Problematic platelets Phase 2a Lecture 11th December 2015 University of Sheffield Medical School Dr Stephanie Stone Spr Haematology Sheffield Teaching Hospitals www.ifm.liu.se

Overview Normal physiology of platelets Platelet abnormalities – focus on thrombocytopenia Clinical features of thrombocytopenia and platelet disorders Important causes of thrombocytopenia

Platelets Anucleate cells formed by fragmentation of megakaryocyte (MK) cytoplasm in bone marrow Disc shape allows them to flow close to endothelium Important role in primary haemostasis Life span 7-10 days Old platelets are phagocytosed by splenic macrophages in red pulp www.studyblue.com Platelets have no nucleus and are formed when mature megakaryocyte cytoplasm fragments within the bone marrow. Platelets are shaped like a disc, which allows them to flow more slowly in blood vessels, close to the endothelial surface. This means they are ready to aid in stopping bleeding if damage to the endothelium occurs at the site of injury. Their life span is much shorter than red blood cells, and older platelets are removed by the spleen. In patients who have had their spleen removed, you often see a rise in their platelet count as older platelets are allowed to remain in circulation for longer. Similarly, if a patient has an enlarged spleen, they may have a reduction in their platelet count, as more platelets are being removed.

Control of platelet production TPO Thrombopoietin (TPO): produced mainly by liver Stimulates production of platelets by megakaryocytes Binds to platelet and MK receptors ↓plts = less bound TPO = ↑ free TPO able to bind to MK = ↑ Plt prodn TPO Thrombopoietin is a hormone mainly produced by the liver. It stimulates the production of platelets. In patients with significant liver disease, the production of this hormone will be reduced, resulting in one mechanism for why they may have thrombocytopenia. www.studyblue.com

Platelet activation & role in primary haemostasis Ca Collagen vWF Release of granule contents Fibrin clot Xa-Va Ca2+ fibrinogen Spreading ADP Aggregation thrombin Platelet physiology - following damage to endothelium: 1) Platelets adhere to vascular endothelium via collagen & vWF (von Willebrand factor) 2) Binding of platelets to collagen stimulates cytoskeleton shape change within the platelets, and they ‘spread’ out 3) This increases their surface area and results in their activation, leading to the release of platelet granule contents including ADP, fibrinogen, thrombin and calcium. These components facilitate the clotting cascade ending with the production of fibrin. 4) Aggregation of platelets then occurs, which involves the cross-linking of activated platelets by fibrin 5) Activated platelets also provide a negatively charged phospholipid surface, which allows coagulation factors to bind and enhance the clotting cascade. www.ifm.liu.se

Fibrin Clotting cascade APTT PT Va Xa P Thrombin Prothrombin (II) XII XI VII IX VIII Va Xa Prothrombin (II) Platelet granules contain contents that help the formation of fibrin, and provide a surface for clotting factors Va and Xa to function better. Thrombin Fibrin Fibrinogen (I) P

Other important platelet receptors & functions Thromboxane A2 (TXA2) synthesized from Arachidonic acid in platelets via cyclooxygenase (COX)-1 induces platelet aggregation & vasoconstriction P2Y12: receptor on platelets activated by ADP amplifies activation of platelets & helps activate Gp IIbIIIa Gp IIbIIIa: acts as a receptor for fibrinogen and vWF aids platelet adherence and aggregation

Medications affecting platelet function Tirofiban Clopidogrel P2Y12 Gp IIbIIIa Arachidonic acid Aspirin This diagram shows the mechanism of action for three antiplatelet agents. PGH2 = prostaglandin H2. PGI2 = Prostaglandin I2 PGH2 COX 1 TXA2 COX 2 PGI2

Platelet dysfunction: clinical features Mucosal bleeding Epistaxis, gum bleeding, menorrhagia Easy bruising Petechiae, purpura Traumatic haematomas (inc subdural) www.petechialrash.net www.gumsbleeding.com www.pediatrics.wisc.edu:

Causes of platelet dysfunction: Reduced platelet number: thrombocytopenia <150 x 109/l Normal numbers (150-400 x 109/l) but reduced function: - Congenital abnormality in platelet function - Medication e.g aspirin - von Willebrand disease (reduced VWF activity) - Uraemia Decrease in production Increase in destruction

Thrombocytopenia: decreased production Congenital thrombocytopenia Absent / reduced / malfunctioning megakaryocytes in BM Infiltration of bone marrow Leukaemia, metastatic malignancy, lymphoma, myeloma, myelofibrosis

Bone marrow infiltration Metastatic malignancy: breast, prostate Leukaemia / lymphoma / myeloma Myelofibrosis

Thrombocytopenia: decreased production B12 Folate Reduced platelet production by bone marrow Low B12 / folate Reduced TPO (e.g. liver disease) Medication: Methotrexate, chemotherapy Toxins: e.g. Alcohol Infections: e.g. viral (e.g. HIV) TB Aplastic anaemia (auto immune) Dysfunctional production of platelets in BM Myelodysplasia DNA synthesis HIV inhibits plt production and causes immune destruction.

Thrombocytopenia: Increased destruction Autoimmune: Immune thrombocytopenia (ITP) Primary, or secondary Hypersplenism: Portal hypertension, splenomegaly Drug related immune destruction: E.g. Heparin induced thrombocytopenia

Thrombocytopenia: Increased destruction Consumption of platelets: Disseminated intravascular coagulopathy (DIC) Thrombotic thrombocytopenic purpura (TTP) Haemolytic uraemic syndrome (HUS) Haemolysis, elevated liver enzymes and low platelets (HELLP) Major haemorrhage

Case 1: 14 year old girl. Easy bruising after viral infection Hb 130 Plt 6 Wbc 5.2 U+E normal LFTs normal Clotting normal Examination: bruises but no LN or hepatosplenomegaly Blood film: reduced platelets but nil else Most likely diagnosis? Mechanism of thrombocytopenia? Immune thrombocytopenia Increased destruction www.180degreehealth.com

Immune thrombocytopenia IgG antibodies form to platelet and megakaryocyte surface glycoproteins Opsonized platelets are removed by reticuloendothelial system Primary: May follow viral infection / immunisation esp in children Secondary: Occurs in association with some Malignancies, such as Chronic Lymphocytic Leukaemia (CLL) Infections e.g. HIV / Hep C

Immune thrombocytopenia Investigations: Any underlying cause? Diagnosis of exclusion Treatment: Immunosuppression e.g. steroids / IVIG Treat underlying cause If bleeding – give platelets - but will disappear quickly Tranexamic acid Inhibits breakdown of fibrin Good for mucosal bleeding but NOT if urinary tract bleeding (clot retention)

Case 2: 3 year old with easy bruising and gum bleeding Hb 80 Plt 6 Wbc 2 Neut 0.3 Clotting / LFTs / UE normal Small cervical lymph nodes palpable Widened mediastinum on CXR Possible diagnoses? Mechanism of thrombocytopenia? Acute Lymphoblastic Leukaemia Reduced production

Acute lymphoblastic leukaemia https://en.wikipedia.org/wiki/Acute_lymphoblastic_leukemia

Case 3: 44 year old diabetic on HDU with gram –ve sepsis Hb 102 Plt 54 Wbc 28 Neut 22 Crp 380 PT 18 APTT 40 Fib 0.9 D-dimer 50,000 Blood film: ‘toxic’ neutrophils Diagnosis? Mechanism of thrombocytopenia? Disseminated Intravascular Coagulopathy Increased consumption

Pathophysiology of DIC Cytokine release in response to SIRS (Sepsis, trauma, pancreatitis, obstetric emergency, malignancy) Systemic activation of Consumption of platelets and clotting cascade clotting factors Microvascular thrombosis Bleeding Organ failure www.coolfacts.in Adapted from BCSH guidelines: DIC 2009 www.storybird.com

Disseminated Intravascular Coagulation Investigations: Underlying cause Thrombocytopenia, prolonged PT + APTT, low fibrinogen, high d-dimers +/- evidence of organ failure Treatment: Treat underlying cause Supportive provision of: Platelets FFP: contains clotting factors Cryoprecipitate: contains fibrinogen and some clotting factors

Case 4: 68 year old man. Platelets fall to 46 on day 5 after CABG Hb 105 Plt 46 Wbc 10 Clotting normal Crp 50 Creatinine 120 LFTs normal Noted to have a new swollen left leg: DVT found Differential diagnosis? Mechanism of thrombocytopenia? www.medpagetoday.com Heparin Induced Thrombocytopenia Increased destruction

Heparin Induced Thrombocytopenia Development of an IgG antibody against a complex formed between Platelets + Heparin IgG/PF4/Heparin complexes bind to and activate platelets Platelet consumption Thrombosis (arterial or venous), skin necrosis www.anaesthesiamd.blogspot.com

Heparin Induced Thrombocytopenia Most at risk: After cardiac bypass surgery Unfractionated Heparin treatment Usual presentation: Sharp fall in platelets 5-10 days after starting Heparin treatment Life threatening – need to stop UFH / LMWH Heparin immediately Alternative anticoagulation (even if platelets low) Never re-expose patient to Heparin

Medications affecting platelet function Tirofiban Clopidogrel P2Y12 Gp IIbIIIa Arachidonic acid Aspirin PGH2 COX 1 TXA2 COX 2 PGI2

Summary Platelets – highly active cells Life cycle: BM - circulation - spleen Production under influence of TPO Dysfunction in platelet activity: Too few around: thrombocytopenia Decreased production Increased consumption or destruction Reduced function: E.g. Aspirin www.ftalk.us

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