Written by Clay Smith.
POCUS had reasonable diagnostic accuracy for the diagnosis of constipation by measuring the transrectal diameter and using a cutoff of 3.8cm, and it performed better than x-ray.
Why does this matter?
We often do an abdominal x-ray with suspected constipation - not that this does any good. See this Radecki review - No Pictures of Poop Needed. Ultrasound would be a way to obtain information with no radiation, but it is difficult to see bowel, as ultrasound doesn't image air-filled structures well. So would this work?
POCUS for Poo 💩
This was a prospective study of the diagnostic accuracy of point-of-care ultrasound (POCUS) for constipation compared to a gold standard, the Rome III questionnaire*. Abdominal x-rays were at the discretion of the faculty. Fifty patients were enrolled. Of these, 25/50 had an x-ray ordered. Using the transrectal diameter (TRD), they determined that 3.8cm was the best cutoff for constipation. Sensitivity was 86%, specificity 71%. For those who had an abdominal x-ray, the sensitivity and specificity were similar to POCUS: sensitivity 87%, specificity 40%. They estimated that 88% of x-rays could have been avoided by using POCUS.
For the ultrasound gurus, here is how they did it (see figure). They used the phased array probe 2cm above the pubic symphysis, aimed caudal, with the indicator to the right. The rectum is just posterior to the bladder (arrows). An empty bladder did not degrade the ability to measure TRD. They measured the outer edges of the crescent-shaped posterior shadowing (+ signs). This chid was very constipated, with a TRD of 8.08cm.
*Rome III, at least 2 of the following for 3 months:
- Fewer than 3 bowel movements per week
- Lumpy or hard stools
- Sensation of anorectal obstruction
- Sensation of incomplete defecation
- Manual maneuvering required to defecate
Measuring the Transrectal Diameter on Point-of-Care Ultrasound to Diagnose Constipation in Children. Pediatr Emerg Care. 2018 Mar;34(3):154-159. doi: 10.1097/PEC.0000000000000775.
Peer reviewed by Thomas Davis.