Methemoglobinemia
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| ICD-10 | D74 |
|---|---|
| ICD-9 | 289.7 |
| DiseasesDB | 8100 |
| eMedicine | med/1466 |
| MeSH | D008708 |
Methemoglobinemia, also known as "met-Hb", is a blood disorder characterized by the presence of a higher than normal level of methemoglobin in the blood.
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[edit] Types
[edit] Congenital methemoglobinemia
Due to a deficiency of the enzyme diaphorase, the blood of met-Hb victims has reduced oxygen-carrying capacity. Instead of being red in colour, the arterial blood of met-Hb victims is brown. This results in the skin of Caucasian sufferers gaining a bluish cast (thus making them "blue people"). 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.
[edit] Acquired/Toxic methemoglobinemia
Methemoglobin is a form of hemoglobin that does not bind oxygen. When its concentration is elevated in red blood cells a functional anemia and tissue hypoxia may occur. Normally methemoglobin levels are <1%, as measured by the co-oximetry test. Elevated levels of methemoglobin in the blood are caused when the mechanisms that defend against oxidative stress within the red blood cell are overwhelmed and the oxygen carrying ferrous ion (Fe2+) of the heme group of the hemoglobin molecule is oxidized to the ferric state (Fe3+). This converts hemoglobin to methemoglobin, a non-oxygen binding form of hemoglobin that binds a water molecule instead of oxygen. Spontaneous formation of methemoglobin is counteracted by protective enzyme systems cytochrome-b5 reductase (major pathway) and NADPH methemoglobin reductase (minor pathway). These pathways normally maintain methemoglobin levels at less than one percent of the total hemoglobin in healthy people. Exposure to exogenous oxidizing drugs and their metabolites (such as benzocaine, dapsone and nitrates) may accelerate the rate of formation of methemoglobin up to one-thousandfold, overwhelming the protective enzyme systems and acutely increasing methemoglobin levels.
Infants under 6 months of age are particularly susceptible to methemoglobinemia caused by nitrates ingested in drinking water, dehydration usually caused be gastroenteritis with diarrhea, sepsis and topical anesthetics containing benzocaine. Nitrates that are used in agricultural fertilizers leach 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.
[edit] Treatment
Methemoglobinemia is treated with supplemental oxygen and methylene blue 1% solution (10mg/ml) 1-2mg/kg administered intravenously slowly over five minutes followed by IV flush with normal saline. Methylene blue restores the hemoglobin to its normal oxygen-carrying state. Genetically induced chronic low-level methemoblobinemia may be treated with oral methylene blue daily.
[edit] Symptoms
Signs and symptoms of methemoglobinemia (methemoglobin >1%) include shortness of breath, cyanosis, mental status changes, 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 methmeoglobinemia (methemoglobin >50%) patients have dysrhythmias, seizures, coma and death. 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%).
[edit] Carriers
The Fugate family, a family which lived in the hills of Kentucky, are the most well known example of this hereditary chromosomal error. Known as the Blue Fugates, Martin Fugate, settled near Hazard, Kentucky circa 1800. His wife was a carrier of the recessive methemoglobinemia (met-H) gene, as was a nearby clan with whom the Fugates intermarried. As a result many descendants of the Fugates were born with met-H.
The 'blue men of Lurgan' were a pair of Lurgan men suffering from what was described as 'familial idiopathic methaemoglobinaemia' that were treated by Dr. James Deeny in 1942. Deeny, who would later become the Chief Medical Officer of the Republic of Ireland, prescribed a course of ascorbic acid and sodium bicarbonate. In Case one, by the eighth day of treatment there was a marked change in appearance and by the twelfth day of treatment the patient's complexion was normal. In case two, the patient's complexion reached normality over a month long duration of treatment. Reference to these cases is found in the British Medical Journal , June 12, Vol. 1 ,pg. 721 written by J. Deeny, E.T. Murdock and J.J. Rogan and appears also in the book of essays 'The End of an Epidemic' by James Deeny ISBN I 899047 06 9.
[edit] External links
- Straight Dope article on the Fugates of Appalachia, an extended family of blue-skinned people
- The Blue People of Troublesome Creek
[edit] References
- Ash-Bernal R, Wise, R. Acquired methemoglobinemia: 138 cases at 2 teaching hospitals. Medicine 2003 83(5):265-73. http://www.md-journal.com
