The first and most common use of the thumb sign refers to a radiographic finding seen in epiglottitis. It is seen on a lateral x-ray and refers to the shape the epiglottis takes when it becomes immense and swollen, because it looks like a big thumbprint. The board question may not specifically use the term “thumb sign,” but it may show the lateral x-ray and ask you to make the diagnosis of epiglottitis.
Another use of the thumb sign refers to a physical exam finding seen in Marfan syndrome. It is seen when the patient with Marfan syndrome makes a closed fist over the top of the thumb. When looking at the closed fist, the thumb will extend beyond the fifth digit (pinky finger) and will be able to be seen poking out. A way this question could be tested is to show a picture of closed fists with long thumbs extending out though the ulnar side and asking you to make the diagnosis of Marfan syndrome.
The thumb sign referring to epiglottitis is well known to medical students, but this second clinical scenario is a good tidbit to know to completely bury this buzzword. If you suspect Marfan syndrome, the thumb sign is another physical exam finding that can be helpful.
Sources:
Eagle, Kim, and Rodney H. Falk. "The “Thumb Sign” in Marfan's Syndrome." New England Journal of Medicine 333.7 (1995): 430. NEJM.org. Web. 6 Dec. 2011. <http://www.nejm.org/doi/full/10.1056/NEJM199508173330706>.
Grover, Casey. "“Thumb Sign” of Epiglottitis." New England Journal of Medicine 365.5 (2011): 447. NEJM.org. Web. 6 Dec. 2011. <http://www.nejm.org/doi/full/10.1056/NEJMicm1009990>.
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