Nonalcoholic fatty liver disease and elastography: Incremental advances but work still to be done

Nonalcoholic fatty liver disease (NAFLD) is the leading cause of chronic liver disease in developed countries, with an estimated prevalence of 20%-30% and increasing to 70% in obese individuals and as high as 90% in diabetics. Given the pathophysiological link with metabolic syndrome, rates of NAFLD are likely to continue to rise with the obesity pandemic. Within a community cohort, prevalence of patients with NAFLD who have nonalcoholic steatohepatitis (NASH) has been estimated at 3%-4% and rising as high as 12.2% in middle-aged patients. For diabetic patients, prevalence of NASH may be as high as 56%. Whereas advanced fibrosis has the highest risk of mortality, patients with NASH are 6 times more likely to develop liver-related mortality over 20 years than those with non-NASH NAFLD. It is precisely these individuals with NASH and fibrosis who have the greatest risk of progression to advanced disease, cirrhosis, hepatocellular carcinoma, and death. Therefore, it is critical to distinguish between patients with fibrosis, NASH, and nonNASH NAFLD. The current gold standard for diagnosis and histological assessment of NAFLD is a liver biopsy. Unfortunately, liver biopsy is invasive, subject to complications, costly, and limited by sampling errors and intraobserver variability. Furthermore, given the high prevalence of NAFLD, it is neither practical nor cost effective to perform liver biopsies in all patients who are at risk for NASH and fibrosis. Recently, several noninvasive imaging modalities based on elastography have been developed. To date, these imaging modalities have proven promising in the assessment of liver fibrosis in chronic liver disease, though rigorous comparative studies in patients with NAFLD are lacking. Furthermore, distinction of NASH from non-NASH NAFLD using noninvasive imaging remains elusive because the diagnosis of NASH historically relies on the observation of ballooning degeneration of hepatocytes as well as steatosis and inflammation—not simply steatosis and fibrosis. The most validated, commonly used elastography method is vibration controlled transient elastography (VCTE), which is also known as transient elastography (TE) and marketed as the FibroScan (Echosens, Paris, France). VCTE is ultrasound based, but is not used with image guidance. The velocity of a lowamplitude shear wave is measured in a region of interest and is converted into a measure of liver stiffness. Both point quantification shear wave elastography (pSWE) and two-dimensional shear wave elastography (SWE) use acoustic radiation force impulse (ARFI) technology to generate shear waves in the liver, and the shear wave speed is calculated in meters Abbreviations: ARFI, acoustic radiation force impulse; AUROC, area under the receiver operating curve; CAP, controlled attenuation parameter; LSM, liver stiffness measurement; NAFLD, nonalcoholic fatty liver disease; NASH, nonalcoholic steatohepatitis; pSWE, point quantification shear wave elastography; SSI, supersonic shear imaging; SWE, shear wave elastography; TE, transient elastography; VCTE, vibration controlled transient elastography. The views expressed are those of the authors and should not be construed as official views of the United States Air Force, United States Army, or Department of Defense.