Methemoglobinemia is an uncommon cause of cyanosis in infants and children, but can cause significant morbidity and even death Oxidant stress under physiologic conditions produces MHb that is reduced back to hemoglobin by cellular mechanisms In normal individuals, MHb exists in a steady state of about 1% of total hemoglobin; this may be higher in those chronically exposed to tobacco smoke Congenital forms of methemoglobinemia present in the neonatal period or early infancy and result from defects in or absence of the endogenous reductase systems Defects in globin chains (αor β-chain) can also alter the oxidation state of the heme iron resulting in cyanosis Termed M hemoglobins, they are inherited in an autosomal dominant fashion and typically not require treatment Acquired forms of methemoglobinemia are more common Ingestion or topical exposure to oxidizing drugs or chemicals (e.g., benzocaine, dapsone, chloroquine, nitrates, paraquat) occurs most commonly Ingestion of high levels of nitrates, such as through well water, can cause MHb Systemic acidosis in infants may also result in MHb because of the relative immaturity of NADHdependent enzyme system early in life Clinical Recognition Consider the diagnosis of methemoglobinemia when cyanosis occurs in the absence of cardiac or pulmonary disease and does not improve with oxygen therapy Symptoms depend on the actual concentration of MHb ( Table 93.4 ) At low levels, patients may present with cyanosis only As the level rises, symptoms of headache, fatigue, anxiety, and light-headedness develop An MHb level greater than 30% is considered life threatening, and these patients may exhibit altered mental status, dyspnea, tachypnea, tachycardia, seizures, respiratory depression, and arrhythmia Coma may occur at MHb levels above 50%