CITW 11: The Bouncing Baby
Welcome back to another Clinical Image of the Week from the case files of the Brown EM Residency!
HPI: A 2-year old boy presents after being “bounced” by his 12-year old brother in a “bouncyhouse” at a birthday party. He had the immediate onset of knee pain and refused to weight bear on his right leg. He has never had an injury before and has no prior medical problems.
VS: HR 140 (crying), BP 94/50, RR 40, SpO2 98%, T 98.8
PE: Agitated and screaming whenever you enter the room. Apparent right knee effusion, but no erythema. He will not tolerate ROM well, but you note that he flexes his right hip when his parents lower his feet to the floor to avoid putting his right foot down. It is difficult to assess for point tenderness due to patient agitation.
X-rays are obtained of the right hip, femur, knee and tib/fib:
What’s the Diagnosis?
To truly understand the FAST exam, you need to be familiar with the common structures and well-known findings, but you also have to know the FAST “fake-outs”. This post will discuss one common fake-out, the “Double-Line Sign,” and after reading it, you’ll be better armed to avoid a false positive study.
Think FAST, what do you need for a complete study?
Quick review: the FAST is comprised of a series of 4 “views”, each requiring visualization of key structures to be considered complete.
- Hepatorenal – Look at 5 locations:
- Pleural space – Watch for anechoic fluid with lung floating in it and a visible spine (aka “spine sign” indicating supra-diaphragmatic fluid). Lung tissue which appears to be liver is actually normal; this is called “multi-path reflection”, “duplication” or “mirror” artifact, and it’s due to magic/wave reflection).
- Sub-diaphragmatic space
- Morison’s Pouch (between liver and kidney)
- Tip of Liver – Easy to miss a positive FAST if you don’t check here
- Inferior pole of Kidney
- Splenorenal –Similar to the 5 locations in the RUQ, but substituting the spleen for the liver The most dependent position is peri-splenic recess, so don’t just stop at the kidney. (Below, #1: Free fluid in the Pleural Space. Below, #2: Normal RUQ – note the lack of anechoic fluid above the diaphragm, no visible spine, and the appearance of liver above the diaphragm due to artifact).
- Pelvic – Both axial and longitudinal views are needed, as well as a full bladder for a complete scan. Watch for sharp areas of anechoic fluid. (Below: Positive FAST, showing pelvic free fluid)
- Pericardial – Subxiphoid views for pericardial effusion. (Below: Pericardial effusion as seen from subxiphoid view)