Atypical Hemolytic Uremic Syndrome

End-Stage Kidney Disease Resulting from Atypical Hemolytic Uremic Syndrome after Receiving AstraZeneca SARS-CoV-2 Vaccine: A Case Report

Hemolytic uremic syndrome (HUS) is classically described as a triad of nonimmune hemolytic anemia, thrombocytopenia, and acute kidney injury. Atypical HUS (aHUS) is a rare variant of the disease, and it accounts for 5-10% of the cases. It has a poor prognosis, with a mortality rate exceeding 25% and a more than 50% chance of progressing into end-stage kidney disease (ESKD). Genetic or acquired dysregulation of the alternative complement pathway is highly implicated in the pathogenesis of aHUS. Multiple triggers for aHUS have been described in the literature, including pregnancy, transplantation, vaccination, and viral infections. Herein, we report a case of a previously healthy 38-year-old male who developed microangiopathic hemolytic anemia and severe kidney impairment one week after receiving the first dose of AstraZeneca SARS-CoV-2 vaccine. A diagnosis of aHUS was made after excluding other causes of thrombotic microangiopathies. Treatment with plasma exchange, prednisone, and rituximab (375 mg/m2) once weekly for four doses resulted in improvement of his hematological parameters. However, he progressed to ESKD.

Thrombotic microangiopathies in critically ill children: The MATUCIP registry in Spain

INTRODUCTION

Thrombotic microangiopathies (TMA) are rare diseases usually presenting with renal, haematological, neurologic and cardiovascular involvement and nonspecific but severe symptoms. A registry of TMA cases managed in Spanish paediatric intensive care units (the MATUCIP Registry) was established with the aim of gaining knowledge on their clinical characteristics, diagnosis and acute-phase treatment.

METHODS

We conducted a prospective multicentre observational study in 20 paediatric intensive care units (PICUs) in Spain from January 2017 to December 2021 in children aged more than 1 month with TMAs, who were followed up through the discharge from the PICU.

RESULTS

The sample included 97 patients (51.5% female) with a median age of 2.6 years (interquartile range [IQR], 1.6-5.7). The initial manifestations were gastrointestinal (74.2%), respiratory (14.4%), fever (5.2%), neurologic (3.1%) and other (3.1%). At admission, 75.3% of patients had microangiopathic haemolytic anaemia, 95.9% thrombocytopenia and 94.8% acute kidney injury. Of the total sample, 57.7% of patients received a diagnosis of Shiga toxin-associated haemolytic uraemic syndrome (HUS), 14.4% of Streptococcus pneumoniae-associated HUS, 15.6% of atypical HUS, 10.3% of secondary TMA and 2.1% of thrombotic thrombocytopenic purpura. Eighty-seven patients (89.7%) developed arterial hypertension, and 49.5% gastrointestinal, 22.7% respiratory, 25.8% neurologic and 12.4% cardiac manifestations. Also, 60.8% required renal replacement therapy and 2.1% plasma exchange. Twenty patients received eculizumab. The median PICU stay was 8.5 days (IQR, 5-16.5). Two children died.

CONCLUSIONS

The MATUCIP registry demonstrates the clinical variability of TMA cases requiring admission to the PICU. Knowledge of the presentation and outcomes of TMAs can facilitate early aetiological diagnosis. This registry can help improve our understanding of the clinical spectrum of these diseases, for which there is a dearth of published data.

The treatment of atypical hemolytic uremic syndrome with eculizumab in pediatric patients: a systematic review

BACKGROUND

The atypical hemolytic uremic syndrome (aHUS) is a rare form of thrombotic microangiopathy associated with high morbidity and high mortality. Eculizumab, a humanized anti-C5 monoclonal antibody, was the first medication approved for treating aHUS in 2011.

OBJECTIVE

The objective of this study is to evaluate the efficacy and safety of eculizumab treatment in pediatric patients with aHUS.

DATA SOURCES

We consulted PubMed, Scopus, SciELO, and Cochrane Library databases in July 2021. The descriptors were as follows: "Atypical Hemolytic Uremic Syndrome," "aHUS," "eculizumab," "Pediatrics," "Pediatric," "Child," "Children," "Adolescent."

STUDY ELIGIBILITY CRITERIA

The study eligibility criteria are as follows: clinical trials and observational studies that included pediatric patients with aHUS diagnosis and who were treated with eculizumab.

PARTICIPANTS AND INTERVENTIONS

The participants are pediatric patients, up to 18 years old, with aHUS. The intervention was eculizumab treatment.

STUDY APPRAISAL

For quality assessment, we used the Newcastle-Ottawa Scale, the National Institutes of Health (NIH) quality assessment tool for case series studies, and the Risk of Bias In Non-Randomized Studies of Interventions (ROBINS-I) tool.

RESULTS

The initial search retrieved 433 studies, from which 15 were selected after complete assessment: 9 cohorts, 4 case series, and 1 clinical trial. The publication date ranged from 2015 to 2021. In total, 940 pediatric patients were included, and 682 received eculizumab. All studies reported improvements in renal and hematological parameters in most of the patients treated with eculizumab. The mortality rate was 1.6% for all patients treated with eculizumab.

LIMITATIONS

The number of studies is limited, and the included studies were methodologically heterogeneous. The studies were mostly observational and many had small sample sizes.

CONCLUSIONS

Eculizumab appears to be safe and effective for the treatment of aHUS in pediatric patients. More research is necessary to establish long-term efficacy, safety, and time of discontinuation.

SYSTEMATIC REVIEW REGISTRATION NUMBER

CRD42021266255.

Trends and Outcomes of Hospitalizations Due to Hemolytic Uremic Syndrome: A National Perspective

BACKGROUND

Hemolytic uremic syndrome (HUS) is a rare but challenging disease with varying degrees of mortality and prognosis. We aim to evaluate the trends and outcomes of hospitalizations due to HUS by utilizing a large population-based dataset.

METHODS

We derived a study cohort from the Nationwide Inpatient Sample (NIS) for the years 2007-2018. Our primary outcomes were in-hospital mortality, discharge disposition, and predictors of poor outcomes. We then utilized the Cochran Armitage trend test and multivariable survey logistic regression models to analyze the trends, outcomes, and predictors.

RESULTS

A total of 8043 hospitalizations ranging from age zero to above 65 years of age occurred due to HUS from 2007-2018. The number of hospitalizations with HUS increased steadily from 528 in 2007 to 800 in 2013, but afterwards, we noticed a steady decline to 620 in 2018. Additionally, trends of in-hospital mortality slowly increased over the study period but we noticed a decline in the rate of discharge to skilled nursing facilities (SNFs). Furthermore, in multivariable regression analysis, predictors of increased mortality in hospitalized HUS patients were advanced age (95%CI: 1.221-1.686; p-value <0.0001) and requirement for dialysis (95%CI: 1.141-4.167; p-value: <0.0001). Advanced age >65 years (OR: 2.599, 95%CI: 1.406-4.803; p-value: 0.0023), as well as comorbidities such as diabetes mellitus and pulmonary circulatory diseases, which are under vascular events (OR: 1.467, 95%CI:1.075-2.000; p-value: 0.0156), were shown to have a higher rate of discharge to SNFs. Moreover, patients needing intravenous immunoglobulin (IVIG) and plasmapheresis had high odds of discharge to SNFs ((OR: 1.99, 95%CI: 1.307-3.03; p-value: 0.0013) and (OR: 5.509, 95%CI: 2.807- 10.809; p-value <0.0001), respectively), as well as smaller hospital bed size and hospital type (OR: 1.849, 95%CI: 1.142-2.993; p-value: 0.012).

CONCLUSION

In this national representative study, we observed a total decrease in hospitalizations as well as discharge to SNFs; however we saw an increase in inpatient mortality. We also identified multiple predictors significantly associated with increased mortality, some of which are potentially modifiable and can be points of interest for future studies.

Ravulizumab 100 mg/mL formulation reduces infusion time and frequency, improving the patient and caregiver experience in the treatment of atypical haemolytic uraemic syndrome

WHAT IS KNOWN AND OBJECTIVE

The C5 inhibitor eculizumab is the standard of care for treatment of atypical haemolytic uraemic syndrome (aHUS). Ravulizumab, a next-generation C5 inhibitor, was engineered to have a longer terminal half-life than eculizumab. We describe practical benefits of the advanced ravulizumab 100 mg/mL formulation.

COMMENT

Use of ravulizumab results in fewer maintenance infusions per year (25%-50%) compared with eculizumab. Maintenance infusion time of ravulizumab 100 mg/mL is 2-4 times shorter than ravulizumab 10 mg/mL in all weight cohorts and approximately half that of eculizumab for patients weighing <40 kg. Ravulizumab 100 mg/mL requires fewer vials annually than eculizumab in most weight cohorts.

WHAT IS NEW AND CONCLUSION

With ravulizumab 100 mg/mL, patients and caregivers experience fewer infusions per year and decreased annual infusion times, improving infusion experience. Infusion centres can expect corresponding decreases in resource utilization.

Long-Term Care of the Pediatric Kidney Transplant Recipient

Pediatric kidney transplant recipients are distinguished from adult recipients by the need for many decades of graft function, the potential effect of CKD on neurodevelopment, and the changing immune environment of a developing human. The entire life of an individual who receives a transplant as a child is colored by their status as a transplant recipient. Not only must these young recipients negotiate all of the usual challenges of emerging adulthood (transition from school to work, romantic relationships, achieving independence from parents), but they must learn to manage a life-threatening medical condition independently. Regardless of the age at transplantation, graft failure rates are higher during adolescence and young adulthood than at any other age. All pediatric transplant recipients must pass through this high-risk period. Factors contributing to the high graft failure rates in this period include poor adherence to treatment, potentially exacerbated by the transfer of care from pediatric- to adult-oriented care providers, and perhaps an increased potency of the immune response. We describe the characteristics of pediatric kidney transplant recipients, particularly those factors that may influence their care throughout their lives. We also discuss the risks associated with the transition from pediatric- to adult-oriented care and provide some suggestions to optimize the transition to adult-oriented transplant care and long-term outcomes.

Complement Activation Fragments Are Increased in Critically Ill Pediatric Patients with Severe AKI

Background

Children who are critically ill with AKI suffer from high morbidity and mortality rates, and lack treatment options. Emerging evidence implicates the role of complement activation in AKI pathogenesis, which could potentially be treated with complement inhibitors. The purpose of this study is to evaluate the association between complement activation fragments and severity of AKI in children who are critically ill.

Methods

A biorepository of samples from children who are critically ill from a prior multisite study was leveraged to identify children with stage 3 AKI and matched to patients without AKI on the basis of PELOD-2 (illness severity) scores. Specimens were analyzed for plasma and urine complement activation fragments of factor B, C3a, C4a, and sC5b-9. The primary outcomes were MAKE30 and severe AKI rates.

Results

In total, 14 patients with stage 3 AKI (five requiring RRT) were matched to 14 patients without AKI. Urine factor Ba and plasma C4a levels increased stepwise as severity of AKI increased, from no AKI to stage 3 AKI, to stage 3 AKI with RRT need. Plasma C4a levels were independently associated with increased risk of MAKE30 outcomes (OR, 3.2; IQR, 1.1-8.9), and urine Ba (OR, 1.9; IQR, 1.1-3.1), plasma Bb (OR, 2.7; IQR, 1.1-6.8), C4a (OR, 13.0; IQR, 1.6-106.6), and C3a (OR, 3.3; IQR, 1.3-8.4) were independently associated with risk of severe stage 2-3 AKI on day 3 of admission.

Conclusions

Multiple complement fragments increase as magnitude of AKI severity increases. Very high levels of urine Ba or plasma C4a may identify patients at risk for severe AKI, hemodialysis, and MAKE30 outcomes. The fragments may be useful as a functional biomarker of complement activation and may identify those patients to study complement inhibition to treat or prevent AKI in children who are critically ill. These findings suggest the need for further specific investigations of the role of complement activation in children who are critically ill and at risk of AKI.

[Thrombotic Microangiopathy (TMA) after Gene Replacemant Therapy (GRT) due to Spinal Muscular Atrophy: Case Summary and Recommendations for Treatment]

INTRODUCTION

5q-associated spinal muscular atrophy is one of the most severe and common genetic diseases. In the last few years, innovative methods of therapy have been developed based on SMN2 gene modification, such as splicing, or replacement of the damaged SMN1 gene (gene replacement therapy, GRT). GRT is known to be accompanied by off target effects like temporary elevation of liver and cardiac enzymes usually without serious clinical relevance. We report a 4-year-old girl suffering from thrombotic microangiopathy (TMA) after GRT due to 5q- SMA.

CASE SUMMARY

A 4-year-old girl developed TMA indicated by haemolytic anemia and thrombocytopenia in conjunction with renal failure 7 days after GRT with onasemnogene abeparvovec. The latter was characterized by a rise in serum creatinine, oliguria, hypertension, protein- and haematuria, and oedema. The patient was started on eculizumab and antihypertensives resulting in normalization of haemolytic activity, platelet count, kidney function and blood pressure within one week.

RECOMMENDATION AND CONCLUSION

SMA patients receiving GRT should undergo close monitoring for early detection of TMA. Adequate measures for TMA including eculizumab or plasmapheresis as well as renal replacement therapy should be available without delay in order to avoid progressive kidney disease or other severe complications in these patients. Careful follow-up including assessment of proteinuria and blood pressure is recommended since patients may require antihypertensive/nephroprotective treatment to avoid chronic kidney disease in later life. Therefore, GRT in SMA patients should only be performed at centers with neuropediatric and paediatric nephrology expertise.

An Imperfect Marker: SLE and TTP-Like MAHA Without Low ADAMTS13

Thrombotic thrombocytopenic purpura (TTP) is a potentially fatal condition that can be challenging for clinicians to identify in the setting of autoimmune diseases such as systemic lupus erythematosus (SLE). This difficulty is compounded when a patient presents with all of the clinical signs of a TTP-like microangiopathy, however, with near normal ADAMTS13. This case report describes a 44-year-old female with a history of SLE who was hospitalized with acute on chronic anemia, thrombocytopenia, and altered mental status. The patient's ADAMTS13 was mildly low; hence, she was initially treated for SLE-associated immune thrombocytopenic purpura without any clinical response. The patient then underwent plasmapheresis (plasma exchange [PLEX]) for treatment of a suspected TTP-like microangiopathy. She responded to PLEX with improvement in her platelet count and mental status. This case illustrates the importance of considering TTP-like microangiopathic hemolytic anemia in the differential for patients with a history of SLE presenting with clinical signs of TTP even in the setting of near-normal ADAMTS13, thus warranting prompt treatment with PLEX.

Thrombotic Microangiopathy After Kidney Transplantation: An Underdiagnosed and Potentially Reversible Entity

Thrombotic microangiopathy is a rare but serious complication that affects kidney transplant recipients. It appears in 0.8–14% of transplanted patients and negatively affects graft and patient survival. It can appear in a systemic form, with hemolytic microangiopathic anemia, thrombocytopenia, and renal failure, or in a localized form, with progressive renal failure, proteinuria, or arterial hypertension. Post-transplant thrombotic microangiopathy is classified as recurrent atypical hemolytic uremic syndrome or de novo thrombotic microangiopathy. De novo thrombotic microangiopathy accounts for the majority of cases. Distinguishing between the 2 conditions can be difficult, given there is an overlap between them. Complement overactivation is the cornerstone of all post-transplant thrombotic microangiopathies, and has been demonstrated in the context of organ procurement, ischemia-reperfusion phenomena, immunosuppressive drugs, antibody-mediated rejection, viral infections, and post-transplant relapse of antiphospholipid antibody syndrome. Although treatment of the causative agents is usually the first line of treatment, this approach might not be sufficient. Plasma exchange typically resolves hematologic abnormalities but does not improve renal function. Complement blockade with eculizumab has been shown to be an effective therapy in post-transplant thrombotic microangiopathy, but it is necessary to define which patients can benefit from this therapy and when and how eculizumab should be used.

Thrombotic Microangiopathy Following Onasemnogene Abeparvovec for Spinal Muscular Atrophy: A Case Series

Spinal muscular atrophy is treated with onasemnogene abeparvovec, which replaces the missing survival motor neuron 1 gene via an adeno-associated virus vector. As of July 1, 2020, we had identified 3 infants who developed thrombotic microangiopathy following onasemnogene abeparvovec. Early recognition and treatment of drug-induced thrombotic microangiopathy may lessen mortality and morbidity.

Neonatal Atypical Hemolytic Uremic Syndrome in the Eculizumab Era

The atypical hemolytic uremic syndrome (aHUS) in the newborn is a rare disease, with high morbidity. Eculizumab, considered a first-line drug in older children, is not approved in neonates and in children weighing less than 5 kg. We present a 5-day-old female newborn, born at 36 weeks' twin gestation, by emergency cesarean section due to cord prolapse, with birth weight of 2,035 g and Apgar score of 7/7/7, who develops microangiopathic hemolytic anemia, thrombocytopenia, and progressive acute renal failure. In day 5, after diagnosis of aHUS, a daily infusion of fresh frozen plasma begins, with improvement of thrombocytopenia and very slight improvement in renal function. The etiologic study (congenital infection, Shiga toxin, ADAMTS13 activity, directed metabolic study) was normal. C3c was slightly decreased. On day 16 for maintenance of anemia and severe renal failure, she started 300 mg/dose eculizumab. Anemia resolves in 10 weeks and creatinine has normal values after 13 weeks of treatment. The genetic study was normal. In this case, eculizumab is effective in controlling microangiopathy and in the recovery of renal function. Diagnosis of neonatal aHUS can be challenging because of phenotypic heterogeneity and potential overlap with other manifestations that may confound it, such as perinatal asphyxia or sepsis/disseminated intravascular coagulation.

Atypical Hemolytic Uremic Syndrome: New Challenges in the Complement Blockage Era

Atypical hemolytic uremic syndrome (aHUS) is a rare cause of thrombotic microangiopathy (TMA), characterized by microangiopathic hemolytic anemia, consumptive thrombocytopenia, and multisystem end organ involvement, most commonly affecting the kidney. Diagnosis is clinical, after exclusion of other TMA causes. Primary aHUS arises from genetic abnormalities, resulting in uncontrolled complement activity, while a variety of clinical scenarios cause secondary aHUS, including infection, pregnancy, malignancy, autoimmune disease, and medications. They can also induce a temporary complement deregulation with an overlap between both scenarios, which can make differential diagnosis difficult. Primary aHUS can be sporadic or familial and is associated with a high rate of progression to ESRD. Many aHUS patients relapse in the native or transplanted kidneys, leading to kidney failure. The introduction of eculizumab has changed the prognosis of aHUS, by inducing hematologic remission, improving or stabilizing kidney functions, and preventing graft failure. The early institution of appropriate therapy can prevent multiorgan damage, so is essential to recognize and differentiate the TMA syndromes. Eculizumab is considered now the first-line treatment, and it is recommended lifelong therapy. However, the high cost of therapy has led to make efforts to develop precise complement functional and genetic studies that help physicians to determine the appropriate duration of eculizumab therapy. Nowadays, more studies are needed to select candidates to adjustment of therapy.

Whole exome sequencing revealed a novel homozygous variant in the DGKE catalytic domain: a case report of familial hemolytic uremic syndrome

Background

Atypical hemolytic uremic syndrome (aHUS) is a rare disease characterized by microangiopathic hemolytic anemia caused by small vessel thrombosis, thrombocytopenia, and renal failure. The common cause of aHUS is a dysregulation in the alternative complement pathway. Mutations in none complement genes such as diacylglycerol kinase epsilon (DGKE) can also result in this syndrome.

Case presentation

Here, we report on a 19-year-old female with the clinical diagnosis of aHUS, who has unaffected consanguineous parents and an older sibling who was deceased from aHUS when she was seven months old.

We performed whole exome sequencing (WES) followed by evaluation of detected variants for functional significance, using several online prediction tools. Next, in order to confirm the detected pathogenic variant in proband and segregation analysis in her family, Sanger sequencing was done. The novel variant was analyzed in terms of its impact on the protein 3-dimensional structure by computational structural modeling.

The results revealed that the proband carried a novel homozygous missense variant in DGKE located in exon 6 of the gene (NM_003647.3, c.942C > G [p.Asn314Lys]), and in silico analysis anticipated it as damaging. Protein computational study confirmed the influence of potential pathogenic variant on structural stability and protein function.

Conclusion

We suggest that some variations in the catalytic domain of DGKE like p.Asn314Lys which can cause alterations in secondary and 3-D structure of protein, might lead to aHUS.

Nephrotic syndrome associated with primary atypical hemolytic uremic syndrome

Primary atypical hemolytic-uremic syndrome is a rare disease characterized by non-immune microangiopathic hemolytic anemia, thrombocytopenia, and renal dysfunction; it is related to alterations in the regulation of the alternative pathway of complement due to genetic mutations. The association with nephrotic syndrome is unusual. We present here a pediatric patient diagnosed with primary atypical hemolytic-uremic syndrome associated with nephrotic syndrome who responded to eculizumab treatment.

CFH and CFB mutations in Shiga toxin-associated haemolytic uraemic syndrome in a 6-year-old boy

Haemolytic uraemic syndrome (HUS) is most commonly associated with Shiga toxin-producing Escherichia coli (STEC) while the recurrent hereditary atypical (aHUS) form secondary to complement system control protein mutations is relatively rare. A 6-year-old boy with complement factor H (CFH) and factor B (CFB) mutations and a history of bloody diarrhoea and PCR positivity for Shiga toxin was initially diagnosed as STEC+HUS. Acute kidney injury resolved with Eculizumab but he remains with chronic renal failure. Although the exact role of STEC in the pathogenesis of aHUS in this patient is not certain, there seems to be a relationship. However, several issues remain to be explained including the effect of genetic and environmental factors in modifying susceptibility to develop aHUS in some patients following STEC infection.Abbreviations: aHUS: atypical haemolytic uraemic syndrome; ANA: anti-nuclear antibody; ANCA: anti-neutrophil cytoplasmic antibody; ASO: anti-streptolysin O; BUN: blood urea nitrogen; CFB: complement factor B; CFH: complement factor H; EHEC: enterohaemorrhagic Escherichia coli; MCP: membrane co-factor protein; PD: peritoneal dialysis; STEC: Shiga toxin-producing Escherichia coli; STX 1-2: Shiga toxins 1-2.

Case Report: Denys-Drash Syndrome With WT1 Causative Variant Presenting as Atypical Hemolytic Uremic Syndrome

The WT1 variant is confirmed to be pathogenic for Denys-Drash syndrome (DDS), a rare disorder characterized by early-onset nephrotic syndrome and renal failure, pseudo-hermaphroditism, and a high risk of Wilms' tumor. Several cases of DDS presenting with atypical hemolytic uremic syndrome (aHUS) have been reported. Here we report the case of a 2-year-old child who was diagnosed with WT1 missense variant, associated with DDS and initial presentation of aHUS. Complement factor H autoantibodies were negative. Complement regulatory system-related gene variants were not found, but a de novo heterozygous c.754G>A missense variant in exon 9 of WT1 gene was detected, resulting in a p. Asp252Asn substitution, by next-generation sequencing. The patient was a female morphologically but proved to be a genetic male because of karyotype 46, XY with normally developed female external genitalia. Bilateral nephrectomy and renal transplantation were performed 1 year later, and there was no recurrence of aHUS at 10 months after transplantation.

A severe case of bevacizumab-induced thrombotic microangiopathy

Bevacizumab is a human monoclonal antibody against VEGF. Many adverse reactions about bevacizumab are identified including venous and arterial thromboembolism, nausea, fatigue, alopecia, proteinuria, gastrointestinal system (GIS) perforation, exfoliative dermatitis, and peripheral edema and thrombocytopenia. In this paper, we report a case of bevacizumab-induced dose-dependent, toxicity-mediated drug-induced thrombotic microangiopathy.

The chemical and laboratory investigation of hemolysis

The abnormal breakdown of circulating red blood cells (RBCs), also known as hemolysis, is a significant clinical issue that can present as a primary disorder or arise secondary to another disease process. The evaluation for pathologic hemolysis (and the establishment of a hemolytic disorder) is heavily dependent on assays performed and overseen by the divisions of Hematology, Blood Bank/Transfusion Medicine, Clinical Chemistry, and Immunology in the clinical laboratory. Because of the wide variety of assays used across the spectrum of clinical pathology and potential pitfalls/limitations associated with this testing, the decision of which assay to choose and, perhaps more importantly, how to interpret results, can both be quite challenging. Thus, the aim of this manuscript is to provide a comprehensive review on the laboratory investigation of pathologic forms of hemolysis and hemolytic disorders. This chapter will: (1) introduce basic concepts on the pathophysiology of hemolysis and (2) examine assays available for hemolysis on a laboratory-by-laboratory basis, with a particular emphasis on the strengths, limitations, and clinical interpretations of each of these assays.

Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is the clinical triad of thrombocytopenia, anemia, and acute kidney injury. Classically associated with enterocolitis from Shiga toxin-producing Escherichia coli, HUS is also associated with Streptococcus pneumoniae infections; genetic dysregulation of the alternative complement pathway or coagulation cascade; and, rarely, a hereditary disorder of cobalamin C metabolism. These share a common final pathway of a prothrombotic and proinflammatory state on the endothelial cell surface, with fibrin and platelet deposition. Much work has been done to distinguish between the different mechanisms of disease, thereby informing the optimal therapeutic interventions for each entity.