"Spontaneous" Heparin-Induced Thrombocytopenia and Thrombosis After Total Knee Arthroplasty: A Report of 2 Cases

Spontaneous Heparin-Induced Thrombocytopenia Presenting as Concomitant Bilateral Cerebrovascular Infarction and Acute Coronary Syndrome

Background: Spontaneous heparin-induced thrombocytopenia is a pro-thrombotic syndrome in which anti-heparin antibodies develop without heparin exposure.

Case presentation: A 78-year-old man who underwent a successful lumbar laminectomy presented to the hospital 5 days after discharge for stroke-like symptoms and was found to have acute infarcts of the bilateral frontal lobes. The patient was found to be severely thrombocytopenic and was incidentally found to have an inferior wall myocardial infarction. Further investigation led to the diagnosis of bilateral lower extremity deep vein thromboses. His overall clinical presentation prompted a detailed hematologic workup that indicated positivity for heparin-induced thrombocytopenia despite no previous exposure to heparin products.

Conclusions: This case illustrates a patient with no prior lifetime heparin exposure who underwent laminectomy with subsequent development of acute infarcts of the bilateral frontal lobes, an inferior wall myocardial infarction, and bilateral lower extremity deep vein thromboses, with concern for sequelae of spontaneous heparin-induced thrombocytopenia.

Limb ischemia due to spontaneous heparin-induced thrombocytopenia as the primary presentation of acute COVID-19 infection

Heparin-induced thrombocytopenia (HIT) occurs with the development of IgG antibodies that bind complexes of heparin and platelet factor 4 (PF4), which activate platelets and result in a profoundly prothrombotic condition. In rare instances, this syndrome develops in the absence of proximate heparin administration, referred to as spontaneous HIT, for which less than three dozen cases have been reported. Spontaneous HIT is considered a subtype of "autoimmune HIT" (aHIT), characterized by platelet activation in the serotonin release assay (SRA) without the addition of exogenous heparin. Here, we report spontaneous HIT as the presenting feature in a patient with 2019 coronavirus disease infection (COVID-19).A 66-year-old male presented with progressive leg pain and was found to have a platelet count of 39 × 109/L and multiple lower extremity arterial thromboses requiring fasciotomy and thrombectomy. He had no recent hospitalization, heparin exposure, vaccinations, or known thrombophilia. He had a strongly positive IgG-specific enzyme-linked immunosorbent assay for heparin-PF4 antibodies, and the SRA was strongly positive both with and without the addition of heparin. He was treated successfully with bivalirudin, intravenous immunoglobulin, and apixaban.

Spontaneous HIT syndrome: Knee replacement, infection, and parallels with vaccine-induced immune thrombotic thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is characterized clinically by thrombocytopenia, hypercoagulability, and increased thrombosis risk, and serologically by platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies. Heparin-"induced" acknowledges that HIT is usually triggered by a proximate immunizing exposure to heparin. However, certain non-heparin medications (pentosan polysulfate, hypersulfated chondroitin sulfate, fondaparinux) can trigger "HIT". Further, naturally-occurring polyanions (bacterial lipopolysaccharide, DNA/RNA) can interact with PF4 to recapitulate HIT antigens. Indeed, immunologic presensitization to naturally-occurring polyanions could explain why HIT more closely resembles a secondary, rather than a primary, immune response. In 2008 it was first reported that a HIT-mimicking disorder can occur without any preceding exposure to heparin or polyanionic medications. Termed "spontaneous HIT syndrome", two subtypes are recognized: (a) surgical (post-orthopedic, especially post-total knee arthroplasty, and (b) medical (usually post-infectious). Recently, COVID-19 adenoviral vector vaccination has been associated with a thrombotic thrombocytopenic disorder associated with positive PF4-dependent enzyme-immunoassays and serum-induced platelet activation that is maximal when PF4 is added. Vaccine-induced immune thrombotic thrombocytopenia (VITT) features unusual thromboses (cerebral venous thrombosis, splanchnic vein thrombosis) similar to those seen in spontaneous HIT syndrome. The emerging concept is that classic HIT reflects platelet-activating anti-PF4/heparin antibodies whereas spontaneous HIT syndrome and other atypical "autoimmune HIT" presentations (delayed-onset HIT, persisting HIT, heparin "flush" HIT) reflect heparin-independent platelet-activating anti-PF4 antibodies-although the precise relationships between PF4 epitope targets and the clinical syndromes remain to be determined. Treatment of spontaneous HIT syndrome includes non-heparin anticoagulation (direct oral Xa inhibitors favored over direct thrombin inhibitors) and high-dose immunoglobulin.