What causes water lily sign in hydatid cysts on imaging modalities?
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Water lily sign is a pathognomonic radiologic finding observed in hydatid cyst infection, particularly during the transitional stage between active and inactive disease. The sign occurs when the laminated endocyst (inner membrane) becomes detached or free-floating within the ectocyst (outer fibrous cyst wall). This detachment typically indicates partial degeneration or the beginning of involution of the cyst. Hydatid disease is a parasitic infection caused by Echinococcus species, most commonly affecting the liver but capable of involving multiple organ systems including the spinal muscles, heart, and other tissues. The cyst develops through parasitic transmission, often from exposure to contaminated food or water in endemic regions. The internal membranes floating within the dense fibrous cyst wall create the characteristic appearance on imaging.
Why is it called so?
The sign is named for its visual resemblance to a water lily floating on water. The detached laminated membranes create a curvilinear, floating appearance within the cyst cavity that resembles the appearance of a water lily bloom suspended in water on radiographic and cross-sectional imaging. The name provides an intuitive visual descriptor for recognizing this pathognomonic finding.
Pathophysiology
The water lily sign develops through a specific sequence of cyst evolution. Initially, the hydatid cyst contains intact laminated membranes attached to the fibrous outer wall. As the cyst progresses through its natural history, particularly during the transitional stage between active and inactive disease, the endocyst begins to separate from the ectocyst. This separation occurs due to partial degeneration, host immune response, or internal pressure changes within the cyst. Once detached, the floating laminated membrane or membranes create the characteristic curvilinear structure visible on CT, MRI, ultrasound, and other imaging modalities. The degree of membrane detachment and the presence of internal fluid create the distinctive appearance that resembles a water lily floating freely within the cyst cavity rather than being adhered to the cyst wall.
Alternative names: Camalote sign
Other associated named signs: Picture frame vertebra (when hydatid cysts affect vertebral bodies)
Access all radiology signs posted so far: http://radiogyan.com/radiology-signs/
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