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a Division of Gastroenterology, Hepatology, and Nutrition, Department of Pediatrics
b Department of Family and Preventive Medicine
d Department of Pathology, University of California, San Diego School of Medicine, San Diego, California
c Office of the Medical Examiner, County of San Diego, San Diego, California
| ABSTRACT |
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METHODS. We conducted a retrospective review of 742 children between the ages of 2 and 19 years who had an autopsy performed by a county medical examiner from 1993 to 2003. Fatty liver was defined as
5% of hepatocytes containing macrovesicular fat.
RESULTS. Fatty liver was present in 13% of subjects. For children and adolescents age 2 to 19 years, the prevalence of fatty liver adjusted for age, gender, race, and ethnicity is estimated to be 9.6%. Fatty liver prevalence increases with age, ranging from 0.7% for ages 2 to 4 up to 17.3% for ages 15 to 19 years. Fatty liver prevalence differs significantly by race and ethnicity (Asian: 10.2%; black: 1.5%; Hispanic: 11.8%; white: 8.6%). The highest rate of fatty liver was seen in obese children (38%).
CONCLUSIONS. Fatty liver is the most common liver abnormality in children age 2 to 19 years. The presence of macrovesicular hepatic steatosis in
1 of every 10 children has important ramifications for the long-term health of children and young adults. The influence of the risk factors identified should be taken into consideration in the development of protocols designed to screen at-risk children and adolescents.
Key Words: obesity nonalcoholic fatty liver disease steatohepatitis Hispanic American metabolic syndrome
Abbreviations: SCALEStudy of Child and Adolescent Liver Epidemiology
The growing epidemic of childhood obesity has prompted studies of the prevalence of obesity-related conditions, such as hypertension, hypercholesterolemia, and the metabolic syndrome.14 Reports of pediatric fatty liver disease and steatohepatitis in obese children have been increasing57 and include cases of cirrhosis811 and liver transplantation.12 However, despite the potentially serious nature of fatty liver in children, the prevalence is unknown.
Efforts to determine the prevalence of fatty liver in children have been limited to indirect measures, such as blood tests or ultrasound, to predict a histologic outcome. In 1989, based on ultrasound, fatty liver prevalence was estimated to be 2.6% in school-aged children in northern Japan.13 Projections for children in the United States are considerably higher,14 although no population-based estimates are available. All of the other studies have been limited to children selected for the conditions of obesity. Taken together, the prevalence of fatty liver in obese children in China, Italy, Japan, and the United States has been reported to be between 10% and 77%.1518 The wide range in estimates of prevalence is attributable in part to the difficulty in determining the presence or absence of fatty liver using noninvasive tests. Therefore, we conducted the Study of Child and Adolescent Liver Epidemiology (SCALE) to determine the prevalence of fatty liver as diagnosed by histology in a population-based sample of children and adolescents.
| METHODS |
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We included all children age 2 through 19 years at the time of death who had an autopsy performed by the medical examiner from 1993 to 2003. We set a minimum age of 2 years, because this is the youngest age reported for biopsy-proven steatohepatitis.10 The upper age limit was set at 19 years to mirror the age brackets used by the US Census Bureau. Exclusion criteria were insufficient data (missing clinical data or the absence of a liver slide for review) or identification of a factor that may unduly influence liver histology (inpatient clinical care or alcohol or drug use). SCALE was conducted with approval from the county of San Diego and the University of California, San Diego.
Clinical Data Collection
Medical examiner records were reviewed for age, gender, race, ethnicity, weight, height, toxicology, and the mode and cause of death. BMI was calculated as the weight (kilograms) divided by the height (meters) squared. Weight status was defined according to the Institute of Medicine as underweight (BMI
5th percentile), normal weight (BMI > 5th and < 85th percentile), overweight (BMI
85th and < 95th percentile), and obese (BMI
95th percentile).19 BMI varies with age and gender; therefore, to compare subjects independently of age and gender, BMI Z-scores were calculated. The BMI z score represents the number of SDs from the national reference mean that a subject is for a given age and gender.
Assessment of Liver Histology
A hepatopathologist (C.B.) with expertise in pediatric fatty liver reviewed a section of liver tissue stained with hematoxylin and eosin for all of the subjects in a blinded fashion. The severity of steatosis from any 1 sample obtained at autopsy has been shown to correlate well with the severity of steatosis for the liver overall.20 Steatosis was recorded as the percentage of hepatocytes containing macrovesicular fat (fat droplet equal to or larger than the size of the nucleus) or microvesicular fat (numerous small fat droplets surrounding a centrally located nucleus). Fatty liver was defined as
5% of hepatocytes containing macrovesicular fat.21 Steatohepatitis was defined as reported previously for children; fatty liver along with
1 of the following features: ballooning degeneration, perisinusoidal fibrosis, portal inflammation, or portal fibrosis.11 Based on these features, those biopsies with steatohepatitis were subclassified as type 1, type 2, or overlap.11 Type 1 steatohepatitis is defined as fatty liver with ballooned hepatocytes and/or perisinusoidal fibrosis in the absence of portal features. Type 2 steatohepatitis is defined as fatty liver with portal inflammation and/or fibrosis and the absence of ballooning degeneration and perisinusoidal fibrosis.
Data Analysis
Means and SEs of the mean are reported for continuous variables, and frequencies and percentages are reported for categorical variables. To estimate the prevalence of fatty liver in the county of San Diego, fatty liver was defined as
5% steatosis versus <5% steatosis. To allow for direct comparisons between the study population and county population data, the prevalence of fatty liver, overall and for each subgroup, was directly standardized to the age group, gender, race, and ethnicity distribution of the county. We obtained 95% confidence interval estimates for prevalence using estimation for proportions based on normal distribution assumptions or simulation using 2000 samples when normal distribution assumptions did not hold. Based on our previous work suggesting that age, gender, race, ethnicity, and weight influence fatty liver,10,22 the independent relationship of each of these predictors with fatty liver was tested using 2-tailed t tests for continuous variables and Pearson
2 tests for categorical variables. In addition, an a priori logistic regression model was developed, and odds ratios for fatty liver were estimated using the lowest-risk subgroup as the reference category. All of the reported P values are 2-sided.
| RESULTS |
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The SCALE population (Table 1) had a mean age of 14 years, and the majority of subjects were boys. The racial and ethnic distribution is noted to closely represent that of children in San Diego as determined by the US Census Bureau's 2000 Census.23 The distribution of weight category in the SCALE population (4% were underweight; 54% were normal weight; 17% were overweight; and 26% were obese) also closely matched the weight distribution for children age 5 to 19 years in the county of San Diego as estimated by the Centers for Disease Control and Prevention Pediatric Nutrition Surveillance Study.24
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Also shown in Table 1, steatohepatitis was observed in 23% (22 of 97) of the subjects with fatty liver. This represents 3% (22 of 742) of the total study population. Steatohepatitis was characterized as type 1 in 4 subjects, type 2 in 17 subjects, and as overlap in 1 subject. The major features of pediatric steatohepatitis were present in the following percentages: portal inflammation (68%), portal fibrosis (59%), perisinusoidal fibrosis (14%), and ballooning degeneration (18%). Bridging fibrosis or cirrhosis was seen in 9% of those with steatohepatitis.
Estimate of Fatty Liver Prevalence
After standardizing for age, gender, race, and ethnicity, the estimated prevalence of fatty liver is 9.6%, which represents
70000 children age 2 to 19 years in the county of San Diego (Table 2). Fatty liver prevalence increases with age ranging from 0.7% for ages 2 to 4 years up to 17.3% for ages 15 to 19 years. Fatty liver is present in 11.1% of boys and 7.9% of girls. As shown in Table 3, after controlling for age and gender adjusted BMI z score, race and ethnicity significantly predict the presence of fatty liver. Fatty liver is most prevalent in children and adolescents of Hispanic ethnicity and least prevalent among children and adolescents of black race. As such, the odds of a Hispanic child having fatty liver are 5 times greater than for a black child. The prevalence of fatty liver is estimated to be 10.2% in children and adolescents of Asian race. However, there was substantial variability among the subgroups of Asian children. For example fatty liver was present in 20% of Filipino-American children but only in 4% of children of Cambodian or Vietnamese background.
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| DISCUSSION |
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Our finding that 38% of obese children have fatty liver is consistent with reports from clinical populations of obese children.15,18 Because the majority of children with fatty liver are obese, effective prevention and treatment of obesity will be important components of addressing the prevalence of fatty liver. However, despite an increased focus on obesity prevention, the rates of pediatric obesity in the United States continue to rise.25 Therefore, we must also consider secondary prevention strategies targeted toward preventing the development of fatty liver in children who are overweight. The current data suggest that a key age group to target for secondary prevention is children age 5 to 9 years.
Despite black children having high rates of risk factors for fatty liver, such as obesity and insulin resistance, few black children are included in pediatric clinical series of fatty liver.2629 This discrepancy could be because of either lower rates of fatty liver in black children or, alternatively, that fatty liver in black children is more likely to remain undiagnosed. Two recent studies of obese children and adolescents observed much lower rates of a putative surrogate marker of fatty liver, elevated serum alanine aminotransferase activity, in blacks than in whites.22,30 The histologic data in the current study strongly demonstrates that black children, even when obese, are much less likely to have fatty liver. Furthermore, these data refute the contention of a race-based selection bias in clinical studies and support the need for studies to explore the biological basis of the existing differences.
Efforts to curtail obesity in the Hispanic community with a special emphasis on mitigating the development and progression of fatty liver disease should be a public health imperative. In the most recent national report,
40% of Mexican-American children were either overweight or obese.25 A high rate of obesity is one important risk factor for fatty liver. However, our data demonstrate that, even after controlling for the severity of obesity, Hispanic boys and girls have higher rates of fatty liver than non-Hispanic peers. Not only are Hispanic children at increased risk for fatty liver, but clinical data suggest they are more prone to advanced liver fibrosis.11 The high rates of fatty liver in Hispanic adolescents may contribute to the higher rate of liver-related morbidity and mortality experienced by Hispanic-American adults compared with white or black Americans.31,32
SCALE is the first study to provide information on fatty liver prevalence within Asian Americans. We estimate that 10% of Asian children and adolescents in the county of San Diego have fatty liver. This is much higher than was reported previously by Tominaga et al13 for the prevalence of fatty liver in Japanese children living in northern Japan. The difference may be explained in part by the high false-negative rate for ultrasound in the context of mild steatosis. In addition, the environments of children living in northern Japan and those living in the United States are likely to be different. Moreover, despite being considered as one common race, there may be considerable differences among children with ancestors from different Asian nations. The Asian subgroup at greatest risk may be Filipino American children, which is consistent with previous reports of a propensity for lipid abnormalities in normal-weight Filipino adolescents33 and for type 2 diabetes mellitus in Filipino-American adults.34 Whether differences between Asian subgroups in the development of fatty liver are related to genetic or environmental factors merits further investigation.
We acknowledge that it is uncertain to what extent the SCALE population mirrors all of the children in the community. In strong support, however, is the finding that the distributions of race and ethnicity, as well as weight classification within the SCALE population, closely match the distributions of these features within the county of San Diego. A limitation of the study design is that the specific cause of fatty liver cannot be determined. In the absence of a liver biopsy, the differential diagnosis for a child with suspected fatty liver is rather broad. However, because we based the diagnosis on histology, the possibilities are substantially reduced. By restricting the definition of fatty liver to require macrovesicular steatosis, we eliminated many possible diagnoses, such as inborn errors of metabolism. The inclusion of only those children who experienced a rapid death outside of the hospital setting eliminated possible causes, such as parenteral nutrition. The possible explanations for fatty liver were further reduced by excluding all of the subjects with any drug or alcohol use. Therefore, the remaining differential diagnosis is rather narrow and can be focused on nonalcoholic fatty liver disease, cystic fibrosis, and Wilson's disease. No children with cystic fibrosis were knowingly included in the data set. The prevalence of Wilson's disease is 1 in 30000, making it an unlikely cause. Thus, although we cannot prove cause on a case-by-case basis, it is likely that most of the cases of fatty liver in the study population are attributable to nonalcoholic fatty liver disease.
| CONCLUSIONS |
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| ACKNOWLEDGMENTS |
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We thank Susan Korosy for her assistance with data collection.
| FOOTNOTES |
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Address correspondence to Jeffrey B. Schwimmer, MD, Division of Gastroenterology, Hepatology, and Nutrition, Department of Pediatrics, University of California, San Diego, 200 West Arbor Dr, San Diego, CA 92103-8450. E-mail: jschwimmer{at}ucsd.edu
The authors have indicated they have no financial relationships relevant to this article to disclose.
This work was presented in abstract form at the annual meeting of the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition; October 21, 2005; Salt Lake City, UT.
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