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

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

3. 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
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.

4. 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.

5. Platelet activation & role in primary haemostasisCa
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.

6. 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

7. 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

8. 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

9. Platelet dysfunction: clinical features
Mucosal bleeding
Epistaxis, gum bleeding, menorrhagia
Easy bruising
Petechiae, purpura
Traumatic haematomas (inc subdural)

10. Causes of platelet dysfunction:
Reduced platelet number:
thrombocytopenia <150 x 109/l
Normal numbers ( 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

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

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

13. Thrombocytopenia: decreased production
B 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.

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

15. 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

16. Case 1: 14 year old girl. Easy bruising after viral infection
Hb Plt 6 Wbc 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

17. 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

18. 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)

19. 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

20. Acute lymphoblastic leukaemia

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

22. 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
Adapted from BCSH guidelines: DIC 2009

23. 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

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

25. 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

26. 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

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

28. 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

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