Clinical case: A 40 year old man presents to the ED complaining of severe lower back pain that radiates into his left leg. He tells you that the pain started suddenly about 12 hours ago. On exam you find that he has reduced anal tone with normal perianal sensation. Voluntary anal contraction is present.
You decide to do a bladder scan to measure post-void residual volumes
So how does this test help us?
Cauda equina Syndrome (CES) can present with a myriad of symptoms. Unfortunately no single or combination of clinical features can rule in or out CES. This means that it is frequently poorly managed and often misdiagnosed. Recent studies show that a post-void residual (PVR) of ≥ 200 ml correlates well with cauda equina compression on MRI. In a prospective study from a Spinal Unit in the UK, the sensitivity of a PVR >200mLs or CES was 94.1%, specificity 66.8%, PPV 29.9% and NPV 98.7% . That is, a patient with a PVR of <200ml was unlikely to have findings of cauda equina on their MRI scan. This suggests that PVR, along with a thorough history and clinical exam, can be used to risk-stratify patients presenting to ED with back pain.
When exactly do you measure the PVR? It’s really important to measure the PVR immediately after the patient performs a voluntary void. Even short delays can affect the result significantly. PVR can be measured using a bladder scanner or a bedside ultrasound. Gold standard for measurement is urine collection through urinary catheterisation, but that’s not routinely performed in practice.
How do you do this using US?
Use the curvilinear probe for this assessment. Ideally, lie your patient down in a supine position. Place the ultrasound scanner over the suprapubic area. You should obtain two views of the bladder - sagittal and transverse.
The ultrasound machines in the department have a “volume” measurement function that can be used to calculate the PVR. Use the callipers to measure the maximal AP, superior-inferior and transverse diameters. The machine will calculate a bladder volume see below
If you are using first principles to calculate bladder volume, the formula is
Bladder volume = (L x H x W)* 0.52 (you may also see the co-efficient of 0.7 used)
Measure bladder dimensions in 2 planes - transverse and sagittal
What does RCEM say about back pain in the ED?
What other clinical signs are reliable?
Anal tone in itself is a poor predictor
Our man had an emergency MRI. He had a paracentral disc tear without compression of the cauda equina and multilevel degenerative changes throughout the lumbar intervertebral discs.
Learning points from this case
PVR is useful along with other tests
Keep cauda equina in your mind as a differential but remember that other things are possible also - think AAA, renal pathology, abscess, TB....the list continues!
Peer review - Dr Cian McDermott
Some Further Reading
A prospective study of the role of bladder scanning and post-void residual volume measurement in improving diagnostic accuracy of cauda equina syndrome. Katzouraki G, Zubairi AJ, Hershkovich O, Grevitt MP. Bone Joint J., 2020.
Evaluation and management of cauda equina syndrome in the emergency department. Long B, Koyfman A, Gottlieb M. Am. J. Emerg. Med., 2020.
Bladder Post Void Residual Volume. Levi Ballstaedt; Blair Woodbury. Stat Pearls 2020