Acquired Methemoglobinemia Following Herbal Decoction Ingestion Presenting With Refractory Hypoxemia and Associated Generalized Seizure: A Case Report

Acquired methemoglobinemia is an uncommon but potentially life-threatening disorder characterized by oxidation of hemoglobin iron into the ferric state, resulting in impaired oxygen delivery and functional anemia. Patients may present with cyanosis, persistent hypoxemia, and discordance between pulse oximetry and arterial oxygen saturation (SaO₂). Although medications and industrial chemicals are well-recognized causes, herbal and traditional remedies remain underreported aetiologies. A 35-year-old man presented with a sudden-onset generalized tonic-clonic seizure followed by persistent hypoxemia and peripheral cyanosis after ingestion of a home-prepared herbal (Ayurvedic) decoction. The blood sample demonstrated characteristic chocolate-brown discoloration. Oxygen saturation remained approximately 85% despite supplemental oxygen therapy. Arterial blood gas (ABG) analysis demonstrated preserved arterial oxygen tension (PaO₂ 167 mmHg) with a calculated SaO₂ of 97.3%, indicating a characteristic saturation gap. Co-oximetry demonstrated a methemoglobin level of 19%. The diagnosis of methemoglobinemia was supported by refractory hypoxemia, cyanosis, chocolate-brown blood discoloration, saturation gap, and co-oximetry-confirmed methemoglobinemia (19%). The patient improved rapidly following treatment with intravenous methylene blue, anticonvulsants, and supportive care. There was a rapid disappearance of cyanosis; oxygen saturation returned to normal with full neurological recovery. This example highlights the importance of checking for methemoglobinemia in patients with symptoms of unexplained cyanosis, refractory hypoxemia, and a mismatch between pulse oximetry and arterial oxygen levels, particularly following exposure to herbal or traditional preparations. Recognition of these findings facilitated prompt diagnosis and treatment in our patient.