Recommendations for diagnosis and treatment of Atypical Hemolytic Uremic Syndrome (aHUS): an expert consensus statement from the Rare Diseases Committee of the Brazilian Society of Nephrology (COMDORA-SBN)

Abstract

Atypical hemolytic uremic syndrome (aHUS) is a rare cause of thrombotic microangiopathy (TMA) caused by the dysregulation of the alternative complement pathway. The diagnosis of TMA is made clinically by the triad: microangiopathic hemolytic anemia, thrombocytopenia, and organ damage (mainly acute kidney injury). The heterogeneity of clinical manifestation and the lack of a gold standard diagnostic test makes the precise diagnosis of aHUS a challenging process that may impact patient management. Until one decade ago, there was no specific treatment for aHUS and patients were submitted to plasma therapy (plasma exchange and/or plasma infusion) and/or liver transplantation, procedures that are not free of serious complications and that do not address the underlying pathophysiology of the disease. Since 2011, an anti-C5 complement monoclonal antibody has been approved by the Food and Drug Administration (FDA) for aHUS patients beginning a new era in treatment. Clinical trials on new complement inhibitors may also add to the treatment portfolio in the future. The Brazilian population is a mixed race with a unique genetic and clinical profile. This consensus aims to offer recommendations for the diagnosis and treatment of patients with aHUS in this population based on expert experience, data from the aHUS Brazilian Registry and literature review. The GRADE system was used to classify the quality of the evidence.

Figure 1. Practical classification of thrombotic microangiopathies. Modified from Genest et al, 2023 .

Figure 2. Acute, subacute, and chronic thrombotic microangiopathy in the glomeruli. Acute: A) H&E staining showing fibrin thrombus (arrows) in some glomerular tufts. There is diffuse endothelial edema and recruitment of leukocytes around the glomerular capillary loops. B) Masson’s trichrome staining revealing fibrin thrombus (arrows) obliterating the capillary loops together with endothelial edema and leukocyte permeation in the capillary loops. C) H&E showing duplicated capillary loops, endothelial edema, red blood cell fragmentation, foamy macrophages, and some fibrin thrombi obliterating the lumen of several glomerular tufts. D) Masson’s trichrome stain with congested capillary loops, duplication of the basement membrane, endothelial edema, and red blood cell fragmentation (schistocytes – arrow). Subacute: E) H&E showing a glomerulus with mesangial expansion with vacuolated matrix (mesangiolysis - arrows - resulting from thrombotic process and vascular repair). Chronic: F) Jones’ silver methenamine staining displaying a glomerulus with duplication of the glomerular basement membrane, endothelial edema, collapsed capillary loops with “podocyte hyperplasia” - asterisks. G) Jones’ silver methenamine staining identifying an interlobular arteriole with fibrin thrombus obstructing the vessel lumen. Downstream, the glomerulus with corrugated, ischemic/anemic capillary loops are visible. H) PAS staining revealing shrunken, ischemic, anemic glomeruli, with dilation of the urinary space and “podocyte hyperplasia” (resulting from upstream thrombotic vascular injury - asterisks). I) PAS displaying glomeruli with diffusely duplicated capillary loops, edematous endothelial cells (glomerular and vascular) with arterioles showing vascular lumen narrowing (arrows).

Figure 3. Acute, subacute, and chronic thrombotic vasculopathy. Acute A) Masson’s trichrome staining showing fibrin mesh adhered to the vascular endothelium and extending into the arterial vascular lumen. B) H&E staining showing the arterial vessel lumen with obliteration by a fibrin thrombus housing leukocytes, platelets, and whole and fragmented red blood cells. C) Masson’s trichrome staining displaying arteries with fibrin thrombi (in red) obliterating the vascular lumen. Subacute: D) H&E staining exhibiting an arterial vessel with diffusely edematous walls and fragmented red blood cells. E) H&E staining of arterial vessel with mucoid edema (pale/light blue) and obstruction of the vascular lumen. Chronic: F) PAS staining indicating an arterial vessel with onion skin lesion, characteristic of chronic endothelial/vascular damage.

Figure 4. Thrombotic microangiopathic endothelial damage – ultrastructural analysis – A and B) Semi-thin section stained with Toluidine Blue showing glomeruli with edematous and duplicated capillary loops, endothelial edema, and narrowing of the vascular lumen. C to F Ultrathin sections analyzed by transmission electron microscopy contrasted with osmium tetroxide, uranyl acetate, and ruthenium red. C and D) Diffusely duplicated capillary loops, with expansion of the internal rarefied lamina by electronelucent material and hint of a newly formed basement membrane. E) Widening of the subendothelial space with deposition of fibrin tactoids (arrow). F) In detail, the expansion of lamina rara interna with lucent material and effacement of endothelial fenestrae (arrow) due to endothelial lesion.

Figure 5. Thrombotic microangiopathy (immunophenotypic profile). A) Granular staining by immunofluorescence for IgM in the walls and lumen of arterial vessels with thrombotic vascular lesion (likely trapping within the thrombus mesh). B) Granular staining by immunofluorescence for fibrinogen in the walls of arterial vessels with thrombotic vascular lesion (resulting from endothelial damage and plasma/fibrin extravasation). C) Granular staining by immunofluorescence for C3 in vascular walls and vascular lumen (due to complement activation). D) Immunohistochemistry with deposition of membrane attack complex (C5b9) in the vascular lumen and wall. E) Immunohistochemistry with coarse granular staining in the walls of arterial vessels and vascular lumen (C4d complement fragment produced by classical and lectin pathway activation with high tissue stability). F) Immunohistochemistry for CD61 platelet aggregation marker with staining in walls and lumen.

Figure 6. Diagnostic and therapeutic approach of TMA.

Figura 1. Classificação prática das microangiopatias trombóticas. Modificado de Genest et al., 2023.

Figura 2. Microangiopatia trombótica aguda, subaguda e crônica nos glomérulos. Aguda: A) Coloração H&E mostrando trombo de fibrina (setas) em alguns tufos glomerulares. Há edema endotelial difuso e recrutamento de leucócitos ao redor das alças capilares glomerulares. B) Coloração de tricrômico de Masson revelando trombo de fibrina (setas) obliterando as alças capilares, juntamente com edema endotelial e permeação de leucócitos nas alças capilares. C) H&E mostrando alças capilares em duplicidade, edema endotelial, fragmentação de hemácias, macrófagos espumosos e alguns trombos de fibrina obliterando o lúmen de vários tufos glomerulares. D) Coloração de tricrômico de Masson com alças capilares congestas, duplicação da membrana basal, edema endotelial e fragmentação de hemácias (esquistócitos - seta). Subaguda: E) H&E mostrando glomérulo com expansão mesangial com matriz vacuolada (mesangiólise - setas - resultante de processo trombótico e reparo vascular). Crônica: F) Coloração de prata metenamina de Jones exibindo um glomérulo com duplicação da membrana basal glomerular, edema endotelial, alças capilares colapsadas com “hiperplasia de podócitos” - asteriscos. G) Coloração de prata metenamina de Jones identificando uma arteríola interlobular com trombo de fibrina obstruindo o lúmen do vaso. A jusante, é visível o glomérulo com alças capilares corrugadas, isquêmicas/anêmicas. H) Coloração PAS revelando glomérulos encolhidos, isquêmicos e anêmicos, com dilatação do espaço urinário e “hiperplasia de podócitos” (resultante de lesão vascular trombótica a montante - asteriscos). I) PAS exibindo glomérulos com alças capilares difusamente duplicadas, células endoteliais edematosas (glomerulares e vasculares) com arteríolas mostrando estreitamento do lúmen vascular (setas).

Figura 3. Vasculopatia trombótica aguda, subaguda e crônica. Aguda: A) Coloração de tricrômico de Masson mostrando malha de fibrina aderida ao endotélio vascular e estendendo-se para o lúmen vascular arterial. B) Coloração H&E mostrando o lúmen do vaso arterial com obliteração por um trombo de fibrina que abriga leucócitos, plaquetas e glóbulos vermelhos inteiros e fragmentados. C) Coloração de tricrômico de Masson exibindo artérias com trombos de fibrina (em vermelho) obliterando o lúmen vascular. Subaguda: D) Coloração H&E exibindo um vaso arterial com paredes difusamente edematosas e glóbulos vermelhos fragmentados. E) Coloração H&E de vaso arterial com edema mucoide (claro/azul claro) e obstrução do lúmen vascular. Crônica: F) Coloração PAS indicando um vaso arterial com “lesão em casca de cebola”, característica de dano endotelial/vascular crônico.

Figura 4. Dano endotelial de microangiopatia trombótica - análise ultra estrutural – A e B) Seção semifinas corada com Azul de Toluidina mostrando glomérulos com alças capilares edematosas e duplicadas, edema endotelial e estreitamento do lúmen vascular. C a F) Seções ultrafinas analisadas por microscopia eletrônica de transmissão contrastadas com tetróxido de ósmio, acetato de uranila e vermelho de rutênio. C e D) Alças capilares difusamente duplicadas, com expansão da lâmina rara interna por material elétron-lúcido e indício de uma membrana basal recém-formada. E) Alargamento do espaço subendotelial com deposição de tactóides de fibrina (seta). F) Em detalhe, a expansão da lâmina rara interna com material lucente e o apagamento das fenestrações endoteliais (seta) devido à lesão endotelial.

Figura 5. Microangiopatia trombótica (perfil imunofenotípico). A) Coloração granular por imunofluorescência para IgM nas paredes e no lúmen de vasos arteriais com lesão vascular trombótica (provável aprisionamento na malha do trombo). B) Coloração granular por imunofluorescência para fibrinogênio nas paredes dos vasos arteriais com lesão vascular trombótica (resultante de dano endotelial e extravasamento de plasma/fibrina). C) Coloração granular por imunofluorescência para C3 nas paredes vasculares e no lúmen vascular (devido à ativação do complemento). D) Imunohistoquímica com deposição do complexo de ataque à membrana (C5b9) no lúmen e na parede vascular. E) Imunohistoquímica com coloração granular grosseira nas paredes dos vasos arteriais e no lúmen vascular (fragmento do complemento C4d produzido pela ativação da via clássica e da lectina com alta estabilidade tecidual). F) Imunohistoquímica para marcador de agregação plaquetária CD61 com coloração nas paredes e no lúmen.

Figura 6. Abordagem diagnóstica e terapêutica da MAT.