Pediatric emergency medicine trisk 2198 2198

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 do 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%.