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Pathogenesis[ edit ] No single mechanism leading to steatosis exists; rather, a varied multitude of pathologies disrupt normal lipid movement through the cell and cause accumulation.
Failure of lipid metabolism can also lead to the mechanisms which would normally utilise or remove lipids becoming impaired, resulting in the accumulation of unused lipids in the cell. Certain toxins, such as alcohols, carbon tetrachloride , aspirin , and diphtheria toxin , interfere with cellular machinery involved in lipid metabolism. Protein malnutrition, such as that seen in kwashiorkor , results in a lack of precursor apoproteins within the cell, therefore unused lipids which would normally participate in lipoprotein synthesis begin to accumulate.
Macrovesicular steatosis[ edit ] Macrovesicular steatosis is the more common form of fatty degeneration and may be caused by oversupply of lipids due to obesity, obstructive sleep apnea OSA ,  insulin resistance, or alcoholism.
Nutrient malnutrition may also cause the mobilisation of fat from adipocytes and create a local oversupply in the liver where lipid metabolism occurs.
Excess alcohol over a long period of time can induce steatosis. The breakdown of large amounts of ethanol in alcoholic drinks produces large amounts of chemical energy in the form of NADH , signalling to the cell to inhibit the breakdown of fatty acids which also produces energy and simultaneously increase the synthesis of fatty acids. This "false sense of energy" results in more lipid being created than is needed. Microvesicular steatosis[ edit ] Microvesicular steatosis is characterized by small intracytoplasmic fat vacuoles liposomes which accumulate in the cell.
Histology[ edit ] Histologically , steatosis is physically apparent as lipid within membrane bound liposomes of parenchymal cells. As such, samples prepared this way will appear to have empty holes or vacuoles within the cells where the lipid has been cleared. Special lipid stains, such as Sudan stains and osmium tetroxide are able to retain and show up lipid droplets, hence more conclusively indicating the presence of lipids.
Other intracellular accumulations, such as water or glycogen , can also appear as clear vacuoles, therefore it becomes necessary to use stains to better decide what is accumulating. Grossly, steatosis causes organ enlargement and lightening in colour. In severe cases, the organ may become vastly enlarged, greasy, and yellow in appearance. Micrograph of fatty liver showing lipid steatosis.
Medical imaging[ edit ] Liver steatosis fatty liver disease as seen on CT On X-ray computed tomography CT , the increased fat component will decrease the density of the liver tissue, making the image less bright. Typically the density of the spleen and liver are roughly equivalent. In steatosis, there is a difference between the density and brightness of the two organs, with the liver appearing darker. The combination of liver steatosis being dark on CT and bright on ultrasound is sometimes known as the flip flop sign.
On magnetic resonance imaging , multiecho gradient echo images can be used to determine the percent fat fraction of the liver. Acquisition of echoes in "in phase" and "out phase" conditions pertaining to the relative phases of the fat and water proton contingents enables to obtain a signal proportional to the water and fat contingent, or a signal proportional to the water minus the fat contingent.
These signal intensities are then algebraically combined into a percent fat. More recent techniques take into account experimental noise, signal decay and spectroscopic properties of fat. Numerous validation studies have demonstrated excellent correlations between the steatosis level quantified at MRI and the steatosis levels semi-quantitavely and quantitatively determined on liver biopsies reference methods.
Several MRI vendors offer automated calculation of percent fat with acquisition sequences no longer than a single breath hold. On abdominal ultrasonography , steatosis is seen as a hyperechoic liver as compared to the normal kidney.
Liver steatosis fatty liver disease as seen on MRI. Multiecho MR sequence in a healthy liver top row and a liver with severe steatosis bottom row are shown. In the healthy liver, the signal does not vary much in the different echoes. In the steatotic liver, the signal varies greatly between in and out phase echoes. Algebraic combination of these images can be used to accurately quantify liver steatosis. Abdominal ultrasonography of focal steatosis.
It is distinguished from a tumor by not compressing the hepatic vein. The scans also found that 2.
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