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)
Why do a FAST?
There are many indications for FAST exam, including any unstable patient with blunt or penetrating abdominal, thoracoabdominal, or chest trauma. The utility of FAST is that it can reduce time to the OR by bypassing the CT scanner in unstable patients. However, the FAST also has limitations, including ability to visualize the retroperitoneum, smaller volumes of free fluid, and inability to complete studies secondary to large body habitus, bowel gas, or other barrier.
How good is a FAST exam at finding free fluid (compared to CT or DPL)?
Pretty darn good.
FAST is a beneficial tool that can change management, but it can be a double-edged sword. There are multiple reasons for a falsely-positive FAST, including massive resuscitation, ascites, gastric fluid, psoas muscle misinterpretation, and others. False-negative FAST exams occur too; just remember it’s possible!
A 40-year old man presents to Anderson ED after MVC. Primary survey is intact, secondary survey reveals chest wall and abdominal tenderness. RUQ FAST view is shown below. The patient’s blood pressure is 100/64 and HR is 110. Does he go to CT or the OR?
In patients with a positive FAST, the most common location to see free intraperitoneal fluid is the RUQ. However, when you see hypoechoic or anechoic, shaply-demarkated areas in Morison’s pouch, think twice! The above images show “double-line sign”, a frequently-encountered false-positive finding. In this case, the perinephric fat pad mimics free fluid as a hypoechoic region with sharp borders due to fascial planes. When visualizing what might be free fluid in Morison’s pouch, look for echogenic borders; if they are present on both hepatic and renal sides, you are most likely looking at a fat pad. In a recent abstract in Annals of EM, a prospective series of 47 FAST exams demonstrated a Double-Line sign in over 30% of exams! Remember, always correlate clinically to patient’s presentation!
Now that we’ve broken our FAST fast, here’s a quick quiz: Free fluid, Fat pad or Phat pad?
1 – Free fluid
2 – Fat pad, double-line sign. Note dual hyperechoic lines!
3 – Phat pad, obvi.
Thanks to Drs. Liebmann and George for their input and edits!
- American Institute of Ultrasound in Medicine – http://www.aium.org/resources/guidelines/fast.pdf
- EM Docs Blog – http://www.emdocs.net/category/ultrasound/
- ICU Sonography – http://www.criticalecho.com/content/tutorial-1-basic-physics-ultrasound-and-doppler-phenomenon
- Journal of Emergency Medicine – http://www.jem-journal.com/article/S0736-4679%2809%2900657-X/abstract
- org – http://www.trauma.org/archive/radiology/FASThowgood.html
- POSITIVE FAST WITHOUT HEMOPERITONEUM DUE TO FLUID RESUSCITATION IN BLUNT TRAUMA, Jonathan E. Slutzman et al.
- J Pediatr Surg. 2013 Jun;48(6):1377-83. doi: 10.1016/j.jpedsurg.2013.03.038.The role of focused abdominal sonography for trauma (FAST) in pediatric trauma evaluation.
- False-Negative FAST Examination: Associations With Injury Characteristics and Patient Outcomes. Annals of Emergency Medicine Volume 60, Issue 3, September 2012, Pages 326–334.e3
- J Ultrasound 2015 Aug;34(8):1429-34. doi: 10.7863/ultra.34.8.1429. Focused Assessment With Sonography for Trauma Examination: Reexamining the Importance of the Left Upper Quadrant View.
- Prevalence of the “Double-Line” Sign When Performing Focused Assessment With Sonography For Trauma Exams Patwa AS, Lomibao A, Aziz-Bose R, Abo A, Nelson M, Bramante R, Modayil V, Haines C, Raio CC/North Shore-LIJ Health System, Manhasset, NY. Annals of EM, Vol 60, November 2012.