Hemostasis, Platelets, and Primary Hemostasis Disorders

Fundamentals of Hemostasis and Thrombosis

Definition: Hemostasis is a complex physiologic process that maintains circulating blood in a fluid state and stops bleeding at the site of injury. The term is derived from "Hemo" meaning blood and "Stasis" meaning stop.
The Hemostatic Balance: The body must maintain a continuous balance between:
- Coagulation (clotting): The process of forming a physical barrier to stop blood loss.
- Hemorrhage (bleeding): The state where blood escapes the circulatory system.
Response to Injury: When a blood vessel is injured, three events occur:
1. A clot is produced to stop the bleeding.
2. The clot is confined to the specific site of injury.
3. The clot is dissolved as the wound heals.
Functional Properties of the System:
- Procoagulant: Substances that initiate and promote clotting.
- Anticoagulant: Substances that prevent undesirable or spontaneous clotting.
- Fibrinolytic: Proteins involved in the breakdown of the fibrin clot.

The Three Interacting Processes of Hemostasis

Primary Hemostasis: Focuses on the formation of the platelet plug. It is activated by desquamation and small injuries to blood vessels. It involves the vascular intima and platelets. The response is rapid and short-lived.
Secondary Hemostasis: Focuses on the formation of a stable fibrin clot. It is activated by large injuries to blood vessels and surrounding tissues. It involves platelets and the coagulation system. The response is delayed and long-term. The activator, tissue factor, is exposed on cell membranes.
Fibrinolysis: The process of breaking down the fibrin clot after the vessel is repaired.

Roles of Blood Vessels and Cellular Components

Blood Vessel Structure:
- Tunica Intima: The inner layer consisting of vascular endothelium. It normally prevents spontaneous clotting by releasing anticoagulant proteins but can release procoagulants upon injury.
- Tunica Media (M): The middle layer containing smooth muscle cells that contract to cause vasoconstriction, reducing blood flow to the injured area.
- Tunica Adventitia (A): The outer layer. The sub-endothelial space supports a surface protein called Tissue Factor (TF), which is critical for the initiation of coagulation.
Key Vascular Proteins and Receptors:
- ADAMTS13: A disintegrin and metalloprotease with a thrombospondin type 1 motif, member 13.
- EPCR: Endothelial cell protein C receptor.
- PAI-1: Plasminogen activator inhibitor-1.
- PGI2: Prostacyclin or prostaglandin I2.
- TAFI: Thrombin activatable fibrinolysis inhibitor.
- TFPI: Tissue factor pathway inhibitor.
- TPA: Tissue plasminogen activator.
- VWF: von Willebrand factor.
Blood Cell Roles:
- Red Blood Cells (RBCs): Add bulk and structural integrity to the final fibrin clot.
- White Blood Cells (WBCs): Help stimulate wound healing. Monocytes and lymphocytes have Tissue Factor on their surfaces that can trigger coagulation.
- Platelets (PLTs): Purely procoagulant. They do not prevent clotting or break down clots; they adhere, aggregate, and secrete granule contents to form the initial plug.

Plasma Components and Control Proteins

Zymogens: Inactive enzyme precursors that include Prekallikrein, Factor XII ( $FXII$ ), Factor XI ( $FXI$ ), Factor IX ( $FIX$ ), Factor X ( $FX$ ), Factor VII ( $FVII$ ), Prothrombin, and Factor XIII ( $FXIII$ ).
Cofactors: Accelerate the reaction rates of zymogens, including HMWK (High Molecular Weight Kininogen), Tissue Factor, Factor VIII ( $FVIII$ ), Factor V ( $FV$ ), Protein Z, Protein S, and Thrombomodulin.
Substrate: Fibrinogen is the primary substrate for the final clot.
Control Proteins / Inhibitors: Regulate the process to prevent excessive clotting. This group includes Antithrombin, Heparin cofactor II, TFPI, Protein C, $\alpha_2$ -macroglobulin, $\alpha_1$ -antitrypsin, and ZPI.

Megakaryocytopoiesis and Endomitosis

Platelet Production: Platelets are produced by megakaryocytes (MKs) in the bone marrow.
Endomitosis: A unique process where DNA is duplicated without cell division.
- Megakaryocytes become polyploid.
- Most have a ploidy of $16N$ , though the range can be $4N$ to $64N$ .
- They are the largest normally occurring cells in the marrow ( $30-100\,\mu m$ ).
- A single MK ( $8N$ or $16N$ ) can produce $2000-4000$ platelets.
Maturation Sequence:
- CFU-GEMM $\rightarrow$ Megakaryocyte-erythrocyte progenitor $\rightarrow$ BFU-Meg $\rightarrow$ CFU-Meg $\rightarrow$ LD-CFU-Meg $\rightarrow$ Megakaryoblast $\rightarrow$ Megakaryocyte.
Thrombopoiesis Process:
- Demarcation System (DMS): The plasma membrane invades the cytoplasm, creating a series of channels that form the basis for fragmentation into individual platelets.
- Proplatelet Processes: Tubules develop and squeeze through or between endothelial cells, shedding into the central vein of the bone marrow.
- The remaining megakaryocyte nucleus is reabsorbed by macrophages.
Control of Production: Thrombopoietin (TPO) is the primary hormone controlling this process. It is produced by the liver, kidney, and smooth muscle cells. When platelet counts decrease ( $\downarrow$ PLTs), circulating TPO increases ( $\uparrow$ TPO) to stimulate production.
Cell Markers: Markers for identifying MKs and PLTs include MPL, CD34, CD41, CD42, PF4, VWF, and Fibrinogen.

Platelet Structure and Function

Internal Structure:
- Surface-Connected Canalicular System (SCCS): Channels leading from the surface to the interior.
- Dense Tubular System (DTS): Storage site for calcium.
- Organelles: Microtubules, Mitochondria, Glycogen, and Perisoxomes.
Platelet Granules:
- Alpha ( $\alpha$ ) Granules: (50-80 per platelet) Contain Fibrinogen, Factor V, VWF, $\beta$ -thromboglobulin, HMWK, PAI-1, and Plasminogen.
- Dense Bodies/Granules: (2-7 per platelet) Contain ADP, ATP, serotonin, $Ca^{2+}$ , and $Mg^{2+}$ .
Platelet Receptors:
- ADP, Serotonin, Thrombin, Epinephrine.
- Collagen: GP VI, GPIa/IIa.
- vWF: GP Ib/IX/V.
- Fibrinogen: GP IIb/IIIa.
Activated State: Characterized by the formation of lamellipodia and pseudopodia.
The Hemostatic Response:
1. Adhesion: Platelets bind to the site of injury. They bind collagen directly (GPVI and GPIa/IIa) or indirectly through VWF (vWF binds GPIb/IX/V on the platelet).
2. Aggregation: Platelets bind to each other through the GPIIb/IIIa receptor. This involves shape change and membrane phospholipid "flip-flopping."
3. Secretion: Release of granular contents (mostly coagulation proteins) to act as a platform for secondary hemostasis.

Disorders of Primary Hemostasis

Qualitative vs. Quantitative: Can involve abnormal function or abnormal counts. These can be congenital (inherited) or acquired.
General Clinical Signs (Mucocutaneous Bleeding):
- Bruising, Petechiae (pinpoint red spots), Purpura (purple spots), Ecchymosis (large bruises), Epistaxis (nosebleeds), and Gingival bleeding.
Quantitative Disorders:
- Thrombocytosis: Increased platelets (> 450 \times 10^9/L).
  - Reactive: Secondary to surgery, splenectomy, iron deficiency anemia (IDA), or inflammation. Usually not associated with thrombosis.
  - Myeloproliferative: Examples include Essential Thrombocythemia or Polycythemia Vera. Can cause both thrombosis and bleeding.
- Thrombocytopenia: Platelet count < 150 \times 10^9/L. Significant bleeding risk usually occurs when counts are < 100 \times 10^9/L.
  - Causes: Decreased production, increased destruction (immunologic or mechanical), or abnormal splenic sequestration (due to splenomegaly).
Immune Thrombocytopenic Purpura (ITP):
- Destruction of platelets by antibodies against GPIIb/IIIa, GPIb/IX/V, or GPIa/IIa.
- Acute form often follows viral infections in children (recovery in weeks).
- Chronic form is more common in adult women.
Qualitative Disorders:
- Glanzmann's Thrombasthenia: Autosomal recessive; mutation in GPIIb/IIIa (fibrinogen receptor). Normal count/morphology but no aggregation with ADP, collagen, or thrombin. Ristocetin response is normal.
- Bernard Soulier Syndrome: Autosomal recessive; mutation in GPIb $\alpha$ of the VWF receptor. Features giant platelets and thrombocytopenia. Shows normal aggregation with most agonists but no response to ristocetin.
- May-Hegglin Anomaly: Autosomal dominant; characterized by large/giant platelets and basophilic inclusions in granulocytes called Döhle-like bodies. Function is usually normal.

Laboratory Diagnosis of Platelet Function

Sample Collection Requirements:
- Anticoagulant of choice: Citrate.
- Avoid short draws, specimen clots, hemolysis, and lipemia.
- Tourniquet time must be < 1 minute.
- Hold samples at room temperature ( $15-25^\circ C$ ); chilling destroys platelet activity.
Platelet Function Analysis (PFA-100):
- Replaces Bleeding Time. Measures the time required for a platelet plug to occlude an aperture coated with Collagen/Epinephrine (CEPI) or Collagen/ADP (CADP).
Optical Aggregometry:
- Uses Platelet Rich Plasma (PRP). A photometer measures light transmission.
- As platelets aggregate, the plasma becomes clearer, and light transmission increases.
- Agonist Responses:
  - ADP: Normal ( $N$ ) in BSS, decreased ( $\downarrow$ ) in GT.
  - Collagen: Normal in BSS, decreased in GT.
  - Ristocetin: Decreased in BSS, normal in GT.
Storage Pool Defects: Characterized by deficiencies in dense granules, often leading to abnormal PFA-100 results and decreased aggregation.

Clinical Cases and Discussion

Case 1 Discussion

Patient: 35-year-old woman with petechiae, purpura, and ecchymoses.
Lab Data: Platelet count $35 \times 10^9/L$ ; MPV $13.2\,\text{fL}$ ; large platelets (> 6\,\mu m); bone marrow shows 10-12 small unlobulated megakaryocytes per LPF.
Questions:
1. Do these signs and symptoms indicate mucocutaneous (systemic) or anatomic bleeding?: The presentation of petechiae and purpura indicates mucocutaneous bleeding.
2. What is the probable cause of the bleeding?: The severe thrombocytopenia ( $35 \times 10^9/L$ ).
3. Does the patient’s bleeding result from altered platelet production in the bone marrow?: No, the presence of megakaryocytes in the marrow suggests the issue is likely peripheral destruction (e.g., ITP).

Case 2 Discussion

Patient: 55-year-old man with epistaxis and inflammatory hemarthroses. Physical exam shows swollen knees, mild jaundice, and hepatosplenomegaly.
Lab Data: Anemia; thrombocytopenia ( $74,400/\mu L$ ); PT $18\,\text{s}$ (RI: $12-14\,\text{s}$ ); aPTT $43\,\text{s}$ (RI: $25-35\,\text{s}$ ).
Questions:
1. What is the most likely diagnosis?: The presence of jaundice, hepatosplenomegaly, prolonged PT/aPTT, and thrombocytopenia suggests end-stage liver disease or cirrhosis leading to multiple coagulation factor deficiencies.
2. What treatment does the patient need?: Treatment focus would be on the underlying liver disease and possible replacement of factors/platelets.

Case 3 Discussion

Patient: 19-year-old woman with easy bruising and heavy periods, worsened after aspirin ingestion. Family history of similar symptoms.
Aggregometry: Near normal ristocetin/ADP; absent arachidonic acid aggregation; decreased collagen; epinephrine shows only primary wave.
Further Testing: Firefly luciferin-luciferase assay shows marked decrease in ATP release stimulated by thrombin.
Questions:
1. Possible explanations for test results?: Aspirin effect, Storage Pool Disease, or Release defect.
2. Likely cause based on ATP release?: Storage Pool Disease (specifically Dense Granule deficiency), as indicated by the lack of ATP release.

Case 4 Discussion

Patient: 73-year-old woman with cirrhosis, DIC, Hepatorenal syndrome. Average platelet count in the low $20\text{s} \times 10^6/\mu L$ .
Clinical Problem: Platelet transfusions failed to increase the count for a central line placement.
Questions:
1. Likely cause of thrombocytopenia?: A combination of decreased production, consumption (DIC), and splenic sequestration due to cirrhosis.
2. Tests to order?: HLA antibody screening or crossmatching for platelets to determine Refractoriness.
3. Are specialized platelets better?: If the patient has developed HLA antibodies due to previous pregnancies/transfusions (Refractoriness), HLA-matched platelets are required for successful increments.