Hemofilia

01Pathophysiology

Hemophilia is a disorder of secondary hemostasis, meaning the problem lies in the coagulation cascade rather than in platelet plug formation. This distinction is fundamental to understanding every aspect of the disease and is repeatedly tested.

In Hemophilia A, the patient lacks functional factor VIII, which serves as a cofactor for factor IXa in the intrinsic pathway tenase complex. Factor VIII accelerates the conversion of factor X to factor Xa by several orders of magnitude. Without adequate factor VIII, this reaction proceeds at a profoundly slow rate, leading to insufficient thrombin generation and unstable fibrin clot formation. Hemophilia B involves deficiency of factor IX itself, the serine protease in that same complex, and produces a clinically indistinguishable phenotype.

Because the intrinsic pathway is impaired, the activated partial thromboplastin time (aPTT) is prolonged. However, the extrinsic pathway (measured by PT/INR) remains intact because factor VII and tissue factor function normally. Platelets are unaffected, so the platelet count and bleeding time are normal. This is why hemophilia patients do not develop petechiae or mucosal bleeding (those are hallmarks of platelet disorders). Instead, they develop deep tissue bleeding: hemarthrosis, intramuscular hematomas, and retroperitoneal hemorrhage.

The inheritance pattern is X-linked recessive. Mothers are typically carriers (heterozygous), and sons have a 50% chance of being affected. Daughters of an affected father are obligate carriers. This is why the classic vignette describes a young boy with bleeding symptoms, often with a maternal uncle who had similar problems. However, remember that approximately one-third of cases arise from spontaneous new mutations, so absence of family history does not exclude the diagnosis.

Recurrent bleeding into joints causes hemophilic arthropathy through a destructive cycle: blood in the synovial space triggers inflammation, synovial hypertrophy, and cartilage degradation. These damaged joints become "target joints" that are prone to repeated bleeding episodes, creating a self-perpetuating loop of damage.

02Classification and Clinical Manifestation

Severity Classification

Clinical Manifestations by System

Hemophilia causes delayed bleeding, not immediate bleeding. The initial platelet plug forms normally (primary hemostasis is intact), but the fibrin clot that stabilizes it is deficient. A vignette may describe a patient who stops bleeding initially after a cut, then re-bleeds hours later.

03Diagnostic Workup

The best initial test when you suspect a bleeding disorder is a basic coagulation panel: aPTT, PT, platelet count, and bleeding time. In hemophilia, the hallmark pattern is a prolonged aPTT with everything else normal. This pattern immediately tells you the defect is isolated to the intrinsic pathway.

The next step after finding an isolated prolonged aPTT is the mixing study. You combine the patient's plasma 1:1 with normal pooled plasma. If the aPTT corrects (returns to normal), it means the patient is simply missing a factor that normal plasma can supply. This confirms a factor deficiency. If the aPTT does not correct, it means something in the patient's plasma is actively inhibiting coagulation, pointing toward an inhibitor (such as a lupus anticoagulant or an acquired factor VIII inhibitor).

Once the mixing study corrects, the confirmatory test is a quantitative factor assay. You measure factor VIII activity (for Hemophilia A) and factor IX activity (for Hemophilia B). The absolute factor level determines the severity classification. Hemophilia A accounts for roughly 80% of all hemophilia cases, so it is more commonly tested.

An important nuance: von Willebrand Disease (vWD) can also cause a prolonged aPTT because von Willebrand factor (vWF) serves as a carrier protein that stabilizes factor VIII in circulation. In type 3 vWD (severe), factor VIII levels can be very low, mimicking hemophilia. This is why a vWF antigen level and vWF activity (ristocetin cofactor assay) should also be checked when evaluating a prolonged aPTT, especially if the patient has mucosal bleeding in addition to deep tissue bleeding.

For patients already diagnosed with hemophilia who develop new, unexplained bleeding or whose factor replacement therapy becomes less effective, you must suspect the development of inhibitors (alloantibodies against the infused factor). The mixing study will not correct, and the Bethesda assay quantifies the inhibitor titer, which directly guides management.

04Management and Treatment

Desmopressin (DDAVP) is the first-line treatment for mild Hemophilia A only. It works by stimulating endothelial cells to release stored von Willebrand factor, which in turn stabilizes and raises circulating factor VIII levels by 2 to 5 fold. The standard dose is 0.3 mcg/kg IV infused over 20 to 30 minutes, or it can be given intranasally (150 mcg per spray, one spray for patients under 50 kg, two sprays for those over 50 kg). DDAVP is ineffective in moderate to severe Hemophilia A (insufficient stored factor VIII to release) and completely ineffective in Hemophilia B (factor IX is not stored in endothelial cells). Tachyphylaxis develops with repeated dosing, so it should not be used more than once every 24 to 48 hours over consecutive days. Hyponatremia from water retention is a notable adverse effect, particularly in children.

Factor replacement therapy is the cornerstone of treatment for moderate to severe disease and for acute bleeding episodes. The dosing principle relies on the fact that 1 IU/kg of factor VIII raises plasma levels by approximately 2%, while 1 IU/kg of factor IX raises levels by approximately 1% (due to the larger volume of distribution of factor IX). For example, to raise factor VIII from 0% to 50% in a 70 kg patient:

$$\text{Dose}=\frac{\text{Desired rise (\%)}\times \text{Weight (kg)}}{2}=\frac{50\times 70}{2}=1750\text{ IU}\backslash text\{Dose\}\; =\; \backslash frac\{\backslash text\{Desired\; rise\; (\backslash \%)\}\; \backslash times\; \backslash text\{Weight\; (kg)\}\}\{2\}\; =\; \backslash frac\{50\; \backslash times\; 70\}\{2\}\; =\; 1750\; \backslash text\{\; IU\}$$

Target factor levels depend on the clinical scenario: minor bleeds require 30 to 50%, major bleeds require 50 to 80%, and life-threatening or surgical situations require 80 to 100%.

Prophylactic regimens are indicated for all patients with severe hemophilia to prevent spontaneous bleeding and the development of hemophilic arthropathy. Prophylaxis should ideally begin before the onset of joint damage, typically before age 2 or after the first hemarthrosis. For Hemophilia A, standard prophylaxis is recombinant factor VIII 25 to 40 IU/kg administered every other day or three times per week. Extended half-life factor VIII products allow less frequent dosing (every 3 to 5 days).

Emicizumab (Hemlibra) is a bispecific monoclonal antibody that bridges factor IXa and factor X, mimicking the cofactor function of factor VIII. It is administered subcutaneously (loading dose of 3 mg/kg weekly for 4 weeks, then maintenance at 1.5 mg/kg weekly, 3 mg/kg every 2 weeks, or 6 mg/kg every 4 weeks). It is approved for prophylaxis in Hemophilia A regardless of inhibitor status. A critical testing pearl: emicizumab normalizes the aPTT (because it bypasses the need for factor VIII in the aPTT reagent assay), so a normal aPTT in a known Hemophilia A patient on emicizumab does not mean they have adequate factor VIII. Standard factor VIII assays also give falsely elevated results; chromogenic bovine factor VIII assays are needed for accurate monitoring.

For patients who develop inhibitors (occurring in ~25 to 30% of severe Hemophilia A patients and ~3 to 5% of Hemophilia B patients), management depends on the inhibitor titer. Low-titer inhibitors (< 5 Bethesda units) can be overwhelmed with high-dose factor concentrate. High-titer inhibitors (> 5 Bethesda units) require bypassing agents: activated prothrombin complex concentrate (aPCC / FEIBA, 50 to 100 U/kg every 8 to 12 hours) or recombinant activated factor VII (rFVIIa / NovoSeven, 90 mcg/kg every 2 to 3 hours). Immune tolerance induction (ITI) is the long-term strategy to eradicate inhibitors, involving daily high-dose factor infusion over months to years.

Antifibrinolytic agents (tranexamic acid 25 mg/kg PO every 8 hours, or aminocaproic acid 50 to 100 mg/kg PO every 6 hours) are used as adjunctive therapy for mucosal bleeding, particularly during dental procedures and oral surgery. They stabilize clots by inhibiting plasmin. They are contraindicated in urinary tract bleeding (risk of obstructive uropathy from clots in the renal collecting system).

Absolute contraindications to remember: avoid aspirin, NSAIDs, and any antiplatelet agents (they impair the already-critical platelet function that provides initial hemostasis). Avoid intramuscular injections (risk of hematoma).

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most commonly tested concept in hemophilia is the coagulation lab pattern: prolonged aPTT with normal PT, normal platelets, and normal bleeding time. Students who have not committed this pattern to memory will confuse hemophilia with platelet disorders or combined coagulopathies. If a vignette describes petechiae, think platelets, not hemophilia. If a vignette describes hemarthrosis in a boy, think hemophilia, not anything else.

The second major trap involves the mixing study. Many students forget this step or misinterpret the result. A mixing study that corrects means factor deficiency. A mixing study that does not correct means an inhibitor is present. The exam loves to present a patient with hemophilia whose factor replacement therapy "stopped working" and test whether you recognize inhibitor development and order a Bethesda assay.

A classic distractor vignette places a young girl with bleeding symptoms and a prolonged aPTT. Students reflexively choose hemophilia, but hemophilia in females is exceedingly rare (requires homozygous inheritance or Turner syndrome 45,X with a carrier X chromosome). The more likely answer in a female patient with prolonged aPTT and mucosal bleeding is von Willebrand Disease. The giveaway will be bleeding time or PFA-100 abnormalities and a mucosal bleeding pattern.

Another frequently tested scenario is life-threatening bleeding (intracranial hemorrhage) in a known hemophilia patient. The next best step is immediate factor replacement, not imaging. You treat before you scan. The exam wants to see that you will not delay definitive therapy to obtain a CT scan.

Finally, know that DDAVP only works for mild Hemophilia A. A vignette describing a patient with severe Hemophilia A or any form of Hemophilia B who is given DDAVP is testing whether you recognize this as an inadequate intervention. The answer in those cases is always factor concentrate replacement.