Platelet Disorders

Platelets are small anucleate cells produced predominantly by the bone marrow megakaryocytes as a result of budding of the cytoplasmic membrane. Megakaryocytes are derived from the haemopoietic stem cell, which is stimulated to differentiate to mature megakaryocytes under the influence of various cytokines, including thrombopoietin. Platelets play a key role in securing primary haemostasis.

Once released from the bone marrow, young platelets are trapped in the spleen for up to 36 hours before entering the circulation, where they have a primary haemostatic role. Their normal lifespan is 7–10 days and the normal platelet count for all age groups is 150–450 × 10^9/L. The mean platelet diameter is 1–2 µm, and the normal range for cell volume (mean platelet volume; MPV) is 8–11 fL. Although platelets are non-nucleated cells, those that have recently been released from the bone marrow contain RNA and are known as reticulated platelets. They normally represent 8–16% of the total count and they indirectly indicate the state of marrow production.

Normal haemostasis

The platelet membrane containss integral glycoproteins essential in the initial events of adhesion and aggregation, leading to the formation of the platelet plug during haemostasis (Fig. 7.1).

Glycoprotein receptors react with aggregating agents, such as collagen on the damaged vascular endothelial surface and fi brinogen and von Willebrand factor (VWF), to facilitate platelet–platelet and platelet–endothelial cell adhesion. The major glycoproteins are the Ib/IX complex, the main binding protein of which is VWF, and IIb/IIIa, which specifically binds fibrinogen. Storage organelles within the platelet include the ‘dense’ granules, which contain nucleotides, 

Figure 7.1 Normal platelet function

calcium and serotonin, and αgranules containing fibrinogen, VWF, platelet-derived growth factor and many other clotting factors. Following adhesion, the platelets are stimulated to release the contents of their granules, essential for platelet aggregation. The platelets also provide an extensive phospholipid surface for the interaction and activation of clotting factors in the coagulation pathway.

Congenital abnormalities

Congenital abnormalities of platelets can be divided into disorders of platelet production and those of platelet function. All are very rare. In general, they cause moderate to severe bleeding problems. 

Increasingly, the molecular basis for these disorders has been characterized and therefore can be used as a diagnostic tool, and may facilitate antenatal diagnosis.

Fanconi’s anaemia

Fanconi’s anaemia is an autosomal recessive preleukaemic condition, which often presents as thrombocytopenia with skeletal or genitourinary abnormalities. The cardinal laboratory feature is abnormal chromosomal fragility. The condition can be cured with bone marrow transplantation (BMT).

Thrombocytopenia with absent radii

Thrombocytopenia with absent radii (TAR) syndrome presents with the pathognomonic sign of bilateral absent radii (Fig. 7.2) and with severe (< 10 × 10^9/L) neonatal thrombocytopenia, although this often improves after the first year of life. This should be distinguished

Figure 7.2 Amegakaryocytic thrombocytopenia with absent radii (TAR syndrome) 

from amegakaryocytic thrombocytopenia, another leukaemia predisposition syndrome, in which severe neonatal thrombocytopenia is present with orthopaedic or neurological abnormalities in 10–30% of children. The underlying genetic abnormality, located to the cmpl gene on chromosome 1, affects the thrombopoietin receptor.

Wiskott–Aldrich syndrome

This is an X-linked disorder with a triad of thrombocytopenia, eczema and T-cell immunodeficiency. The platelet count is usually 20–100 × 10^9/L, and the platelets are small and functionally abnormal. 

The diagnosis can be confirmed by analysis of the WASgene (Xp11). 

Like Fanconi’s anaemia, this condition can only be cured with BMT.

MYH9-related thrombocytopenias

MYH9-related thrombocytopenias, including the May Hegglin anomaly, are autosomal dominant conditions associated with macrothrombocytopenia. The genetic abnormality is in the MYH9 gene, located on chromosome 22. Variants of Alport’s syndrome are also characterized by giant platelets, associated with progressive hereditary nephritis and deafness (Fig. 7.3).

Disorders of the surface membrane

Disorders of the surface membrane are characterized by absence or abnormalities of platelet membrane glycoproteins, resulting in defective platelet adhesion and aggregation. These disorders include Bernard–Soulier syndrome, an autosomal recessive condition, and macrothrombocytopenia, a lack of VWF-dependent platelet agglutination, linked to genetic lesions of the glycoprotein (Gp)Ib/IX/V complex. Glanzmann’s thrombasthenia is associated with abnormalities of the GpIIb/IIIa complex. In platelet-type von Willebrand’s disease, spontaneous binding of plasma VWF to enlarged platelets results from mutations of GpIbα(Fig. 7.4).

Platelet storage pool diseases 

Deficiencies in either the αor dense granules cause poor secondary platelet aggregation. Absence of αgranules in Grey Platelet Syndrome, an autosomal dominant inherited condition, results in large, pale platelets on blood films.

Figure 7.3 Giant granular platelets in peripheral blood fi lm as seen in Bernard-Soulier syndrome or May Hegglin anomaly 

Figure 7.4  Site of abnormality in congenital platelet disorders.

Other conditions 

There are also a variety of further specific surface membrane defects and internal enzyme abnormalities, which, although difficult to define, can cause troublesome chronic bleeding problems (Fig. 7.5).

Acquired abnormalities 

Decreased production of platelets 

Decreased platelet production caused by suppression or failure of the bone marrow is the commonest cause of thrombocytopenia. In aplastic anaemia, leukaemia and marrow infiltration, and after chemotherapy, thrombocytopenia is usually associated with a failure of red and white cell production, but may be an isolated finding secondary to drug toxicity (penicillamine, cotrimoxazole), alcohol, or viral infection (human immunodeficiency virus, infectious mononucleosis). Viral infection is the most common cause of mild transient thrombocytopenia (Box 7.1).

Figure 7.5  Bleeding around  the eye in a patient  with Bernard-Soulier syndrome.

Increased consumption of platelets 

Increased platelet consumption may be due to immune or non-immune mechanisms. 

Idiopathic thrombocytopenic purpura 

Idiopathic thrombocytopenic purpura is a relatively common disorder and is the most frequent cause of an isolated thrombocytopenia without anaemia or neutropenia. In adults, it often presents insidiously, most frequently in women aged 15–50 years, and can be associated with other autoimmune diseases, in particular systemic lupus erythematosus or the primary antiphospholipid syndrome. In children, the onset is more acute and often follows a viral infection. The autoantibody produced is usually IgG, directed against antigens on the platelet membrane. Antibody-coated platelets are removed by the reticuloendothelial system, reducing the lifespan of the platelets to a few hours. The platelet count can vary from <5 × 10^9/L to near normal. The severity of bleeding is less than that seen with comparable degrees of thrombocytopenia in bone marrow failure, owing to the predominance of young, larger and functionally supe- rior platelets (Figs 7.6 and 7.7, Box 7.2).

Figure 7.6  Spontaneous skin purpura  in severe autoimmune thrombocytopenia.

Figure 7.7  Bone marrow aspirate showing increased megakarocytes in immune thrombocytopenia

Post-transfusion purpura

Post-transfuson purpura is a rare complication of blood transfusion. It presents with severe thrombocytopenia 7–10 days after the transfusion and usually occurs in multiparous women who are negative for the human platelet antigen (HPA)1a. Antibodies to HPA1a develop and, in some way, this alloantibody is responsible for the immune destruction of autologous (patient’s own) platelets

Neonatal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia is similar to haemolytic disease of the newborn except that the antigenic stimulus comes from platelet specific antigens rather than red cell antigens. In 80% of cases, the antigen is HPA1a and mothers who are negative for this antigen (about 5% of the population) form antibodies when sensitized by a fetus positive for the antigen. Fetal platelet destruc- tion results from transplacental passage of these antibodies, and severe bleeding, including intracranial haemorrhage, can occur in utero. Firstborns are frequently affected and successive pregnancies are equally or more affected.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia occurs during unfractionated heparin therapy in up to 5% of patients, but is less frequently associated with low molecular weight heparins. It may become manifest when arterial or venous thrombosis occurs during a fall in the platelet count and is thought to be due to the formation of antibodies to heparin that are bound to platelet factor 4, a platelet granule protein. The immune complexes activate platelets and endothelial cells, resulting in thrombocytopenia. Heparin-induced thrombocytopenia carries an appreciable morbidity and mortality, especially from resulting thrombosis, if the diagnosis is delayed.

Thrombotic thrombocytopenic purpura

The hallmarks of thrombotic thrombocytopenic purpura are thrombocytopenia and microangiopathic haemolytic anaemia with clinical symptoms affecting any organ, but primarily manifesting as neuro- logical symptoms, resulting from microvascular thrombosis. The condition is associated with deficiency of ADAMTS 13, a metalloprotease enzyme responsible for cleaving the ultra-high molecular weight multimers of VWF. The condition is suspected clinically by thrombocytopenia, red cell fragmentation on the blood film and a reticulocytosis. The demonstration of an abnormal pattern of von Willebrand multimers makes the diagnosis highly likely and the complete absence of the cleaving protease caused by an inhibitory anti- body can be proven in some specialized laboratories (Fig. 7.8).

Figure 7.8  Red cell fragmentation in patient  who presented with confusion and lethargy in whom thrombotic thrombocytopenic purpura  was diagnosed. She responded well to large volume plasma exchange  for one week.

Microangiopathic thrombocytopenia

Microangiopathic thrombocytopenia includes disorders such as preeclampsia or HELLP (haemolysis, elevated liver enzymes, low platelets) syndrome in pregnancy, haemolytic uraemic syndrome, disseminated intravascular coagulation (DIC) and catastrophic antiphospholipid syndrome. The blood films may be similar in all these disorders, with thrombocytopenia, anaemia and fragmented red blood cells. 

Disseminated intravascular coagulation 

DIC usually occurs in critically ill patients as a result of catastrophic activation of the coagulation pathway, often due to sepsis. Widespread platelet consumption occurs, causing thrombocytopenia.

Massive blood transfusion 

In patients with life-threatening bleeding, transfusion of 8–10 units of red blood cells without replacing clotting factors or platelets may result in prolonged clotting screen and thrombocytopenia.

Massive splenomegaly 

The spleen normally pools about a third of the platelet mass, but in massive splenomegaly this can increase up to 90%, resulting in apparent thrombocytopenia.

Drugs 

Aspirin, non-steroidal anti-inflammatory agents and glycoprotein IIb/IIIa antagonists are the most common causes of acquired platelet dysfunction (Box 7.3). For this reason, aspirin and the IIb/IIIa antagonists are used therapeutically as antiplatelet agents. Aspirin acts by irreversibly inhibiting cyclo-oxygenase activity in the platelet, resulting in impairment of the granule release reaction and defective aggregation. The effects of a single dose of aspirin last for the lifetime of the platelet (7–10 days). Recently, clopidogrel, a thienopyridine derivative, has been introduced as an oral antiplatelet agent that inhibits adenosine diphosphate binding to the platelet membrane and is useful in patients who are intolerant or resistant to aspirin. It is becoming widely used as a prophylactic agent for myocardial ischaemia and related coronary syndromes.

Bleeding in uraemic patients 

Bleeding most commonly results from defects in platelet adhesion or aggregation, although thrombocytopenia, severe anaemia with packed cell volume < 20% or coagulation defects can also contrib- ute.

Essential (primary) thrombocytosis and reactive (secondary) thrombocytosis

In these conditions, the platelet count is raised above the upper limit of normal. A wide range of disorders can cause a raised platelet count (> 800 × 10^9/L) but patients are normally asymptomatic, except in essential thrombocytosis, when excessive spontaneous bleeding may develop when the count exceeds 1000 × 10^9/L. Antiplatelet drugs can be useful to prevent thrombosis in high risk patients, for example, postoperatively. Some myelodysplastic syndromes may be complicated by an acquired storage pool-type platelet disorder (Box 7.4).

History and examination of patients

Abnormal bleeding associated with thrombocytopenia or abnormal platelet function is characterized by spontaneous skin purpura and ecchymoses, mucous membrane bleeding and protracted bleeding after trauma. Prolonged nosebleeds can occur, particularly in children, and menorrhagia or postpartum haemorrhage is common in women. Rarely, subconjunctival, retinal, gastrointestinal, genitourinary and intracranial bleeds may occur. In thrombocytopenic patients, severe spontaneous bleeding is unusual with a platelet count ≥ 20 × 10^9/L, unless there is associated platelet dysfunction.

Investigations

The investigations in a suspected platelet disorder will depend on the presentation and history in each patient. If the bleeding is severe, the patient may need urgent hospital referral for prompt evaluation, diagnosis and treatment, which may entail blood product support. All patients should have a full blood count, coagulation and biochemical screen, and then further investigations depending on the results of these. A thorough review of the blood film can help in the diagnosis or exclusion of many disorders associated with thrombocytopenia (Fig. 7.9).

Thrombocytopenia can be artefactual and due to platelet clumping or a blood clot in the sample, which should be excluded in all cases. The skin bleeding time, which is invasive, variable and not reliable in screening mild platelet disorders, has been replaced by devices that perform an in vitro bleeding time on small volumes of citrated blood and simulate platelet function in a high shear rate situation. The sensitivity of these devices for all platelet disorders is still under investigation.

Figure 7.9  Investigation of suspected platelet disorder

Management

All serious bleeding due to a platelet disorder needs haematological assessment and treatment. Mild or trivial bleeding due to a transient postviral thrombocytopenia or aspirin ingestion needs no active treatment and can be managed on an outpatient basis (Box 7.5).

Congenital disorders

A neonate or small infant with bleeding must be referred for evaluation as the inherited bleeding disorders (e.g. haemophilia or von Willebrand’s disease) and platelet disorders can present at a very young age.

Severe bleeding episodes in all the congenital thrombocytopenias and platelet function disorders require filtered human leucocyte antigen-compatible platelet transfusions to secure haemostasis, although, in minor episodes in the dysfunctional syndromes, desmopressin (1-deamino-8-d-arginine vasopressin; DDAVP) given intravenously or intranasally with antifibrinolytics (tranexamic acid) may be sufficient. There is increasing evidence that, in selected patients with congenital disorders, recombinant factor VIIa (Novoseven®; Novartis) may be of use in the treatment or prevention of bleeding. It is licensed for use in patients with Glanzmann’s thrombasthenia who have antibodies to the missing glycoprotein. This avoids exposure to blood products, but is expensive and the response may be variable. Bone marrow transplantation can potentially offer a cure in a number of these conditions.

Acquired disorders

In thrombocytopenia due to bone marrow failure or marrow in filtration, for example leukaemia or cancer, prophylactic platelet transfusions are given to keep the platelet count above 10 × 10^9/L, although the threshold is higher in infected or bleeding patients or to cover invasive procedures.

In childhood idiopathic thrombocytopenic purpura, spontaneous recovery is common, and treatment, such as corticosteroids, intravenous immunoglobulin or anti-D immunoglobulin, is given only in life-threatening bleeding. In adults, the condition rarely remits without treatment and is more likely to become chronic. Initial treatment is prednisolone 1mg/kg daily (80% of cases remit) and/

or intravenous immunoglobulin (0.4 g/kg for 5 days or 1 g/kg for 2 days), or both. More recently, the use of anti-D in Rh(D) positive patients has been associated with a 75% response rate. In refractory patients, splenectomy has a 60–70% chance of long-term cure, although the use of rituximab, an anti-CD20 monoclonal antibody, has proven equally effective, without the need for surgical inter- vention. Other potential therapies including azathioprine, danazol, vinca alkaloids and high-dose dexamethasone have all been tried with variable success.

Post-transfusion purpura may respond to intravenous immunoglobulin (at the doses given above), or plasma exchange may be required. Platelet transfusions should be avoided.

Patients in whom heparin-induced thrombocytopenia is suspected are often inpatients with ongoing thrombosis and may have complex medical problems. It is essential to stop heparin and treat thrombosis with other anticoagulants; further use of heparins should be avoided. Warfarin, synthetic heparinoids or ancrod can be used. Platelet transfusions are contraindicated in heparin-induced thrombocytopenia and in thrombotic thrombocytopenic purpura. If the latter is suspected clinically and on the basis of laboratory tests, large volume plasma exchange should be started immediately and continued daily until substantial clinical improvement occurs and all the results of haematological tests have normalized. Aspirin can be started once the platelet count is > 50 × 10^9/L.

With DIC, it is essential to treat the underlying cause in addition to aggressive replacement of depleted clotting factors and platelets with blood products. In patients requiring massive blood transfusion, replacement with fresh frozen plasma (15 mL/kg) and a pool of platelets should be given with every 8–10 units of red cells received. In pronounced bleeding or risk of bleeding due to the acquired dis- orders of platelet function, platelets usually have to be transfused to provide normally functioning platelets, although desmopressin and tranexamic acid can also be of value. Usually treatment may only be necessary to cover surgical procedures or major haemorrhage.