HLH Secondary to Influenza
Authors: Trivedi Krunal , Shaaban Hamid , Abo Stephen , Khan Nibras yar 
Published: December 2023

Introduction
Hemophagocytic lymphohistiocytosis (HLH) is a pathological condition characterized by excessive inflammation and tissue damage resulting from an aberrant immune system response. HLH can be classified into two distinct categories: primary HLH and secondary HLH. Secondary hemophagocytic lymphohistiocytosis (HLH) is an acquired condition that can be precipitated by various infections, malignancies, or rheumatological disorders. Herein, we present a rare case of HLH secondary to influenza B infection.
Case Presentation
A 37-year-old who routinely vapes and smokes cannabis presented to the emergency department with subjective fevers, body pains, fatigue, productive cough, shortness of breath, nausea, non-bloody, non-bilious vomiting, and watery diarrhea for one week.
He was tachypneic, tachycardic, and febrile (101.1 F). He had loud bibasilar crackles on auscultation. Hb 14.8, total WBC 2.2, platelets 170, AKI with serum creatinine 1.9, and transaminitis with AST/ALT/ALP 130/56/122 U/L were the patient's initial lab results. Rapid influenza, strep, and COVID-19 antigen tests were negative. He exhibits hypoxemia with pH 7.46, pCO2 32.5, pO2 52.1, HCO3 22.9, and FIO2 40%. Urinalysis showed nonsignificant. CRP was 8.3 mg/dL. Negative procalcitonin. chest x-ray and ct showed bilateral diffuse patchy airspace infiltration, suggesting infectious process with mild pleaural effusion.
In the critical care unit, he received Antibiotics and inhaled therapy for acute hypoxic respiratory failure with severe ARDS. After deteriorating, he was intubated. The respiratory pathogen panel detected Influenza B. Blood cultures and remaining pneumonia workup were negative. The lab results suggested HLH (table 1). Autoimmune tests were negative. Oseltamivir was given for 5 days. Repeat chest x-rays showed stable pulmonary infiltrates in the right middle and lower lobes, but the patient decompensated clinically with increasing ventilator requirements and dyssynchrony despite heavy sedation, paralytics, and proning and was transferred to a different hospital for ECMO evaluation on day 12.
Discussion
Primary and secondary HLH: Secondary HLH is caused by infections, malignancies, or rheumatological illnesses, while primary HLH is hereditary.
The mortality outcome of HLH is extremely dependent on early detection and treatment 1.
Due to the rarity of this disease entity, difficult diagnostic criteria, and concern for alternate diagnoses, internists may be less aware of it. Further complicating the diagnosis is the fact that the first bone marrow biopsy may not always show hemophagocytosis 2. This is why it is important to be cognizant of this diagnosis and it's diagnostic criteria.
Our patient was suspected of HLH based on HScore for reactive hemophagocytic syndrome and the newest updated HLH diagnostic criteria (5 of 8 criteria should be met) 6.
EBV is the most common viral infection which can cause HLH. CMV, Parvo, HSV, and H1N1 are other infections. HLH-Influenza B connection is poorly documented 7 Only one case of HLH induced by influenza B in a healthy patient was found in literature. To our knowledge, Our 38-year-old patient is the second known case of secondary HLH secondary to Influenza B in a healthy patient. Influenza B seldom causes HLH. This connection in adulthood is poorly researched.
Patients with influenza virus infections have significant intestinal apoptosis and reactive hemophagocytic activity in bone marrow, lungs, and liver3. Influenza variants reduce CD8+ T cell cytotoxicity and perforin expression, likely causing HLH syndrome 4.
A study suggests that virus-associated hemophagocytic syndrome (VAHS) is a common consequence of severe 2009 influenza A (H1N1) infection and contributes to multiorgan failure and mortality. 9 (36%) of 25 severely ill patients with confirmed 2009 influenza A (H1N1) infection developed VAHS, and 8 (89%) died 5.
Conclusion:
We present this case to raise awareness among physicians regarding the potential occurrence of hemophagocytic lymphohistiocytosis (HLH) as a complication in adult patients who present with influenza B virus infection accompanied by fever and pancytopenia. The aim is to emphasize the importance of considering HLH as a differential diagnosis in order to facilitate timely identification and initiation of appropriate treatment for this condition.