure his urinary retention improves.
Pearls and Pitfalls on next page
Pearls & Pitfalls
for Ultrasound Evaluation of the Bladder and Prostate
1. Finding the Bladder: The urinary bladder is located in the pelvis, but with retention may protrude into the lower abdomen. Start with your probe just above the symphisis pubis and aim slightly inferiorly initially in case the bladder is decompressed. Urine in the bladder will appear as an area of dark black fluid (anechoic), and there will usually be some degree of hyperechoic posterior enhancement behind a fluid filled structure such as the bladder (see image). Measure the bladder size in three different planes.
2. Bladder Volume = (A)(B)(C)/2
Post-void residual bladder volume should be measured immediately after the patient completes a spontaneous void or at least an attempt to do so. To estimate the bladder volume, measure in 3 different planes (height, width, depth) and divide the product of these 3 measurements by 2.
Bladder volume = A x B x C/2, where A, B and C are the height, width and depth. A normal post-void residual urine should be less than 50-100ml.
3. Finding the Prostate Gland: The normal prostate gland is hypoechoic (gray) and is located deep and inferior to the bladder. It is usually round or apple-shaped and may have minor internal irregularities and occasionally calcifications that may shadow. Although it is not within our scope of practice to use bedside ultrasonography to assess the prostate gland, it is important to know what normal and abnormal prostates look like so that the appropriate follow up can be obtained. In situations where an obvious abnormality is visualized, it is important to arrange definitive evaluation of the prostate and surrounding structures. Many urologists will obtain a trans-rectal ultrasound for a more comprehensive evaluation of what you saw on your bedside scan. Note that bladder masses and intraluminal clots can often appear similar to an enlarged prostate abutting the bladder wall and that further evaluation or imaging is warranted when an abnormal mass is seen on bedside ultrasonography.
4. Prostate Size = (A)(B)(C)/2 or (A)(A)(C)/2 The normal prostate size in a young male is approximately 20 grams or 20cc which correlates to two finger-breadths or less. In an older male, a normal prostate size is approximately 30 grams. Urinary retention is rare if the prostate is under 40 grams and degree of urinary symptoms tends to correlate with size. However, this is not always true. If you assume the density of the prostate is similar to water, then the size of this patient’s prostate is approximately
5. Practice: With bedside ultrasound, there is no substitute for experience. The more scans you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is. Find more image-based case studies at www.epmonthly.com
Brady Pregerson manages a free on-line EM Ultrasound Image Library and is the author of the Tarascon Emergency Department Quick Reference Guide. For more information visit www.EMresource.org.
Teresa S. Wu is the Associate Residency Director, and Director of Ultrasound and Simulation Based Training for the Maricopa Emergency Medicine Program in Phoenix, Arizona.