What causes the starry sky appearance of the liver on ultrasound?
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Answer:
The starry sky liver is the appearance on ultrasound of a hypoechoic hepatic parenchyma against which the walls of the portal venules stand out as bright specks, like stars in a night sky. It is classically described in acute hepatitis, where diffuse hepatocellular swelling and oedema reduce the background echogenicity and thereby increase the conspicuity of the normally echogenic periportal connective tissue. The finding is not specific and has limited diagnostic value on its own: it is described in right heart failure with hepatic congestion, in toxic shock and diffuse infiltrative disease, in leukaemic infiltration and in children with various acute illnesses, and the same appearance may be simulated by technical factors such as an inappropriately low gain setting or the use of a high-frequency transducer in a thin patient. Its practical role is as a prompt: when the liver looks unusually dark with prominent portal triads, the study should be completed with assessment of gallbladder wall thickening and pericholecystic fluid, which accompany acute hepatitis and must not be misread as cholecystitis, hepatic and portal vein patency with Doppler, spleen size, ascites and biliary calibre. The diagnosis of hepatitis is biochemical and serological, and imaging serves mainly to exclude biliary obstruction and vascular causes.
Why is it called so?
The bright portal triads scattered through the dark parenchyma resemble stars scattered across a night sky.
Pathophysiology
In acute hepatitis, hepatocytes swell with intracellular oedema and inflammatory cells infiltrate the lobules, increasing the water content of the parenchyma. Water is a poor scatterer of ultrasound, so the background echogenicity of the liver falls. The portal triads, composed of collagenous connective tissue surrounding the portal vein, hepatic artery and bile duct, retain their strong reflectivity because their composition does not change. The contrast between the darkened parenchyma and the unchanged periportal tissue is therefore increased and the triads become conspicuous. The same swelling reduces hepatic compliance and can produce a gallbladder that is thick-walled and contracted through reduced bile flow and adjacent inflammation, which is why the two findings coexist.
Alternative names: Starry sky appearance; starry night liver
Other associated named signs: Periportal cuffing on CT and the gallbladder wall thickening that accompanies acute hepatitis
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