Methemoglobinemia

Methemoglobinemia is a disorder characterized by the presence of a higher than normal level of methemoglobin in the blood. Methemoglobin is an oxidized form of hemoglobin that has almost no affinity for oxygen, resulting in almost no oxygen delivery to the tissues. When its concentration is elevated in red blood cells, tissue hypoxia can occur.

TYPES

Congenital methemoglobinemia

 

Due to a deficiency of the enzyme diaphorase I (NADH methemoglobin reductase), methemoglobin levels rise and the blood of met-Hb patients has reduced oxygen-carrying capacity. Instead of being red in color, the arterial blood of met-Hb patients is brown. This results in the skin of Caucasian patients gaining a bluish hue. Hereditary met-Hb is caused by a recessive gene. If only one parent has this gene, offspring will have normal-hued skin, but if both parents carry the gene there is a chance the offspring will have blue-hued skin.
Another cause of congenital methemoglobinemia is seen in patients with abnormal hemoglobin variants such as hemoglobin M (HbM), or hemoglobin H (HbH), which are not amenable to reduction despite intact enzyme systems. Methemoglobinemia can also arise in patients with pyruvate kinase deficiency due to impaired production of NADH – the essential cofactor for diaphorase I. Similarly, patients with Glucose-6-phosphate dehydrogenase (G6PD) deficiency may have impaired production of another co-factor, NADPH.

Acquired methemoglobinemia

Methemoglobinemia can also be acquired.The protective enzyme systems normally present in red blood cells maintain methemoglobin levels at less than one percent of the total hemoglobin in healthy people. Exposure to exogenous oxidizing drugs and their metabolites may accelerate the rate of formation of methemoglobin up to one-thousandfold, overwhelming the protective enzyme systems and acutely increasing methemoglobin levels. Other classical drug causes of methemoglobinaemia include antibiotics, local anaesthetics, and others such as aniline dyes, metoclopramide, chlorates and bromates. Ingestion of compounds containing nitrates can also cause methemoglobinemia. Infants under 6 months of age are particularly susceptible to methemoglobinemia caused by nitrates ingested in drinking water (called blue-baby syndrome), dehydration usually caused by gastroenteritis with diarrhea, sepsis, and topical anesthetics containing benzocaine or prilocaine. Nitrates used in agricultural fertilizers may leak into the ground and may contaminate well water. The current EPA standard of 10 ppm nitrate-nitrogen for drinking water is specifically designed to protect infants.

TREATMENT

Methemoglobinemia can be treated with supplemental oxygen and methylene blue 1% solution 1 to 2 mg/kg administered intravenously slowly over five minutes followed by IV flush with normal saline. Methylene blue restores the iron in hemoglobin to its normal (reduced) oxygen-carrying state.
This is achieved by providing an artificial electron acceptor (such as methylene blue, or flavin) for NADPH methemoglobin reductase (RBCs usually don't have one; the presence of methylene blue allows the enzyme to function at 5x normal levels) The NADPH is generated via the hexose monophosphate shunt.
Diaphorase II normally contributes only a small percentage of the red blood cells reducing capacity but is pharmacologically activated by exogenous cofactors, such as methylene blue, to 5 times its normal level of activity. Genetically induced chronic low-level methemoglobinemia may be treated with oral methylene blue daily. Also, vitamin C can occasionally reduce cyanosis associated with chronic methemoglobinemia but has no role in treatment of acute acquired methemoglobinemia.

SYMPTOMS

Signs and symptoms of methemoglobinemia (methemoglobin >1%) include shortness of breath, cyanosis, mental status changes (~50%), headache, fatigue, exercise intolerance, dizziness and loss of consciousness. Arterial blood with elevated methemoglobin levels has a characteristic chocolate-brown color as compared to normal bright red oxygen containing arterial blood. Severe methemoglobinemia (methemoglobin >50%) patients have dysrhythmias, seizures, coma and death (>70%). Healthy people may not have many symptoms with methemoglobin levels < 15%, however patients with co-morbidities such as anemia, cardiovascular disease, lung disease, sepsis, or presence of other abnormal hemoglobin species (e.g. carboxyhemoglobin, sulfehemoglobin or sickle hemoglobin) may experience moderate to severe symptoms at much lower levels (as low as 5-8%).