Soundings: Chronic back pain? Check the ultrasound

You are working the Sunday overnight shift when you pick up a chart that sends your mood south. You immediately recognize the patient’s name. He’s a frequent flyer on NMN (Need More Norco) airlines. 

 

You are working the Sunday overnight shift when you pick up a chart that sends your mood south. You immediately recognize the patient’s name. He’s a frequent flyer on NMN (Need More Norco) airlines. Chief compliant: “Chronic back pain and can’t urinate.” You’ve seen him before for back pain, but the “can’t urinate” tag line is new. Better take a closer look into things this time. Your patient is a 38-year-old male with 9 visits in the past 6 months for back pain, but he has a slightly different look on his face this time. He states that his pain has been this bad before, but this is the first time he ever had trouble urinating. He remembers being told more than once that he should return immediately if he couldn’t urinate normally. He says he’s “had trouble peeing for two days” and last night in his sleep he actually wet himself. He says he can go, but he has to push and it seems like “it won’t all come out”. Of course he also wants a shot of pain medications.

He won’t allow you to perform a thorough physical exam, but his strength seems slightly diminished in L5 and S1, without that give-away stuttering effort. He also has three beats of clonus bilaterally. He refuses a rectal exam and responds, “You’ve go to be out of your mind!” when you tell him the nurse is going to need to insert a catheter through his penis into his bladder. You do your best to convince him, even telling him that he may end up paralyzed, but he won’t change his mind. Somehow his nurse is able to convince him, but as soon as she starts to insert the Foley, he screams, “No way – I’m not doing it! You people are sick!”

You’re running out of options here. This guy may actually have a cauda equina syndrome, and you don’t want to let the fact that he is infamous for crying wolf to stand between you and the correct diagnosis. Of course there are other considerations. Perhaps it’s just narcotic induced urinary retention. It could be a prostate problem, but he’s a bit young for that. Nevertheless, you know that your radiologist is probably not going to approve a middle-of-the-night MRI, where the tech has to come in from home, if you don’t have some hard findings to substantiate your hunch. A post-void residual (PVR) urine which is abnormally high would sure help you out here. Then it dawns on you; you can measure the PVR without a Foley catheter if you use your department’s ultrasound machine.
Image 1

Image 2

What does the image show? How do you calculate the Post Void Residual? What is the normal value for a PVR?

see next page for results and discussion
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Image 1 shows the bladder in the transverse plane as a fluid filled structure with anechoic contents and a hyperechoic wall. The electronic calipers measure 7.9cm by 5.7cm. Image 2 shows the bladder in transverse along with electronic calipers measuring 7.5cm by 5.2cm. Using the 7.9 instead of 7.5cm as the depth, 5.2cm as height and 5.7cm as width the volume formula becomes (7.9)(5.2)(5.7)/2, which rounds off to (8)(5)(6)/2. This equals about 120ml, which is definitely abnormal. With this ammunition in hand you call your radiologist who calls in the MRI tech. An hour later the radiologist calls you and tells you the patient has a large midline disc herniation at L4 with compression of the cauda equina.
Image 1

Image 2

You next call your neurosurgeon and tell the story. His response is short but sweet. “I’ll be there in 10 minutes after I call the OR so they can start getting set up.” 

Indications: Urinary retention, decreased urine output, Foley catheter obstruction, checking for adequate urine volume prior to catheterization of an infant or young child.
Getting Started: Place the probe in the midline just above the symphisis pubis. A full bladder appears as a anechoic fluid collection surrounded by the hyperechoic bladder wall. If the bladder is not full, you may need to aim inferiorly, down into the pelvis to find it.

Be Thorough: Scan the bladder in both the transverse and longitudinal planes. Use a fanning motion and ensure that you see the entire organ. Abnormal findings may include diverticuli and bladder stones

Beware of Enhancement: If you are looking for fluid behind the bladder, you will need to turn down the gain as the fluid filled bladder will cause posterior enhancement making small amounts of fluid easier to miss.

Estimate Bladder Volume: The volume of a sphere is (4/3) (pi)(radius)3. If you round pi down to 3 and use diameter instead of radius, the volume formula simplifies to ½ (diameter)3. To estimate the volume of the bladder you can use the ABC/2 formula. Where A, B and C are the measurements of maximum diameters in the three orthogonal planes. Of course this is only an estimate. Expect an error of up to 25%, but it should be good enough to let you know if there is an emptying problem.

Practice Makes Perfect: With bedside ultrasound there is no substitute for experience. The more ultrasounds 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. It helps immensely to check out the image library of normal and abnormal ultrasounds.

 

For more examples of ultrasounds, check out the Ultrasound Library

Brady Pregerson runs the QE Emergency Medicine Ultrasound Course, has a free Ultrasound Image Library on-line and writes the Emergency Medicine Pocketbook series. For more info go to ERPocketBooks.com. Teresa Wu completed her ultrasound fellowship at Stanford and is the Director of Simulation Education and Training for Graduate Medical Education, and Ultrasound Faculty at Orlando Regional Medical Center in Orlando, FL.