Water lily sign | Radiology Signs

Water lily sign is a pathognomonic radiologic finding observed in hydatid cyst infection, typically following partial cyst rupture and detachment of the membrane. The sign occurs when the laminated endocyst (inner membrane) becomes detached and collapses, floating freely within the ectocyst (outer fibrous cyst wall/pericyst). 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 lung, 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, membrane detachment occurs due to partial rupture, 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: Cumbo sign (double-arc/dome sign of air or fluid separating the pericyst from a detaching endocyst membrane) and cartwheel appearance (multivesicular, spoke-like daughter-cyst pattern on ultrasound) – both distinct hydatid-disease-specific signs. Note: the “picture frame vertebra” is NOT a hydatid disease sign – it is a sign of vertebral body cortical thickening seen classically in Paget disease of bone (and vertebral squaring in ankylosing spondylitis), and has been removed from this list as an incorrect association.

References

1. Pedrosa I et al. Hydatid disease: radiologic and pathologic features and complications. Radiographics. 2000.