Review Question - QID 216437

This patient who immigrated from Brazil and presents with fever, malaise, neuropathy, and hypopigmented skin lesions most likely has leprosy. Standard treatment of leprosy includes dapsone, which can cause methemoglobinemia by oxidizing the Fe2+ in hemoglobin to Fe3+, thus inducing symptoms of headache, fatigue, and cyanosis (bluish lips and nail beds).

Methemoglobinemia is a condition that involves elevated levels of methemoglobin in the blood. Methemoglobin is an oxidized form of hemoglobin that contains ferric iron (Fe3+) instead of ferrous iron (Fe2+). Methemoglobinemia can be inherited or caused by certain medications, such as nitrates, benzocaine, and dapsone. Ferric iron binds oxygen much less readily than ferrous iron. In addition, the remaining ferrous iron in tetrameric hemoglobin molecules has increased O2 affinity (left-shiting of the oxygen-hemoglobin curve). Together, this leads to functional anemia. Clinically, this manifests as signs of hypoxia (headache, lightheadedness, fatigue) and "chocolate-colored" blood. Treatment for methemoglobinemia includes methylene blue and vitamin C, which induce the reduction of methemoglobin to hemoglobin. Of note, dapsone has a long half-life, and retreatment with methylene blue and vitamin C is often needed.

Ludlow et al. discuss the epidemiology, pathophysiology, and treatment of methemoglobinemia. The article describes classic findings and evaluation of a patient with methemoglobinemia. The authors recommend treatment of methemoglobinemia with methylene blue and high-flow oxygen delivered by a non-rebreather mask.

Figure/Illustration A is a picture of well-demarcated, hairless, hypopigmented plaques (black circles) on the skin characteristic of leprosy.

Incorrect Answers:
Answer 1: Consolidation of lung parenchyma describes pneumonia, which can present with headache, fatigue, and signs of hypoxia, as this patient does. However, pneumonia is not typically associated with exposure to medication, and is associated with rales, dullness to percussion, and decreased breath sounds.

Answer 2: Foreign body aspiration and bronchial obstruction can present in pediatric patients with wheezing and stridor, as well as unilaterally decreased breath sounds, none of which are present in this patient. Aspiration can also cause voice changes or suprasternal retractions.

Answer 3: Improper formation of the aorticopulmonary septum describes transposition of the great vessels (TGV), a congenital heart defect that causes the aorta to emerge from the right ventricle and the pulmonary trunk from the left ventricle, presenting as early cyanosis and tachypnea in infants. However, TGV does not typically present at this patient's age and is not associated with exposure to medications.

Answer 4: Increased pressure in the pulmonary circulation describes pulmonary hypertension, which presents with dyspnea, fatigue, and cyanosis, similar to this patient. Pulmonary hypertension is not generally associated with headaches or exposure to medications.

Bullet Summary:
Methemoglobinemia is caused by the oxidation of Fe2+ in hemoglobin to Fe3+ which can be the result of exposure to medications such as dapsone.