Dr Avik Bhattacharyya, Consultant Vascular & Interventional Radiologist, The Calcutta Medical Research Institute and BM Birla Heart Research Center, Kolkata gives an overview on the usage of ultrasonography in detecting fatty liver diseases and outlines its advantages
Fatty liver or hepatic steatosis is the term used to describe a spectrum of conditions in which triglyceride accumulates within hepatocytes. The most common condition associated with fatty liver is alcohol abuse. In addition, a variety of etiologic factors are associated with fatty liver, including obesity, diabetes, hepatitis, and drug toxicity. The prevalence of non-alcoholic fatty liver disease in the general population is estimated to be 6–35 per cent1. This prevalence is also partly related to the growing epidemic in childhood obesity during the past decades.
Although fatty liver is well-known in developed countries and is typically associated with the ‘industrialised’ or ‘Western’ lifestyle of low physical activity, alcohol abuse, calorically dense diet, and obesity; the prevalence of this disease is also rising in the developing world. A population based study by Das2 and colleagues showed 8.5 per cent of the population in rural West Bengal had fatty liver without being either alcohol consumers or overweight or obese. They have concluded that a non-obese, even non-overweight, supposedly physically active individual, with or without a bulging waistline, can have a fatty liver.
Many patients with fatty liver have no symptoms or signs of liver disease at the time of diagnosis, and are often found incidentally on cross sectional imaging. In other patients, fatty liver can be the cause of hepatomegaly and elevated liver enzyme levels, prompting a sonographic study of the liver. Here, we need to know that fatty liver encompasses a spectrum of histopathology ranging from simple steatosis to steato-hepatitis and has the potential to progress to fibrosis, cirrhosis, and end-stage liver disease and even hepatocellular carcinoma2,3. Fatty liver disease is an independent risk factor of cardiovascular disease and also prompts the clinician to search for associations with diabetes mellitus, hypertension, and hypertriglyceridemia4. Thus, the diagnosis is of great clinical interest and it is certainly a challenge.
Even though liver biopsy is considered to be the ‘gold standard’ for the diagnosis of fatty liver, it has its limitations, including risk, high cost, and invasiveness. We always prefer to assess fatty liver in a needle-free, non-invasive way by using ultrasonography. It is the first choice for screening for fatty liver1. However, it is operator-dependent, and the sonographic evaluation of fatty liver is based mainly on the subjective impression of hepatic echogenicity and posterior attenuation of the ultrasound beam.
Liver echogenicity normally equals or slightly exceeds renal cortical echogenicity, but this factor relies on the visual perception of the radiologist. We use a grading of fatty liver into mild, moderate, and severe degrees of steatosis based on an increase in hepatic echogenicity, impaired visualisation of hepatic vessels and diaphragm, and poor penetration of the posterior aspects of the liver as below.
Mild: Slight increase in liver echogenicity and with clear visualisation of intrahepatic vessel wall and diaphragm.
Moderate: Slightly impaired visualisation of intrahepatic vessel wall and the diaphragm.
Severe: Marked increase in hepatic echogenicity, poor penetration of the posterior segment of the right lobe of liver and poor or no visualisation of the hepatic vessel wall and diaphragm. The reported sensitivity and specificity to diagnose fatty liver by sonography is >90 per cent and 77–95 per cent respectively.
Estimation of liver fibrosis is also important in patient with advanced fatty liver disease because the prognosis and treatment decision often depends on its severity. Non-invasive methods are developed nowadays for evaluation of liver fibrosis. One important method is Acoustic Radiation for Impulse (ARFI) elastography. This is a type of elastography using acoustic radiation force to generate images of the mechanical properties (stiffness) of soft tissue. This method has many advantages like non-invasiveness, shows immediate result and generally comparable with other investigating method for assessing fibrosis.
We perceive that the prevalence of fatty liver disease is rising in our country as a result of economic prosperity and the Westernisation of diet and lifestyle. This is an emerging problem and expected to be responsible for consumption of a significant proportion of healthcare facilities in near future6. In this situation, we hope that improvement and refinement of ultrasonography as a diagnostic modality will make it suitable for objective and reliable assessment of fatty liver in near future.
References:
1. Koplay M, Sivri M, Erdogan H. Importance of imaging and recent developments in diagnosis of nonalcoholic fatty liver disease. World J Hepatol 2015 Apr 18; 7(5): 769-776.
2. Das K, Das K, Mukherjee PS et Al. Nonobese population in a developing country has a high prevalence of nonalcoholic fatty liver and significant liver disease. Hepatology. 2010 May; 51 (5): 1593-602.
3. Das K, Chowdhury A. Lean NASH: distinctiveness and clinical implication. Hepatol Int. 2013 Dec;7 Suppl 2:806-13.
4. Ajmal MR, Yaccha M, Malik MA, Rabbani MU, Ahmad I, Isalm N, Abdali N. Prevalence of nonalcoholic fatty liver disease (NAFLD) in patients of cardiovascular diseases and its association with hs-CRP and TNF-a. Indian Heart J. 2014 Nov-Dec; 66(6):574-9.
5. Guerra JA, Trippia M, Pissaia A et Al. Acoustic radiation force impulse is equivalent to liver biopsy to evaluate liver fibrosis in patient with chronic hepatitis C & nonalcoholic fatty liver disease. Arq Gastroenterol. 2015; 52(3):234-8.
6. Majumdar A, Misra P, Sharma S, Kant S, Krishnan A, Pandav CS. Prevalence of nonalcoholic fatty liver disease in an adult population in a rural community of Haryana, India. Indian J Public Health. 2016 Jan-Mar; 60(1):26-33.
