Acetaminophen (Paracetamol) toxicity: Clinical sciences

Acetaminophen toxicity is the most common cause of acute liver failure in the United States. Acetaminophen, also known as paracetamol or N-acetyl-para-aminophenol or APAP for short, is a medication commonly used to treat pain and fever in children and adults.

Normally, acetaminophen is metabolized in the liver, but if consumed in large amounts, the toxic byproduct of acetaminophen metabolism called NAPQI can lead to hepatotoxicity, which can eventually result in liver failure.

The management of acetaminophen toxicity is based on the amount of time that has passed since the drug was ingested, either within 4 hours, 5 to 24 hours, or longer than 24 hours ago.

If a patient presents with signs and symptoms suggestive of acetaminophen toxicity, the first step is to perform an ABCDE assessment to determine if the patient is stable or unstable. If the patient is unstable, stabilize the airway, breathing, and circulation. This means that you might need to intubate the patient. Next, obtain IV access and put the patient on continuous vital sign monitoring, including heart rate, blood pressure, and pulse oximetry. Finally, if needed, don’t forget to provide supplemental oxygen.

Alright, now let’s go back to the ABCDE assessment and discuss how to manage stable patients. If the patient is stable, first perform a focused history and physical exam. The patient will typically report acetaminophen ingestion, as well as loss of appetite, fatigue, and malaise. Additionally, they might report abdominal pain, nausea, vomiting, and confusion. The physical exam might reveal tachycardia, right upper quadrant abdominal tenderness, and altered mental status.

Now, here’s a high-yield fact! In adults, acute ingestion of 12 grams of acetaminophen or 150 milligrams per kilogram is considered a toxic dose. But, keep in mind that patients might be asymptomatic for hours after ingesting acetaminophen, so a lack of symptoms doesn’t mean the patient won’t have acetaminophen toxicity.

Generally speaking, clinical manifestations of acetaminophen toxicity can be subdivided into 4 main stages!

Stage I of acetaminophen toxicity covers the first 24 hours after the ingestion, and typically presents with non-specific findings like pallor and diaphoresis, as well as gastrointestinal manifestations like nausea and vomiting. In this stage, there’s still no liver injury, so labs are usually normal.

Next up is stage II, which occurs between 24 and 72 hours after the ingestion, and is associated with initial damage to the liver! In this stage, the symptoms from stage I typically resolve, but, your patient could develop right upper abdominal pain and hepatomegaly. Regarding labs, as hepatocytes start to die off, they begin to release AST and ALT in the bloodstream! Additional lab findings that are supportive of hepatic dysfunction include elevated bilirubin levels, as well as prolonged PT and INR.

Next up is stage III, which occurs between 72 and 96 hours after ingestion! In this stage, there’s significant damage to the liver, which in some cases can even lead to fulminant hepatic failure. Some patients may experience hepatic encephalopathy, which is characterized by altered mental status and confusion. In extreme cases, patients may even progress to a state of coma! Lab findings reveal hepatic injury and dysfunction, including metabolic acidosis, abnormal PT and INR, as well as hypoglycemia, and findings of renal failure, such as elevated creatinine.

Finally, there’s stage IV, which begins on day 4 of the overdose and can last up to 4 weeks. Stage IV is a period of recovery, during which the patient’s liver function may gradually improve, leading to a subsiding of symptoms. However, it's important to note that certain patients who have experienced extensive liver damage or delayed medical intervention might face lasting sequelae, such as the development of liver failure or the need for a transplant.

Let’s follow this with a clinical pearl! In the context of evaluating for liver failure, prolonged PT/INR is referred to as synthetic dysfunction, meaning, the liver’s failing at its role of producing proteins involved in the clotting cascade.

Thus, a commonly used phrase you might hear in the clinical setting is "AST and ALT are elevated, but there's no synthetic dysfunction". What this means is that, although there’s elevated levels of AST and ALT, which are markers of liver damage, the liver's ability to synthesize clotting proteins as indicated by the PT/INR is not yet affected. On the flip side, if labs reveal synthetic dysfunction, you’re facing severe liver damage or liver failure.