Non - Alcoholic Fatty Liver Disease

NON - ALCOHOLIC FATTY LIVER DISEASE

Author: Nicolene Naidu

Bachelor of Biological Science (Cellular Biology), Bachelor of Medical Science (Medical Microbiology) (Honours)

Non - Alcoholic Fatty Liver Disease (termed NAFLD for short) is a condition characterized by the significant accumulation of lipids in the hepatocytes of the liver parenchyma. While non - alcoholic fatty liver disease and alcoholic liver disease are pathologically similar, unlike alcoholic liver disease, non - alcoholic fatty liver disease occurs in patients who do not have a history of excessive alcohol intake.

Another term closely related to NAFLD but more histologically and clinically specific is Non - Alcoholic Steatohepatitis(NASH). It was coined by Ludwig et al in 1980 and is characterised by a fatty liver accompanied by inflammation and hepatocyte injury (“Ballooning”). Fibrosis may or may not be present. It should be noted that the histological appearance of non – alcoholic steatohepatitis is identical to that of alcoholic liver disease. Distinction between the two is based on the amount of alcohol intake.

Non - alcoholic fatty liver disease is the loose term used to describe a wide spectrum of liver damage ranging from hepatic steatosis (simple benign fatty liver), non - alcoholic steatohepatitis, chronic fibrosis and cirrhosis (P Angulo, 2002).

Figure 1: Showing a drawing of the various stages of liver damage, including a normal liver; fatty liver in which deposits of fat cause liver enlargement; liver fibrosis in which scar tissueforms and more liver cell injury occurs and cirrhosis in which scar tissue makes the liver hard and unable to work properly.(www.liverfoundation.org).

Pseudo-alcoholic liver disease, non - alcoholic Laennec’s disease, alcohol-like hepatitis, diabetic hepatitis and steatonecrosis are among the terms that have been used to refer to non – alcoholic fatty liver disease (Shethet al, 1997).

While non - alcoholic fatty liver diseasehas no age or sex predilectionand affects all racial and ethnic groups, overweight and obese individuals, individuals with type 2 diabetes mellitus, those with elevated blood lipids like triglyceride and cholesterol (hyperlipidemia)and those with ‘metabolic syndrome’’ are most likely at risk of developing non – alcoholic fatty liver disease(Vernon et al, 2011).Recently, patients with hypothalamic or pituitary dysfunction (Adams et al, 2004), impaired glucose tolerance and dyslipidemia were also found to progressively develop non – alcoholic fatty acid liver disease.

Epidemiologically, it has been reported worldwide with evidence of geographic variations in prevalence and is estimated to be the most common liver disease in the Western world.

PATHOGENESIS

While the exact mechanisms involved in the pathogenesis of non – alcoholic liver disease are not fully elucidated (Ramesh et al, 2005), scientists propose a ‘2 hit’ theory (Day et al, 1998). One “hit” that causes an accumulation of excess fat in the liver and then a second “hit” or further injury that causes hepatic steatosis to lead to hepatic inflammation.

(Rouviere O, Yin M, Dresner MA, et al. MR elastography of the liver: preliminary results. Radiology 2006; 240:440–448)

THE FIRST HIT – The accumulation of fats in the hepatocytes (hepatic steatosis):  Closely related to the metabolic changes associated with insulin resistance and obesity (Hubsher, 2006).  Impaired fatty acid metabolism in the liver cells and an increase in free fatty acid delivery to the liver lead to the accumulation of trigylcerides in the liver hepatocytes (Hubsher, 2006).When the supply of fatty acids to the liver is greater than the demand for the synthesis of cholesterol, triglyceride and phospholipid as well as mitochondrial oxidation, hepatic steatosis occurs.  In the presence of insulin resistance, the amount of free fatty acids available exceeds the mitochondrial capacity for oxidation. It is thought that the impaired mitochondrial β-oxidation function that results in insulin resistance results in decreased fatty acid disposal (Reid, 2001).  The hepatic steatosis that occurs in this stage is a form of ectopic lipid accumulation.

NOTE:It hasrecently been recognised that a fatty liver may exacerbate insulin resistance. The accumulation of fat in the liver interferes with phosphorylation of insulin receptor substrates (Samuel et al, 2004) which may be mediated by increased microsomal cytochrome P450 2E1expression at the hepatocellular level (Schattenberg et al,2005). This process could potentiate a cycle whereby fatty change in the liver could be initiated by the metabolic syndrome and vice versa (Adams et al, 2005).

THE SECOND HIT – Oxidative stress  The second hit involves oxidative stress causing the lipids accumulated in the hepatocytes to undergo peroxidation (Day et al, 1998). A process which is promoted by the formation of reactive oxygen species as a result ofmitochondrial dysfunction, along with the induction of microsomal cytochrome P450 2E1(Chitturi et al, 2001).  Oxidative stress also causes disease progression by resulting in cytokine production (Pessayre et al,2001).  Disease progression which is also mediated byelevated levels of pro- inflammatory cytokines like Tumour Necrosis Factor α and a decrease in anti- inflammatory cytokines like adiponectin (Diehl et al, 2005 ), which are bothcharacteristic of the metabolic syndrome, further suggest the influence of the metabolic syndrome in non – alcoholic fatty liver disease progression. Both the first and second “hit” (Day et al, 2002).  Biopsies of livers with non- alcoholic steatohepatitis (NASH) have shown a significant reduction the expression of the cytokine adiponectin and its receptors in NASH as compared to simple steatosis (Kaser et al, 2005).

 Progression to fibrosis and cirrhosis among the above factors requires an interplay between inflammatory cells, Kuppfer cells, fatty liver cells and hepatic stellate cells (while the exact role of iron in NAFLD is uncertain, iron can lead to stellate cell activation and collagen deposition – Younossi et al, 1999).  We will not go into detail on the exact interactions involved between the above mentioned cells, but will note that leptin, interleukin – 10 (pro-inflammatory cytokine), transforming growth factor β (also a pro-inflammatory cytokine) are among the elements that play a role in activating and regulating this process.

Table 1: Shows the numerous conditions associated with fatty liver disease.

(David A. Sass MD,* Parke Change, MD,†and Kapil B. Chopra, MD. Nonalcoholic Fatty Liver Disease:A Clinical Review. Digestive Diseases and Sciences, Vol. 50, No. 1 (January 2005), pp. 171–180)

SIGNS AND SYMPTOMS

Because most patients with non – alcoholic fatty liver disease present with no symptoms (asymptomatic), it is often called a “silent killer”. Some patients do however present with non- specific symptoms like vague upper right quadrant abdominal pain, malaise and fatigue.

DIAGNOSIS

Patients suffering from non –alcoholic fatty liver disease are often discovered by “accident” during routine laboratory testing, when the liver function panel shows elevated liver enzyme levels or when abdominal imaging incidentally detects a fatty liver.

 Generally the most common and usually the first abnormality found in non – alcoholic fatty liver disease patients is a slight elevation of the liver enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST) (Angulo P, 2002).

 Patients with non – alcoholic fatty liver disease usually present with an AST/ALT ratio of < 1. This differs from patients with alcoholic liver disease who typically exhibit increases in ALT and AST that are disproportionate to each other.However,there is not enough diagnostic difference between alcoholic and non- alcoholic fatty liver disease for elevated liver enzymes alone to be used to make a positive diagnosis for either disease.

 It should be noted that the degree of serum aminotransferase elevation does not have a significant histological correlation with the severity of hepatic inflammation or fibrosis (Sonsuz et al, 2000).

 Other laboratory findings may include elevated serum alkaline phosphatase (AlkP), bilirubin, glucose and lipid levels, although these may only present once the disease is established.

 A physical exam may show hepatomegaly due to fatty infiltration of the liver.

 Because Hepatitis C (HCV) can cause histological changes that closely resemble those associated with non-alcoholic fatty liver disease, serological testing to

exclude viral hepatitis should be a prerequisite for the diagnosis of non – alcoholic fatty liver disease.

Before a diagnosis of non- alcoholic fatty liver disease is even considered, the patient must consume less than 20 - 30 grams (approx. 2 drinks) of alcohol a day and all the other possible causes for liver disease must first be eliminated (eg. viral, metabolic, autoimmune markers, drugs or toxins, hepatobiliary obstructions, tumours, etc. )

Imaging techniques are a valuable tool in the diagnosis of non – fatty liver disease. Techniques that can identify hepatic steatosis include ultrasonography (US), Computed Tomography (CT), and Magnetic Resonance Imaging (MRI).

ULTRASONOGRAPHY

 Most commonly used  least expensive method  Can be used to subjectively diagnose moderate to severe steatosis.  This technique ishowever, poor at detecting smaller amounts of fat in the liver.  Findings of diffuse fatty liver change include increased acoustic attenuation, vascular blurring and an increased echogenicity of the liver (ie. hyperechogenicity).  The accumulation of intracellular lipid vesicles leads to a higher number of acoustic interfaces, which aides in hyperechogenicity.  The liver is said to have a hyperechoicechotexture. In simple terms, fatty liver shows as a “bright liver”.  While it is impossible to accurately quantify hepatic fat, using this imaging technique, severe steatosis is generally diagnosed when visualization of the walls of the hepatic and portal veinsare obscured by hepatic parenchymal echogenicity.

COMPUTED TOMOGRAPHY

 A more sensitive imaging technique at detecting fatty change in the liver than ultrasonography.  Grey scales that represent the amount of absorbed radiation are measured of the liver and spleen. These are then expressed in Houndsfield Units (HU). The difference in grey scale between the liver and spleen can be measured in HU and used to quantify the amount of hepatocytes that have becomefat- infiltrated.Fatty change is suspected when the density of liver is more than 10 H below that of the spleen.

 Shows fatty change by a reduced attenuation of liver density. A liver that has been infiltrated by fat produces a low-density hepatic parenchyma on CT scanning

MAGNETIC RESONANCE IMAGING

 Magnetic resonance spectroscopy is a newer radiologic technique that allows one to examine the resonance frequencies of all proton species within the region of interest being investigated.  Has the potential to both qualitatively and quantitatively diagnose fatty infiltration of the liver.  The principle of is based on the differences in protonresonance frequencies. Slightly different frequencies for the protons of fat and water are produced in a magnetic field. These can be used diagnose fatty infiltration.  The most commonly usedquantitative method is theso-called “in- and out-of- phase imaging “orchemical shift imaging.  A superior methodto ultrasonography in assessing liver fat but is not as widely available.  Particularly useful in patients with small amounts of hepatic steatosis.

It should be noted that in a recent study neither ultrasonography, computed tomography nor magnetic resonance imaging were able to detectthe presence ofMallory’s hyaline,hepatocyte ballooning or fibrosis, all of which are important features in the diagnosis of non – alcoholic fatty liver disease (Saadeh et al, 2002).

Also because of the inability to distinguish between simple steatosis, steatohepatitis and the severity of liver injury, the best diagnostic test for steatohepatitis remains a liver biopsy.

Imaging or radiographic techniques may be sufficient to diagnose non- alcoholic fatty liver disease if the imaging is consistent with fatty infiltration, the patient does not belong to a high risk group for advanced fibrosis or cirrhosis and also does not have signs and symptoms associated with liver scarring. Also other causes for liver disease must have already been ruled out.

So what would be the appropriate protocol to follow when non – alcoholic liver disease is suspected?

 A patient may present with hepatomegaly upon physical exam or routine liver panel tests might indicate slight elevations in the liver enzymes, particularly ALT and or AST.  An ALT/AST ratio of < 1 would be typical for non- alcoholic liver disease (the ratio of ALT/AST in alcoholic liver disease is greater than this, typically >2).  Ferritin and iron levels may be elevated  AlkP levels may be elevated and patients may test positive for serum auto- antibodies eg. Anti-nuclear antigen.

Based on the above findings, the physician might suspect non – alcoholic fatty liver disease, but would first have to eliminate other potential causes for liver injury.

The patient’s history, associated symptoms, alcohol consumption, genetic possibilities based on the patient’s family history would first have to be considered. Some of these factors are mentioned in Table 1.

Then patients with hepatic steatosis would undergo more specific laboratory tests to rule out other conditions. These tests could include:

 Tests for viral hepatitis C infection (anti – hepatitis C antibody)  Sometimes hepatitis A and B are also tested for to rule out these infections when aminotransferase levels are elevated.  An iron profile  Serum levels of antinuclear antibody, anti-smooth muscle antibody, anti- liver/kidney microsomal antibody -1, and gammaglobulin levels.

Once these tests have ruled out causes other than non – alcoholic liver disease, then the imaging techniques already mentioned (eg, ultrasonography) may be used to detect the disease. Sometimes however none of these tests may prove conclusive in making the diagnosis of non-alcoholic fatty liver disease, and the gold standard for diagnosing this disease, a liver biopsy, may be required to reach a conclusive diagnosis and differentiate between NAFLD and NASH.

THE LIVER BIOPSY

The only way to distinguish between non – alcoholic fatty liver disease (NAFLD) andnon - alcoholic steatohepatitis(NASH) is a liver biopsy. It is also the only way to assess the severity of liver damage. While it remains the gold standard in diagnosis, it is often not the first choice in the investigative process due to its invasive nature.

It involves the use of a fine, hollow needle to withdraw a small piece of sample tissue from the liver. The insertion is made through the skin, directly into the liver, usually under local anaesthetic. The tissue is then examined under a microscope. A biopsy would also allow the physician to determine whether scaring of the liver has occurred and the degree of scarring if so.

Figure 2: Showing the biopsy needle being inserted through a man’s torso into the liver to remove a small slender core of tissue that will be examined under a microscope. (www.liverfoundation.org)

In simple terms if the liver biopsy reveals fat in the liver in the absence of inflammation and damage to the hepatocytes, then NAFLD or simple fatty liver is diagnosed.

However, if the liver biopsy reveals a fatty liver in the presence of inflammation as well as hepatocyte damage then NASH is diagnosed.

While scans and blood tests can help the physician infer diagnosis (and this is usually enough), a liver biopsy is the only way to reliably reach a diagnostic conclusion.Also note that the histological features of non – alcoholic fatty liver disease are indistinguishablefrom those of alcohol-induced liver disease.

TREATMENT

Treatment of non – alcoholic fatty liver disease is usually therapeutic and involvestreating risk factors associated with the disease (obesity, diabetes, high cholesterol and associated cardiovascular disease), as no treatment thus far has effectively altered the natural history of the disease.

Note that up to a certain point, liver damage is reversible (mainly because of the livers uncanny ability to “heal” itself or regenerate)…but only to a certain point.

As part of the therapeutic measure, often physicians will recommend that patients that are obese or overweight reduce their weight and implement lifestyle changes, like following a balanced diet or increasing the amount of daily exercise. Diabetic patients would also be encouraged to manage their disease better, avoid medications that are not necessary, etc.

Studies show that gradual sustained weight lossin obese or overweight patients, can lead to improved liver histology and liver function tests (Ueno et al, 1997).

One of the most reproducible pre-disposing factors for non – alcoholic fatty liver disease appears to be insulin resistance. While insulin sensitizing agents(eg.Thiazolidinediones, Metaformin, Rosiglitazone) have been shown to improve liver function tests and the grade of liver inflammation in patients with NASH (Caldwell et al, 2001), concern surrounds the long term safety of such agents.

Because hypertriglyceridemia is another common factor associated with non-alcoholic fatty liver disease, lipid lowering agents may also be used to manage the disease, however there is at present no significant histological data available and the effects of different lipid lowering agents seem to vary between the different drugs used eg. after one year of treatment with 2 g/day of Clofibrate, no beneficial effect in liver function tests or histology was noted (Laurin et al, 1996) whilst another drug gemfibrozil administered at 600 mg/day over a 4 week period, showed a significant improvement in ALT levels when compared to the placebo group (Basaranoglu et al, 1999).

Ursodeoxycholic acid (UDCA), and the anti-oxidants betaineand vitamin E, are thought to offer hepatocyte protection mainly by reducing oxidative stress.

Other drugs like lecithin, β-carotene, selenium, and Nacetylcysteine have also been suggested to provide this protection however insufficient randomized controlled data exists for these substances to be conclusive.

LIVER TRANSPLANTS

Patients diagnosed with end-stage liver disease, can be evaluated for a liver transplant, however, however studies have shown that non-alcoholic fatty liver disease can recur in such patients, with the progression from steatosis to steatohepatitis being rapid (Charlton et al, 2001).

This suggests that many different factors plays a role in the disease (as already mentioned - obesity, diabetes mellitus, “metabolic syndrome” associated conditions, etc) and that a liver transplant does not necessarily “cure” the underlying metabolic derangements associated with the disease.

HOPE THROUGH TRIALS

To make much needed headway in understanding this disease and how to best treat patients and evaluate prognosis, more biopsy controlled, long term studies are required to fully elucidate this disease and provide much needed information.

REFERENCES

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