The term nonalcoholic steatohepatitis (NASH) was first coined by Dr. Ludwig 3 decades ago to describe a unique entity characterized by fatty changes with lobular hepatitis in the absence of a history of alcoholism (see the first image below).  The main lesions described in that study—namely, steatosis, liver cell injury, and the unique zone 3 “chicken wire” fibrosis (see the second image below)—remain central in establishing the diagnosis.
Since nonalcoholic steatohepatitis (NASH) was first identified in 1980, it has been increasingly recognized. At present, nonalcoholic fatty liver disease (NAFLD) has become the most common liver disease in the United States and, indeed, worldwide. The prevalence of NAFLD is rising rapidly because of ongoing epidemics of obesity and type 2 diabetes. NAFLD is also an emerging problem in the Asia-Pacific region, where it is likely to increase in the future. 
The estimated prevalence of NAFLD in the general US population is currently in the range of 20%, [3, 4, 5] and that of nonalcoholic steatohepatitis (NASH) is about 3.5-5%. [6, 7, 8] The prevalence in the morbidly obese population has been estimated as 75-92%, and up to 17% of children and adolescents may be affected with NAFLD. 
Although the mechanism is complex and incompletely understood, a 2-hit hypothesis has been proposed,  in which the first hit involves an imbalance of fatty acid metabolism that leads to hepatic triglyceride accumulation (steatosis). The second hit may be oxidative or metabolic stress and dysregulated cytokine production resulting from efforts to compensate for altered lipid homeostasis, leading to subsequent inflammation and fibrosis. Data show that hepatic mitochondrial dysfunction is crucial to the pathogenesis of NAFLD. 
In recent years, the 2-hit hypothesis has been the subject of debate. In view of the current understanding of the potentially deleterious effects of peripheral and hepatic insulin resistance, multiple hits may be possible. Although the specific pathways leading to inflammation and fibrosis are not clearly pinpointed, evidence supports a role for dysregulated lipid partitioning mediated by insulin resistance and concomitant altered cytokine profiles.
Oxidative stress is thought to be obligatory. Along with lysosomal cathepsin release, mitochondrial dysfunction, endoplasmic reticulum stress, and apoptosis, it leads to further inflammation, and stimulation of transforming growth factor (TGF)-beta and activation of hepatic stellate cells with collagen deposition within the hepatic sinusoids become apparent.  Furthermore, the ability of the liver to repair and recover from injury appears variable and may impact the rate of progression, as well as the severity of liver disease. 
Nonalcoholic fatty liver disease (NAFLD) is a multifaceted metabolic disorder that represents a spectrum of disease, ranging from steatosis without specific injury (which is considered relatively benign) to nonalcoholic steatohepatitis (NASH) and to NAFLD-associated cirrhosis, as well as infrequently to hepatocellular carcinoma [5, 7, 10, 15, 16, 17, 18, 19] in the absence of alcohol intake, sometimes necessitating a liver transplant.
Nonalcoholic steatohepatitis (NASH)–associated cirrhosis accounts for approximately of 13% of all cases of hepatocellular carcinoma.  Well-known risk factors are central obesity, hyperglycemia, type 2 diabetes, arterial hypertension, and hypertriglyceridemia. 
Affected patients are generally asymptomatic but come to medical attention because of elevated transaminase levels, increased alkaline phosphatase levels, hepatomegaly, or some combination thereof.  NAFLD remains a diagnosis of exclusion, made by eliminating other liver diseases. [10, 21]
Nonalcoholic steatohepatitis (NASH) in adults is characterized as a zone 3 injury pattern with lobular-based inflammation. When disproportionately dense portal inflammation is present, concomitant liver disease should be considered, such as viral hepatitis, autoimmune hepatitis, or chronic biliary disease. Portal inflammation may also characterize nonalcoholic steatohepatitis (NASH) in the pediatric, rather than the adult, population.
The injury pattern of nonalcoholic steatohepatitis (NASH) in the adult population is central zonal, with steatosis, liver cell injury, and inflammation accentuated in zone 3. However, a different injury pattern has been documented in children, centered in zone 1. 
The following conditions are also considered part of the differential diagnosis:
Hepatic Radiologic Studies
Ultrasonography, computed tomography, magnetic resonance scanning and magnetic resonance spectroscopy have been used increasingly for detecting fatty liver. However, liver ultrasonography is perhaps the most popular modality for the diagnosis of nonalcoholic fatty liver disease (NAFLD), mainly because of its lower cost and its wide availability in medical practice.
Ultrasonographic findings of "bright" liver, with increased echogenicity in comparison with the kidneys, vascular blurring, and deep attenuation, are suggestive of liver steatosis. The sensitivity and specificity of liver ultrasonography for detecting liver steatosis are 60-94% and 88-95%, respectively.
Nevertheless, none of these imaging modalities can clearly distinguish between fatty liver alone and nonalcoholic steatohepatitis (NASH), nor can they predict the degree of liver fibrosis. [7, 21, 23]
The diagnosis of nonalcoholic steatohepatitis (NASH) is not dependent on a single histologic feature; rather, it involves an overall assessment of multiple independent features. The generally accepted minimal criteria for the diagnosis of nonalcoholic steatohepatitis (NASH) are steatosis, ballooning degeneration, and lobular inflammation. [24, 25]
Steatosis is predominantly macrovesicular, with lobular inflammation and ballooned hepatocytes in zone 3 (see the image below).
Patchy microvesicular steatosis is a manifestation of severe mitochondrial dysfunction and is commonly associated with acute fatty liver of pregnancy, alcoholic foamy degeneration, and certain drug toxicities (eg, valproic acid or Reye syndrome).
Hepatocellular damage in the form of hepatocytic ballooning is an important finding in nonalcoholic steatohepatitis (NASH). Studies have shown it to be a significant histologic feature for distinguishing progressive nonalcoholic fatty liver disease (NAFLD) from less aggressive forms.  Hepatocytic ballooning is characterized by swelling and enlargement of the hepatocytes, resulting in loss of their normal hexagonal shape, along with cytoplasmic alterations that sometimes contain Mallory-Denk bodies (see the following image).
The cytoplasm is clumped, rarefied, or finely reticulated, possibly as the result of accumulation of intracellular fluid due to liver cell injury. The nuclei may be enlarged, hyperchromatic, or both. The Mallory-Denk bodies consist of ropy and clumped dense eosinophilic material that can be highlighted by p62 or ubiquitin immunostains.
The inflammatory infiltrates are mixed and predominantly mononuclear, including lymphocytes and Kupffer cells (see the images below), or they may have foci of "satellitosis" consisting of aggregates of neutrophils in the immediate vicinity of injured hepatocytes.
The portal inflammation is usually minimal to mild in adult NAFLD or nonalcoholic steatohepatitis (NASH); however, disproportionately accentuated portal inflammation has been documented in the pediatric population.  Alternatively, portal inflammation may represent more progressive disease, as shown by Brunt and colleagues. [27, 28]
In adult nonalcoholic steatohepatitis (NASH), fibrosis develops over time, initially in the centrilobular location (zone 3) as pericellular/perivenular fibrosis—a pattern referred as "chicken wire" fibrosis—and eventually leading to portal-central bridging fibrosis and cirrhosis (see the image below). This pattern of fibrosis is well highlighted by trichrome staining as strands of collagen deposits within the sinusoidal spaces and surrounding the hepatocytes.
In nonalcoholic steatohepatitis (NASH), hepatocytes may also contain megamitochondria (giant mitochondria), apparent as eosinophilic round or needle-shaped intracytoplasmic inclusions 2 to 10 microns in size. These are swollen mitochondria with multilamellar membranes and paracrystalline inclusions, with loss of cristae on electron microscopy, and may reflect injury or adaptive change of the mitochondria. However, megamitochondria are not specific to nonalcoholic steatohepatitis (NASH) and can also be seen in alcoholic steatohepatitis.
Mild to moderate iron deposition in the form of stainable iron in hepatocytes, Kupffer cells, and sinusoidal endothelial cells is a common finding in nonalcoholic steatohepatitis (NASH), especially in cirrhosis; however, large amounts of iron may indicate coexisting hereditary hemochromatosis.
Other microscopic findings that are common but that are not necessary to establish the diagnosis include acidophil bodies, glycogenated nuclei, pigmented macrophages, and lipogranulomas.
The Mallory-Denk bodies can be highlighted by p62 or ubiquitin immunostains. Loss of keratin 8/18 immunostaining has been shown in the ballooned hepatocytes of patients with nonalcoholic steatohepatitis (NASH); however, this finding is not specific and may also be seen in the ballooned hepatocytes of patients with alcoholic steatohepatitis or in the swollen hepatocytes of patients with cholestasis or ischemia. 
Long term prognosis of nonalcoholic fatty liver disease (NAFLD) depends critically on the histologic stage at presentation. [30, 31] However, progression of fibrosis stage may also be associated with certain clinical parameters such as diabetes, weight gain, and increased insulin resistance. [32, 33] Data also suggest that increased portal chronic inflammation may be associated with certain clinical and pathologic features of progressive NAFLD including advanced fibrosis.