Non-alcoholic fatty liver disease (NAFLD) is a broad term used to cover a spectrum of conditions which are characterized by evidence of hepatic steatosis on imaging or histology (macro-vesicular steatosis), and absence of secondary causes of hepatic steatosis such as significant alcohol consumption, chronic use of medications that can cause hepatic steatosis or hereditary disorders. The definition of significant alcohol consumption has not been consistent. For non-alcoholic steatohepatitis (NASH) clinical trials, it has been defined as ongoing or recent consumption of more than 14 standard drinks on average per week in women and more than 21 standard drinks on average per week in men. Non-alcoholic fatty liver disease is most often diagnosed incidentally on imaging or when it presents with complications. The prevalence of NAFLD in western countries is around 20 to 30%. NAFLD is considered to be the liver manifestation of metabolic syndrome. 50 to 70% of people with diabetes are found to have NAFLD.
NAFLD has several phases of progression, which include simple steatosis, steatohepatitis, fibrosis, cirrhosis, and ultimately could even progress to hepatocellular carcinoma. The disease has a benign course; it is a silent liver disease when the only histological finding is steatosis. The presence of hepatic injury with inflammation with or without fibrosis constitutes non-alcoholic steatohepatitis (NASH). Please refer to the chapters on hepatitis, non-alcoholic steatohepatitis under transplant hepatology section for a detailed discussion of this topic.
Obesity, diabetes, dyslipidemias, insulin resistance, and metabolic syndrome are known to be associated with the development of non-alcoholic fatty liver disease. A temporal association has also been shown between inorganic arsenic exposure and the development of NAFLD reflected by elevated alanine transferase (ALT). Due to its close association with metabolic syndrome, NAFLD correlates with cardiovascular risk factors, which also contributes to mortality in these patients in addition to end-stage liver cirrhosis and hepatocellular carcinoma.
Non-alcoholic fatty liver disease (NAFLD) incidence is rapidly increasing, especially in Western countries. Rising obesity levels, increasing incidence of childhood obesity, sedentary lifestyles, consumption of unhealthy quick eats, and a longer lifespan are some of the likely contributors. The incidence and prevalence of NAFLD are underestimated as ultrasonography is commonly used to screen for fatty liver disease. The prevalence of NAFLD is 80% to 90% in obese adults, 30% to 50% in patients with diabetes mellitus, 90% or more in patients with hyperlipidemia, 3 to 10% in children, and as high as 40% to 70% among obese children.
Both environmental and genetic factors are contributing factors in the development of non-alcoholic fatty liver disease and its progression. First-degree relatives of patients with NAFLD are at higher risk than the general population. Histone amino-terminal ends, maintain the chromatin structure and gene expression that is cAMP-responsive element-binding protein H (CREBH) or sirtuin (SIRT1). Genetic studies have shown that activation of SIRT1 is thought to play a role in the development of NAFLD. The trigger of the progression of NAFLD to cancer is via abnormal DNA methylation.
Day and James proposed a two-hit model of pathogenesis in 1998. The first hit is caused by insulin resistance, which leads to the accumulation of fat droplets that is triglycerides in the cytoplasm of hepatocytes leading to the development of steatosis. Insulin resistance causes excess delivery of free fatty acid and triglycerides to the liver and decreased excretion leading to accumulation. Also, excess carbohydrates are a stimulus for de no fatty acid synthesis in the liver
The second hit causing hepatocellular injury and the development of NASH is multifactorial. Excessive fatty acids in the liver make the liver more vulnerable to injury. Peroxisomal fatty acid oxidation, reactive oxygen species (ROS) production from the mitochondrial respiratory chain, cytochrome P450 metabolism of fatty acids, hepatic metabolism of gut-derived alcohol is hypothesized to cause the injury. Obesity also contributes to the second hit as adipose tissue releases inflammatory mediators such as leptin, tumor necrosis factor (TNF)- alpha and interleukin (IL)-6, causing hepatocyte damage. The hepatocytes undergo ballooning, cytoskeletal aggregation, apoptosis, and necrosis.
Insulin resistance is also a part of the second hit. The sinusoidal collagen deposition caused by the activation of hepatic stellate cells and the portal fibrosis caused by the ductular proliferation leads to the development and progression of NASH. These changes have correlated with insulin resistance, which is now believed to cause the progression of steatosis to NASH and progressive fibrosis.
Non-alcoholic fatty liver disease is more than 10% of hepatocytes with fat droplets on liver biopsy. Functionally, the liver subdivides into three zones; the classification is made based on the oxygen supply. Zone 1 has the highest oxygenation (oxygenated blood from hepatic arteries) and encircles the portal tracts, and zone 3 encircles the central veins where the oxygenation is poor.
The American Association for the Study of Liver Diseases (AASLD) defined the histopathological abnormalities required in the diagnosis of NASH, which includes steatosis (macro more than micro), lobular inflammation, and hepatocellular ballooning is seen most apparently in the zone 3 steatotic liver cells. Fibrosis, although not necessary for the diagnosis, is usually present. Some other findings seen are Mallory-Denk bodies (MDB, eosinophilic intracytoplasmic inclusions), megamitochondria, glycogenated nuclei, and iron deposition.
Fibrosis starts in the acinar zone 3 and has the appearance of chicken wire from the deposition of collagen and other extracellular matrices along the sinusoids. NASH-related cirrhosis is macronodular or mixed. When cirrhosis develops, the other histological features may not be evident.
Patients with non-alcoholic fatty liver disease could present with many non-specific symptoms way before the diagnosis is made, although most patients are asymptomatic. Fatigue is one of the most common presenting symptoms. Sharp or dull aching upper abdominal pain, thirst, bloating, and sleep disturbances. Patients who develop NASH-associated cirrhosis, end-stage liver disease, or hepatocellular carcinoma (HCC) present with symptoms like:
The most common clinical sign is mild to moderate hepatomegaly. Advanced stages of the spectrum can demonstrate signs of end-stage liver disease such as:
Mildly elevated serum aminotransferases are the primary abnormality in non-alcoholic fatty liver disease, although the liver enzymes are normal in the majority of patients. The ratio of aspartate aminotransferase (AST) to alanine aminotransferase (ALT) is less than 1. Gamma-glutamyl transferase (GGT), when elevated in NAFLD, can be a marker of increased mortality. With the progression of the disease hypoalbuminemia, hyperbilirubinemia, thrombocytopenia, and prolonged prothrombin time present due to hepatic synthetic dysfunction.
Ultrasound of the abdomen is routinely used to evaluate for fatty liver, but a liver biopsy is considered the gold standard for the diagnosis of NAFLD. A non-invasive clinical scoring system called NAFLD in metabolic syndrome (MS) score was developed to predict the development of NAFLD in patients with metabolic syndrome. The clinical predictors included are BMI greater than or equal to 25, AST/ALT greater than or equal to 1, type 2 diabetes mellitus, and obesity. The positive likelihood ratio of developing NAFLD is 2.32 (low when the score is less than 3), and the risk is 7.77 (high when the five or more). Some of the other scoring systems are NAFLD fibrosis score (NFS), FIB-4 (Fibrosis-4) Index, original ELF (enhanced liver fibrosis) test, AST-to-platelet ratio index (APRI), AAR, FibroMeter, NAFLD-MS score.
Lifestyle changes are recommended for all patients with non-alcoholic fatty liver disease even without NASH as these patients have metabolic derangements and are at risk for the development of cardiovascular diseases. A weight loss of 3 to 5% in simple steatosis and a weight loss of 7% to 10% in NASH is the recommendation. Adequate control of risk factors like hyperlipidemia with statins (it also protects from cardiovascular risks), hypertension, adequate glycemic control is required. Please refer to the chapters on hepatitis, non-alcoholic steatohepatitis under transplant hepatology section for a detailed discussion of this topic.
Patients with NASH are to followed by hepatologists or gastroenterologists. NASH with cirrhosis require hepatocellular carcinoma surveillance with an ultrasound every six months. Several clinical trials are being conducted using anti-fibrotic, anti-apoptotic, and immune therapies for the treatment of NAFLD.
The differential diagnosis of non-alcoholic fatty liver disease (conditions that can also cause hepatic steatosis) include:
Patients with non-alcoholic fatty liver disease exhibit increased mortality rates when compared to the general population. These patients have a high risk of mortality from cardiovascular causes as these patients have metabolic derangements. Cardiovascular causes of mortality are higher in these patients over liver causes. NAFLD is a slowly progressive disease; simple steatosis is reversible and non-progressive, whereas NASH can progress to cirrhosis. Over a 13 year follow up, the progression of cirrhosis presented in 41% in a study done by Ekstedt et al. A meta-analysis was done by White et al., showed that in cohorts of NADLF or NASH with few or no cases of cirrhosis the risk of developing HCC was minimal at 0 to 3% over 20 years and in cohorts with NASH with cirrhosis the risk was high at 2.4 % over seven years.
The most important complications in the descending order are cardiovascular disease, hepatocellular carcinoma, end-stage liver disease. The severity of these complications is proportional to the severity of the histological stage and grade of liver disease.
The education of patients is crucial to obtain the best patient outcomes. Some of the resources include:
The incidence and prevalence of non-alcoholic fatty liver disease are rising and will continue to grow due to increasing obesity and lifestyle changes. The management of NAFLD will require an interprofessional team that consists of a primary care physician, nurse practitioner, hepatologist or gastroenterologist, nutritionist, endocrinologist, bariatrician. Primary prevention with adequate management of metabolic derangement is essential to prevent the rising incidence of NAFLD and its associated complications. To lower the risk of heart disease, patients should be urged to reduce body weight, discontinue smoking, eat a healthy diet, and participate in regular exercise.
Management of NAFLD should is optimal with an interprofessional health care team that includes physicians, specialists, specialty-trained nurses, and where appropriate, pharmacists. These different disciplines need to engage in open communication about the patient to ensure the best possible outcomes.
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