其他血紅素病變

Buzzwords → Dx

Diagnostic Criteria

• Methemoglobinemia: co-oximetry MetHb >1 % (symptomatic at >15–20 %).
• Hb electrophoresis or HPLC: identifies HbC, HbE, others.
• Hb stability test (heat / isopropanol): detects unstable variants.
• P50 (oxygen affinity): abnormal in high/low-affinity variants.
• DNA sequencing for unusual variants.

Workup

• CBC + smear (target cells, Heinz bodies — supravital stain).
• Reticulocyte count, LDH, haptoglobin, indirect bilirubin (hemolysis).
• Hb electrophoresis or HPLC.
• Co-oximetry if methemoglobinemia suspected (don't trust pulse oximeter).
• G6PD activity before methylene blue treatment.
• Family history.

Treatment Algorithm

flowchart TD
  A[Acquired methemoglobinemia] --> B[Discontinue offending agent]
  B --> C{MetHb level + symptoms}
  C -- mild <20% asymptomatic --> D[Observation, supportive]
  C -- symptomatic >20% --> E{G6PD status?}
  E -- normal --> F[Methylene blue 1-2 mg/kg IV<br>over 5 min<br>repeat in 1 h if needed]
  E -- deficient or unknown high-risk --> G[Ascorbic acid 300-1000 mg IV<br>or hyperbaric oxygen<br>or exchange transfusion]
  C -- severe / refractory --> G
  A --> H[Hb electrophoresis] --> I{Hb variant identified}
  I -- HbC trait / disease --> J[Mild hemolysis<br>folate; rarely transfusions]
  I -- HbE trait --> K[Asymptomatic; counseling]
  I -- HbE-β-thal --> L[Severe transfusion-dependent thalassemia<br>treat as β-thal major]
  I -- unstable Hb --> M[Avoid oxidant drugs<br>folate; splenectomy if severe]
  I -- high-affinity Hb (erythrocytosis) --> N[Phlebotomy if symptomatic<br>distinguish from PV]

Pearls / Pitfalls

• Pulse oximeter is unreliable in methemoglobinemia — typically reads ~85 % regardless of true SaO₂. Use co-oximetry.
• Methylene blue in G6PD deficiency: CONTRAINDICATED — paradoxical hemolysis. Use ascorbic acid instead.
• Dapsone-induced methemoglobinemia is common in HIV/PJP prophylaxis or leprosy — co-prescribe vitamin C or use cimetidine adjunct.
• Topical benzocaine (e.g., teething gel, throat spray) can cause severe methemoglobinemia in children — FDA boxed warning.
• Congenital methemoglobinemia types: type I (RBC-only, asymptomatic + cyanosis since birth, easy to miss), type II (generalized, severe, neurologic).
• HbC trait — asymptomatic; HbC homozygous → mild hemolysis with target cells. HbS-HbC compound (HbSC) behaves like SCD with retinopathy and AVN risk.
• HbE — most common Hb variant globally (~30 % of Thai population). HbE trait asymptomatic. HbE-β-thal → severe transfusion-dependent thalassemia (similar to β-thal major).
• Unstable Hb triggered by oxidants → Heinz bodies + hemolysis. Avoid oxidant drugs (similar list to G6PD).
• High-affinity Hb (erythrocytosis without ↓EPO) — don't misdiagnose as PV. Check P50 + family history.
• CO poisoning is a separate entity (carboxyhemoglobin) — also low SpO₂ with normal PaO₂; treat with 100 % O₂ ± hyperbaric.

Cross-references

• G6PD — methylene blue contraindication
• SCD — HbSC variant
• Thalassemia — HbE-β-thal
• PV — high-affinity Hb DDx

相關題目

• Q-130 — Methemoglobinemia recognition
• Q-131 — High-affinity hemoglobin — distinguishing from PV
• Q-132 — HbSC compound heterozygosity

Sources