Mimic of massive pulmonary embolism: heparin-induced thrombocytopaenia (HIT)-associated anaphylactoid reactions following subcutaneous low molecular weight heparin.

A female in her 60s was admitted via the emergency department following a respiratory collapse at home. She was profoundly hypoxic and hypertensive. For 1 week prior, she had been self-administering subcutaneous (SC) low molecular weight heparin (LMWH) for a small postoperative pulmonary embolism (PE) and reported worsening, self-resolving episodes of breathlessness 1 hour post-injection. Repeat imaging revealed a new small PE and blood tests showed a significant platelet drop. Her heparin-induced thrombocytopaenia (HIT) antibody test was positive. A diagnosis of HIT-associated anaphylactoid reaction was made. She was switched to a direct oral anticoagulant and fully recovered. This condition is well described, often presenting with profound respiratory compromise that can be misdiagnosed as 'massive PE'. A key differentiation is that there is often hypertension instead of hypotension. This case, however, is unusual as it was triggered by SC LMWH not intravenous heparin as previously described in the literature.