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The Speed of Sound

The Speed of Sound

Symptoms: Arm, Back Pain, and the Seat Belt Sign

Butts, Christine MD

Author Information
doi: 10.1097/01.EEM.0000550372.20567.b1
    double-line sign
    double-line sign:
    Image 1. The double-line sign, which is indicative of perinephric fat.

    EMS brought in a 45-year-old woman after a motor vehicle crash. She was the restrained driver in a high-speed, front-end collision. She complained of severe right arm and back pain.

    Her vital signs were stable, and her physical exam was significant for an obvious deformity to her right humerus and a linear ecchymosis across her mid-abdomen consistent with the seatbelt sign.

    She had moderate, diffuse tenderness to palpation of her abdomen, and a FAST exam was performed. The right upper quadrant view is shown below, and the remainder of the views were negative. What does this view demonstrate?

    Find the diagnosis and case discussion on the next page.

    Diagnosis: The Double-Line Sign

    The double-line sign refers to the two hyperechoic lines surrounding the kidney and liver. (Image 1.) These represent fascial planes and are visible because of perinephric fat in the hepatorenal space. Anatomically, the liver and kidney should lie next to each other with little space in between. The presence of perinephric fat in patients with a larger habitus, however, can create the appearance of space in the area. This hypoechoic (dark gray) area can cause confusion about whether this space represents a positive FAST. When free fluid is present, this double-line sign will not be apparent. (Image 2.)

    Image 2. Positive FAST in the right upper quadrant. Note only one hyperechoic line can be seen surrounding the kidney.

    This finding was first described in The Journal of Emergency Medicine in 2011, and holds true as a means to prevent false-positive FAST exams. (J Emerg Med 2011;40[2]:188.) Other normal structures, such as bowel loops or blood vessels, can also be confused with free fluid and cause a false-positive FAST exam.

    It is essential when performing your FAST exam to evaluate all of the potential spaces carefully, particularly the inferior tip of the liver where free fluid can hide. Also keep your eyes peeled for the double-line sign to avoid being fooled by perinephric fat.

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