Complicated hydatid cyst and "air bubble" sign: a stepping-stone to correct diagnosis

Hydatid cyst, or Echinococcosis, is an important helminthic zoonotic disease in humans that commonly affects the liver and lungs. Uncomplicated hydatid cysts, seen as round opaque lesions on chest radiography, are easily diagnosed, whereas complicated cysts (infected and or perforated) may change the radiographic appearance of the hydatid cyst, causing an incorrect diagnosis and delayed treatment. Although in radiology many signs have been described, the "air bubble" sign, seen in the mediastinal window of CECT as a single or multiple small rounded radiolucent areas with sharp margins within the periphery of a solid mass lesion, is being recognized as a sign with high sensitivity and specificity in the diagnosis of complicated hydatid cysts.

A 32-year-old female on anti-tubercular treatment for the past 3 months without any improvement was admitted to our hospital. CECT of the chest revealed a mass-like lesion with the "air bubble" sign. After 15 days the patient had a vigorous bout of coughing, leading to expectoration of pieces of whitish yellowish gelatinous membrane for the next 3 days. The ELISA result for Echinococcus was highly positive. On the basis of the "air bubble" sign, positive serology, and expectorated pieces of the membrane, the patient was diagnosed as having a complicated hydatid cyst.

Due to the varied presentations of complicated hydatid cyst, the knowledge and awareness of various signs in radiology associated with the hydatid cyst, in particular the "air bubble" sign, is imperative in making a prompt and accurate diagnosis of a complicated hydatid cyst.