Section 4 - Disorders of Primary Hemostasis (Vasculature & Platelets

Hereditary Hemorrhagic Telangiectasia (HHT)

Impact on Coagulation

• Pathophysiology: Single-layer endothelium with poor structural support → fragile vessels

• Bleeding risk: Recurrent bleeding from vessel fragility, not platelet or factor defects

• Telangiectasia: Blanching, superficial dilated vessels (vs. petechiae) → vascular origin

• Clinical signs: Epistaxis, GI/UG bleeding, oral/mucocutaneous lesions; any organ

• Coagulation labs: Normal PT, aPTT, platelet count/function; defect is structural

Hereditary Vascular Disorder

Ehlers-Danlos Syndrome (EDS)

Impact on Coagulation

• Pathophysiology: Inherited collagen disorder; variable connective tissue fragility

• Bleeding risk: Varies by type; vascular EDS may bleed easily, others do not

• Clinical signs: Hypermobile joints, stretchy skin, easy bruising in some subtypes

• Coagulation labs: Typically normal; bleeding due to tissue fragility, not platelet/coag defects

Hereditary Vascular Disorder

Allergic and Drug-Induced Purpuras

Impact on Coagulation

• Pathophysiology: Autoimmune vascular injury or drug-induced anti-vessel wall antibodies

• Bleeding risk: Increased due to immune-mediated vessel damage

• Telangiectasia: Absent; lesions appear as non-blanching purpura or petechiae

• Clinical signs: Palpable purpura, often in dependent areas; may be drug- or autoimmune-triggered

• Coagulation labs: Typically normal; bleeding from vascular inflammation, not factor deficiency

Acquired Vascular Disorder

Henoch–Schönlein Purpura (HSP)

Impact on Coagulation

• Pathophysiology: Post–upper respiratory infection; IgA immune complexes deposit in small vessels

• Bleeding risk: Immune-mediated vasculitis causes vessel fragility and organ bleeding

• Clinical signs: Typically affects children; palpable purpura, joint pain, abdominal pain, hematuria/proteinuria

• Progression: Small percentage may develop chronic renal disease

• Coagulation labs: Usually normal; bleeding due to vascular inflammation, not factor or platelet defects

Acquired Vascular Disorder

Scurvy

Impact on Coagulation

• Pathophysiology: Vitamin C deficiency → impaired collagen synthesis → fragile capillary walls

• Bleeding risk: High due to structural vessel weakness, especially in skin and mucosa

• Clinical signs: Easy bruising, perifollicular hemorrhages, bleeding gums, poor wound healing

• Coagulation labs: Normal; bleeding from defective vascular support, not platelet/coag factor issues

Acquired Vascular Disorder

Senile Purpura

Impact on Coagulation

• Pathophysiology: Age-related loss of collagen and subcutaneous fat → reduced vessel support

• Bleeding risk: Mild to moderate; bruising from minor trauma due to capillary fragility

• Clinical signs: Elderly, especially on forearms and hands; dark, irregular purpura

• Coagulation labs: Normal; bleeding is mechanical, not hematologic

Acquired Vascular Disorder

Bernard–Soulier Syndrome

Defect in Platelet Adhesion

• Primary defect: Absent or dysfunctional glycoprotein Ib (GPIb) on platelet surface

• Mechanism: Platelets cannot bind von Willebrand factor (vWF) → impaired adhesion to subendothelium

• Lab findings:

  • ↑ Bleeding time/PFA

  • ↓ Platelet count, large platelets

  • Normal aggregation with all agents except ristocetin

Bernard–Soulier Syndrome
Laboratory Diagnosis

• Platelet count: ↓ Mild thrombocytopenia

• Platelet morphology: Giant platelets seen on peripheral smear

• Bleeding time / PFA-100: Prolonged (defective adhesion)

• Platelet aggregation studies:

  • Normal with ADP, collagen, epinephrine

  • Absent response to ristocetin (does not correct with vWF addition)

• Flow cytometry: Shows ↓ or absent GPIb expression

von Willebrand Disease (vWD)
General Defect Overview

• Inherited bleeding disorder due to deficiency or dysfunction of vWF

• vWF is critical for platelet adhesion and carries factor VIII

• Affects both primary and secondary hemostasis

• Stored in alpha granules of plts & present in plasma

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vWD Type I – Classic

• Most common type

• Quantitative deficiency (↓ all multimers)

• vWF structure is normal

• Labs: ↓ vWF, ± ↓ FVIII:C, Prolonged APTT possible

vWD Type II – Qualitative Defects

Subtypes: abnormal structure or function of vWF

• 2A: ↓ HMW multimers, impaired platelet binding

• 2B: ↑ affinity for GPIb → platelet clumping & ↓ platelet count

• 2M: ↓ platelet binding (normal multimers)

• 2N: ↓ FVIII binding (mimics mild hemophilia A)

• APTT: often prolonged, FVIII:C low or normal

vWD Type III – Severe

• Near absence of all vWF multimers

• Severe bleeding, very low FVIII:C

• APTT: prolonged, bleeding resembles hemophilia A

Platelet-Type vWD

• Gain-of-function defect in platelet GPIb

• Platelets bind vWF spontaneously → thrombocytopenia

• Multimers are normal, but appear consumed

vWD Type I – Lab Findings

• PT: Normal

• APTT: ↑ (due to ↓ factor VIII)

• PFA: ↑ (no longer routinely used)

• Other tests: Needed to distinguish subtypes (e.g., vWF antigen, activity, multimer analysis)

vWF:Ag – Antigen Level

• Purpose/Principle: Measures quantity of von Willebrand factor (vWF) protein in plasma.

• Diagnostic Utility: Distinguishes quantitative (Type I, III) vs. qualitative (Type II) defects.

• Result Interpretation:

  • ↓ in Type I (partial deficiency)

  • ≈ 0 in Type III (near-total absence)

  • Normal or slightly ↓ in most Type II subtypes

• Contextual Notes:

  • Does not assess vWF function

  • Measured via ELISA, latex agglutination, chemiluminescence

vWF:RCo – Ristocetin Cofactor Activity

• Purpose/Principle: Measures functional activity of vWF by its ability to agglutinate donor platelets in the presence of ristocetin.

• Diagnostic Utility: Confirms functional impairment of vWF.

• Result Interpretation:

  • ↓ in all types except possibly 2N

  • Disproportionately lower than Ag in Type II

  • Corrects with normal plasma (Type I), not in Type II or III

• Contextual Notes:

  • Most important functional test

  • Bernard-Soulier will have normal RCo with abnormal platelets

vWF Multimer Analysis

• Purpose/Principle: Separates vWF multimers by size using agarose gel electrophoresis or immunoelectrophoresis.

• Diagnostic Utility: Identifies structural abnormalities in vWF multimers.

• Result Interpretation:

  • ↓ in all multimers: Type I

  • ↓ or absent HMW: Types 2A, 2B

  • Normal multimers: Type 2N, 2M

  • Absent or nearly absent multimers: Type III

• Contextual Notes:

  • Useful for subtyping Type II disorders

  • Not routinely done in all labs

Platelet Aggregation Patterns: vWD vs Bernard-Soulier vs Glanzmann

• von Willebrand Disease

  • Normal aggregation w/ all agonists except ristocetin

  • Ristocetin abnormal (corrects w/ normal plasma)

• Bernard-Soulier Syndrome

  • Normal aggregation w/ all agonists except ristocetin

  • No correction with normal plasma (due to GPIb defect)

• Glanzmann Thrombasthenia

  • Absent aggregation with all agonists except ristocetin

  • Ristocetin: normal (does not require GPIIb/IIIa)

Key Mechanisms:

• vWD: ↓ vWF (defective ligand)

• Bernard-Soulier: ↓ GPIb (defective receptor)

• Glanzmann: ↓ GPIIb/IIIa (defective fibrinogen receptor for aggregation)

Glanzmann’s Thrombasthenia – Defect

Defect:

• Platelets lack or have dysfunctional GPIIb/IIIa complex (integrin αIIbβ3)

• Prevents fibrinogen bridging between platelets

• Results in impaired platelet aggregation, despite normal adhesion

Clinical Type:

• Most common inherited aggregation disorder

Genetics:

• Autosomal recessive

Glanzmann’s Thrombasthenia – Laboratory Diagnosis

Platelet Function Analyzer (PFA):
↑ Prolonged — due to defective aggregation.

Prothrombin Time (PT):
Normal — coagulation cascade unaffected.

Activated Partial Thromboplastin Time (APTT):
Normal — intrinsic pathway intact.

Platelet Count:
Normal — platelet number unaffected.

Platelet Aggregation Studies:

• Abnormal with all agonists except ristocetin

• Normal aggregation with ristocetin — ristocetin tests adhesion (via vWF/GPIb), not aggregation.

von Willebrand Disease – DDAVP Therapy

Mechanism:
DDAVP (1-deamino-8-d-arginine vasopressin) stimulates endothelial cells to release stored von Willebrand factor (vWF) from Weibel–Palade bodies, increasing circulating vWF levels.

Best Use:

• Effective for Type 1 vWD (mild to moderate forms)

• Not recommended for Type 2B (risk of thrombocytopenia) or Type 3 (vWF absent)

Note:
Not a replacement for vWF; boosts endogenous stores.

Bleeding in Plasma Cell Dyscrasias (MM/Waldenstrom’s)

Mechanism:
Excess IgG or IgM coats:

• Platelet membranes → impairs aggregation

• Collagen fibers → interferes with platelet adhesion

Result:
→ Impaired primary hemostasis
→ Prolonged bleeding time/PFA despite normal platelet count and coag factors

Contextual Note:
This mimics platelet adhesion disorders (like vWD or Bernard–Soulier) but is due to paraprotein interference, not intrinsic platelet defects.

Effect of Aspirin and Ibuprofen on Platelet Function

Aspirin irreversibly inhibits cyclooxygenase (COX), blocking thromboxane A₂ (TXA₂) synthesis → impaired platelet aggregation
• Ibuprofen reversibly inhibits COX, causing transient platelet dysfunction (less potent than aspirin)
• Aspirin Resistance:
 • Up to 22% of patients have reduced aspirin response
 • Leads to higher risk of MI and stroke
 • Detect with platelet aggregation studies—arachidonic acid curve is most sensitive
• Aggregation studies can also track aspirin use/effectiveness

Platelet Antibodies

• IgG antibodies target platelet membrane receptors (e.g., GPIIb/IIIa, GPIb/IX)
• Lead to platelet destruction by splenic macrophages → ↓ platelet count
• Some antibodies block receptor function → impaired aggregation or adhesion
• Result: thrombocytopenia and/or platelet dysfunction → mucocutaneous bleeding

Hermansky-Pudlak Syndrome

Dense granule deficiency
• Dilated canalicular system on platelet surface
• Electron microscopy shows "Swiss cheese platelets"
• Defective release reaction → abnormal aggregation

Gray Platelet Syndrome

Marked ↓ in platelet alpha granules
• Platelets appear hypogranular or agranular
• Impairs platelet aggregation and clot formation

Wiskott-Aldrich Syndrome

• Microplatelets
• ↓ alpha and dense granules
• Platelet sequestration → ↓ platelet count
• Immunodeficiency: recurrent infections, ↓ serum IgM

Chediak-Higashi Syndrome

Platelets lack normal dense granules
• Giant, abnormally formed lysosomal granules in WBCs
• Impairs platelet function and immune cell activity

Storage Pool Diseases (SPD)

• Deficiency of platelet dense or alpha granules
• Impaired release reaction
• Results in abnormal platelet aggregation

Thrombocytopenia

Low platelet count
• Caused by:
 – Decreased production → IPF (Immature Platelet Fraction) low
 – Increased destruction → IPF high (↑ immature “retic” platelets)

Thrombocytosis

Reactive: due to blood loss, surgery, childbirth, necrosis, inflammation, or exercise
• Platelets ↑ but function normally
• Myeloproliferative: seen in PV, CML, primary myelofibrosis, ET

Essential Thrombocythemia (ET)

• PLT >600K, often >1 million
• Bone marrow: ↑ megakaryocytes
• Peripheral smear: giant, bizarre, and small platelets; large aggregates
• Spontaneous aggregation
• Platelet dysfunction → ↑ risk of thrombosis and bleeding

Four causes of thrombocytopenia

• Decreased production (e.g. Fanconi syndrome, May-Hegglin, marrow failure)
• Increased destruction (e.g. ITP, drug-induced antibodies)
• Platelet sequestration (e.g. Wiskott-Aldrich)
• Ineffective thrombopoiesis (e.g. May-Hegglin: large, bizarre plts, Dohle-like bodies)

3 Causes of congenital platelet hypoplasia

Fanconi syndrome – congenital aplasia
• Wiskott-Aldrich – platelet sequestration, small plts, ↑ apoptosis signaling
• May-Hegglin – ineffective thrombopoiesis, giant plts, Dohle-like bodies

List 9 Causes of acquired platelet hypoplasia

• Irradiation

• Drugs

• Ethanol

• Early aplastic anemia

• Pernicious anemia and folate deficiency (DNA synthesis defects impair megakaryocyte function)

• Viruses

• Bacterial infections

• Malignancies

• Myelodysplastic syndromes (may also cause increased platelet destruction)

Idiopathic Thrombocytopenic Purpura (ITP)

• Autoimmune platelet destruction via anti-platelet antibodies
  • Most common mechanism: IgG-mediated opsonization → splenic phagocytosis
  • Isolated thrombocytopenia (<100,000) without another underlying cause
  • Can be acute (self-limited) or chronic (relapsing/persistent)

Laboratory Findings in ITP

• PLT count often < 20,000
• Peripheral blood: large platelets with variable size/shape
• Bone marrow: megakaryocyte hyperplasia
• IPF: low to very low
• Bleeding time or PFA: prolonged due to low PLT
• Deficient clot retraction

Acute vs Chronic ITP

Acute ITP
• Most common in children
• Often follows a viral infection (2–21 days prior)
• May occur post-immunization
• Usually self-limited
 • 80% spontaneous remission

Chronic ITP
• Most common in adults (20–50 yrs)
• Fluctuating disease course
• Bleeding episodes may last days or weeks
• Spontaneous remissions are rare
• May be early manifestation of AIDS

Drug-Induced Immune Thrombocytopenia

• Caused by formation of drug-related antibodies that affect platelets

• True autoantibody may form that targets platelets even without drug presence

• Hapten mechanism: drug binds platelet → antibody forms to drug-platelet complex

• Drug-antibody complex mechanism: Ab-drug complex attaches to platelet

• Common drugs: Heparin, Quinine, Quinidine

Heparin-Induced Thrombocytopenia (HIT) Mechanism

• PF4 is released from platelet alpha granules upon activation

• Heparin binds to PF4 → forms PF4/heparin complex

• IgG antibody binds to this complex

• Fc portion of IgG binds Fc receptor on platelet

• This activates platelets → aggregation, granule release

• Results in thrombocytopenia and procoagulant microparticles

• These microparticles are thought to drive thrombosis

• Removal of heparin leads to improvement

Heparin-Associated Thrombocytopenia (HAT)

• Direct non-immune activation of platelets by heparin

• Develops within 1–3 days of treatment

• PLT rarely falls below 100 × 10³/mm³

• Benign and transient

• No thrombosis risk

• Not antibody mediated

Heparin-Induced Thrombocytopenia (HIT)

• Immune-mediated drop in PLT count (~30–50%)

• Develops 5–10 days after heparin initiation

• Antibody to PF4/heparin complex forms

• PLT activation via Fc receptor causes consumption and thrombosis

• Remove heparin → PLT count rebounds

• May lead to severe thrombocytopenia and hypercoagulable state

HIT Type II (HITTS)

• Subset of HIT with thrombotic complications

• PLT can fall as low as 20 × 10³/mm³

• PF4/heparin complex + IgG → strong platelet activation

• Produces procoagulant microparticles

• May occlude microvasculature → thrombosis

• Pathophysiology not fully understood

• HIT Ab may bind heparan on endothelium → TF expression

• Patients may develop only thrombocytopenia or also thrombosis

Heparin Induced Platelet Aggregation (HIPA) Test

• Detects heparin-induced antibodies in patient plasma

• Patient must be off heparin for ≥ 8 hours

• Uses platelet aggregometer

• Mix: normal donor platelets + patient platelet-poor plasma + heparin dilutions

• If Ab present, platelets aggregate (measure %T – transmission)

• Negative control: donor platelets + patient PPP + saline

• Interpretation:

  • < 20%T → unable to demonstrate (not reported as “negative”)

  • 20–40%T → weak positive

  • 40%T → positive

Neonatal alloimmune thrombocytopenia (NAIT)

• Caused by maternal IgG antibodies against fetal platelet antigens (usually HPA-1a)

• Mother lacks antigen; fetus inherits it from father

• Fetal platelets enter maternal circulation → antibody formation

• Maternal antibodies cross placenta and destroy fetal platelets

• Newborn appears healthy at birth → develops petechiae/purpura

• Mechanism is similar to HDFN

Post-transfusion purpura (PTP)

• Alloimmune thrombocytopenia developing 3–12 days post transfusion

• Caused by antibodies (usually anti-HPA-1a) against platelet antigens not present in the recipient

• Initial sensitization from pregnancy or prior transfusion

• Re-exposure → anamnestic response → rapid thrombocytopenia

• Can also impact future pregnancies if antibody crosses placenta

• Confirm via antibody ID (e.g., ELISA) and antigen phenotype (e.g., Flow)

ITP in Pregnancy

• Autoantibodies target maternal platelets → thrombocytopenia

• IgG autoantibodies cross placenta → fetal platelet destruction

• Risk of neonatal thrombocytopenia and intracranial hemorrhage

• Pregnant patient may require treatment (e.g., IVIG, steroids) to maintain safe PLT count

• Platelet transfusion typically ineffective unless combined with immune suppression

Context: Differentiate ITP from incidental or non-immune causes (e.g., HELLP, TTP, HUS) — ITP is immune-mediated and can affect both mom and fetus.

HELLP Syndrome Overview

• Variant of preeclampsia w/ Hemolysis, Elevated Liver enzymes, Low Platelet count

• Labs:

  • ↓ Hgb/Hct, ↓ haptoglobin, ↑ LDH, ↑ bilirubin (hemolysis)

  • ↑ AST, ↑ ALT (hepatic injury)

  • PLT < 200,000/mm³ (thrombocytopenia)

  • Schistocytes on smear

• Onset: 27–36 wks (can be postpartum)

• Tx: Delivery, transfusion, corticosteroids (for lung maturity)

HELLP Pathophysiolog

• Endothelial damage + vasospasm → platelet activation

• Thromboxane A₂ → vasoconstriction

• Liver microvascular injury → ↑ AST/ALT

• Risk: DIC due to systemic clotting

HELLP Syndrome Symptoms

• Hypertension, edema

• RUQ/epigastric pain, malaise, nausea

• Proteinuria, oliguria (↓ urination)

• Visual/cerebral disturbances

• Symptoms overlap w/ preeclampsia but often more severe

Thrombotic Thrombocytopenic Purpura (TTP)

• Pentad: thrombocytopenia, microangiopathic hemolytic anemia (MAHA), neurological changes, renal dysfunction, fever

• Labs: ↓ platelets, schistocytes, ↑ LDH, ↑ bilirubin, ↓ haptoglobin

• Platelet function normal on aggregometry

Hemolytic Uremic Syndrome (HUS)

• Triad: MAHA, thrombocytopenia, acute renal failure

• Often post-E. coli O157:H7 infection (children 6 mo–4 yrs)

• Labs: schistocytes, hematuria, proteinuria, casts

• Minimal/no neurologic symptoms (vs. TTP)

TTP Mechanism

• ↓/inhibited ADAMTS13 → uncleaved ultra-large vWF multimers

• → platelet microthrombi in small vessels

• Acquired: anti-ADAMTS13 autoantibody

• Congenital: inherited ADAMTS13 deficiency

TTP vs. HUS

• Both: MAHA + thrombocytopenia

• TTP: prominent neuro symptoms, mild renal impairment, adults

• HUS: severe renal failure, minimal/no neuro signs, children (esp. post-E. coli)

The hallmark finding of Essential Thrombocythemia (ET):

• Sustained thrombocytosis

• Platelet count >600,000/µL, often >1 million/µL

• No reactive cause (e.g., inflammation, iron deficiency)

• Bone marrow shows increased megakaryocytes, often large and atypical