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Platelets have ________ , which are released during platelet activation. They help to recruit other platelets, and platelets do not work very well if they don't have these.
What are the two types of platelet granules?
- Dense Granules
- Alpha Granules
What are the two contractile proteins in Platelets?
Platelet granules are released to the exterior via what system? When activated, the platelet granules move to the center of the platelet and then release their contents into this system.
What hemostatic system is related to immediate bleeding and to mucocutaneous bleeding (petechiae, epistaxis)?
Primary Hemostatic System
What hemostatic system is linked to delayed bleeding and to deep visceral hemotomas or hemarthroses?
Secondary Hemostatic System
What is the platelet surface receptor for von Willebrand Factor?
What is the platelet surface receptor for Fibrinogen?
What molecule is a potent platelet agonist and vasoconstrictor?
Thromboxane A2 (TxA2)
In platelet prostaglandin metabolism, platelet membrane phospholipid is converted to arachidonic acid, then the arachidonic acid is eventually converted to thromboxane A2. What enzyme catalyzes the first reaction (Phospholipid --> Arachidonic Acid)?
In platelet prostaglandin metabolism, platelet membrane phospholipid is converted to arachidonic acid, then the arachidonic acid is eventually converted to thromboxane A2. What enzyme catalyzes the second reaction (Arachidonic Acid --> Prostaglandin Endoperoxides)?
In platelet prostaglandin metabolism, platelet membrane phospholipid is converted to arachidonic acid, then the arachidonic acid is eventually converted to thromboxane A2. Cyclooxygenase catalyzes the second reaction (Arachidonic Acid --> Prostaglandin Endoperoxides). What inhibits cyclooxygenase, and thus thromboxane A2, and thus platelet function?
In platelet prostaglandin metabolism, platelet membrane phospholipid is converted to arachidonic acid, then the arachidonic acid is eventually converted to thromboxane A2. What enzyme catalyzes the third reaction (Prostaglandin Endoperoxides --> Thromboxane A2)?
What inherited disorder of platelet function is caused by a lack of von Willebrand factor receptor?
What inherited disorder of platelet function is caused by a lack of a fibrinogen receptor?
What two inherited disorders of platelet function are caused by a lack of platelet granules?
- Grey Platelet Syndrome
- Storage Pool Disease
What is likely the most common acquired disorder of platelet function? What are a couple others?
- Uremia, & Cardiopulmonary Bypass
Which are more common, inherited or acquired disorders of platelet function?
What are two common and well-recognized offenders in acquired disorders of platelet function? What are some other of these agents?
- NSAIDS & Clopidogrel (Plavix)
- Ticlopidine, Prasugrel, RheoPro
What is an important mechanism of Platelet Defects in Uremia? In this subtype of platelet defect, the frequency of bleeding in unclear, and the cause is not completely understood. This is a disorder of ___________ metabolism, and there are poorly characterized substances in uremic plasma.
- Anemia: defective clearance of NO by RBCs (NO is a platelet antagonist & vasodilator)
What are the four treatment options for platelet defects in uremia?
- Erythropoietin: to raise hemoglobin
- Dialysis: to get rid of uremic toxins
- Cryoprecipitate: contains FVIII & vWF; helps, but don't know why
- DDAVP: causes epithelium to release FVIII & vWF
What is it called when the platelet count reaches the upper limit of the reference range?
Thrombocytosis is pretty common, but generally the cause is pretty obvious and by itself this is no big deal. What are the three variants of this?
- Artifactual: RBC or WBC fragments
- Secondary (reactive) Thrombocytosis
- Myeloproliferative Neoplasms (essential thrombocythemia (ET))
Which non-artifactual variant of Thrombocytosis doesn't cause any problems (the person doesn't appear to be predisposed to getting thrombosis)? Which type can be associated with a significant risk of thrombosis and can therefore be significant?
- Secondary (Reactive) Thrombosis
- Myeloproliferative Neoplasm-related Thrombocytosis
Virtually any type of __________ or __________ can be associated with (or a cause of) secondary thrombocytosis.
Inflammation or Infection (infections can also be a cause of thrombocytopenia)
What is a platelet count below the reference range referred to as? People do not generally start having problems with this until platelet count reaches what level?
- around 5,000-10,000/ microliter
Thrombocytopenia is common, particularly in hospitalized patients. It is clinically significant because it can predispose to what?
What are the three mechanisms of Thrombocytopenia?
- Impaired production
- Accelerated destruction or utilization
- Increased sequestration (spleen) or abnormal distribution
Normally about 1/3 of platelets are sequestered in the spleen -- this is in equilibrium with circulating platelets. However, the proportion of platelets in the spleen increases with __________. Is this associated with bleeding? What two other phenomenons is this often associated with?
- Leukopenia & Anemia
What type of Thrombocytopenia is usually due to platelet clumping or clotting in the specimen (or, less often, platelet satellitism around neutrophils)?
Artifactual Thrombocytopenia (Pseudothrombocytopenia)
Congenital or inherited thrombocytopenias are rare, but do occur. What are they often associated with?
_____________ must always be excluded before starting on workup or treatment for thrombocytopenia!
Platelet clumping is due to agglutination of platelets in blood anticoagulated with _______. The IgG antibody aggregates platelets in presence of extremely low calcium. To combat this, a repeated platelet count should be done on specimen anticoagulated with what?
What are four causes of Thrombocytopenia that is neonatal but not inherited?
- In Utero Infections -- rubella, CMV, varicella, hepatitis, etc
- Other Infections
- Maternal Drug Consumption -- thiazides, oral hypoglycemic agents, ethanol, others
- Maternal Antibodies to Fetal Platelets -- immune thrombocytopenic purpura (ITP), other immune diseases
This is a common cause of thrombocytopenia. It can occur as an idiopathic phenomenon, as part of a systemic autoimmune disease (SLE), due to drugs, or others. In most cases an antibody coats the platelets in the circulation, then the spleen phagocytizes and chews up the platelets. In some cases the antibodies can react with the megakaryocytes in the bone marrow, which can result in decreased platelet production as well as increased platelet destruction.
What are some of the common causes of Immune Thrombocytopenia?
- Idiopathic Immune Thrombocytopenic Purpura (ITP)
- Infections -- especially HIV
- Autoimmune Diseases -- SLE
- Lymphoproliferative Disorders
A large number of drugs are implicated in drug-related immune thrombocytopenia. What are the most commonly associated drugs?
- Trimethoprim-sulfamethoxazole (Bactrim)
How is drug-related immune thrombocytopenia diagnosed?
It is a diagnosis of exclusion -- careful drug and medication history are required
How is drug-related thrombocytopenia treated?
- STOP THE DRUG: recovery is usually prompt after (platelet count normalizes in about 1 week)
- IVIG or plasmapheresis may be helpful for acute bleeding
- Thrombocytpenia due to gold compounds may be prolonged
Which type of Heparin-Induced Thrombocytopenia is relatively common, has its onset within the first 1-4 days, and is not associated with any significant sequela?
Which type of Heparin-Induced Thrombocytopenia is rare, has an onset that occurs after 5-7 days, and is a more severe form, often associated with thrombosis?
Type 2 (HIT; HITT)
Thrombosis is a major complication of Type 2 Heparin-Induced Thrombocytopenia (HIT). Which is more common, venous or arterial thrombosis? What lab tests can be used to diagnose this?
- Venous -- PE common; large arteries of lower limbs may be affected
- HIT Antibody & Serotonin Release Assay (SRA)
Does a positive HIT antibody test mean that the patient has Heparin-Induced Thrombocytopenia?
No, it is a "rule out" test only
What is the treatment for Heparin-Induced Thrombocytopenia?
- Stop Heparin
- Direct Thrombin Inhibitor -- Argatroben, Lepirudin, Bivalirudin
What is an isolated thrombocytopenia with no clinically apparent associated disorder? This is a diagnosis of exclusion. What must first be ruled out?
- Idiopathic (immune) Thrombocytopenic Purpura (ITP)
- Must exclude -- infection, drug-related, autoimmune, & primary hematologic disease
Which type of Idiopathic (immune) Thrombocytopenic Purpura (ITP) is more common in children, and usually follows an infection, often viral? This type is usually transient and self-limited, with rare recurrences.
Acute: duration < 6 months
Which type of Idiopathic (immune) Thrombocytopenic Purpura (ITP) is more common in adults and is of an insidious onset? This tends to be chronic, with frequent relapses.
Chronic: duration > 6 months
What is the most common cause of isolated thrombocytopenia in young, otherwise healthy adults? The ratio of females is greater than males for this disease, and the peak age of onset is typically ~18-40 years. The majority of cases of this are chronic, with frequent recurrences, and the onset is usually insidious. What are two common signs of this?
- Idiopathic (immune) Thrombocytopenic Purpura (ITP)
- Petechiae & Ecchymoses (also, heavy menstrual bleeding common in women)
The diagnosis of Idiopathic (immune) Thrombocytopenic Purpura (ITP) is largely clinical, and one of exclusion. PE may reveal modest __________, with no __________. CBC and blood smear reveal no immature or abnormal _______, and no ____________.
What is the first line of defense for treating Idiopathic (immune) Thrombocytopenic Purpura (ITP)? What are the two next options?
- Corticosteroids (prednisone) -- most relapse after tapering
- Rituximab & Splenectomy
This is a heterogenous group of conditions characterized by a combination of: microangiopathic hemolysis, thrombocytopenia, plus or minus systemic manifestations -- renal failure, fever, neurologic symptoms. There is a variety of causes, and a variable severity and outcome.
The presence of schistocytes on a blood smear is term _________ _________.
This is a systemic disease associated with the deficiency of an enzyme called the vWF cleaving enzyme (also called ADAMTS13). when vWF is released from endothelial cells it is in the form of extremely large multimers, which are then normally cleaved into smaller multimers by this enzyme. The extremely large vWF multimers have the capability to induce spontaneous platelet aggregation, so they aggregate in small blood vessels all over the body. This is idiopathic and familial.
Thrombotic Thrombocytopenic Purpura (TTP)
This is most often caused by an infection produced by a bacteria which produces a Shiga toxin. This can be produced by several different species of bacteria, including certain strains of E. coli, Shigella, and others. The Shiga toxin is absorbed into the blood stream from the gut; it binds to and damages endothelial cells all over the body, but particularly the endothelial cells in capillaries of glomeruli of the kidney. Thus, it features prominent renal involvement.
It is a syndrome of renal abnormalities and microangiopathic hemolysis. It is most common in childhood. It frequently follows an episode of bloody diarrhea, and may occur in epidemics.
Hemolytic Uremic Syndrome (HUS)
What are the diagnostic criteria for Thrombotic Thrombocytopenic Purpura (TTP)?
- Microangiopathic Hemolysis (schistocytes)
- No Obvious Precipitating Cause
- Largely a diagnosis of exclusion
What is the treatment for Thrombotic Thrombocytopenic Purpura (TTP)?
What is the treatment for Hemolytic Uremic Syndrome (HUS) for children and adults?
- Children: primarily supportive -- hemodialysis to control BP; usually recover spontaneously
- Adults: Plasma Exchange
This is a mild drop in platelets that is common in pregnancy. It usually occurs during the third trimester. The platelet count usually remains above 100,000/cmm, and is almost never below 70,000/cmm. This is of no clinical significance.