Acquired methaemoglobinaemia related to phenazopyridine ingestion

Abstract

SummaryMethaemoglobin is an altered state of haemoglobin in which the ferrous ions of haeme are oxidised to the ferric state. This results in increased affinity to the bound oxygen and decreasing its availability to tissues. Most cases of methaemoglobinaemia are acquired, resulting from an increased methaemoglobin formation by various exogenous agents. The authors report an elderly patient presenting to the emergency department with a 1-month history of shortness of breath. Around the same time she had started using over-the-counter (OTC) phenazopyridine tablets for urinary symptoms. The patient was hypoxic and cyanotic; however, lacked evidence of hypoxaemia on the arterial blood gas. The presence of abnormal haemoglobin was suspected and confirmed by elevated levels of methaemoglobin. Phenazopyridine was proposed to be the likely aetiology of the methaemoglobinaemia, which the patient was not aware of. This case highlights the importance of always inquiring the OTC drug use especially in geriatric population. BACKGROUND

A Rare but Important Clinical Presentation of Induced Methemoglobinemia

“…Nevertheless, the epidemiology is unknown, presentation quite rare, and less than five cases have been reported in PubMed in over 35 years. 1 – 4 We present a case with a different set of patient characteristics than seen in the few recent case reports, and an approach to treatment that validates further uniqueness, justifying reporting the case in the literature. In particular, the patient was a young otherwise-healthy adult, with the initial diagnosis and decision to treat based on clinical grounds versus laboratory values.…”

“…It is often taught as such and seen in board review courses. Nevertheless, the epidemiology is unknown, presentation quite rare, and less than five cases have been reported in PubMed in over 35 years 1–4. We present a case with a different set of patient characteristics than seen in the few recent case reports, and an approach to treatment that validates further uniqueness, justifying reporting the case in the literature.…”

“…Conversely, methemoglobinemia induced from phenazopyridine ingestion is rarely seen in the ED. The epidemiology is unknown and very few cases are cited in the literature, with most in the remote past 1–4. Cases of methemoglobinemia are most frequently cited in the literature after use of dapsone and local anesthetics (i.e.…”

“…The epidemiology is unknown and very few cases are cited in the literature, with most in the remote past. 1 – 4 Cases of methemoglobinemia are most frequently cited in the literature after use of dapsone and local anesthetics (i.e. benzocaine and lidocaine), and rarely reported in the emergency medicine literature.…”

Methemoglobinaemia with chronic phenazopyridine ingestion

“…Methemoglobinaemia results when iron in the haemoglobin is oxidised from ferrous to ferric state. Ferric iron has lower affinity for oxygen, thereby more oxygen binds to ferrous form in methemoglobin, resulting in shift of oxygen haemoglobin dissociation curve to the left 2. If symptomatic or severe, methemoglobinaemia is usually treated with methylene blue.…”