Hemoglobin FM-Fort Ripley: another lesson from the neonate

In this issue of Pediatrics, Priest and co-workers have described a new fetal hemoglobin abnormality (Hb FM-Fort Ripley) which causes neonatal methemoglobinemia. Methemoglobin is the form of hemoglobin in which iron is oxidized (Fe3+) instead of reduced (Fe2+), and in this state hemoglobin does not transport oxygen. Methemoglobin normally accounts for less than 1% of the total hemoglobin, although small amounts of hemoglobin continually are being oxidized by exogenous and endogenous agents (including oxygen itself). Elevated methemoglobin levels occasionally occur in pediatric patients, and when this happens they are due to one or more of the following causes: exposure to chemicals that oxidize hemoglobin iron (eg, nitrates, nitrites, aniline derivatives); hereditary deficiency of methemoglobin reductase (also known as cytochrome b5 reductase), an erythrocyte enzyme that normally reduces methemoglobin; transient red blood cell deficiency of methemoglobin reductase, a normal neonatal event that persists until 3 to 4 months of age; the inheritance of an M hemoglobin, such as Hb FM-Fort Ripley.