The pathologic findings would be consistent with any of the clinicopathologic categories of TMA, including:
As in the patient under consideration, TMA may be misdiagnosed clinically as rapidly progressive glomerulonephritis because it often presents with hematuria (sometimes with RBC casts), proteinuria (rarely in the nephrotic range), hypertension, rapidly rising serum creatinine and occasionally hypocomplementemia. When these renal manifestations are accompanied by purpura, a TMA may be misdiagnosed as a small vessel vasculitis (e.g. microscopic polyangiitis).
The presence of a microangiopathic hemolytic anemia (MAHA) and thrombocytopenia are the best clues to the appropriate diagnosis. Nephrologists, however, see a disproportionate number of patients with TMA who do not have a well defined MAHA, but rather manifest primarily renal dysfunction. A possible explanation for this predominantly renal phenotype is that some patients have microangiopathic changes confined to the kidneys, and these lesions progress so rapidly that perfusion of the kidneys is rapidly curtailed resulting in very little destruction of blood cells and platelets by the injured vessels.
In retrospect, given the clinical data available at the time of biopsy, the most appropriate diagnosis in the patient under consideration appears to be HUS, especially if the anemia was found to have the characteristics of a MAHA. Although the patient had headaches, there was no evidence for other CNS problems and no purpura to support a diagnosis of TTP. Although there was hypertension, it was not in the malignant range, therefore PMH is not an appropriate diagnosis. At least at the time of biopsy, there was no recognized evidence for systemic sclerosis (including a negative ANA), although some patients will present with acute TMA and subsequently be found to have systemic sclerosis.
HUS in adults is often idiopathic, but may be secondary to recognized causes, such as pregnancy (e.g., acute postpartum renal failure), drugs (e.g., mitomycin C), autoantibodies (e.g., anti-phospholipid antibodies), toxins (e.g., bee venom), heredity (i.e., familial HUS), or infections (e.g., Shiga toxigenic E. coli infection).
Of the categories of HUS, post-diarrheal childhood HUS has the best prognosis, with over 80% of patients recovering with only supportive care (e.g., fluid and electrolyte control, hypertension control, maintenance of hematocrit, and dialysis if required). The treatment of other forms of HUS, and recalcitrant post-diarrheal HUS is controversial, but some investigators advocate treatment with plasma infusion or plasma exchange.
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