3. Hemolytic Uremic Syndrome (HUS)

Quick Recap

Hematology System, Protocol 3/8. Closely related to TTP (prior protocol) — shares the platelet-transfusion-contraindication principle but diverges completely in definitive therapy (complement-directed vs plasma exchange).

1. Definition & Classification

HUS = a thrombotic microangiopathy causing renal-predominant injury, presenting with the classic triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury, often with difficult-to-control hypertension. Two major forms with ENTIRELY DIFFERENT mechanisms and treatments:

• Typical (Shiga-toxin-associated) HUS: the diarrheal form, most common overall
• Atypical HUS (aHUS): complement-mediated, genetic/acquired dysregulation of the alternative complement pathway

This distinction is the single most important branch point in this protocol — typical HUS is managed with supportive care alone, while aHUS has a specific, disease-modifying complement-blocking therapy (eculizumab) that dramatically changes outcomes when correctly identified and used.

2. Shared Features and Precipitants with TTP

Both TTP and HUS share several precipitating factors: HIV infection, malignancy, calcineurin inhibitors, pregnancy, and chemotherapeutic agents. Ticlopidine, and less commonly clopidogrel, are more closely associated with TTP specifically than HUS — a useful drug-history clue when trying to lean toward one diagnosis over the other in an ambiguous presentation. If TTP is suspected, an ADAMTS13 level should be obtained (see TTP protocol) — in HUS, ADAMTS13 activity is typically normal/preserved, distinguishing it mechanistically from TTP even though the clinical pictures overlap substantially.

3. Typical (Shiga-Toxin-Associated) HUS

Mechanism: in the diarrheal form, Shiga-like toxin (from Shiga toxin-producing E. coli, STEC — classically E. coli O157:H7, though other STEC serotypes cause it too) enters the circulation through compromised colonic epithelium -> endothelial injury, inflammation, and thrombosis specifically concentrated in the RENAL microvasculature.

Clinical recognition pearl: many patients with STEC enteric infection are NOT febrile at presentation — painful, BLOODY diarrhea in the ABSENCE of fever should specifically raise STEC/HUS suspicion, rather than being reassuring against a severe bacterial process. Any patient suspected of STEC infection should have a peripheral blood smear evaluated for schistocytes, along with close monitoring of cell counts and renal indices specifically to watch for evolving HUS — proactive surveillance in at-risk diarrheal-illness patients, not waiting for renal failure to declare itself.

Stool testing: cultures or PCR for enteropathogenic bacteria (Salmonella, Shigella, STEC, Campylobacter, Yersinia) warranted in patients with diarrhea PLUS hemodynamic compromise, abdominal pain/tenderness, bloody/mucoid stools, and/or fever.

Treatment: SUPPORTIVE CARE ONLY — no proven efficacy for antibiotics, anticoagulation, immunoglobulin, or plasmapheresis in Shiga-toxin-associated HUS. As a general rule, empiric antibiotic treatment for enteropathogenic bacteria is NOT indicated except in severe symptoms/immunocompromise — ideally defer treatment until a pathogen is isolated, and antibiotic treatment of STEC infection specifically has been associated with INCREASED HUS risk in some studies (theorized to result from increased toxin release with bacterial lysis), reinforcing the general antibiotic-avoidance stance in this specific scenario rather than treating it as a routine bacterial enteritis.

4. Atypical HUS (aHUS) — The Complement-Mediated Form

Mechanism: dysregulation of the ALTERNATIVE COMPLEMENT PATHWAY (genetic mutations in complement regulatory proteins, or acquired autoantibodies against them) -> uncontrolled complement activation -> endothelial injury and thrombotic microangiopathy, without the Shiga toxin trigger of the typical form.

Treatment: eculizumab (humanized monoclonal antibody inhibiting complement factor C5) has been shown to be BENEFICIAL — a disease-modifying, targeted therapy fundamentally different from the supportive-care-only approach to typical HUS, making the typical-vs-atypical distinction genuinely consequential rather than academic.

Diagnostic workup: patients with suspected/confirmed aHUS should undergo genetic testing for mutations in the alternative complement pathway — both to confirm the diagnosis and to inform prognosis/family counseling given the genetic/hereditary component in many cases.

When to suspect aHUS over typical HUS: absence of a preceding diarrheal illness/STEC exposure, recurrent or relapsing thrombotic microangiopathy episodes, family history of similar presentations, or onset in a context (post-transplant, pregnancy, complement-triggering drug exposure) without a clear Shiga-toxin source — though genetic/complement testing is ultimately needed for definitive confirmation.

5. Immediate Stabilization (ABCDE)

Circulation: aggressive management of hypertension, which is often SEVERE and difficult to control in HUS — treat per standard hypertensive emergency principles (see Hypertensive Emergencies protocol, Cardiovascular System) while the underlying microangiopathic process is addressed.

Renal: monitor closely for AKI progression — renal replacement therapy per standard AKI/CRRT indications if renal failure develops (see AKI and CRRT Indications protocols, Renal System); AKI in HUS is often the dominant, most severe organ manifestation, distinguishing it from TTP's relatively greater CNS-predominant pattern.

CRITICAL RULE (shared with TTP): PLATELET TRANSFUSION IS GENERALLY CONTRAINDICATED in thrombotic microangiopathy including HUS — same rationale as TTP (providing substrate for ongoing microthrombosis, risking vaso-occlusive complications) — reserve for life-threatening bleeding only, not for the thrombocytopenia itself.

Checklist:

6. Investigations

CBC with peripheral smear (schistocytes), LDH, haptoglobin, reticulocyte count, direct Coombs test (negative, as in TTP), renal function (often the dominant abnormality), ADAMTS13 activity (to help exclude TTP), stool studies for STEC/enteropathogenic bacteria if diarrheal prodrome present, complement pathway genetic testing if aHUS suspected, urinalysis.

7. Organ Support

Renal replacement therapy per standard AKI indications (a common need given HUS's renal-predominant severity); aggressive antihypertensive therapy; supportive care for typical HUS; eculizumab for confirmed/strongly suspected aHUS; standard ICU supportive care; avoid platelet transfusion absent life-threatening bleeding.

8. Consultation Matrix

9. Monitoring Framework

Serial CBC/platelet count, LDH trend, renal function trend (the primary severity marker in HUS), blood pressure (often severely elevated and difficult to control), urine output, watch for progression to RRT-dependence.

10. Complications

Severe AKI/RRT-dependence, severe/refractory hypertension, CNS involvement (less prominent than TTP but can occur), progression to chronic kidney disease even after acute recovery (particularly in aHUS or severe typical HUS), complications of inappropriate platelet transfusion if given. Prevention: correct typical-vs-atypical distinction driving appropriate therapy, antibiotic avoidance in suspected STEC enteritis, aggressive BP control, avoiding platelet transfusion absent bleeding. Rescue: RRT for renal failure, eculizumab for confirmed aHUS, standard hypertensive emergency management for severe BP elevation.

11. Escalation & De-escalation

Escalate: progressive renal failure -> RRT per standard indications; suspected aHUS not responding to supportive care alone -> eculizumab initiation.

De-escalate: renal function stabilizing/improving, platelet count normalizing, LDH trending down -> wean RRT if applicable per recovery trajectory, continue/complete eculizumab course if aHUS confirmed, transition to nephrology follow-up given CKD risk.

12. ICU Discharge Criteria

Renal function stable or on an established RRT/recovery trajectory, hypertension controlled, platelet count and LDH normalized/normalizing, typical vs atypical classification established with an appropriate treatment/follow-up plan, nephrology and hematology follow-up arranged.

13. Documentation & Medicolegal Checklist

14. Key Guidelines

George JN, Nester CM. Syndromes of thrombotic microangiopathy. N Engl J Med. 2014;371(7):654-666. Loirat C, Fakhouri F, Ariceta G, et al. An international consensus approach to the management of atypical hemolytic uremic syndrome in children. Pediatr Nephrol. 2016;31(1):15-39 (principles extrapolated to adult management given shared complement biology).

15. Controversies

The association between antibiotic treatment of STEC enteritis and increased HUS risk, while supported by some studies, is not universally accepted or proven causal, and practice varies in how strictly antibiotic avoidance is applied in mild/early STEC-suspected presentations. The precise duration of eculizumab therapy in confirmed aHUS (indefinite vs time-limited with monitoring for relapse) remains an evolving area of clinical practice and ongoing study.

16. References

1. Glomerular and Microvascular Renal Disorders (HUS section). Washington Manual of Critical Care, 4th ed, 2025 (Ch. 61).
2. Enteric Infections (STEC/HUS surveillance section). Washington Manual of Critical Care, 4th ed, 2025 (Ch. 45).
3. George JN, Nester CM. Syndromes of thrombotic microangiopathy. N Engl J Med. 2014;371(7):654-666.
4. Loirat C, Fakhouri F, Ariceta G, et al. An international consensus approach to the management of atypical hemolytic uremic syndrome. Pediatr Nephrol. 2016;31(1):15-39.
5. Legendre CM, Licht C, Muus P, et al. Terminal complement inhibitor eculizumab in atypical hemolytic-uremic syndrome. N Engl J Med. 2013;368(23):2169-2181.

See also: TTP (Hematology System) for the essential differential diagnosis and shared platelet-transfusion-avoidance principle; Acute Kidney Injury and CRRT Indications (Renal System) for the renal support framework this protocol frequently requires.