Rare case of acquired methemoglobinemia reported in NEJM

Dr Otis U. Warren and Dr Benjamin Blackwood at Miriam Hospital, Providence, RI have reported a rare case of acquired methemoglobinemia. The case has appeared in the New England Journal of Medicine.

Acquired methemoglobinemia is usually due to the ingestion of drugs or toxic substances including topical anaesthetic agents such as benzocaine, through metabolic pathways that appear to vary from person to person, which may account for the unpredictability of this complication. Exposure to such substances in amounts that exceed the enzymatic reduction capacity of RBCs precipitates symptoms. Acquired methemoglobinemia is more frequent in premature infants and infants younger than 4 months, and the following factors may have a role in the higher incidence in this age group. Acquired methemoglobinemia is a medical emergency, and its prompt recognition and treatment can avoid catastrophic complications including death.

A 25-year-old woman presented to the emergency department with a 1-day history of generalized weakness, fatigue, shortness of breath, and skin discolouration. She had a respiratory rate of 22 breaths per minute and an oxygen saturation of 88% while breathing ambient air. The oxygen saturation value displayed on the pulse oximeter did not improve with the administration of supplemental oxygen. The patient appeared cyanotic (Panel A), and she had dark arterial and venous blood (Panel B, smaller tube and larger tube, respectively).

Measurements of arterial blood gases included a partial pressure of arterial oxygen of 120 mm Hg and calculated oxygen saturation of 100%; however, when measured by CO-oximetry, the oxygen saturation was 67%. The percentage of methemoglobin was 44%. She was treated with intravenous methylene blue and had a considerable improvement in her breathing and reduction in skin discolouration. She reported having used large amounts of topical benzocaine the night before for a toothache.

The patient had complete resolution of her symptoms, and she was referred for outpatient dental follow-up.