Indeterminate

Historically, a specific diagnosis was not established in over 50% of PALF and these children were categorized as indeterminate (IND-PALF). Likely, the indeterminate group consisted of some patients who underwent an incomplete diagnostic evaluation, and within the PALFSG cohort, inadequate assessments for autoimmune hepatitis (AIH), mitochondrial disorders, and other metabolic diseases were the most common deficiencies [9]. Comprehensive viral testing in PALF has also been found to often be incomplete [10]. Recent efforts have looked to address this shortcoming. While almost 43% of the overall PALFSG participants were categorized as indeterminate (Figure 4.2), the incorporation of standardized diagnostic test recommendations into electronic medical record (EMR) order sets enabled a significant reduction in the percentage of IND-PALF, from 48% during the first two phases of the study (1999–2010) to 30.8% when looking at phase three (2011–2014) [1]. In smaller studies, the utility of next generation sequencing and un-biased whole exome sequencing has expanded diagnostic capabilities as well [11, 12]. Still, “indeterminate” remains the most common categorization of these critically ill children.

The presentation of IND-PALF mirrors that of most other children with acute liver failure (Figure 4.1). IND-PALF can occur at any age and cases do not segregate by sex or race. Temporal clustering of IND-PALF supports a potential role for a community-acquired viral infection, but a novel or previously known pathogenic virus has not been identified [6]. Injury secondary to overzealous inflammatory responses and immune dysregulation have been suspected in many cases of IND-PALF and the degree of increasing inflammatory network connectivity in these patients has been shown to be associated with poorer outcomes [13, 14]. Some patients with IND-PALF present with a hemophagocytic lymphohistiocytosis (HLH)-like phenotype including elevated soluble IL-2 receptor levels [15, 16]. However, the majority of patients with evidence of immune activation do not meet criteria for HLH but do exhibit characteristic features that suggest a common pathogenesis [17]. This sub-set of IND-PALF patients have distinctive liver histology which includes a dense CD103⁺CD8⁺ T-cell infiltrate further suggesting an immune mediated liver injury [16, 17]. PALF patients with this “activated” CD8 T cell hepatitis are more likely to receive liver transplantation than patients with other causes of PALF. Based on these observations, there has been a growing trend toward treatment of PALF associated with activated CD8⁺ T cell hepatitis with immunosuppression. However, the therapeutic benefit of this practice remains unknown [18]. The clinical course of IND-PALF can be variable, yet the trajectory of commonly obtained clinical measures such as international normalized ratio (INR), total bilirubin, and the presence of HE has been shown to have important prognostic value [19]. When considering transplant, having an indeterminate diagnosis appears to influence decisions toward listing [20]. Additional co-morbidities, such as transient bone marrow suppression and aplastic anemia, can be seen in these children, even after successful liver transplantation [6].

Acetaminophen

Acetaminophen (N-acetyl-p-aminophenol (paracetamol)) is widely used in children for management of fever and pain. It is available without prescription and is commercially available as a single formulation or can be compounded with decongestants or narcotics. Acetaminophen (APAP) is safe and well tolerated when dosing instructions are strictly followed. However, it has a low therapeutic index, and in certain individuals or clinical scenarios, chronic administration of therapeutic dosages of acetaminophen can result in significant hepatotoxic effects [21]. Notably, APAP toxicity is the most commonly identified cause of PALF in over 1,100 children enrolled in the PALFSG [1]. Two clinical scenarios are associated with acetaminophen hepatotoxicity.

The most common scenario follows an intentional single ingestion of a hepatotoxic dose of 15–25 g in adults (or >100 mg/kg in children) in an attempt at suicide or attention-seeking behavior [22]. Plasma acetaminophen at four and 24 hours after a single ingestion will assist in determining the relative toxicity of the ingestion [23]. Females over ten years of age represent the most common demographic associated with intentional overdose in children, but it should be considered in all age groups outside the newborn period [5]. Immediately following ingestion, patients may experience non-specific symptoms of nausea and vomiting. While a liver biopsy is not generally indicated in the setting of known acetaminophen overdose, centrilobular hepatic necrosis is the hallmark finding and should raise the consideration of acetaminophen toxicity even without a clear history of exposure. N-acetylcysteine (NAC) given enterally or intravenously can successfully reverse the toxic injury if given shortly after the ingestion, ideally within 24 hours. Mechanistically, NAC has demonstrated hepatoprotective effects in acetaminophen toxicity by antagonizing the damage-associated molecular pattern (DAMP) molecule high mobility group box 1 (HMGB1) which drives a pro-inflammatory program in PALF [24] and by enhancing hepatic and mitochondrial glutathione levels (scavenging of reactive oxygen and peroxynitrite) [25]. If treatment is delayed beyond 24 hours following ingestion, the patient is at increased risk of having irreversible liver injury. Regardless of the interval between ingestion and presentation, NAC should be administered when a toxic ingestion has occurred. Serum aminotransferase levels can reach over 10,000 IU/mL and the total bilirubin is generally lower than might be expected given the degree of liver injury, typically <10 mg/dL. Early metabolic acidosis and serum lactate elevations can be seen along with co-morbidities of acute renal failure (10–50%) and pancreatitis (0.3–5%) [2]. If treatment is not initiated, jaundice develops within 48 to 72 hours, with death occurring in the most severe cases by five to seven days following the ingestion.

In the second most common presentation, patients inadvertently partake in a “therapeutic misadventure” whereby they unintentionally overdose by taking therapeutic doses of multiple APAP-containing medications [22]. Risk factors for developing severe hepatotoxicity secondary to a therapeutic misadventure include concomitant use of other medicines that alter hepatic metabolism, delayed medical care, younger age, and prolonged periods of fasting [26]. In both adults and children, polymorphic variants in UDP-glucuronosyltransferase (UGT) 1A, a main metabolizing enzyme of APAP, have been shown to affect the risk of developing APAP toxicity [27, 28]. The presence of acetaminophen adducts in the serum may indicate unsuspected hepatotoxicity or underreported acetaminophen (ab)use [29]. Additionally, the presence of adducts in indeterminate cases of PALF suggests occult APAP toxicity may play a role in approximately 10% of children where a distinct diagnosis for ALF cannot be established [30]. Although the magnitude of this clinical problem in children is not well defined, between 16% and 48% of acetaminophen-induced ALF in adults results from unintentional overdosing, reinforcing the concept that accidental misuse of this medication leading to serious liver injury is relatively common [22]. Similar to children with a single intentional overdose, alanine aminotransferase levels can reach into the many thousands with a relatively low total bilirubin level.

Medication (Non-Acetaminophen) or Toxin

Liver injury caused by drugs, herbals, or toxins other than acetaminophen was identified in a little more than 3% of cases in the PALFSG registry, the vast majority occurring in children over ten years of age [1, 5]. Anti-epileptics (valproic acid, phenytoin, carbamazepine, felbamate) are the most common offenders in children [5, 31]. Valproic acid, particularly in children with unsuspected mitochondrial disease, may precipitate ALF. The list of xenobiotics associated with liver failure is extensive and expanding; a partial list is found in Table 4.2. Hepatotoxic agents, such as industrial solvents and mushroom toxin, are dose dependent and will predictably result in liver injury or failure. The diagnosis of hepatotoxic liver injury is often one of exclusion, based on latency, clinical features, laboratory values, histology, and a response to removal of the offending agent [32]. However, idiosyncratic drug reactions remain a common etiology. Given the complexity of drug-induced liver injury, the Drug-Induced Liver Injury Network (DILIN), established by the National Institutes of Health in 2003, has and continues to standardize the nomenclature and causality assessment of drug-induced liver injury (DILI) including those associated with liver failure. In April of 2012 the website LiverTox® () was launched as a joint effort of the Liver Disease Research Branch of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and the Division of Specialized Information Services of the National Library of Medicine (NLM), National Institutes of Health. The purpose of LiverTox® is to provide up-to-date, accurate, and easily accessed information on the diagnosis, cause, frequency, patterns, and management of liver injury attributable to medications, herbals and dietary supplements.

A careful history of exposure to hepatotoxins should be obtained from the family of any child presenting with ALF, including prescription and non-prescription drugs in the home that could have been ingested accidentally. In teenagers, a history should include evidence of depression, recreational drug use (e.g., cocaine, ecstasy), or solvent sniffing. Any exposure to hepatotoxic drugs, chemicals, or herbals should be considered possibly related to the liver injury. Ingestion of the Amanita mushroom is clearly traced to ALF.

Liver biopsy can assist in the diagnosis of drug-induced idiosyncratic liver injury. The histologic pattern of injury observed should be that expected from the drug to which the patient has been exposed. The patterns seen are hepatitis (hepatocellular necrosis), cholestasis, mixed cholestasis and hepatitis, and steatosis. Drugs that cause hepatitis (e.g., isoniazid, propylthiouracil, and halothane) have the greatest potential for causing PALF. Drugs associated with steatosis (e.g., sodium valproate, amiodarone) may also cause liver failure. Drugs that cause cholestasis (oxacillin) rarely produce liver failure, whereas drugs that cause mixed cholestasis and hepatitis (sulfa drugs) sometimes do. If histology differs from that expected by the drug in question, another cause should be sought. Exposure to a drug or toxin should not preclude a thorough search for other causes of liver injury.

Immune Mediate Liver Injury

Gestational Alloimmune Liver Disease

Gestational alloimmune liver disease (GALD), results from an intrauterine alloimmune liver injury and is the single most common cause of neonatal ALF [39]. The phenotype of neonatal liver disease in association with siderosis of various extrahepatic tissues has been termed neonatal hemochromatosis (NH) for over 20 years. Recently, it has been determined that GALD accounts for 60–90% of NH cases. The remaining patients with the NH phenotype have been found to have a variety of disorders including mitochondrial DNA depletion syndrome, bile acid synthetic defects, Down syndrome with myelodysplasia, and severe perinatal infection. While HLH may also present with extra-hepatic iron deposition, iron is present in the reticuloendothelial system (e.g., spleen and macrophages) which is spared in the NH phenotype. In GALD-NH, maternal immunoglobulin G appears to activate fetal complement, which leads to the formation of the membrane attack complex and results in liver cell injury. The resultant pathologic siderosis seen in both the liver and various extrahepatic tissue results from severe liver injury causing iron overload due to poor regulation of maternofetal iron flux. The degree of liver injury can be so profound that death from liver failure can occur within the first few weeks of life [39, 40]. Therefore, liver failure associated with GALD-NH is technically a terminal event of a chronic intrauterine liver disease. However, the phenotype of a family’s index case of NH confronting the clinician is one of ALF and thus deserves to be included in this section for clinical purposes.

Characteristic clinical features of GALD-NH include an ALF presentation usually at birth and almost always in the first days of life. The majority (70–90%) of affected infants are born premature and a history of maternal sibling death is common. Medical history often reveals intrauterine growth restriction and oligohydramnios. Refractory hypoglycemia, severe coagulopathy, hypoalbuminemia, elevated serum ferritin (>1000 μg/L), and ascites are often noted. Strikingly, serum aminotransferase levels are normal or near normal and should alert the clinician to the possibility of GALD-NH [39]. Extrahepatic iron deposition is a hallmark finding. Hemosiderin deposition in the minor salivary glands obtained by a buccal mucosal biopsy is often seen. Alternatively, MRI of the abdomen would suggest the diagnosis with the finding of reduced T₂-weighted intensity of the liver and/or the pancreas relative to the spleen. Exchange transfusion and high-dose intravenous immunoglobulin (IVIG) is the preferred treatment to remove offending antibodies and block their action, including activation of complement [39].

Inherited Metabolic Disease

Metabolic diseases may not fit the definition of ALF precisely as the condition was certainly present prior to presentation. However, a number of conditions will present acutely in a child who is not known to have the condition until the diagnosis is established and ALF may be the initial presentation. Overall, metabolic diseases account for just under 10% of PALF [1]. While some conditions, such as mitochondrial disease, may present at any age, many metabolic conditions presenting as liver failure segregate within age groups. Metabolic conditions that should be considered in these age groups are listed in Table 4.3. Details of the specific conditions can be found in other sections of this textbook. Highlighting those which have been associated with a presentation of ALF is the purpose of this section.

Metabolic conditions affecting infants in the first few months of life include galactosemia, tyrosinemia, Niemann–Pick type C, mitochondrial hepatopathies, hereditary fructose intolerance, urea cycle defects, and the defects associated with recurrent PALF [41–43]. Galactosemia should be considered in a child consuming breast milk or other lactose-containing formula and developing liver failure associated with reducing substances in the urine. Hepatosplenomegaly is common and profound coagulopathy can be seen with only modest aminotransferase elevations [2]. Similarly, tyrosinemia can present with a profound coagulopathy and normal or near normal serum aminotransferase levels. Both galactosemia and tyrosinemia can present in association with gram-negative sepsis. Niemann–Pick type C is a lysosomal storage disease and marked splenomegaly is often noted. Hereditary fructose intolerance more classically presents in an older child after the introduction of fructose and/or sucrose, however, several newer infant formulas contain fructose and ALF in the neonatal period has been reported [42]. Mitochondrial hepatopathies are increasingly recognized as an important cause of liver failure due to deficiencies in respiratory complexes I, III, or IV or mitochondrial DNA depletion. With rare exceptions, mitochondrial hepatopathies have associated systemic mitochondrial dysfunction characterized by progressive neurologic deficiencies, cardiomyopathy, or myopathy. Multisystem mitochondrial dysfunction has historically served as a relative contraindication to liver transplant [44]. However, patients with mitochondrial diseases (excluding POLG-related disease) have more recently been shown to tolerate solid-organ transplant with post-transplant survival similar to non-mitochondrial disease patients [45]. Unfortunately, multi-system involvement may not be apparent at the time of liver transplant, and progressive extra-hepatic disease can occur. Lactic acidosis and an elevated molar ratio of lactate to pyruvate (>25 mol/mol) have historically been used to alert the clinician to the possibility of a mitochondrial hepatopathy; however, recent analysis of the PALFSG cohort found that neither an elevated serum lactate ≥ 2.5 mmol/L nor an elevated lactate:pyruvate (L:P) ratio were specific for mitochondrial disease in the setting of PALF and elevation did not predict clinical outcome [46]. Thus it appears that secondary mitochondrial dysfunction, independent of the cause of acute liver failure, may drive lactate and L:P abnormalities in PALF and that neither diagnosis nor clinical decisions (including considerations for liver transplant) should be based solely on these findings [46]. Defects in fatty acid oxidation, a primary function of mitochondria, may become clinically apparent during a period of fasting, as a consequence of anorexia associated with an acute illness, or when the infant begins to sleep through the night. With these defects, hyperammonemia is common as are extra-hepatic manifestations involving the neurologic, cardiac, and muscular symptoms [2]. Recurrent PALF episodes have been associated with the infantile liver failure syndrome type I (LARS mutations) and NBAS deficiency. Importantly, ALF in the neonatal period may be the initial presentation of these rare disorders and a high clinical suspicion is indicated to make the diagnosis [43, 47].

In older infants and young children up to five years of age, mitochondrial diseases, tyrosinemia, hereditary fructose intolerance, and urea cycle defects can be identified. Mitochondrial hepatopathies, particularly disorders of fatty acid oxidation, occur commonly in this age group [48]. Hereditary fructose intolerance outside the neonatal period presents when fructose-containing foods, such as fruits, are introduced into the diet. Urea cycle defects typically present with hyperammonemia, mental status changes and seizures, but without liver synthetic dysfunction. However, PALF has been associated with arginosuccinate synthetase deficiency (citrullinemia type 1) and ornithine transcarbamylase deficiency, although the mechanism of liver injury is uncertain [2, 49, 50].

Wilson disease is the most common metabolic condition associated with PALF in children over five years of age [1]. The presence of a Coombs-negative hemolytic anemia, marked hyperbilirubinemia, low serum ceruloplasmin, and a low serum alkaline phosphatase should prompt consideration of Wilson disease, but confirming the diagnosis remains a challenge. Findings in a predominately adult population suggests that the combination of an alkaline phosphatase to total bilirubin ratio of <4 and an aspartate aminotransferase to alanine aminotransferase ratio of >2.2 provided a rapid and accurate method for diagnosis of Wilson disease presenting as ALF [51]. However, these findings have not yet been confirmed in a pediatric population. Fatty acid oxidation defects can also present with ALF in older children and adolescents.

Infectious Diseases

A non-specific prodrome consisting of fever, nausea, vomiting, and abdominal discomfort will precede many cases of ALF in children regardless of etiology. Therefore, it is not surprising that early accounts of ALF often attributed the cause to a virus or infection. As the ability to identify specific infectious agents through serology, culture, and polymerase chain reaction technology has been applied, the burden of viral associated-PALF has become clearer. Although a comprehensive infectious diagnostic work-up is often not performed [10], acute viral infection as the cause of PALF was identified in only 8.4% of subjects [1]. The common hepatitis viruses have not often been found in ALF unless they were endemic to the region or in association with a community outbreak. Although a yet unidentified infectious agent may account for some unexplained cases of ALF in children, efforts to identify rare infectious agents in adults have not been fruitful. Therefore, a reasonable alternative is to classify as indeterminate any patient without an identifiable cause for the ALF until a more specific diagnosis can be established.

Other Rare Causes

Liver failure may be the presenting manifestation of a systemic condition. For example, leukemia can present as liver failure [1]. Cardiovascular shock associated with systemic hypotension, as seen in patients with trauma, sepsis, hemorrhage, cardiomyopathy or left heart failure (e.g., hypoplastic left heart) or following cardiac bypass may also develop liver failure. Budd–Chiari syndrome is a rare cause of PALF as obstruction of the hepatic venous outflow can lead to hepatomegaly, ascites, and evidence of hepatocellular injury. Liver failure may be the presenting feature of celiac disease and, if recognized, is potentially treatable following institution of a gluten-free diet [56].

Hepatic Immunology

The liver is a central immunological organ with high exposure to circulating antigens and endotoxins [57]. In addition to hepatocytes, an estimated 20–40% of the liver cell mass consists of endothelial cells, Kupffer cells, or hepatic macrophages, lymphocytes, biliary cells, and stellate cells. The immunologic milieu that exists within the liver is notably different than that of the peripheral blood compartment. The balance between CD8⁺ and CD4⁺ T cells favors CD8⁺ cells with effector/memory cells more frequent in liver parenchyma than peripheral blood. Natural killer T-cells (NKT cells) as well as NK cells are more abundant in the liver where there is also a large source of gamma–delta T cells. Antigen-presenting cells may be conventional (e.g., Kupffer cells, liver sinusoidal endothelial cells, and dendritic cells) or unconventional (e.g., hepatocytes).

Both innate and adaptive immune responses are generated within the liver. However, given the hepatic enrichment by NK cells, NKT cells, and Kupffer cells, it is not surprising that the innate immune response predominates. Under normal circumstances, the lymphocyte-driven innate response provides a nimble but temperate reaction to pathogens that present to the liver. Kupffer cells and other immune cells express pattern-recognition receptors that detect and bind pathogen-associated molecular patterns expressed on the presumptive pathogen, which is then phagocytosed and quietly eliminated. Activated CD8⁺ cells residing in the liver direct the immune response against bacterial and viral invasion. Priming of intrahepatic T cells may occur within the liver without having to circulate through draining lymph nodes, thus expediting the immune response. Hepatic NK cells serve to modulate the inflammatory response by mediating the balance between proinflammatory (T helper 1) and anti-inflammatory (T helper 2) cytokines that are generated [57].

Adaptive immune responses involve both B and T cells. Very few B cells reside in the normal liver and thus little is known about their role except under pathologic conditions such as HBV and hepatitis C infections. Antigen-specific CD8⁺ T cells generate an effector response that includes rapid proliferation coupled with the production of proinflammatory cytokines as well as initiation of cytolytic mechanisms such as perforin and granzyme. Not unexpectedly, the redundancy of the human immune response cannot be easily divided into separate and distinct components. For example, findings suggest that NK cells may have features associated with the adaptive immune response such as “memory,” with rapid secondary expansion upon re-exposure to antigen that is associated with degranulation and cytokine production. Therefore, while missteps in the liver’s overall tolerance of real or perceived pathogens rarely result in liver failure, understanding these events will provide opportunities to extend our knowledge of the interactions between the liver and its environment.

Drug- and Toxin-Induced Injury

Drug-induced liver injury causes approximately 16% of PALF, with acetaminophen toxicity accounting for 80% of the drug-induced cases. The mechanisms by which drugs cause liver injury vary. Early mechanistic studies in mice demonstrated the formation of a reactive metabolite, which is responsible for hepatic glutathione depletion and initiation of the toxicity. This insight led to the rapid introduction of NAC as a clinical antidote as it is known to promote glutathione restoration. However, more recently, substantial progress was made in further elucidating the detailed mechanisms of APAP-induced cell death. Mitochondrial protein adducts trigger a mitochondrial oxidant stress, which requires amplification through a MAPK cascade that ultimately results in activation of c-jun N-terminal kinase (JNK) in the cytosol and translocation of phospho-JNK to the mitochondria. The enhanced oxidant stress is responsible for the membrane permeability transition pore opening and the membrane potential breakdown. The ensuing matrix swelling causes the release of intermembrane proteins such as endonuclease G, which translocate to the nucleus and induce DNA fragmentation. These pathophysiological signaling mechanisms can be additionally modulated by removing damaged mitochondria by autophagy and replacing them by mitochondrial biogenesis. Although recruitment of inflammatory cells is necessary for removal of cell debris in preparation for regeneration, these cells have the potential to aggravate the injury [58].

Additional aspects of hepatic injury resulting in drug-induced, herbal-induced, and herbal and dietary supplement-induced liver injury constitute an important and growing area of active research, though much remains to be discovered in terms of mechanism. Areas that have recently garnered interest include bile acid homeostasis, oxidative stress, mitochondrial dysfunction, immune and inflammatory dysregulation, and lipophilicity [59].

Vascular Injury

Acute liver failure can result from reduced or absent arterial blood flow to the liver. The insult can occur because of hemorrhagic shock, trauma, sepsis, coarctation of the aorta, congenital heart defects associated with low cardiac output, congestive heart failure, and liver transplant. Myriad processes are initiated following liver ischemia that include the release of pro- and anti-inflammatory cytokines and chemokines, complement activation, along with the activation of both the innate and the adaptive immune response. Mitochondrial injury resulting in ATP depletion and surge in toxic reactive oxygen species contribute to the injury.

Liver failure associated with acute obstruction of hepatic outflow is rare but has been described in patients later found to have a coagulation disorder leading to a hypercoagulable state; in patients with Behçet syndrome or idiopathic Budd–Chiari syndrome; and in women immediately postpartum [2].