Fat and fiber: how the controlled attenuation parameter complements noninvasive assessment of liver fibrosis.

Abstract

Nonalcoholic fatty liver disease (NAFLD) is an evergrowing problem in the developed and developing worlds, affecting 15–40 % of the population [1]. Once regarded as a benign condition, NAFLD is the leading cause of cryptogenic cirrhosis and cryptogenic hepatocellular carcinoma (HCC) [2], and the third most frequent indication for liver transplantation [3]. A key challenge faced by clinicians who manage NAFLD is to establish the diagnosis and to differentiate simple steatosis from advanced disease [4]. Furthermore, steatosis often coexists in other chronic liver diseases such as chronic hepatitis C [5]. Trans-abdominal ultrasonography (USG) is the commonest imaging tool used in the diagnosis of hepatic steatosis, but only with confidence when the amount of fatty infiltration exceeds 33 % [6]. Although proton magnetic resonance spectroscopy (H-MRS) is an accurate and reproducible noninvasive means of quantifying hepatic fat, its availability is limited in many areas [1]. Transient elastography (Fibroscan, Echosens, Paris, France), originally developed for the diagnosis of liver fibrosis and cirrhosis, has been validated in patients with NAFLD [7]. The fundamental technology employed by Fibroscan involves a plastic shear wave that propagates through liver tissue combined with a pulse-echo ultrasound used to follow and measure the velocity of the propagation of the shear wave, which is directly related to tissue stiffness and hence severity of liver fibrosis [8]. Its latest iteration includes the measurement of the controlled attenuation parameter (CAP), a measurement of the attenuation of the forward and return ultrasound signal at 3.5 MHz using signals acquired by the regular M probe of Fibroscan. At a given frequency, the ultrasound-attenuation coefficient can be expressed in dB/m. Since the attenuation of ultrasound signals is greater in fat than in water, CAP may be used to estimate the degree of hepatic steatosis [9]. The diagnostic attractiveness of CAP is that it may be measured concurrently with liver stiffness using the same instrument enabling the simultaneous assessment of hepatic steatosis and fibrosis [8]. Data concerning the accuracy of CAP to diagnosis different degrees of hepatic steatosis are evolving. CAP efficiently detected low-grade steatosis ([10 %), with a sensitivity of 91 % and specificity of 81 % at a cutoff value of 238 dB/m in a retrospective cohort of patients of mixed etiology chronic liver diseases [9]. The accuracy of CAP was confirmed in two prospective studies of mixed etiology liver disease [10, 11], and in single etiology populations, including chronic hepatitis B (CHB), chronic hepatitis C, NAFLD, and alcoholic liver disease [12]. The suggested diagnostic performance and cutoff values for different degrees of steatosis are summarized in Table 1. Nonetheless, data concerning its performance in healthy subjects and patients with CHB were either lacking or insufficient. In this issue of the Digestive Diseases and Sciences, two groups of investigators provide important data to illustrate the accuracy of CAP in these two settings. Chon et al. [16] report CAP data from 264 healthy subjects, either as potential liver donors or in subjects undergoing routine G. L.-H. Wong (&) V. W.-S. Wong Institute of Digestive Disease, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China e-mail: wonglaihung@cuhk.edu.hk