Thrombotic Microangiopathy in Solid Organ Transplantation

Thrombotic Microangiopathy (TMA) is a syndrome characterized by microangiopathic hemolytic anemia (MAHA) and thrombocytopenia. The presence of schistocytes on peripheral smear, a negative Coombs test, elevated lactate dehydrogenase, increased reticulocyte count and low haptoglobin are often the clues for MAHA. The microvascular process often targets vasculature in kidneys, brain, gastrointestinal system, heart, and skin. A timely diagnosis and treatment are often crucial to prevent severe end organ damage and death. TMA is classified into primary and secondary forms. Primary TMA includes TTP and complement mediated or atypical hemolytic uremic syndrome (aHUS), often related to a mutation or deficiency and clinically expressed in the setting of a precipitant condition. Secondary TMA is a manifestation of underlying disorder and can occur in clinical scenarios associated with autoimmune disease, malignancy, infections, SOT (Solid Organ Transplant), pregnancy, HSCT (Hematopoietic Stem Cell Transplantation), medications, or methylmalonic acidemia. Transplant associated TMA (TA-TMA) can be complement mediated or aHUS and could be related to the ischemic reperfusion injury, induction regimen, calcineurin inhibitor (CNI) use, mammalian target of rapamycin (MTOR) inhibitor use, or could be infection related. Cost, access, and turnaround time are often the limitations for certain TTP and complement specific testing. Treatment should not be delayed while waiting for such tests. Treatment must be individualized based on the underlying cause of TMA. Terminal complement blockade utilizing monoclonal antibodies directed against C5 complement is the treatment for complement mediated TMA. C5 inhibitors have also been used successfully in treatment of secondary HUS cases where, unlike aHUS, defects in complement cannot be demonstrated. Such treatment has demonstrated improvement in renal function, MAHA and platelet counts.