Nonalcoholic fatty liver disease: Definitions, risk factors, and workup

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Abbreviations

ALT

alanine aminotransferase

APRI

aspartate aminotransferase‐to‐platelet ratio index

AST

aspartate aminotransferase

BARD

BMI

AST/ALT

DM

CT

computed tomography

ELF

European Liver Fibrosis

FIB‐4

Non‐invasive index to assess fibrosis based on age

AST

ALT

and platelet count

GGT

gamma‐glutamyltransferase

HCV

hepatitis C virus

HIV

human immunodeficiency virus

INR

international normalized ratio

MDB

Mallory‐Denk body

MR

magnetic resonance

MRI

magnetic resonance imaging

MS

metabolic syndrome

NAFL

nonalcoholic fatty liver

NAFLD

nonalcoholic fatty liver disease

NAS

nonalcoholic fatty liver disease activity score

NASH

nonalcoholic steatohepatitis

NASH CRN

Nonalcoholic Steatohepatitis Clinical Research Network

Nonalcoholic fatty liver disease (NAFLD) is defined as the presence of fat in the liver (hepatic steatosis) either on imaging or on liver histology after the exclusion of secondary causes of fat accumulation in the liver (e.g., significant alcohol consumption, certain medications, and other medical conditions). NAFLD is further categorized histologically into nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH). NAFL is defined as hepatic steatosis with no evidence of hepatocellular injury in the form of hepatocyte ballooning. NASH is defined as the presence of hepatic steatosis and inflammation with hepatocyte injury (ballooning) with or without fibrosis. The common terminology used in the literature is summarized in Table 1.

Table 1.

Terminology for NAFLD

Term	Description
NAFLD	Indicates fatty infiltration of the liver
	Defined as a fat concentration > 5%–10% of the liver weight
	Hepatic steatosis > 5% in biopsy specimens
Simple steatosis	Fatty infiltration with no or minimal inflammation and no fibrosis
NASH	Hepatic steatosis with inflammation, ballooned hepatocytes, and/or fibrosis (which may progress to cirrhosis)
Primary NAFLD	Occasionally used in the literature but not uniformly accepted
	Indicates typical disease associated with features of MS but without a specific, additional etiology
Secondary NAFLD	NAFLD associated with a specific cause
	Implies the absence of insulin resistance
	May represent exacerbation of underlying primary NAFLD
	Not a very useful distinction
Presumed NAFLD	Presumptive diagnosis of NAFLD
	Used in epidemiological and pediatric studies
	Based on the following:
	• Abnormal liver enzyme levels
	• Negative results from viral studies
	• Exclusion of other common liver disease etiologies
	• Echogenic or bright liver on imaging (often abdominal ultrasound) consistent with fatty infiltration
Types of NAFLDa
1	Simple steatosis (no inflammation or fibrosis)
2	Steatosis with lobular inflammation but no fibrosis or balloon cells
3	Steatosis, inflammation, and fibrosis of varying degrees (NASH)
4	Steatosis, inflammation, ballooned cells, and Mallory's hyaline or fibrosis (NASH)

^aAccording to Matteoni et al.3

Histology of NAFLD

Histology is important in the evaluation of NAFLD, which is a clinicopathological entity (See Brunt in this issue of CLD).1 Three histological scoring systems for NAFLD have been published: two were reported by Matteoni3 and Brunt4 in 1999, and then a semiquantitative scoring system was validated by the National Institutes of Health–sponsored Nonalcoholic Steatohepatitis Clinical Research Network (NASH CRN) in 2005 (Table 2). The NASH CRN system describes the nonalcoholic fatty liver disease activity score (NAS), which is a composite score of steatosis, lobular inflammation, cytological ballooning, and fibrosis (disease stage). Importantly, the NAS score should not be used as a surrogate for the diagnosis of NASH. The NASH CRN system is presently used only for research. Despite these attempts, there is no consensus on the histological diagnosis of NAFLD or its categories in clinical practice, which remains a gestalt (Tables 3 and 4). The characteristic histological features are briefly described here.

Table 2.

NASH CRN Scoring System: NAS and Fibrosis Score

Steatosis Grade		Lobular Inflammation		Hepatocellular Ballooning		Fibrosis Score
Degree	Description (%)	Degree	Description	Degree	Description	Degree	Description
0	<5	0	None	0	None	0	None
1	5–33	1	<2 foci/20× optical field	1	Mild, few	1a	Mild (delicate) zone 3 perisinusoidal fibrosis
						1b	Moderate (dense) zone 3 perisinusoidal fibrosis
2	34–66	2	2–4 foci/20× optical field	2	Moderate/marked, many	1c	Portal/periportal fibrosis only
						2	Zone 3 perisinusoidal fibrosis with portal/periportal fibrosis
3	>66	3	>4 foci/20× optical field			3	Bridging fibrosis
						4	Cirrhosis

Table 3.

Histological Categorization of Disease States in Patients at Risk for NAFLD

Category	Definition
No Significant evidence of fatty liver disease (Not NAFLD)	Insufficient steatosis for a diagnosis of steatosis (the NASH CRN uses a threshold of 5% of hepatocytes showing steatosis), without other changes (ballooning, fibrosis) that would suggest steatohepatitis
Steatosis	Steatosis witout specific changes to suggest a form of steatohepatitis. This category may include spotty lobular inflammation and/or mild degrees of fibrosis of uncertain significance.
• Steatosis with inflammation
• Steatosis with nonspecific fibrosis
Steatohepatitis	Form of steatopepatitis most common in adults defined as a zone 3 centered injury pattern that includes steatosis, inflammation, ballooning injury (often with Mallory‐Denk bodies) with or without fibrosis. Borderline steatohepatitis has some, but not all of the features that would allow a diagnosis of steatohepatitis.
• Zone 3 borderline Steatohepatitis
Zone 1, Borderline pattern	Form of steatohepatitis that occurs mainly in young children, characterized by zone 1‐centered injury (portal inflammation, portal‐based fibrosis, zone 1 steatosis, ballooning injury in zone 1 if present).
Cryptogenic fibrosis/cirrhosis	Presence of fibrosis (usually advanced) or cirrhosis, with little to no steatosis and no changes (ballooning, Mallory‐Denk bodies) that would suggest borderline or definite steatopatitis. Other explanations for fibrosis (besides steatohepatitis) should be considered.

Table 3 is adapted from Kleiner and Brunt.8

Table 4.

Comparison of the Features of NAFLD Categories

Lesion	Not NAFLD	Steatosis	Steatohepatitis	Zone 1 Borderline Pattern	Cryptogenic Fibrosis/ Cirrhosis
Steatosis	None to minimal (<5%)	Any degree > 5%, often in zone 3 or panacinar	Any degree, often in zone 3 or panacinar when fibrosis is mild	Any degree, often in zone 1 or panacinar	None to minimal
Inflammation	Usually mild	Any degree, usually mainly lobular	Any degree, usually more lobular when fibrosis is mild	Any degree, and portal inflammation may be more prominent	Any degree, and portal inflammation may be more prominent
Ballooning	None	None	Present when the pattern is definite and may be mild or absent in borderline cases when characteristic fibrosis is seen	Usually absent but usually in zone 1 when present	None
Fibrosis	Usually none but there may be stage 1–2 fibrosis	Usually none but there may be stage 1–2 fibrosis	Sometimes none but fibrosis is usually present and often perisinusoidal in zone 3	Usually present as periportal fibrosis (stage 1c) or portal‐portal bridging fibrosis (stage 3)	More than stage 2 fibrosis
NAS (typical range)	0–1	1–5	3–8 (definite) or 2–6 (borderline)	2–6	0–3

Table 4 is adapted from Kleiner and Brunt.8

Steatosis. Steatosis is required for the diagnosis of NAFLD but can be identified in patients with other conditions with elevated liver tests (e.g., alcohol and chronic hepatitis C), and the pathologist should always be alert to other possibilities, particularly when the clinical evaluation is not consistent. The NASH CRN scoring system describes four distribution patterns of steatosis, as shown in Fig. 1.

Figure 1.

Four distribution patterns of steatosis according to the NASH CRN scoring system. The red rectangles represent stylized liver biopsies with triangular portal areas (P) and round central veins (V). The small white circles denote steatotic hepatocytes. The amount of steatosis is the same in each pattern and is relatively mild for demonstration purposes. (A) Zone 3 steatosis, (B) Zone 1 steatosis, (C) Panacinar steatosis, and (D) Azonal steatosis. Table 3 is adapted from Kleiner and Brunt.8

Steatohepatitis. Steatohepatitis is a specific pattern of histological abnormality seen in NAFLD patients. In adults, it is most commonly a zone 3–centered injury pattern that includes steatosis, inflammation, and ballooning injury [often with Mallory‐Denk bodies (MDBs)] with or without fibrosis (Fig. 2). Borderline steatohepatitis has some but not all of the features that would allow a diagnosis of steatohepatitis. The presence of the characteristic ballooning injury is the key to the diagnosis. Ballooning injury results in enlarged vacuolated cells that classically will contain MDBs, which are eosinophilic, ropey cytoplasmic inclusions often found near the nucleus. MDBs are composed of hyperphosphorylated, misfolded cytokeratin 8 and 18 filaments. Thus, steatohepatitis can be definite, borderline, or nonexistent according to histological features.

Figure 2.

Insulin resistance is central to pathophysiology of NAFLD and plays an important role in the development and progression of disease as demonstrated by key histologic changes. It also shows that insulin resistance contributes to disease progression as seen in a simplified schema that illustrates the spectrum of NAFLD and progression of fibrosis to cirrhosis.

Risk Factors Associated With NAFLD

Insulin resistance is central to the development of NAFLD, which represents the hepatic manifestation of metabolic syndrome (MS). As defined by Adult Treatment Panel III, MS is the presence of three or more of the following features: (1) a waist circumference greater than 102 cm in men or greater than 88 cm in women, (2) a triglyceride level greater than or equal to 150 mg/dL, (3) a high‐density lipoprotein cholesterol level less than 40 mg/dL in men and less than 50 mg/dL in women, (4) a systolic blood pressure greater than or equal to 130 mm Hg or a diastolic pressure greater than or equal to 85 mm Hg, and (5) a fasting plasma glucose level greater than or equal to 110 mg/dL. Patients with features of MS are at high risk for NAFLD. Additionally, ethnicity influences the prevalence of NAFLD: even with a lower degree of obesity, Hispanics and Asians are at higher risk than African Americans, who despite features of MS are known to have less severe NAFLD and a lower prevalence of NAFLD. Common conditions associated with hepatic steatosis are noted in Table 5.

Table 5.

Conditions Associated With the Risk of Hepatic Steatosis

Insulin resistance

Obesity

Sedentary lifestyle

Type 2 diabetes mellitus

Hypertension

Dyslipidemia

Drugs

Tamoxifen

Corticosteroids

Amiodarone

Methotrexate

Estrogens

Valproic acid

Antiretroviral medications

Carbohydrate excess (e.g., diet and total parenteral nutrition)

Rapid weight loss

Altered small bowel anatomy

Obesity surgery (e.g., jejunoileal bypass)

Pancreaticoduodenal resection

Short gut

Metabolic diseases (resulting in a NASH‐like histology)

Hypobetalipoproteinemia

Abetalipoproteinemia

Wilson's disease

Lipodystrophies

Andersen disease

Weber‐Christian syndrome

Infections

Chronic hepatitis C virus, mainly genotype 3

Human immunodeficiency virus and acquired immune deficiency syndrome

Emerging associations

Polycystic ovarian syndrome

Hypothyroidism

Obstructive sleep apnea

Hypopituitarism

Hypogonadism

Workup of NAFLD

The diagnosis of NAFLD requires the following: (1) hepatic steatosis according to imaging or histology, (2) no significant alcohol consumption, (3) no competing etiologies for hepatic steatosis, and (4) no coexisting causes for chronic liver disease. It is important to evaluate people at risk for NAFLD and to further determine those at higher risk for steatohepatitis and advanced fibrosis.

NAFLD is the most common cause of abnormal liver chemistry, so other causes should be ruled out. The majority of patients diagnosed with NAFLD are asymptomatic. When symptomatic, it commonly presents as vague right upper quadrant dull ache or discomfort. On examination, hepatomegaly is the most common physical finding. Other clinical symptoms and physical findings are nonspecific, and they most commonly include general malaise, abdominal discomfort, nausea, and other nonspecific symptoms referred to the gastrointestinal tract. Celiac disease should be ruled out in suspected individuals. Common imaging studies such as ultrasound, computed tomography, and magnetic resonance imaging aid in the diagnosis of NAFLD.

Once NAFLD is diagnosed, the next step is to determine the severity, and this information is necessary to understand the prognosis. Clinical examinations and laboratory and imaging studies in combination lack the sensitivity and specificity for distinguishing NAFL from NASH and for determining the presence and stage of fibrosis, which is the most important determinant for the severity and progression of disease. Circulating levels of cytokeratin 18 fragments have been investigated extensively as novel biomarkers for the presence of steatohepatitis in patients with NAFLD, but this testing is not routinely recommended. Other noninvasive tests are emerging; however, these are not yet ready for prime time. Figure 3 outlines the approach to the workup of patients suspected to have NAFLD.

Figure 3.

Workup for NAFLD: incidentally detected abnormal liver chemistry/imaging. Abbreviations: ALT, alanine aminotransferase; APRI, aspartate aminotransferase‐to‐platelet ratio index; AST, aspartate aminotransferase; BARD, BMI, AST/ALT, DM; CT, computed tomography; ELF, European Liver Fibrosis; FIB‐4, Non‐invasive index to assess fibrosis based on age, AST, ALT, and platelet count; GGT, gamma‐glutamyltransferase; HCV, hepatitis C virus; HIV, human immunodeficiency virus; INR, international normalized ratio; MR, magnetic resonance; MRI, magnetic resonance imaging.

Liver biopsy still remains the most reliable approach for identifying the presence of steatohepatitis and fibrosis in patients with NAFLD, but its widespread use is limited by cost, sampling error, and procedure‐related morbidity and mortality. Advanced disease is often associated with older age, obesity, diabetes, hypertension, or multiple features of MS. A small subset of patients may present with stigmata of liver disease with suspected advanced fibrosis or cirrhosis. Therefore, the performance of liver biopsy according to clinical judgment is an important step in the workup of NAFLD.

When should liver biopsy be performed in patients with NAFLD? Liver biopsy remains the gold standard for characterizing liver histology in patients with NAFLD. As mentioned previously, it should be performed in those who would benefit the most from diagnosis, therapeutic guidance, and prognostic perspectives (See Noureddin and Loomba in this issue of CLD).9 The recommendations for liver biopsy are as follows:

1. Patients at increased risk for steatohepatitis and advanced fibrosis according to the presence of features of MS and possibly the NAFLD fibrosis score.

2. Patients with suspected NAFLD for whom competing etiologies of hepatic steatosis and coexisting chronic liver diseases cannot be excluded without liver biopsy.