Academic literature on the topic 'Thrombocytopenia; Malaria; Low Platelets'

Abstract 1. Introduction Since the primary function of platelets is haemostasis, or stopping bleeding, platelet dysfunction is generally seen as an under–or over-expression of this, i.e. in bleeding problems or in various haemostatic or thrombotic disorders. The first task of the clinician faced with such symptoms is to eliminate the possibility of the disorder being produced by a lack or defect in plasma coagulation factors such as factor VIII, von Willebrand factor, fibrinogen, factor IX, factor V, etc. which are commonly affected. Having shown that these lie within the normal range it is necessary to check for platelet-related problems. Basic parameters such as platelet count, platelet volume, and granulosity can be rapidly determined by flow cytometry. Thrombocytopenia, or low platelet count can clearly lead to bleeding problems though there is some controversy about what should be regarded as a threshold value. Normal values lie in the range 150000–400000 platelets/µl but bleeding problems may not appear until values below 10000-20000 platelets/µl are reached. There are many causes for thrombocytopenia which will be dealt with later. When necessary, basic treatment is intravenous platelet concentrates to try to restore platelet levels although the recent cloning of thrombopoietin may, when it becomes generally available, make this a preferred therapy.

Platelet disorders are quantitative or qualitative, or both. Either markedly increased or decreased numbers of platelets can cause harmful sequelae. Platelet disorders can be divided into acquired or hereditary; acquired disorders are much more common than congenital disorders. Platelet disorders can be due to increased platelet destruction or decreased platelet production. Pseudothrombocytopenia, dilutional effects, or possible splenic sequestration should be considered when the platelet count is low for the first time. The recent addition of a drug, commonly used in intensive care units, is also an important consideration for patients with newly acquired thrombocytopenia.

Abstract Heparin-induced thrombocytopenia (HIT) is a rare but potentially devastating complication of heparin therapy. HIT is caused by autoantibodies to platelet factor 4 complexed with heparin that activates platelets. Cardiac surgical patients often have multiple cofounding reasons for low platelet counts, therefore clinical suspicion is the key to diagnosis, based upon history, timing, and laboratory testing. Thrombosis may not be obvious unless you ‘go looking’ for it. Clinicians need a better understanding of tests performed to avoid overdiagnosis and the availability of functional assays is very important to confirm the diagnosis. Cardiac centres should have plans in place for dealing with patients with HIT who require urgent surgery and managing HIT perioperatively.

During the last four decades, there have been major advances in the understanding and management of the haematological disorders associated with pregnancy. This chapter aims to help update the management and knowledge of some of these conditions. The section on anaemia in pregnancy addresses its prevention, diagnosis, and management. Sickle cell disease is discussed, with its associated complications and the role of preconception and antenatal care in the appropriate set-up. The section on thalassemia highlights the advances in antenatal screening and the management of the different types in pregnancy. A concise updated review on antiphospholipid syndrome in pregnancy is included, addressing its diagnosis and management. Early pregnancy loss is the most common pregnancy complication and its occurrence in association with a thrombophilia is discussed in detail. Women with inherited bleeding disorders and disseminated intravascular coagulopathy may face several haemostatic challenges during pregnancy and childbirth. Pregnancy in these women requires specialized and individualized care. This section covers the management of maternal and fetal complications as well as prenatal diagnosis including new advances for haemophilia carriers. The section on thrombocytopenia outlines the management of low platelets in pregnancy, and the association with severe pre-eclampsia, eclampsia, and HELLP syndrome. There is a section devoted to management of haematological malignancies in pregnancy, which is complex and requires a multidisciplinary approach. An attempt has been made to cover as many subjects in a way that will be of interest and value to both obstetricians and haematologists involved in the care of pregnant women.

A family bleeding disorder characterized by a new association between Type IIB von Willebrand's disease (vWD) and a complex platelet disfunction, with an intermittent thrombocytopenia is described in two patients from the same generation. The mother and a maternal aunt died having severe bleeding diathesis. The platelet abnormalities included: borderline or slightly low platelet count but moderate thrombocytopenia coincedent with the acute bleeding episodes, giant platelet site with a very heterogeneous distribution width, large number of vesicles in platelets by electron-microscopy recalling the "Swiss-Cheese" platelets, abnormal platelet aggregation induced by ADP, collagen, epinephrin and slightly, by thrombin, defective release of 14C-Serotonin, von Willebrand factor (vWF) and platelet factor 4 induced by thrombin or ADP. DDAVP was given to both patient and a partial and transitory correction of bleeding time, thrombocytopenia, presence of platelet aggregates on smear besides a brief appearence of larger multimers of vWF and an increase in all Factor VIII/von Willebrand Factor (FVIII/vWF) properties were seen. Binding of labeled vWF using radiolabelled monoclonal anti-vWF antibody showed an enhanced binding of the patients' vWF, induced by ristocetin, to either normal or patients' platelets. In contrast, the binding of labeled purified normal vWF induced by thrombin to patients' platelets was decreased as compared with the correspondent control. Thus, both patients have platelet disfunctions characteristic for more than one specific platelet disorder. Several associations between platelet and FVIII/vWF abnormalities have been described. This is the first family presenting association of Type IIB vWD and a complex thrombocytopathy. The inherited or acquired (induced by the abnormal IIB vWF platelet interaction) nature of the abnormality is discussed.

An original observation of platelet agglutination was found in a 63 y.o. man during 5 years following, without spontaneous bleeding. Low platelet counts (10 × 109/liter) and large agglutinates were observed on blood specimens collected into EDTA (1 to 15 mg/ml whole blood WB), citrate (1/10 volume citrate 0.11 M), sodium heparin (100 units/ml WB), citrate plus acetyl salicylic acid (5 to 100 mg/ml WB), citrate plus prostacyclin (Upjohn 10−5 to 10−6 M)? and buffering anticoagulant (0.8 M citrate, pH 4.65). Low platelet count and large agglutinates were also observed in capillary blood obtained by finger puncture kept at 22° or 37°C. All techniques revealed significant clumping making assessment of overall platelet number impossible. Bleeding time Ivy horizontal incision is longer than 20 minutes.Patient's serum induced normal platelets agglutination up to a 1/512 dilution at 22°and 37°C temperature. Lack of agglutination after treatment of the serum by IgM and indirect immunofluorescence tests identified the IgM nature of the agglutinating factor. This antibody reacted with Glanzmann thrombasthenia platelets ruling out the IIb/IIIa complex as the target of this agglutinin.There was no evidence of platelet activation following agglutination of autologous or alldgenic platelets : electronic microscopy of native blood platelets clumps did not show any sign of activation, and plasma level of B-thromboglobulin was normal in this patient. Normal platelets agglutination by patient's serum was not accompanied with ATP secretion (luciferin-luciferase).This IgM agglutinin was associated with elevation of IgM (700 mg/dl) without clinical or biological signs of auto-immune disease.No similar case with irreversible platelet agglutination in both capillary or venous blood has been reported before. The significance of the observation remains obscure.

Quinine (Qn) can provoke potent antibodies capable of destroying platelets and inducing life-threatening immunologic thrombocytopenia (DITP). A question critical to understanding the mechanism of DITP is why do platelets from all normal individuals express Qn-induced antigens while only relatively few individuals make the destructive drug-dependent antibodies? This problem was investigated in a murine animal model. BALB/c mice (6-12wk) were injected intraperitoneally, every other week, with saline or lOOOpg of Qn (this dose of drug gave serum levels comparable to human therapeutic serum levels). Two wk after the second injection, mice were immunized with a single dose of 108 human platelets and either 0 or 1000μg of Qn. One wk later, mouse serum was screened in the presence and absence of drug for anti-platelet activity by a complement-dependent 51Cr release assay. Results are shown in the table and represent data from at least 2 groups of five mice each.Antibody titers were more than 10-fold lower in mice receiving 1000 |lg Qn than in mice injected with platelets alone. In contrast, mice repeatedly immunized (3-6 injections) with platelets and low doses of Qn (20μg) developed enhanced antibody activity (drug-dependent and nondrug-dependent) that consistently titered two-to four-fold higher than mice injected with platelets alone. Results were confirmed by an enzyme-linked immunosorbent assay which showed that the murine antibodies were IgG. These findings demonstrate that by varying the dose, Qn can either inhibit or enhance production of anti-human platelet antibodies in mice. This suggests the possibility that most individuals fail to respond to Qn because therapeutic doses of the drug inhibit antibody formation; yet in individuals capable of responding, even low doses of Qn (as found in tonic water) can enhance production of antibodies that provoke DITP.

Other investigators have shown that heparin in the usual therapeutic range (0.1-0.5 units/ml) has an enhancing effect on ADP aggregation and an inhibitory effect on collagen and thrombin induced aggregation. The effects of low molecular weight heparin (LMWH)and heparinoids (dermatan sulfate, heparan sulfate) on platelet aggregation have not been as extensivelystudied. We have utilized citrated platelet rich plasma (3.2%citrate-whole blood 1:9) drawn in plastic and adjusted to a final platelet count of 250,000/ul. A Bio-Data 4 channgl aggregometer was utilized with constantstirring at 37 C. The reaction was allowed to run for 20 minutes. Platelet rich plasma was supplemented 1:9 with saline or heparin and various agonists were then added ifno aggregation occurred. ADP, collagen, thrombin, ristocetin and serum from patients with heparin inudced thrombocytopenia (HIT) were utilized as agonists. Heparin was substituted at concentrations of 0.1 to 500 units per ml and various LMWH and heparinoids were substituted in equivalent anti-Xa or gravimetric concentrations. At low concentrations no inhibitory effect on any ofthe agonists was observed with any of the heparins or heparinoids. At concentrations of heparin of 100 u/ml or greater, all agonists were inhibited. At equivalent concentrations of five different LMWH (Cy 216, Cy 222, Pk 10169, Kabi 2165 and pentasaccharide) inhibition did notoccur at all or at very high concentions only. Dermatan sulfate and heparan sulfate inhibited only at high concentrations. HIT serum could not aggregate platelets with dermatan sulfate or pentasaccharide atany concentrations, but it was a good agonist with the other heparins and heparinoids.

A 62 year old man with vWD has suffered from repeated episodes of melaena - his son and daughter have inherited the disorder with few symptoms so far. Laboratory findings in them include consistently prolonged bleeding times, normal factor VIII coagulant activity and decreased ristocetin cofactor activity. Levels of von Willebrand factor (vWf) antigen were lower measured by immunoradiometric assay than by Laurell immunoelectrophoresis. Analysis of vWf structure by SDS agarose gel electrophoresis showed loss of only the large multimers in plasma and the triplet structure of the smaller multimers showed sub-bands more intense than normal. However, the platelets contain the whole series of multimers with a similar pattern to normal. This was suggestive of type IIB vWD but agglutination of the patients' platelet-rich plasma with low concentrations of ristocetin was not enhanced. Agglutination was reduced compared to normal platelet-rich plasma at a final ristocetin concentration of 1 and 2 mg/ml and absent at a final concentration of 0.5 mg/ml. Platelet-type vWD was eliminated as addition of normal plasma or cryoprecipitate to patient's platelet-rich plasma did not produce spontaneous aggregation.The patient has never had thrombocytopenia. We feel that this family demonstrates further the heterogeneity of vWD.

During the past several years, we have initiated studies to determine the role of plasma factors and platelets, and the properties of the blood vessel, which influence the activation of the coagulation mechanism on the subendothelium. Studies were performed by exposing everted segments of de-endothelialized rabbit aorta, mounted in a perfusion chamber, to non-anticoagulated human blood for 5 to 10 minutes under a range of flow conditions, and measuring fibrin and platelet deposition on the subendothelium, and fibrinopepstide A (FPA) levels in post-chamber blood. In normal subjects, platelet deposition increased progressively with increasing shear rates (50-2600 sec-1 ), whereas fibrin deposition and FPA levels decreased sharply at shear rates greater than 650 sec-1 . To examine the role of plasma coagulation factors, we utilized a shear rate of 650 sec-1 to study patients with severe deficiencies of factors XII, XI, IX or VIII. In contrast to the partial thromboplastin time (PTT), which was most strikingly abnormal in patients with factor XII or XI deficiency, fibrin deposition and FPA levels were greater in patients deficient in factor XII or XI than in those with factor VIII or IX deficiency. In addition, we observed smaller platelet thrombi in hemophilia (but not afibrinogenemia), suggesting that thrombin influenced the formation of platelet thrombi under these shear conditions. The findings suggested that tissue factor-Vila activation of factor IX could be important in mediating fibrin deposition on subendothelium and might explain why patients with factor XII deficiency (and some with factor XI deficiency) do not bleed. Initial studies to demonstrate tissue factor activity in subendothelium were inconclusive. More recently, utilizing shorter (1.5, 2 and 3 min) perfusion periods, we have observed decreased fibrin deposition and FPA levels in patients with factor VII deficiency and we have obtained further support for the presence of tissue factor in subendothelium in experiments utilizing a monoclonal antibody to tissue factor. Our studies suggest that activation of factor IX by tissue factor-Vila could account for the results obtained in patients with plasma coagulation defects. Direct experimental verification of this hypothesis will require more extensive studies on the kinetics governing the activation of coagulatjon factors on the subendothelium. In subsequent studies, we examined the role of platelets in mediating fibrin deposition. At a shear rate of 650 sec-1 we found (utilizing patients with thrombocytopenia) that platelets were required for fibrin deposition ; little or no fibrin was deposited on the subendothelium when platelet adhesion was less than 4%, corresponding to blood platelet counts less than 5000/ul. Studies performed in patients with functional platelet disorders provided additional information on the specific platelet properties that contribute to fibrin deposition at this shear rate. Decreased fibrin deposition was observed in a patient with Scott Syndrome, a disorder characterized by an impaired capacity of the platelets to catalyze the conversion of factor X to factor Xa (in the presence of factor IXa and VIII) and prothrombin to thrombin (in the presence of factor Va), the latter defect owing to a decreased factor Xa-binding capacity of the platelets. In contrast to the findings in Scott Syndrome, both fibrin deposition and FPA values were completely normal (and possibly increased) in patients with glycoprotein Ilb/IIIa deficiency. In patients with glycoprotein lb deficiency, the major defect was an impaired association of fibrin with platelets, but not subendothelium. The findings in patients with functional platelet disorders indicate that a monolayer of platelets (including those deficient in glycoprotein Ilb/IIIa) is completely active in promoting fibrin deposition on subendothelium. In addition, they suggest that an agent capable of inducing a platelet defect similar to that observed in Scott Syndrome might prevent platelet-fibrin thrombi at shear rates (200-800 sec-1 ) comparable to those in the coronary circulation. Studies performed at a variety of shear rates in both normal subject^ and patients with platelet disorders suggested that, under the conditions used, platelets were essential for fibrin formation at intermediate (650 sec-1 ), but not low (50 sec-1 ) shear rates. Since platelets have been shown to bind activated coagulation proteins (such as factor Xa, Va, and IXa) to their surface, the presence of adherent platelets on the subendothelium could, with increasing shear rates, serve to maintain activated coagulation proteins in the .boundary layer at a concentration that would otherwise be reduced through convective diffusion in their absence. Thus, at low shear rates (50 sec-1 ), the concentration of activated coagulation factors in the boundary layer might be sufficient to support fibrin deposition despite the absence of platelets, whereas at very high shear rates (2,600 sec-1 and above), even the presence of platelets is insufficient to maintain the required concentration. The shear-dependent defect of fibrin formation that we observed in Scott Syndrome is consistent with such a theory. The results of our various studies demonstrate the complex role of blood flow, plasma coagulation factors, specific platelet properties, and the procoagulant properties (tissue factor) of the vessel in mediating subendothelium-induced coagulation and suggest further experiments for studying the mechanisms involved.

ITP is an autoantibody-mediated disease which would logically be treated by decreasing the level of autoantibody. However, the most exciting developments in understanding the pathophysiology of the thrombocytopenia and its treatment involve a better understanding of the MPS FcR system and ways in which it can be modulated. This work has focussed on phagocytic paralysis or FcR blockade (FcRBl): the slowing of destruction of antibody-coated platelets despite the persistent presence of antibody on the surface of the platelet.Several areas have been explored in learning about the MPS system. Investigation by Kurlander among others have revealed that at least 2 FcR's exist on mononuclear phagocytes: one with high and one with low affinity for monomeric IgG. Study of the high affinity FcR expressed by circulating monocytes, by Schreiber among others, has explored the effect of Danazol to decrease the expression of this FcR. The clinical relevance of this receptor is uncertain however because it is saturated in vitro by physiologic concentrations of IgG. Unkeless defined the properties of the low affinity "immune complex" FcR, expressed on macrophages and neutrophils, via monoclonal antibody 3G8 (see below) which blocks ligand binding to this FcR. The exact roles of these two, and possibly more, FcR's are being explored. Another still unsolved controversy involves whether the interaction Fc portions of antibodies coating particles with FcR's is mediated by a conformational change of the Fc portion or by a multipoint attachment of several Fc parts.Studies by Mollison in the 60's demonstrated that the MPS had a limited capacity for removal of antibody-coated (red) cells. Shulman pursued MPS modulation by exploring the inhibition of thrombocytopenia caused by infusion of ITP plasma into normals. Kelton demonstrated that "compensated" ITP may be caused by a decreased clearance of antibody-coated cells and that the rate of clearance of antibody-coated cells may be correlated with rate of clearance of antibody-coated cells may be correlated with the intrinsic levels of IgG. Stossel investigated FcRBl as a mechanism of effect of corticosteroids and related it clinically. Subsequently intravenous gammaglobulin (IVGG) was introducedas a treatment of ITP and Fehr et al first demonstrated FcRBl as the mechanism of effect of IVGG. Exploration of the mechanism of the FcRBl caused by IVGGled Salama and Mueller-Eckhardt to demonstrate the therapeutic effect of I anti-D, which apparentlycoats RBC with antibody and causes their destruction atthe coats RBC with antibody and causes their destruction at the expense of antibody-coated platelets. A similar degree of FcRBl has been shown for aldometrelated to the development of antibody on RBC.Our studies, including Drs. Clarkson, Kimberly, Nachman, and Unkeless, have focussed on the role of the low affinity or "Immune complex" FcR by using monoclonal antibody 3G8 in vivo. An infusion of 1 mg/kg of 3G8 in chimpanzees caused a reproducible FcRBl demonstrable by a slowing of the destruction of antibody-coated RBC for > 10 days (JEM, 1986). Less effect of 3G8 to inhibit CIC removal was seen using DNA-anti-DNA as the immune complex. In view of the wel1-documented effects of IVGG infusion to create FcRBl, we infused 3G8 into 6 adults with refractory ITP (NEJM, 1986). Specifically these patients were refractory to all forms of conventional therapy including splenectomy, steroids, vinca alkaloid infusion, immunosuppressives and danazol . 3 of the 6 patients had peak platelet responses to >80,000/ul. The other 3 had short-lived platelet increases from 10 to 30,000/ul. These responses confirmed the effect of FcRBl, specifically of the low affinity FcR, to underlie a dramatic platelet increase in therapy of ITP. Surprisingly 3 of the patients had apparent longterm effects of this therapy demonstrable in 2 cases as a maintenance of the platelet count >20,0C0/ul without any further therapy and in 1 case as a clearly enhanced responsiveness to other therapies following 3G8 infusion. Since Natural Killer activity was (transiently) ablated by 3G8 infusion, we speculate that an alternation of regulation of (auto) antibody production by NK cells may be responsible for this effect and that FcR interactions include regulatory roles in addition to their primary function of removal of CIC.