Paediatric acute hepatitis of unknown origin (PAHUO)

### Paediatric Acute Hepatitis of Unknown Origin (PAHUO)

#### Overview:

Paediatric Acute Hepatitis of Unknown Origin (PAHUO) refers to cases of acute liver inflammation in children where no known cause—such as viral hepatitis (A, B, C, D, E), toxins, metabolic disorders, or autoimmune diseases—can be identified. This condition gained global attention in 2022 when clusters of cases were reported in several countries, particularly in Europe and the United States. It poses a significant challenge due to its unknown etiology and potential severity, including cases requiring liver transplantation.

---

#### Key Findings from the 2022 Spanish Outbreak:

Research into PAHUO during the Spanish outbreak in 2022 provided new insights into potential genetic and immunological factors that may contribute to the disease. These findings include:

1. **Genetic Susceptibility:**

• A study of 40 affected children revealed a strong association with specific human leukocyte antigen (HLA) alleles, particularly **HLA-DRB1*04** (subtypes *04:01, *04:03, and *04:07).
• These alleles were present in **84.6% of PAHUO patients**, compared to only **34% of healthy controls**, indicating a potential immunogenetic predisposition.

2. **Autoimmune Basis:**

• The shared epitope theory suggests that certain HLA alleles (*DRB1*01, *04, *10, *15, and *14:02) are linked to autoimmune diseases like rheumatoid arthritis and lupus.
• This raises the possibility that PAHUO may have an autoimmune mechanism, where the immune system mistakenly attacks the liver.

3. **Viral Triggers:**

• Viral infections were frequently detected in PAHUO cases, suggesting that viruses might act as environmental triggers for the disease. Key viruses identified include:
• **Adenovirus:** Found in 42.8% of cases.
• **Adeno-associated virus (AAV):** Present in 19% of cases.
• **Cytomegalovirus (CMV)** and **Epstein-Barr virus (EBV):** Also detected in some patients.
• These infections may provoke an autoimmune-like response in genetically predisposed children, leading to liver damage.

---

#### Clinical Presentation:

Children with PAHUO typically present with:

• **Symptoms:** Jaundice, abdominal pain, vomiting, fatigue, diarrhea, and pale stools.
• **Laboratory Findings:** Elevated liver enzymes (ALT, AST), hyperbilirubinemia, and in severe cases, coagulopathy.
• **Severity:** Some cases progress to acute liver failure, requiring hospitalization and, in rare instances, liver transplantation.

---

#### Diagnosis and Challenges:

• **Exclusion of Known Causes:** Diagnosis of PAHUO is made by ruling out common causes of hepatitis, including viral infections (hepatitis A-E), toxins, drugs, metabolic disorders, and autoimmune hepatitis.
• **Immunological and Genetic Testing:** Emerging research suggests testing for HLA alleles and autoimmune markers may help identify at-risk children.

---

#### Treatment:

Currently, there is no specific treatment for PAHUO. Management focuses on:

• **Supportive Care:** Monitoring liver function, managing symptoms, and preventing complications.
• **Immunosuppressive Therapy:** If autoimmune mechanisms are suspected, corticosteroids or other immunosuppressants may be considered.
• **Liver Transplantation:** In cases of acute liver failure.

---

### Autoimmune Hepatitis in Paediatric Age Groups

#### Definition:

Autoimmune hepatitis (AIH) is a chronic inflammatory liver disease caused by the immune system attacking the liver tissue. It is characterized by elevated liver enzymes, autoantibodies, and histological evidence of inflammation.

#### Causes:

• **Genetic Factors:** Certain HLA alleles, such as DRB1*03 and DRB1*04, are associated with AIH.
• **Environmental Triggers:** Viral infections, toxins, or drugs may initiate autoimmune responses in genetically predisposed children.

#### Symptoms:

• Jaundice, fatigue, abdominal pain, nausea, and growth failure.
• In severe cases, cirrhosis or acute liver failure may develop.

#### Diagnosis:

• Presence of autoantibodies (ANA, SMA, anti-LKM1).
• Elevated liver enzymes (ALT, AST) and immunoglobulin G (IgG).
• Liver biopsy showing interface hepatitis.

#### Treatment:

• **First-line Therapy:** Corticosteroids (e.g., prednisone) to suppress the immune response.
• **Maintenance Therapy:** Azathioprine or other immunosuppressants.
• **Liver Transplantation:** For cases with advanced liver damage.

---

### PAHUO and New Autoimmune Hepatitis

1. **PAHUO and Autoimmune Hepatitis Connection:**

• The genetic predisposition observed in PAHUO (e.g., HLA-DRB1*04 alleles) overlaps with known autoimmune diseases, suggesting a potential autoimmune component in PAHUO.
• Viral infections may act as triggers, initiating immune-mediated damage to the liver.

2. **Emerging Concept of "New Autoimmune Hepatitis":**

• PAHUO might represent a novel form of autoimmune hepatitis triggered by viral infections in genetically susceptible children.
• Unlike traditional autoimmune hepatitis, PAHUO is acute and may lack the classic autoantibody profile.

3. **Implications for Research and Treatment:**

• Further studies are needed to confirm the autoimmune basis of PAHUO and its relationship to HLA alleles.
• If confirmed, immunosuppressive therapy could become a key treatment strategy for PAHUO.

---

### Conclusion:

PAHUO is a complex and poorly understood condition affecting children, with emerging evidence pointing to genetic and autoimmune mechanisms potentially triggered by viral infections. The discovery of HLA associations and shared epitopes linked to autoimmune diseases opens new avenues for understanding and managing this condition. Continued global research is essential to validate these findings, improve diagnostic tools, and develop targeted therapies.