Methemoglobinemia presents with grayish-brown discoloration of the skin that is recognized as cyanosis. Children up to the age of 4 months lack the enzyme activity that normally reduces methemoglobin, thus making them susceptible to oxidant stress-induced methemoglobinemia. Three scenarios occur with some frequency: children with acute gastroenteritis and increased nitrate production from bacteria in the GI tract; children exposed to nitrates in water of agricultural areas where fertilizer runoff contaminates water sources; and overconsumption of nitrogenous vegetables such as spinach.
In drug-induced methemoglobinemia, the slate-gray to blue discoloration of the skin is apparent with levels of 10% to 15%. Symptoms occur in proportion to declining oxygen delivery. Headache, nausea, and fatigue occur at lower levels (20% to 30%). Levels above 50% can cause loss of consciousness, myocardial ischemia, dysrhythmias, seizures, and metabolic acidosis. Levels above 70% may be lethal. In patients with cardiopulmonary disease in which there is impaired oxygen delivery, the symptoms will be manifested at lower methemoglobin levels.