Methemoglobin and Abnormal Hemoglobins

Methemoglobin - Blue Blood Blues

• Definition: Hemoglobin with iron in ferric ($Fe^{3+}$) state, not ferrous ($Fe^{2+}$).

  • Cannot bind $O_2$; causes functional anemia & left-shifted $O_2$ dissociation curve.

• Causes:

  • Congenital: Cytochrome b5 reductase deficiency, HbM disease.
  • Acquired: Drugs (nitrites, dapsone, sulfonamides, local anesthetics), toxins.

• Clinical: "Chocolate cyanosis"; blood is chocolate brown.

  • Symptoms worsen with ↑MetHb levels (e.g., >20% cyanosis, >70% often fatal). ⭐ > Pulse oximetry often reads ~85% despite hypoxia (saturation gap).

• Flowchart Summary:

• Treatment:

  • Methylene Blue: 1-2 mg/kg IV (MetHb >20% / symptomatic).
    • ⚠️ G6PD deficiency: Contraindicated (hemolysis risk).
    • Alternatives if CI/severe: Ascorbic Acid, Exchange Tx.

• 📌 Mnemonic: MetHb = Maroon blood, Methylene blue. A[Oxidants / Genetic Defects] --> B[Hb($Fe^{2+}$) → MetHb($Fe^{3+}$)]; B --> C[↓ $O_2$ Capacity → Hypoxia]; C --> D[Chocolate Cyanosis]; D --> E[Dx: Co-oximetry]; D --> F[Tx: Methylene Blue]; F -.-> G[⚠️ Check G6PD status!];

HbS & Sickle Cell - Crescent Calamity

• Genetics: Autosomal recessive; Point mutation in β-globin gene (GAG → GTG).
  • Substitution: Glutamic acid (hydrophilic) → Valine (hydrophobic) at 6th position of β-globin chain (β6 Glu→Val). This promotes hydrophobic interactions under deoxygenation.
• Pathophysiology: ↓ $O_2$ → HbS polymerization → RBC sickling (initially reversible, then irreversible) → Vaso-occlusion & hemolysis.

• Triggers: Hypoxia, acidosis, dehydration, infection, cold.
• Clinical: Vaso-occlusive crisis (pain), dactylitis (infants), acute chest syndrome, autosplenectomy (↑ infection risk: S. pneumoniae, H. influenzae, N. meningitidis), aplastic crisis (Parvovirus B19).

  ⭐ Howell-Jolly bodies on peripheral smear indicate functional asplenia.

• Diagnosis: Hb electrophoresis (HbS), solubility tests (sickling test), HPLC.
• Management: Hydroxyurea (↑ HbF), hydration, analgesia, oxygen, transfusions.

Other Abnormal Hbs - Rogue Red Roundup

• HbC Disease: ($\beta_6$ Glu→Lys; 📌 C for Lysine, Crystals)
  • HbCC: Mild chronic hemolytic anemia, splenomegaly.
  • Smear: Target cells, HbC crystals (rod-shaped).
  • Electrophoresis: With HbA2 & HbE.
• HbE Disease: ($\beta_{26}$ Glu→Lys; 📌 E for East - SE Asia)
  • HbEE: Mild microcytic anemia.
  • HbE/$\beta$-thal: Severe anemia.
  • Electrophoresis: With HbA2 & HbC.
• Unstable Hemoglobins: (e.g., Hb Köln, Hb Zurich)
  • Autosomal Dominant; mutations → Hb precipitation (Heinz bodies).
  • Congenital Heinz body hemolytic anemia. Oxidant stress ↑.
• Hemoglobins with Altered O₂ Affinity:
  • High Affinity: (e.g., Hb Chesapeake): Familial erythrocytosis; Left shift ODC.
  • Low Affinity: (e.g., Hb Kansas): Familial cyanosis/mild anemia; Right shift ODC.

⭐ HbC crystals are pathognomonic, appearing as tetragonal or rod-shaped intracellular structures, especially after splenectomy or in dehydrated cells.

High‑Yield Points - ⚡ Biggest Takeaways

• Methemoglobin (MetHb): Iron in Fe³⁺ (ferric) state, cannot bind O₂, causing functional anemia.
• Key causes: Oxidant drugs (e.g., dapsone, nitrites), G6PD deficiency.
• Clinical: Cyanosis, chocolate-brown blood, normal PaO₂, saturation gap.
• Treatment: Methylene blue (activates reductase), Vitamin C.
• HbS (Sickle Cell): β-globin gene mutation (Glu6Val). Autosomal recessive. Leads to vaso-occlusion.
• HbC Disease: β-globin mutation (Glu6Lys). Milder hemolysis, HbC crystals, target cells.
• Thalassemias: Quantitative defect in globin chain synthesis (α or β).