Pancytopenia and Methemoglobinemia in a Patient With Multiple Myeloma Presenting With Syncope and Dyspnoea: A Diagnostic Challenge

Methemoglobinaemia is a rare but important cause of hypoxia that may be overlooked, particularly in patients with complex comorbidities. We present a 76-year-old man with Multiple Myeloma undergoing chemotherapy who presented with recurrent syncope and exertional dyspnoea. Initial investigations revealed pancytopenia and hypoxia. Imaging excluded pulmonary embolism and infection. Arterial blood gas analysis demonstrated elevated methemoglobin levels (10.5%), confirming methemoglobinaemia. The patient was managed with supportive therapy, including oxygen, blood transfusion, and filgrastim, resulting in clinical improvement. This case highlights the importance of considering methemoglobinaemia in patients with unexplained hypoxia, especially when conventional investigations are unremarkable. The coexistence of severe anaemia can further exacerbate tissue hypoxia and complicate diagnosis.