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methemoglobinemia

A condition characterized by blood methemoglobin levels of > 1-1.5%. Etiology: 1) hereditary a) defects in activity of NADH cytochrome-B5 reductase [2] -type 4 associated with defects in CYB5A b) hemoglobinopathy predisposing to hemoglobin oxidation 2) acquired: oxidation of hemoglobin by drugs or toxins a) nitrates b) nitrites c) chlorates d) quinones e) dapsone [4] f) acetanilid g) phenacetin h) sulfonamides i) aniline dyes j) large doses of ferrous sulfate Pathology: - methemoglobinemia can compromise oxygen delivery to tissues Clinical manifestations: - cyanosis when methemoglobin levels exceed 1.5 g/dL (> 10% of total hemoglobin, normal is < 1.5%) - degree of cyanosis comparable to hemoglobin < 5 g/dL - at blood methemoglobin levels > 15%, neurologic & cardiac symptoms develop due to hypoxia [4] - blood methemoglobin levels > 70% are generally fatal [4] Laboratory: - blood methemoglobin is quantified spectrophotometrically [3] - arterial blood gas (ABG), pulse oximetry a) normal paO2 b) low SaO2 - complete blood count - reticulocyte count - serum electrolytes - liver function tests - renal function tests - hemoglobin electrophoresis - cyanide in serum/plasma, cyanide in blood Radiology: - ref [4] suggests head, chest & cardiac imaging studies Management: 1) discontinue offending medications 2) supplemental oxygen, IV hydration 3) methylene blue 2 mg/kg IV for severe toxic methemoglobinemia 4) exchange transfusion 5) hyperbaric oxygen [4]

Related

methemoglobin (MetHb, Hi) methemoglobin in blood

Specific

hereditary methemoglobinemia

General

sign/symptom

References

  1. Mayo Internal Medicine Board Review, 1998-99, Prakash UBS (ed) Lippincott-Raven, Philadelphia, 1998, pg 791
  2. Greer FR, Shannon M; American Academy of Pediatrics Committee on Nutrition; American Academy of Pediatrics Committee on Environmental Health. Infant methemoglobinemia: the role of dietary nitrate in food and water. Pediatrics. 2005 Sep;116(3):784-6. PMID: 16140723
  3. Henry's Clinical Diagnosis & Management by Laboratory Methods, 21st edition, McPherson RA & Pincus MR (es), W.B. Saunders Co., Philadelphia, PA. 2007, pg 457
  4. Harmon J, Kapitanyan R Poisoning Clues on the Skin: 10 Cases Medscape. April 6, 2017 http://reference.medscape.com/features/slideshow/acutepoisonings