PE26.7 | Hemophilia — Summary & Reflection

KEY TAKEAWAYS

Hemophilia — key points:

• Hemophilia A = factor VIII deficiency (most common; ~1:5000 males); Hemophilia B = factor IX deficiency (Christmas disease; ~1:30,000 males); both X-linked recessive.
• Severity: severe <1%, moderate 1–5%, mild 5–40% factor activity; severe → spontaneous haemarthrosis and life-threatening bleeds.
• Characteristic lab: prolonged aPTT, normal PT, normal platelet count, normal bleeding time — an isolated intrinsic pathway defect.
• Definitive diagnosis: specific factor VIII or factor IX assay; inhibitor screen in patients failing to respond.
• Haemarthrosis is the hallmark and cause of long-term disability; target joint disease leads to haemophilic arthropathy.
• Dosing formulae: FVIII dose = desired rise (%) × weight (kg) × 0.5; FIX dose = desired rise (%) × weight (kg) × 1.0.
• DDAVP is effective only in mild hemophilia A (not severe A, not hemophilia B).
• Never give IM injections or aspirin/NSAIDs in hemophilia.
• ~30% of hemophilia cases arise from de novo mutations — family history may be absent.

REFLECT

Reflect using the Kolb framework:

Concrete Experience: In the opening case, the boy's aPTT was the only abnormal test. Can you now explain, pathway by pathway, why his PT, platelet count, and bleeding time were all normal — while only the aPTT was prolonged?

Reflective Observation: Think about the X-linked inheritance pattern. If this boy's mother has a brother with hemophilia A, what is the probability that the mother is a carrier? What is the probability that each of her sons will be affected?

Abstract Conceptualisation: The dosing formulae for FVIII (×0.5) and FIX (×1.0) differ because the two factors distribute in plasma differently. FVIII distributes primarily in plasma volume (smaller volume of distribution per unit infused → larger rise per unit/kg), while FIX binds to tissues and subendothelial components (larger volume of distribution → smaller rise per unit/kg). How does this understanding help you avoid a dosing error under exam pressure?

Active Experimentation: Next time you see a child with unexplained bruising or joint swelling, practise adding coagulation tests to your initial investigation plan and interpreting the aPTT/PT pattern before specialist referral.