Thiamethoxam-Induced Acute Methemoglobinemia in a Child.

Methemoglobinemia is a rare and life-threatening condition in which the presence of methemoglobin (MetHb) impairs oxygen delivery to tissues. While hematologic toxicity is well-documented with agents like nitrates, benzocaine, and certain drugs, it is rarely associated with neonicotinoid insecticides. We report a pediatric case of a 9-year-old girl who developed acute methemoglobinemia following ingestion of a cockroach gel containing thiamethoxam. She presented with nausea, vomiting, lethargy, and irritability. In the emergency department, she was in respiratory distress with hypoxia unresponsive to high-flow oxygen. Notably, her venous blood appeared dull brown. Capillary blood gas analysis revealed an elevated MetHb level of 42.1%, low pO₂, and metabolic acidosis. Intravenous methylene blue was administered promptly, resulting in rapid clinical improvement and a reduction in MetHb to 2.9% within one hour. This case highlights a novel manifestation of thiamethoxam toxicity: life-threatening methemoglobinemia. According to current toxicological studies, thiamethoxam, a neonicotinoid primarily associated with neuromuscular and liver toxicity, has not previously been linked to hematologic effects. A plausible mechanism may involve oxidative stress from thiamethoxam metabolites, leading to hemoglobin oxidation. Timely diagnosis was made possible by high clinical suspicion and immediate blood gas analysis. The patient recovered fully with methylene blue treatment. This case expands the known toxicologic profile of thiamethoxam and represents the first reported instance of thiamethoxam-induced methemoglobinemia. Early recognition and treatment are critical. In cases of unexplained hypoxia, especially in children with potential exposure to oxidizing agents like insecticides, clinicians should maintain a high index of suspicion for methemoglobinemia, even if the product appears benign.