Thrombotic microangiopathy and their mimickers

Thrombotic microangiopathy (TMA) is characterized by microangiopathic hemolytic anemia, thrombocytopenia and multiorgan failure. TMA results from diverse etiologies, including Shiga toxin hemolytic uremic syndrome (HUS), thrombotic thrombocytopenic purpura, atypical HUS (abnormalities of the alternative pathway of complement) and secondary TMA (associated with autoimmune diseases, malignant hypertension, infection and drugs).

The kidney is one of the most common organs involved in TMA [1], with the presence of intraluminal fibrin thrombi on biopsy. Other features include endothelial swelling, fragmented red cells in capillaries, microaneurysm formation and mesangiolysis. Immunofluorescence (IF) microscopy is negative, except for fibrinogen. Electron microscopy (EM) shows subendothelial widening by flocculent material (‘subendothelial fluff’) during the acute phase and double contour formation in the chronic phase. While the kidney biopsy findings are helpful in diagnosing TMA, there is no specific feature associated with the underlying etiology [2]. In addition, other non-TMA disease entities may present with similar findings on kidney biopsy, imposing a diagnostic dilemma. Here we discuss the pathology findings of diseases that may pose difficulties on the kidney biopsy and lead to an erroneous diagnosis of TMA (Figure 1).

IMMUNE-MEDIATED GLOMERULONEPHRITIS

These diseases may be confused with TMA, as biopsy findings include intraluminal immunoglobulin (Ig) that may appear as microthrombi (Table 1).

Ig-mediated glomerulonephritis (GN) includes diseases associated with monoclonal Ig (MIg) and diseases with immune complexes, such as autoimmune diseases and infections. IF is critical in accurately identifying immune-mediated GN. Occasionally, pronase (protease) digestion of paraffin-embedded material may be required to demonstrate the Ig.

Cryoglobulins

Cryoglobulins are Igs that reversibly precipitate on cooling of plasma and serum and redissolve on warming. On light microscopy (LM), cryoglobulins appear as eosinophilic, periodic acid–Schiff (PAS) stain–positive, silver-negative, hyaline-like microthrombi within the glomerular capillaries. Cryoglobulinemic GN is typically associated with a membranoproliferative GN (MPGN) pattern of injury, with formation of double contours along the glomerular capillary walls. This may add to the diagnostic conundrum since TMA in the chronic phase also shows an membranoproliferative glomerulonephritis (MPGN) pattern. IF and EM are critical in distinguishing cryoglobulins from thrombi. IF in TMA is negative for Ig. On the other hand, IF in cryoglobulinemic GN is not only positive, but also helps in typing the cryoglobulin (Supplementary data, Figure S1). Thus type I cryoglobulins are MIg that stain for IgG or IgM (usually kappa light chain restricted); type II cryoglobulins are mixed, composed of MIg (usually IgM kappa) complexed to a polyclonal Ig (usually IgG); and type III cryoglobulins are a mixture of polyclonal IgG and IgM. On EM, cryoglobulins appear as large intraluminal electron-dense deposits that usually show a microtubular substructure. In a few cases, cryoglobulins may not be detected on EM due to sampling. In such cases, EM may show only subendothelial deposits and double contour formation.

Crystalglobulins

Crystalglobulins are similar to type 1 cryoglobulins except that instead of hyaline microthrombi, crystals are visible on LM as eosinophilic, trichrome red–positive, weakly PAS-positive and sliver-negative crystals within the glomerular capillaries (Supplementary data, Figure S2). The crystals often occlude the vascular lumen of glomerular capillaries and renal arteries, thus mimicking a TMA. LM shows an MPGN pattern. On IF, the crystals are positive for MIg; lambda light chain restriction is most common [3]. EM shows electron-dense crystals within the glomerular capillaries.

Crystal-storing histiocytosis

Crystal-storing histiocytosis is a rare disorder in which numerous enlarged histiocytes containing MIg are present in the glomeruli/interstitium [4]. On LM, the glomerular capillaries are infiltrated with histiocytes containing eosinophilic, trichrome red–positive, weakly PAS-positive and silver-negative crystalline cytoplasmic inclusions. LM shows an MPGN pattern. Thus the condition is similar to crystalglobulinemia except that the MIg crystals are present within intraluminal and mesangial histiocytes. CD68 stain is helpful to confirm their histiocytic phenotype. IF reveals the nature of the MIg, which typically has kappa light chain restriction. EM identifies the crystals within the histiocyte cytoplasm [3].

ANTINEUTROPHIL CYTOPLASMIC ANTIBODY (ANCA)-ASSOCIATED GN

These diseases may be confused with TMA as biopsy findings include focal necrotizing lesions that may appear as thrombi.

Focal necrotizing GN

The most common cause of a focal necrotizing GN is ANCA -associated GN. Although the histology may be dominated by crescentic and necrotizing GN, occasionally the only lesion present on the biopsy is a focus of fibrinoid necrosis of the glomerular capillary tufts that may be mistaken for TMA. In such cases the main distinguishing feature of glomerular fibrin in focal necrotizing GN versus TMA is the location. The fibrin in focal necrotizing GN ruptures the glomerular capillary walls and is present extending into the urinary space, whereas it is localized within the glomerular capillaries in TMA (Supplementary data, Figure S3). Sometimes, EM may be required to see the break in the capillary walls in a focal necrotizing GN. Focal necrotizing lesions may be seen in other disease entities such as IgA nephropathy and lupus nephritis.

INFECTIONS INCLUDING HISTIOCYTIC LESIONS

These diseases may be confused with TMA, as biopsy findings include massive histiocyte infiltration that may appear as mesangiolysis/endotheliosis. Histiocytes with a foamy appearance may be present in the glomeruli in the setting of TMA. In particular, TMA in pre-eclampsia/eclampsia is characterized by marked endothelial swelling (endotheliosis).

Histiocytic glomerulopathy

Massive histiocytic involvement of the glomeruli (histiocytic glomerulopathy) can be seen in macrophage activating syndrome secondary to a viral illness [5]. Macrophage activating syndrome is a hyperinflammatory response resulting from exaggerated activation and proliferation of nonmalignant macrophages. On LM, numerous foamy histiocytes are present within the glomerular capillary loops, giving the glomeruli a distinct foamy appearance (Supplementary data, Figure S4). Mesangiolysis and endothelial injury with swelling may be present, raising the possibility of a TMA. The lack of subendothelial fluff, well-defined double contours or fibrin thrombi point toward histiocytic glomerulopathy rather than TMA. CD68 stain is helpful in identifying the histiocytes.

Direct fungal infections

In an immunocompromised host, disseminated histoplasmosis and cryptococcus may appear as TMA, but this is rare. On LM, small collections of yeast forms admixed with fibrin are present in glomerular capillaries. Silver and PAS stains are useful in identifying the fungi (Supplementary data, Figure S5) [6].

LIPID DISORDERS AND RARE DISEASES

These diseases may be confused with TMA, as biopsy findings include double contours and an MPGN pattern [7]. Lipid and other rare disorders can also lead to an MPGN pattern (with negative IF findings) and pose a diagnostic dilemma with TMA. These entities include cryofibrinogen glomerulopathy, lipoprotein glomerulopathy and lecithin–cholesterol acyltransferase (LCAT) deficiency. EM is critical in identifying these specific disease entities.

Cryofibrinogen glomerulopathy

Cryofibrinogen is cryoprecipitate that develops after refrigeration of plasma but not serum. LM shows an MPGN pattern and IF is negative for Ig. EM shows organized deposits of large-bore multilayered tubular structures as well as subendothelial fine fibrillary structures in a matrix [8]. Fibrin tactoids may be present. The characteristic multilayered tubular structures may be sparse and a search for these deposits may be required (Supplementary data, Figure S6).

Lipoprotein glomerulopathy

Lipoprotein glomerulopathy is a rare disease characterized by lipoprotein-rich pseudo-thrombi in the glomerular capillaries [9]. It results from mutations in apolipoprotein E (ApoE) and ensuing abnormal processing of triglyceride-rich lipoproteins. LM shows PAS-positive lamellated material in the glomerular capillaries. The material often appears vacuolated and foamy (on hematoxylin and eosin) and distends the glomerular capillaries. IF is negative for Ig. EM shows lamellated material occluding the glomerular capillaries. An oil red O stain is helpful and immunohistochemical staining for ApoE confirms the diagnosis (Supplementary data, Figure S7).

LCAT deficiency

LCAT is involved in cholesterol transport and its deficiency results in accumulation of phospholipids in many organs, including the kidney. LM shows mesangial expansion and capillary wall thickening with negative findings on IF, thus appearing as a chronic TMA. EM is characteristic and shows lipid deposits with lacunae formation within the glomerular basement membranes (Supplementary data, Figure S8). Subendothelial and subepithelial lacunae are also present. The lacunae may be accompanied by membranes surrounding the round lucent spaces or within the lacunae or by amorphous deposits containing thread-like structures with cross-striations [10]. An oil red O stain is useful in confirming the lipid makeup of the deposits.

In summary, a diverse set of diseases may be erroneously diagnosed as TMA on the kidney biopsy. Many of these disease entities are rare and their recognition may be difficult. However, it is imperative that these diseases are correctly diagnosed, as the underlying management and prognosis are different for each disease.

SUPPLEMENTARY DATA

Supplementary data are available at ndt online.

CONFLICT OF INTEREST STATEMENT

L.M.P.P. is a speaker for Alexion Pharma Brazil.

REFERENCES