LAUSR

To the Editor: Bone marrow necrosis (BMN) and embolization of fat and necrotic marrow is a rare, life-threatening condition in patients with sickle cell disorders (SCD). Fat emboli cause multiorgan failure with anemia and thrombocytopenia, often mimicking thrombotic thrombocytopenic purpura (TTP) and posing a diagnostic challenge. In the last 8 years, we have diagnosed BMN/FES in 16 patients with SCD at our institution, some of whom were unaware of their hemoglobinopathy prior to this presentation. Although the etiology and precipitating factors of FES are poorly understood and require further investigation, we discuss important distinguishing features that can facilitate rapid recognition and treatment. We retrospectively reviewed medical records of all BMN/FES patients diagnosed at our institution between April 2010 and January 2018 to obtain histories, laboratory data, treatment, and clinical outcome. We compared each FES patient with 2 TTP patients matched for age, gender and race. We analyzed presenting symptoms and laboratory results aiming at identifying characteristics that differentiate FES from TTP. A univariate exact conditional logistic regression was used to examine significant differences in likelihood of FES for continuous and categorical variables. A Friedman test was used to check for significant differences in schistocytes on admission. Ordinal values were created based off the value of the dependent variable, that is, presence of schistocytes (none = 0, rare = 1, few = 2, occasional = 3, moderate = 4, and many = 5 per high power field). This test accounts for the 1:2 matching and the null hypothesis is that there is no difference in ranks by patient group. We also attempted to calculate fat embolism risk scores (Gurd and Wilson's criteria, and Schonfeld's criteria) for FES patients, to test their applicability in this context. The Institutional Review Board at our institution approved this study and granted a waiver of informed consent. Sixteen patients (8 males and 8 females) with BMN/FES were seen at our institution between April 2010 and January 2018. Their mean age was 40.6 years (SD 12.1) compared with 41.1 years (SD 11.5) for the 32 patients with TTP (P = 0.5834). Ten patients with FES had HbSβ+, 4 HbSC and 2 HbSS disease; 4 patients were unaware of having hemoglobinopathy prior to presentation. Their most common complaints were severe back pain (69%), respiratory compromise (69%) and altered mental status (56%; Supporting Information Figure S1). Laboratory data for both groups are shown in Supporting Information Table S1. On admission, all patients with FES were anemic, with a mean hemoglobin of 9.2 g/dL (SD 2 g/dL) and thrombocytopenia was present in 12 patients (75%). The remainder developed thrombocytopenia during hospitalization. TTP patients had significantly lower platelet count on admission and at the nadir. Six (38%) FES patients had a leukoerythroblastic picture on admission, and seven (total, 81%) developed it during hospitalization. There were no differences in terms of hemoglobin, lactate dehydrogenase (LDH) and creatinine. In terms of nucleated red blood cells (NRBCs), there was a change in the reporting of this parameter in our laboratory in 2015, and only patients seen since that time could be compared. Figure 1A shows that the peak number of NRBCs/100 leukocytes was >100 in 4 of 6 patients with FES but in none with TTP. Conversely,