Non-alcoholic fatty liver disease

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Non-alcoholic fatty liver disease (NAFLD) is a spectrum of liver disease in the absence of excessive alcohol use that begins with fatty infiltration and can proceed to inflammation - Non-alcoholic steatohepatitis (NASH) - and even cirrhosis. At present, estimates are that between 30- 90 million Americans have some degree of NAFLD and 5-6% have NASH. [1]

NAFLD/NASH was first described in 1980 in a series of patients of the Mayo Clinic[2]. Since that seminal description, our understanding of NAFLD/NASH has progressed minimally. [3] The disease is most closely associated with the increasing obesity, insulin resistance, type two diabetes mellitus and hyperlipedmia endemic to the developed world. Roughly half of all patients with NASH, however, do not meet criteria for metabolic syndrome [4].

Prevalence

Estimates are that between 30- 90 million Americans have some degree of NAFLD and 5-6% have NASH. [1] Some have suggested a genetic or sociocultural component to NAFLD spectrum disease.[5] As a part of the Dallas Heart Study,[6] 2,240 patients - 1105 african-americans, 401 hispanics and 734 caucasians - received abdominal MRI's from which we can infer the presence of steatosis. Hepatic steatosis was found in 45% of hispanics (both men and women), 33% of caucasians (42% of men, 24% of women) and 24% of african-american (23% of men, 24% of women). This pattern may hold true in children as well. In a San Diego study of 742 consecutive autopsies of children victims of trauma over 10 years, fatty liver was found in 9.6% of all children, 38% of the obese, 12% of hispanics, 10% of asians, 8.6% of caucasians and 1.5% of african-americans.[7]

Signs and symptoms

Symptoms and associations

By definition, alcohol consumption of over 20 g/day excludes the condition.[8] Most patients with NAFLD have no or few symptoms. Infrequently patients may complain of fatigue, malaise and dull right upper quadrant abdominal discomfort. Mild jaundice can rarely be noticed. More commonly it is diagnosed as a result of abnormal liver function tests during routine blood tests. Often following an asymptomatic course, the disease may present first with cirrhosis and/or the complication of portal hypertension. In that respect, NASH is becoming an increasingly common indication for liver transplantation. [9] As awareness of this condition spreads, it has been regarded as a major cause of cryptogenic cirrhosis of the liver.[10]

Secondary causes

NAFLD can also be caused by the following medications (termed secondary NAFLD):

Diagnosis

Disturbed liver enzymes are common. Typically, one finds a 2-4 fold elevation of the ALT above normal limit and an AST/ALT ratio of less than 1. Another blood test that can be elevated is the ferritin. Typically, and except in very advanced disease, the liver's synthetic function is intact with normal albumin and INR. Imaging is often ordered in the workup of suspected NAFLD. Ultrasound has sensitivities approaching 100%, but a 62% positive predictive value. Problematically, ultrasound of fatty liver reveals a hyperechoic echotexture - a so-called 'bright liver' - that can often be indistinguishable from fibrosis. Computed tomography is reported to be 93% sensitive with a 76% positive predictive value. A biopsy of the liver is considered the gold standard in diagnosis. Classically, biopsy reveals macrovesicular steatosis, inflammation, ballooning degeneration, zone 3 perivenular/periportal/perisinusoidal fibrosis and, finally, mallory bodies.[11] [12][13]


When considering NAFLD, other tests generally performed, including those for associated conditions (e.g. glucose, hemoglobin A1C) and those to distinguish this disease from viral hepatitis. Additionally, autoimmune causes are ruled out with serology. TSH is warranted, as hypothyroidism is more prevalent in NASH patients.[14]

Pathophysiology

NAFLD is considered a spectrum of disease activity. This spectrum begins as fatty accumulation in the liver (hepatic steatosis). A fatty liver can remain without disturbing the function of the liver, but by varying mechanisms and possible insults to the liver, may progress to outright inflammation of the liver. When inflammation occurs in this setting, the condition is then called NASH. Over time, up to 20 percent of patients with NASH may develop cirrhosis.

The exact cause is still unknown. However both obesity and insulin resistance likely play a strong role in this disease process. The exact reasons and mechanisms by which this disease progresses from one entity to the next is a subject of much research and debate. One such debated mechanisim proposes a "second hit", or further injury, enough to cause change that leads from hepatic steatosis to hepatic inflammation. Oxidative stress, hormonal imbalances and mitochondrial abnormalities may be potential causes for this "second hit" phenomenon.[8]

Treatment

Trials are presently being conducted to optimise treatment of NASH. No standard treatment has yet emerged as the "gold standard". General recommendations include improving metabolic risk factors and reducing alcohol intake.[8]

A large number of treatments have been studied for NAFLD. While many may improve biochemical markers, such as alanine transaminase levels, most have not been shown to reverse the histological abnormalities or reduce clinical endpoints:[8]


References

  1. 1.0 1.1 McCullough, AJ. Thiazolidinediones for NASH. NEJM 2006;355(22):2361-2363.
  2. Ludwig J, Viggiano TR, McGill DB, Oh BJ. Nonalcoholic steatohepatitis: Mayo Clinic experiences with a hitherto unnamed disease. Mayo Clin Proc. 1980;55:434-438. PMID 7382552.
  3. Day, CP. Non-alcoholic steatohepatitis (NASH): where are we now and where are we going? Gut. 2002 May; 50(5): 585–588.
  4. Farrell GC, Larter CZ. Nonalcoholic fatty liver disease: from steatosis to cirrhosis. Hepatology. 2006;43:S99–S112.
  5. Caldwell et al. Has natural selection in human populations produced two types of metabolic syndrome (with and without fatty liver). J of Gastroenterology and Hepatology 2007;22(S1):S11-S19
  6. Browning et al. Prevalence of Hepatic Steatosis in an Urban Population in the United States: Impact of Ethnicity. Hepatology 2004;40:1387-1395.
  7. Schwimmer et al. Prevalence of fatty liver in children and adolescents. Pediatrics 2006;118;1388-93.
  8. 8.0 8.1 8.2 8.3
  9. Choudhury J, Sanyal A. Clinical Aspects of Fatty Liver Disease. Semin Liver Dis 2004; 24: 349-362.
  10. Clark JM, Diehl AM. Nonalcoholic fatty liver disease: an underrecognized cause of cryptogenic cirrhosis. JAMA 2003;289:3000-4. PMID 12799409.
  11. Angula P. Nonalcoholic Fatty Liver Disease. NEJM. 2002 346(16):1221-31
  12. Brunt EM, Janney CG, Di Bisceglie AM et al. Nonalcoholic steatohepatitis: A proposal for grading and staging the histological lesions. Am. J. Gastroenterol. 1999; 94(9):2467-2474
  13. Liangpunsakul S, Chalasani N. Is hypothyroidism a risk factor for non-alcoholic steatohepatitis? J Clin Gastroenterol 2003;37:340-3. PMID 14506393

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