Early-Onset Hemophagocytic Lymphohistiocytosis and Inflammatory Neurotoxicity Prior to Post-transplant Cyclophosphamide: A Report of Two Cases.

We report two distinct, early hyperinflammatory toxicities after human leukocyte antigen-mismatched allogeneic hematopoietic cell transplantation with post-transplant cyclophosphamide in adult recipients. Case 1 developed severe cytokine release syndrome with shock, respiratory failure, renal failure, hyperferritinemia (22,771 ng/mL), hypertriglyceridemia (1,024 mg/dL), and multiorgan dysfunction, consistent with secondary hemophagocytic lymphohistiocytosis; he improved rapidly after receiving emapalumab. Case 2 developed fever and abrupt encephalopathy on day +2 with negative infectious and neurologic evaluation and rapidly improved after tocilizumab, consistent with cytokine-mediated neurotoxicity resembling immune effector cell-associated neurotoxicity syndrome. These cases highlight diagnostic overlap with infection and the need for early immunosuppressive intervention.