Methemoglobinemia

Aka: Methemoglobinemia, Blue Baby Syndrome

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II. Pathophysiology

  1. Hereditary Methemoglobinemia
  2. Acquired Methemoglobinemia
    1. Exposure to drugs with amino or nitro group
    2. Chemical exposure results in changed Hemoglobin iron
      1. Oxidizes ferrous Hemoglobin (Fe2+)
      2. Results in ferric Hemoglobin (Fe3+)
  3. Oxyhemoglobin becomes non-Oxygen carrying Methemoglobin
    1. Left Shifts Oxyhemoglobin Dissociation Curve
    2. Impairs oxygen unloading to tissues

III. Causes: Hereditary Methemoglobinemia

  1. Cytochrome b5 Reductase Deficiency
    1. Autosomal Recessive inheritance
    2. Two types
      1. Type I: Lifelong asymptomatic Cyanosis
      2. Type II: Cyanosis with Mental Retardation
    3. Associated conditions in some cases
      1. Mild Polycythemia
  2. M Hemoglobins (5 Hemoglobin variants)
    1. General
      1. Autosomal Dominant inheritance
      2. M Hemoglobins are asymptomatic except for Cyanosis
    2. Alpha chain variants (Cyanosis at birth)
      1. Hemoglobin M Boston
      2. Hemoglobin M Iwate
    3. Beta chain variants (Cyanosis at 4-6 months)
      1. Hemoglobin M Saskatoon
      2. Hemoglobin M Hyde Park
      3. Hemoglobin M Milwaukee

IV. Causes: Acquired Methemoglobinemia Causes (Oxidizing Agents)

  1. Nitrates and Nitrites
    1. Nitrate commonly contaminate well water
    2. Responsible for Blue Baby Syndrome (formula reconstituted in well water)
      1. Distinguish from Congenital Heart Disease
      2. Knobeloch (2000) Environ Health Perspect 108(7):675-8 [PubMed]
  2. Nitroglycerin
  3. Nitroprusside
  4. Aniline
  5. Paints
  6. Varnishes
  7. Inks
  8. Phenacetin
  9. Sulfonamides
  10. Pyridium
  11. Dapsone
  12. Primaquine
  13. Lidocaine
  14. Procaine
  15. Benzocaine
  16. Nitrophenol
  17. Toluidine
  18. Nitrobenzene
  19. Isobutyl nitrate
  20. Sodium Nitrite Ingestion
    1. Large Sodium Nitrite ingestions have been used for Suicide
    2. Also results in Tachypnea, acidosis and seziures

V. Findings: Signs and symptoms

  1. Methemoglobin >15%
    1. Cyanosis
    2. Asymptomatic
  2. Methemoglobin >30%
    1. Fatigue
    2. Headache
    3. Dizziness
    4. Tachycardia
    5. Weakness
  3. Methemoglobin >55%
    1. Dyspnea
    2. Bradycardia
    3. Hypoxia
    4. Acidosis
    5. Seizures
    6. Coma
    7. Arrhythmia
  4. Methemoglobin >70%
    1. Death

VI. Differential Diagnosis

  1. See Cyanosis
  2. See Causes of Dyspnea with Clear Lung Sounds
  3. Congenital Heart Disease

VII. Complications

  1. Hyperkalemia
  2. Renal Failure
    1. Occurs 1-3 days after exposure

VIII. Labs

  1. Arterial Blood Gas (ABG)
    1. Normal arterial pO2
  2. Oxygen Saturation plateaus at 85%
    1. Overestimates the true blood oxygen level
    2. Oxygen Saturation remains 8% regardless of methemoglobin level
  3. Venipuncture
    1. "Chocolate brown" appearance to blood
    2. No change with exposure to oxygen
    3. Color fades with exposure to PotassiumCyanide 10%

IX. Management: Severe Acquired Methemoglobinemia

  1. ABC Management
  2. Oxygen 100%
  3. See Toxin Ingestion Management
  4. Methylene Blue
    1. Dose: 1-2 mg/kg (1% solution) over 5 min
    2. Reduces Methemoglobin by 50% within 1 hour (by reduction back to Hemoglobin)
    3. Indications
      1. Methemoglobin > 20-30 g/L (20-30%)
      2. Methemoglobinemia with Hypoxia or other signficant symptoms (lethargy, confusion, Dyspnea)
    4. Contraindications
      1. G6PD Deficiency
      2. M Hemoglobin (not effective)
  5. Transfuse Packed Red Blood Cells
    1. Goal Hemoglobin: 15 g/dl
  6. Exchange Transfusion

X. References

  1. Wilson (1991) Harrison's Medicine, p. 1549

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