Nonalcoholic steatohepatitis (NASH) is a form of nonalcoholic fatty liver disease (NAFLD). It involves excessive fat accumulation in the liver, accompanied by inflammation and cell damage. A healthy liver typically contains minimal fat, but if more than 5% of its weight is fat, it indicates fatty liver disease. NASH is often considered a “silent” disease in its early stages, as individuals may experience few or no noticeable symptoms.
Initial Evaluation and Risk Factors
NASH diagnosis often begins when a doctor investigates other health conditions or notices abnormalities during routine screenings. Physicians often suspect NASH in individuals presenting with components of metabolic syndrome, a cluster of conditions that collectively heighten the risk. These include type 2 diabetes or prediabetes, characterized by elevated blood sugar levels, and high blood pressure. Additionally, abnormal fat levels in the blood, such as high triglycerides or low levels of high-density lipoprotein (HDL) cholesterol, serve as important indicators.
Obesity is a major risk factor, with many overweight or extremely obese individuals having NAFLD. These imbalances and excess weight contribute to fat accumulation in the liver. Other associated conditions that may raise suspicion include polycystic ovary syndrome, obstructive sleep apnea, and an underactive thyroid.
Blood and Imaging Tests
After an initial evaluation, doctors use non-invasive blood and imaging tests to assess liver health. Routine blood tests often include liver enzyme measurements, specifically alanine transaminase (ALT) and aspartate transaminase (AST). While elevated levels of these enzymes can suggest liver inflammation, individuals with NAFLD or even NASH can have normal ALT and AST readings. Therefore, these tests alone are often not sufficient for a definitive diagnosis of NASH.
Imaging tests, like a standard abdominal ultrasound, are often among the first tools used when liver disease is suspected. An ultrasound can effectively detect the presence of fat in the liver, a condition known as steatosis. However, a key limitation of standard ultrasound, and even more advanced imaging like MRI or CT scans, is their inability to differentiate between simple fatty liver (NAFL) and the more severe, inflammatory NASH. These imaging methods confirm fat accumulation but cannot assess the specific liver cell damage or inflammation indicative of NASH.
The Role of Liver Biopsy
Despite advancements in non-invasive methods, a liver biopsy remains the accepted standard for definitively diagnosing NASH and assessing the extent of liver damage. This procedure involves taking a small sample of liver tissue, typically using a needle inserted between the ribs, which is then examined under a microscope. The biopsy provides direct visual evidence of specific changes within the liver that other tests cannot.
It reveals the presence of inflammation, which is a hallmark of NASH, and hepatocyte ballooning, indicating damage to liver cells. The biopsy also allows pathologists to accurately stage the amount of scarring, or fibrosis, in the liver, which is a significant predictor of disease progression. While highly informative, a liver biopsy is an invasive procedure that carries some risks and can be uncomfortable for the patient.
Staging Fibrosis Without a Biopsy
Given the invasive nature of liver biopsy, non-invasive methods are increasingly used to estimate liver scarring, or fibrosis. These tests primarily measure liver stiffness, which correlates with fibrosis. Transient elastography, often known as FibroScan, is a commonly used ultrasound-based technique that measures the velocity of shear waves through the liver to determine its stiffness. Another advanced imaging method is Magnetic Resonance Elastography (MRE), which combines MRI imaging with sound waves to create a visual map of liver stiffness.
Beyond imaging, blood-based scoring systems offer a less costly and more accessible way to estimate fibrosis. Algorithms like the FIB-4 (Fibrosis-4) index and the NAFLD Fibrosis Score (NFS) calculate a risk score for advanced fibrosis using a combination of routine blood parameters such as age, liver enzyme levels (AST, ALT), platelet count, and albumin. These scores can help clinicians identify patients at higher risk of significant fibrosis, potentially guiding decisions for further evaluation without always requiring a biopsy.