Small infants are at increased risk because they have markedly lower levels of cytochrome-b5 reductase and fetal hemoglobin is more easily oxidized to MHg . The activity of some enzymes involved in these processes is less efficient in the elderly. Methemoglobinemia has also been reported in dialysis patients and may be a result of chloramine toxicity due to inadequate removal by hemodialysis filters.
Patients with methemoglobinemia present with a wide spectrum of symptoms depending on the amount of MHg present in the blood. The initial symptoms of methemoglobinemia, such as visible cyanosis and chocolate brown-coloured arterial blood, typically begin to appear when the level of MHg rises above 10% to 15% and are unresponsive to O2 therapy . As the level of MHg rises past 20%, patients may exhibit anxiety, fatigue, dyspnea, dizziness, tachycardia, headache and syncope . Stupor, coma and convulsions have been reported with MHg levels greater than 50% , and death from cardiac hypoxia has been reported when levels exceed 70% .
The diagnosis of methemoglobinemia is based on clinical and laboratory findings. The cyanosis is generally out of proportion to the respiratory status of the individual and does not respond to O2 therapy. This was clearly seen in both patients, where high-flow O2 did not alter the cyanosis nor did cessation of supplemental O2.