Serpent sign

Last revised by Dr Daniel J Bell on 24 Jun 2021

The serpent sign, a.k.a. snake sign, is described in hydatid disease. The WHO classification (2001) or Gharbi classification (1985) of hydatid disease describe several stages on ultrasound 1.

During the active stage, the cyst is composed of three layers: the outer (pericyst), the middle (ectocyst), and the inner (endocyst) 1. When the pressure increases within the cyst, there may be rupture with dissection of the ectocyst from the pericyst 1. This collapsed ectocyst, within an intact pericyst, can be visible on ultrasound as well as on CT and MRI and looks like a snake or serpent, hence the name 1.

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