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The Speed of Sound

The Speed of Sound

Think Outside Morison's Pouch

Butts, Christine MD

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doi: 10.1097/01.EEM.0000521603.85539.ab
    ultrasound, morison's pouch
    ultrasound, morison's pouch:
    Sagittal image of the RUQ (SD=subdiaphragmatic space, M=Morison's pouch, PG=paracolic gutter). Free fluid is present in Morison's pouch in small amounts and in the paracolic gutter in larger ones. If the inferior tip of the liver is not captured, it's easy to see how the small amount of free fluid in Morison's pouch could be overlooked.

    The FAST exam is one of the oldest and most entrenched applications in bedside ultrasound. Even first-year medical students are often aware of it, and it's typically the first bedside application taught. It's a pretty straightforward exam in most cases, but several nuances to the FAST can lead to mistakes even for more experienced sonographers.

    The right upper quadrant (RUQ) portion of the FAST exam is often thought to be the easiest quadrant to evaluate. The presence of the liver provides a larger acoustic window than the smaller spleen, and its appearance is not as variable as the anatomy in the pelvis. Many sonographers, however, in their rush to get through the FAST, shortchange this quadrant by not fully evaluating all of its potential spaces.

    The RUQ contains several of these spaces: the subdiaphragmatic space (between the liver and diaphragm, typically limited by the coronary ligament), the hepatorenal space (AKA Morison's pouch), and the inferior tip of the liver extending to the inferior pole of the kidney (the superior aspect of the right paracolic gutter). Morison's pouch is easily recognized as the interface between the inferior border of the liver and the right kidney. Not taking the time to examine the other potential spaces can lead to missing free fluid, even surprisingly large amounts.

    The right paracolic gutter is perhaps the most overlooked area of the RUQ. It extends from its most superior aspect, at the inferior tip of the liver, to its inferior aspect beyond the right kidney. Moving the transducer a rib space or two inferiorly (or angling the beam toward the patient's feet) will allow the sonographer to scan quickly through this under-evaluated space.

    Earlier this year, Lobo, et al., published their analysis of positive FAST exams recorded at their institution over an 18-month period. (West J Emerg Med 2017;18[2]:270.) The researchers looked at which quadrants of the FAST were most often positive when free fluid was present, and then further subdivided this analysis to determine which areas of each quadrant were most sensitive. They evaluated 48 positive scans and found, perhaps not surprisingly, that the RUQ was the most positive view (66.7% of patients). Within that quadrant, the area that was most often positive was not Morison's pouch but the area surrounding the tip of the liver and extending to the right paracolic gutter; 93.8 percent of patients had free fluid in the RUQ.

    The takeaway? The next time you are performing your FAST exam, take a few extra seconds to think outside Morison's pouch and check the other potential spaces within the RUQ.

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