"Air - Bubble" Sign - An uncommon presentation of a common disease

Abstract

Sir, A 33-year-old female presented with right-sided pleuritic chest pain and expectoration of mucopurulent, foul-smelling, and salty sputum for one year. There was a history of close contact with a pet dog for last two years. General survey and systemic examination revealed no abnormality. Absolute eosinophil count was 800/cmm. Sputum microscopy for acid fast bacilli was negative. A homogenous opacity in right lower lobe was seen in chest radiogram [Figure 1]. Contrast enhanced computed tomography (CECT) scan of thorax showed two rounded, heterogenous opacities (1 × 1 cm and 1.5 × 2 cm, respectively) with presence of air pockets within the lesions—the “air – bubble sign” [Figure 2]. Ultrasonography (USG) of thorax revealed loculated collection of fluid with echogenic organization inside. USG-guided aspiration of the lesion revealed viscid material. Cytology of the smear showed fragmented membranous structures with lamination along with aggregates of degenerated inflammatory cells in a background of proteinaceous fluid [Figure 3]. Fiberoptic bronchoscopy did not reveal any endobronchial lesion. USG of whole abdomen did not reveal cyst in intra-abdominal organs. Serum anti-echinococcal IgG level was 26.41 U/ml (normal value: <8 U/ml, ELISA method). A diagnosis of infected hydatid cyst in right lung was made and albendazole tablet, 400 mg twice daily, was started. The patient was referred to the department of cardiothoracic surgery for further management and follow up.Figure 1: Chart X-ray—PA and right lateral view showing homogenous opacity in right lower lobe with sparing of right costophrenic angleFigure 2: CECT thorax showing “Air – bubble” signFigure 3: FNAC showing fragmented membranous structures with laminationEchinococcosis, a zoonosis caused by Echinococcus granulosus is endemic in India, mostly involving liver and lungs. Most pulmonary cysts are asymptomatic and become clinically evident as a result of complications like cyst rupture and secondary infection. Pulmonary cysts are characteristically solitary and involve single lobe, mainly the lower lobe, and more common on the right.[1] Structurally, the cysts consist of pericyst, exocyst, and endocyst from outside inwards. Echinococcal cysts carry high risk of rupture with subsequent seeding to the other viscera[2] and secondary infection. The rupture may lead to anaphylactic reaction.[2] Most common respiratory symptoms of ruptured hydatid cyst are cough with fetid expectoration.[2] CECT scan of thorax is the diagnostic modality of choice for pulmonary hydatid cyst. Most of intact pulmonary cysts present as solid lesions of fluid density on CECT thorax. But ruptured cysts almost always present with a variety of radiological appearances due to different combinations of collapsed membrane, air, and fluid, making its diagnosis difficult. Sometimes, secondary bacterial infection supervenes on the ruptured cyst with subsequent increase in attenuation numbers in CECT thorax.[3] Because of solid density of the cyst, the differentiation from a pulmonary neoplasm is usually impossible. Serological test like anti-echinococcal IgG is helpful in most patients for final diagnosis, but measurable immunological response may not develop in few patients. Pulmonary hydatid cyst may rupture through pericyst only or through pericyst, exocyst, and endocyst with expulsion intracystic contents within airways. A number of radiological signs of ruptured cysts are described which include crescent sign, water lily sign, daughter cyst, double arch sign, ring within a ring sign, serpent or snake sign, and spin or whirl sign.[4] When there is dissection of air between the pericyst and parasitic membrane, due to erosion of a bronchiole by an expanding cyst, “airbubble” sign is seen. The “air bubble” sign which is a relatively newly recognized radiological sign is reported to be very sensitive and specific (85.7% sensitivity and 96.6% specificity) in establishing diagnosis of ruptured, infected hydatid cyst.[5] Air bubble sign is best seen in mediastinal window as single or multiple small, rounded radiolucent areas with sharp margins within the periphery of a solid mass lesion.