Protracted methemoglobinemia after phenazopyridine overdose in an infant

Abstract

Acquired methemoglobinemia may be produced by the ingestion or absorption of certain chemicals and xenobiotics. A case of methemoglobinemia in an 8.5-month old infant who ingested approximately 227 mg/kg of phenazopyridine is presented. Although this adverse event is often reversed with a single dose of methylene blue, this patient required three doses of methylene blue (1 mg/kg) over a 25-hour period. It is suggested that the need for repeated doses of methylene blue in this case was not only related to the large dose of phenazopyridine, but also its metabolites (i.e., aniline), which have the potential to produce methemoglobinemia. This case illustrates the need for close observation and serial monitoring of methemoglobin levels in patients who are at increased risk for the development of protracted methemoglobinemia. Integration of knowledge of developmental pharmacology, drug metabolism, and pharmacodynamic properties are critical determinants in the evaluation and treatment of patients with drug-induced methemoglobinemia.