COVID-19 Associated Atypical Hemolytic Uremic Syndrome

Introduction: Atypical hemolytic uremic syndrome (atypical HUS) is a complement-mediated thrombotic microangiopathy (TMA) that affects multiple organs including the kidneys. We present a case of 37-year-old male with recent COVID-19 infection who presented with epistaxis. His blood work was consistent with TMA and acute kidney injury (AKI). Hemodialysis and plasmapheresis (PLEX) were initiated for presumed thrombotic thrombocytopenic purpura (TTP) but after excluding ADAMTS-13 deficiency, eculizumab was started for possible atypical HUS. Renal biopsy and genetic testing ultimately confirmed diagnosis of atypical HUS.

Case presentation: A 37-year-old male with past medical history of recent COVID infection presented to the emergency room with complaint of epistaxis. Physical exam showed mild epistaxis from right naris and bilateral leg swelling. Initial laboratory findings were hemoglobin (Hb) 5 g/dL, platelets 70 B/L, Lactate dehydrogenase (LDH) 581 IU/L, haptoglobin < 10 mg/dL, creatinine (Cr) 36.80 mg/dL, blood urea nitrogen (BUN) 232 mg/dL, potassium (K) 6.6 mmol/L, calcium (Ca) 6 mg/dL and normal prothrombin time (PT) and partial thromboplastin time (PTT). Complement C3 levels were low but normal C4 levels. Peripheral blood smear showed schistocytes confirming TMA. He was started on both PLEX and hemodialysis (HD) along with prednisone 80 mg daily. PLEX was stopped on day 4 after normal ADAMTS-13 levels resulted, helping rule out TTP. Based on history and laboratory findings, diagnosis of atypical HUS was favored and eculizumab was started. He received meningococcal vaccine and prophylactic penicillin while on eculizumab. Hemodialysis was continued with eventual improvement of his labs to Hb 7, platelets 119, LDH 339, haptoglobin < 10, Cr 9.07, BUN 85, K 5.8, and Ca 8.0. He was discharged on bimonthly dose of eculizumab and hemodialysis three times a week. Two months later, his labs were Hb 10.4, platelets 184, LDH 490, haptoglobin < 10, Cr 11.60, BUN 82, K 5.2, and Ca 9.2. PT and PTT remained normal. Given persistence of laboratory findings, renal biopsy and genetic testing for complement proteins was done. Kidney biopsy showed diffuse global renal microangiopathy with negative C3. Genetic testing showed a heterozygous C3 variant, homozygous deletion of CFHR1-CFHR4, negative factor H autoantibodies (FHAA), and single nucleotide variant in C5 gene.

Discussion: Atypical HUS is a disease characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and AKI. The complement system is a part of the immune system that enhances the ability of phagocytic cells to clear microbes and damaged cells. Atypical HUS is caused by dysregulation of the alternative complement pathway that leads to microvascular thrombosis. Various mutations in complement and its regulatory proteins have been discovered in atypical HUS patients. Heterozygous C3 variants result in persistently low C3 levels and cause severe disease. Most individuals with a homozygous deletion of CFHR1 and/or CFHR3, CFHR4 genes have a pathogenic role in the development of the anti-factor H autoantibodies. These autoantibodies interfere with the binding of CFH to the C3 convertase and are associated with defective CFH-dependent cell protection. Some case reports of COVID-19 patients showed that complement activation was significantly stronger in lung alveolar septa, hyaline membranes, and in alveolar lumina via the lectin and alternative complement pathways.

Treatment of atypical HUS is symptomatic but in severe disease eculizumab or ravulizumab have roles by binding to the complement protein C5, inhibiting its cleavage to C5a and C5b, and preventing formation of terminal complement complex C5b-9. Patients on these therapies are at risk of meningococcal disease, thus it is recommended to get meningococcal vaccines and prophylactic penicillin for the duration of therapy.

Our patient had homozygous deletion of CFHR1-CFHR4 genes with no factor H autoantibodies. However, he did have C3 variant with gain of function mutation which caused overt activation of alternative pathway as indicated by his low C3 levels with normal C4 levels. He responded partially to eculizumab and discussion on possible kidney transplant has been started.

No relevant conflicts of interest to declare.