Left ventricular assist device implantation causes platelet dysfunction and proinflammatory platelet-neutrophil interaction.

Blood flow through left ventricular assist devices (LVAD) may induce activation and dysfunction of platelets. Dysfunctional platelets cause coagulation disturbances and form platelet-neutrophil conjugates (PNC), which contribute to inflammatory tissue damage. This prospective observational cohort study investigated patients, who underwent implantation of a LVAD (either HeartMate II (HM II) (n = 7) or HeartMate 3 (HM 3) (n = 6)) and as control patients undergoing coronary artery bypass grafting (CABG) and/or aortic valve replacement (AVR) (n = 10). We performed platelet and leukocyte flow cytometry, analysis of platelet activation markers, and platelet aggregometry. Platelet CD42b expression was reduced at baseline and perioperatively in HM II/3 compared to CABG/AVR patients. After surgery the platelet activation marker ß-thromboglobulin and platelet microparticles increased in all groups while platelet aggregation decreased. Platelet aggregation was more significantly impaired in LVAD compared to CABG/AVR patients. PNC were higher in HM II compared to HM 3 patients. We conclude that LVAD implantation is associated with platelet dysfunction and proinflammatory platelet-leukocyte binding. These changes are less pronounced in patients treated with the newer generation LVAD HM 3. Future research should identify device-specific LVAD features, which are associated with the least amount of platelet activation to further improve LVAD therapy.

Inherited Platelet Disorders: An Updated Overview.

Platelets play a major role in hemostasis as ppwell as in many other physiological and pathological processes. Accordingly, production of about 1011 platelet per day as well as appropriate survival and functions are life essential events. Inherited platelet disorders (IPDs), affecting either platelet count or platelet functions, comprise a heterogenous group of about sixty rare diseases caused by molecular anomalies in many culprit genes. Their clinical relevance is highly variable according to the specific disease and even within the same type, ranging from almost negligible to life-threatening. Mucocutaneous bleeding diathesis (epistaxis, gum bleeding, purpura, menorrhagia), but also multisystemic disorders and/or malignancy comprise the clinical spectrum of IPDs. The early and accurate diagnosis of IPDs and a close patient medical follow-up is of great importance. A genotype-phenotype relationship in many IPDs makes a molecular diagnosis especially relevant to proper clinical management. Genetic diagnosis of IPDs has been greatly facilitated by the introduction of high throughput sequencing (HTS) techniques into mainstream investigation practice in these diseases. However, there are still unsolved ethical concerns on general genetic investigations. Patients should be informed and comprehend the potential implications of their genetic analysis. Unlike the progress in diagnosis, there have been no major advances in the clinical management of IPDs. Educational and preventive measures, few hemostatic drugs, platelet transfusions, thrombopoietin receptor agonists, and in life-threatening IPDs, allogeneic hematopoietic stem cell transplantation are therapeutic possibilities. Gene therapy may be a future option. Regular follow-up by a specialized hematology service with multidisciplinary support especially for syndromic IPDs is mandatory.

Microvesicles generated following traumatic brain injury induce platelet dysfunction via adenosine diphosphate receptor.

BACKGROUND: Traumatic brain injury (TBI) can result in an acute coagulopathy including platelet dysfunction that can contribute to ongoing intracranial hemorrhage. Previous studies have shown adenosine diphosphate (ADP)-induced platelet aggregation to be reduced after TBI. In addition, circulating microvesicles (MVs) are increased following TBI and have been shown to play a role in post-TBI coagulopathy and platelet function. We hypothesized that post-TBI MVs would affect platelet aggregation in a murine head injury model. METHODS: Moderate TBI was performed using a weight-drop method in male C57BL6 mice. Whole blood, plasma, MVs, and MV-poor plasma were isolated from blood collected 10 minutes following TBI and were mixed separately with whole blood from uninjured mice. Platelet aggregation was measured with Multiplate impedance platelet aggregometry in response to ADP. The ADP P2Y12 receptor inhibitor, R-138727, was incubated with plasma and MVs from TBI mice, and platelet inhibition was again measured. RESULTS: Whole blood taken from 10-minute post-TBI mice demonstrated diminished ADP-induced platelet aggregation compared with sham mice. When mixed with normal donor blood, post-TBI plasma and MVs induced diminished ADP-induced platelet aggregation compared with sham plasma and sham MVs. By contrast, the addition of post-TBI MV-poor plasma to normal blood did not change ADP-induced platelet aggregation. The observed dysfunction in post-TBI ADP platelet aggregation was prevented by the pretreatment of post-TBI plasma with R-138727. Treatment of post-TBI MVs with R-138727 resulted in similar findings of improved ADP-induced platelet aggregation compared with nontreated post-TBI MVs. CONCLUSION: Adenosine diphosphate-induced platelet aggregation is inhibited acutely following TBI in a murine model. This platelet inhibition is reproduced in normal blood by the introduction of post-TBI plasma and MVs. Furthermore, observed platelet dysfunction is prevented when post-TBI plasma and MVs are treated with an inhibitor of the P2Y12 ADP receptor. Clinically observed post-TBI platelet dysfunction may therefore be partially explained by the presence of the ADP P2Y12 receptor within post-TBI MVs. LEVEL OF EVIDENCE: Level III.

STIM1 over-activation generates a multi-systemic phenotype affecting the skeletal muscle, spleen, eye, skin, bones and immune system in mice.

Strict regulation of Ca2+ homeostasis is essential for normal cellular physiology. Store-operated Ca2+ entry (SOCE) is a major mechanism controlling basal Ca2+ levels and intracellular Ca2+ store refilling, and abnormal SOCE severely impacts on human health. Overactive SOCE results in excessive extracellular Ca2+ entry due to dominant STIM1 or ORAI1 mutations and has been associated with tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK). Both disorders are spectra of the same disease and involve muscle weakness, myalgia and cramps, and additional multi-systemic signs including miosis, bleeding diathesis, hyposplenism, dyslexia, short stature and ichthyosis. To elucidate the physiological consequences of STIM1 over-activation, we generated a murine model harboring the most common TAM/STRMK mutation and characterized the phenotype at the histological, ultrastructural, metabolic, physiological and functional level. In accordance with the clinical picture of TAM/STRMK, the Stim1R304W/+ mice manifested muscle weakness, thrombocytopenia, skin and eye anomalies and spleen dysfunction, as well as additional features not yet observed in patients such as abnormal bone architecture and immune system dysregulation. The murine muscles exhibited contraction and relaxation defects as well as dystrophic features, and functional investigations unraveled increased Ca2+ influx in myotubes. In conclusion, we provide insight into the pathophysiological effect of the STIM1 R304W mutation in different cells, tissues and organs and thereby significantly contribute to a deeper understanding of the pathomechanisms underlying TAM/STRMK and other human disorders involving aberrant Ca2+ homeostasis and affecting muscle, bones, platelets or the immune system.

Platelet dysfunction in critically ill patients.

: Thrombelastography Platelet Mapping (TEG-PM) allows for measurement of maximal potential clot strength (MA) and strength from stimulation of arachidonic acid (MA-AA) and adenosine disphosphate (MA-ADP) receptors. This study was conducted to assess degree of platelet dysfunction in critically ill adult patients. A retrospective study of critically ill, adult, nontrauma patients in a medical/surgical ICU was conducted from August 2013 to September 2014. All patients who underwent TEG-PM were enrolled. Patients with intracerebral hemorrhage, following cardiac surgery, or without an APACHE II score were excluded. Patients were divided into those with and without aspirin use. Demographics, APACHE II score, and laboratory results were abstracted. Student t test was used to test significance. A total of 79 patients were enrolled (61% male). Average age and APACHE II score were 61 ±â€Š16 years and 18 ±â€Š9, respectively. Factor-associated coagulation measures and MA were normal in all groups but MA-AA and MA-ADP were significantly reduced irrespective of anticoagulant use. Compared to the nonanticoagulated cohort, MA-AA was significantly reduced in those on aspirin. There was no difference in mortality or length of stay in any cohort. Inhibition of the AA and ADP pathways is common in critically ill patients. Clinical correlation with propensity for bleeding and need for transfusion requires further assessment.

Platelet Function Tests in Bleeding Disorders.

Functional disorders of platelets can involve any aspect of platelet physiology, with many different effects or outcomes. These include platelet numbers (thrombocytosis or thrombocytopenia); changes in platelet production or destruction, or capture to the liver (Ashwell receptor); altered adhesion to vascular injury sites and/or influence on hemostasis and wound healing; and altered activation or receptor functions, shape change, spreading and release reactions, procoagulant and antifibrinolytic activity. Procoagulant membrane alterations, and generation of thrombin and fibrin, also affect platelet aggregation. The above parameters can all be studied, but standardization and quality control of assay methods have been limited despite several efforts. Only after a comprehensive clinical bleeding assessment, including family history, information on drug use affecting platelets, and exclusion of coagulation factor, and tissue deficits, should platelet function testing be undertaken to confirm an abnormality. Current diagnostic tools include blood cell counts, platelet characteristics according to the cell counter parameters, peripheral blood smear, exclusion of pseudothrombocytopenia, whole blood aggregometry (WBA) or light transmission aggregometry (LTA) in platelet-rich plasma, luminescence, platelet function analysis (PFA-100) for platelet adhesion and deposition to collagen cartridges under blood flow, and finally transmission electron microscopy to exclude rare structural defects leading to functional deficits. The most validated test panels are included in WBA, LTA, and PFA. Because platelets are isolated from their natural environment, many simplifications occur, as circulating blood and interaction with vascular wall are omitted in these assays. The target to reach a highly specific platelet disorder diagnosis in routine clinical management can be exhaustive, unless needed for genetic counseling. The elective overall assessment of platelet function disorder primarily aims at better management of hemostasis in case of emergency surgery or other interventions and acute bleeding events.

Facing coagulation disorders after acute trauma.

Facing coagulation disorders after acute trauma. PROBLEMS/OBJECTIVES: Trauma is the leading cause of mortality for persons between one and 44 years of age, essentially due to bleeding complications. METHODOLOGY: We screened the PubMed, Scopus and Cochrane Library databases, using specific keywords. Only publications in English were considered. MAIN RESULTS: The pathophysiology of trauma-induced coagulopathy (TIC) is complex and includes the classic "lethal triad" (i.e., haemodilution, acidosis, hypothermia) but may also include activation of protein C, endothelial and platelet dysfunction, and fibrinogen depletion. The time between trauma and treatment of the resultant massive bleeding should be as short as possible using techniques for rapid control of bleeding and avoiding aggravating factors (hypothermia, metabolic acidosis and hypocalcaemia). If given within three hours of injury, tranexamic acid (TXA) reduces all causes of mortality in trauma patients and reduces transfusion requirements. In a bleeding patient, crystalloids are preferred to colloids and the ratio of fresh frozen plasma to packed red blood cells should be at least 1:2. Damage control surgery (DCS) should be considered for patients who present with, or are at risk for developing, the "lethal triad", multiple life-threatening injuries or shock, and in mass casualty situations. DCS can also aid in the evaluation of the extent of tissue injuries and the control of haemorrhage and infection. Finally, there is currently no evidence of the added value of laboratory assays in the management of TIC. CONCLUSIONS: TIC appears quickly after trauma and should be anticipated and detected as soon as possible. TXA plays a central role in the management of such patients. Each institution should establish a local algorithm for the management of bleeding patients.

Update on the inherited platelet disorders.

PURPOSE OF REVIEW: The inherited platelet disorders have witnessed a surge in our understanding of molecular mechanisms of disease in the past few years due in large to part to the introduction of next-generation sequencing for discovery of novel genes. The purpose of this review is to update the reader on the novel discoveries with regard to the inherited platelet disorders, with a particular focus on describing the novel disorders described most recently. RECENT FINDINGS: The description of novel mechanisms of disease including mutations in PRKACG, in a family with severe macrothrombocytopenia, RUNX1 and FLI1 mutations in patients with inherited mild platelet function disorders and CalDAG-GEFI resulting in a severe platelet bleeding phenotype show that there is still much to be learned from studying families and molecular sequencing of patients with well phenotyped platelet disorders. SUMMARY: The implications for clinical practice of the continually growing list of genes described in small numbers of families makes whole exome/genome tempting as an option for evaluation of patients, but use outside of the research setting still needs to be done with extreme caution as interpretation of variants is likely to require additional studies.

Laboratory testing for platelet function disorders.

Platelet function testing is both complex and labor intensive. A stepwise approach to the evaluation of patients with suspected platelet disorders will optimize the use of laboratory resources, beginning with an appropriate clinical evaluation to determine whether the bleeding is consistent with a defect of primary hemostasis. Bleeding assessment tools, evaluation of platelet counts, and review of peripheral blood cell morphology can aid the initial assessment. For patients requiring further laboratory testing, platelet aggregometry, secretion assays, and von Willebrand factor assays are the most useful next steps and will direct further specialized testing including flow cytometry, electron microscopy, and molecular diagnostics. Guidelines and recommendations for standardizing platelet function testing, with a particular focus on light transmission aggregometry, are available and can provide a template for clinical laboratories in establishing procedures that will optimize diagnosis and assure quality results. This review outlines an approach to platelet function testing and reviews testing methods available to clinical laboratories.

Decreased platelet aggregation by shear stress-stimulated endothelial cells in vitro: description of a method and first results in diabetes.

The interaction between platelets and endothelium in vivo is a complex phenomenon. Our aim was to develop an in vitro system that mimics the in vivo environment and investigate platelet function in a common pathological condition. Human umbilical vein endothelial cells were used and platelets from 28 type 2 diabetes patients were studied under shear stress conditions. Mean coefficient of variation of platelet aggregation was 10% in dynamic conditions in the presence of endothelium. Endothelial cells increased the concentration of inductor needed to achieve 50% platelet aggregation to adenosine diphosphate from 2.6 ± 1.3 in static conditions to 3.7 ± 1.3 µM in dynamic conditions. A similar pattern was observed when collagen was used for platelet activation. Incubation of endothelium with a nitric oxide inhibitor abolished this effect, indicating platelet inhibitory effect of endothelial cells is nitric oxide mediated. Platelet reactivity of healthy controls was less influenced by the presence of endothelial cells and displayed reduced basal platelet reactivity compared with platelets from diabetes patients. We show that platelet aggregation in diabetes as commonly reported in vitro may not fully reflect the in vivo pathophysiological process. Future studies are warranted to investigate other pathological conditions and analyse the effects of antiplatelet agents using this system.

Update on the causes of platelet disorders and functional consequences.

Platelets are derived from megakaryocytes in the bone marrow that create the cellular machinery the platelet needs to participate in the different processes of primary hemostasis including adhesion, activation and clot formation at the site of injury. Defects related to megakaryocyte differentiation, platelet formation, and/or platelet function can result in bleeding. Patients with thrombopathies can present with mucous membrane bleeding but may also present with bleeding following trauma or surgery. In this review, we have classified inherited platelet bleeding disorders (IPD) according to their underlying defective pathway: transcription regulation, TPO signaling, cytoskeletal organization, apoptosis, granule trafficking, and receptor signaling. Platelet function testing has provided insights into the underlying molecular defects that can result in bleeding. A major step forward was made during the last 3 years using new-generation genetic approaches that resulted in the discovery of novel genes such as NBEAL2, RBM8A, ACTN1, and GFI1B for the well-known IPD that cause gray platelet syndrome, thrombocytopenia-absent radius syndrome, and autosomal dominant thrombocytopenias, respectively. In the near future, it is expected that a similar approach will identify many novel genes that cause IPD of unknown etiology, which are common. The future challenge will be to use a functional, systems biology approach to study the genes mutated in IPD and determine their roles in megakaryocyte and platelet biology and pathology.

Bleeding gastroduodenal ulcers in patients without Helicobacter pylori infection and without exposure to non-steroidal anti-inflammatory drugs.

BACKGROUND/AIM: A high risk of bleeding in Helicobacter pylori (H. pylori)-negative, non-steroidal anti-inflammatory drugs (NSAID)-negative ulcers highlights the clinical importance of analysis of the changing trends of peptic ulcer disease. The aim of the study was to investigate the risk factors for ulcer bleeding in patients with non-H. pylori infection, and with no NSAIDs use. METHODS: A prospective study included patients with endoscopically diagnosed ulcer disease. The patients were without H. pylori infection (verified by pathohistology and serology) and without exposure to NSAIDs and proton pump inhibitors (PPI) within 4 weeks before endoscopy. After endoscopy the patients were divided into 2 groups: the study group of 48 patients with bleeding ulcer and the control group of 47 patients with ulcer, but with no bleeding. Prior to endoscopy they had completed a questionnaire about demographics, risk factors and habits. The platelet function, von Willebrand factor (vWF) and blood groups were determined. Histopathological analysis of biopsy samples were performed with a modified Sydney system. The influence of bile reflux was analyzed by Bile reflux index (BRI). RESULTS: Age, gender, tobacco and alcohol use did not affect the bleeding rate. The risk of bleeding did not depend on concomitant diseases (p = 0.509) and exposure to stress (p = 0.944). Aspirin was used by 16/48 (33.3%) patients with bleeding ulcer, as opposed to 7/47 (14.9%) patients who did not bleed (p = 0.036). Abnormal platelet function had 12/48 (25.0%) patients who bled, as opposed to 2/47 (4.3%) patients who did not bleed (p = 0.004). Patients with BRI < 14 bled in 79.2%, and did not bleed in 57.4% of the cases (p = 0.023). There was no statistical difference between groups in regards to blood groups and range of vWF. Antrum atrophy was found in 14/48 (29.20%) patients with bleeding ulcer and in only 5/47 (10.6%) patients who had ulcer without bleeding (p = 0.024). CONCLUSION: Abnormal platelet function, aspirin use and antrum atrophy were the risk factors for ulcer bleeding in non-H. pylori, non- NSAIDs ulcer disease.

Sticky platelet syndrome.

Sticky platelet syndrome (SPS) is a thrombophilic thrombocytopathy with familial occurrence and autosomal dominant trait, characterized by an increased in vitro platelet aggregation in response to low concentrations of adenosine diphosphate (ADP) and/or epinephrine (EPI). According to aggregation pattern, three types of the syndrome can be identified (hyperresponse after both reagents, Type I; EPI alone, Type II; ADP alone, Type III). Clinically, the syndrome is associated with both venous and arterial thrombosis. In pregnant women, complications such as fetal growth retardation and fetal loss have been reported. The first thrombotic event usually occurs before 40 years of age and without prominent acquired risk factors. Antiplatelet drugs generally represent adequate treatment. The use of other antithrombotics is usually ineffective and may result in the recurrence of thrombosis. In most patients, low doses of antiplatelet drugs (acetylsalicylic acid, 80-100 mg/d) lead to normalization of hyperaggregability. Combination of SPS with other thrombophilic disorders has been described. Despite several studies investigating platelet glycoproteins' role in platelets' activation and aggregation, the precise defect responsible for the syndrome remains unknown. The aim of this review is to summarize authors' own experience about SPS and the clinical data indexed in selected databases of medical literature (PubMed and Scopus).

Genotyping and phenotyping of platelet function disorders.

The majority of patients with platelet function disorders (PFDs) have normal platelet counts and mild day-to-day bleeding symptoms, but are at risk of major hemorrhage at times of trauma, surgery, or childbirth. This group is challenging to investigate, because the assays are often time-intensive and labour-intensive, and interpretation is difficult, especially in patients with mild disorders. In addition, interuser variability in performance of the assays, including the currently accepted gold standard, light transmission aggregometry, makes the results difficult to compare between laboratories. Furthermore, a similar pattern of mucocutaneous bleeding is seen in disorders in other components of the hemostatic pathway, including type 1 von Willebrand disease (VWD). We have undertaken an extensive investigation of patients with clinically diagnosed excessive bleeding, using a genotyping and platelet phenotyping approach based on lumi-aggregometry, and other specialist tests of platelet function, in combination with Sanger and next-generation sequencing (NGS). We found a functional defect in ~ 60% of patients, the majority being associated with feedback pathways of platelet activation. Function-disrupting mutations were identified in known and novel genes, and coinheritance with other genetic disorders of hemostasis, including type 1 VWD, was shown. A significant number of mutations are heterozygous and unlikely to cause extensive bleeding in isolation, consistent with incomplete penetrance of inheritance of bleeding disorders and a multifactorial etiology for excessive bleeding in many patients. Mucocutaneous bleeding is a complex trait, and this has important implications for NGS in the assessment of a PFD.

Inherited thrombocytopenias: an approach to diagnosis and management.

Inherited thrombocytopenias vary in their presentation, associated features, and molecular etiologies. An accurate diagnosis is important to provide appropriate therapy as well as counseling for the individual and their family members. As the genetic basis of more disorders is understood, it will be possible to diagnose a greater fraction of patients as well as learn more about the process of megakaryopoiesis and platelet production.

What we have learned from inherited platelet disorders.

Identifying the molecular basis of inherited platelet disorders has contributed to our understanding of normal platelet physiology. Many of these conditions are rare, but close observation of clinical and laboratory phenotype, and subsequent identification of the abnormal protein and mutated gene, have provided us with unique opportunities to examine specific aspects of platelet biogenesis and function. Phenotype-genotype association studies are providing a detailed understanding of the structure and function of platelet membrane receptors, the biogenesis and release of platelet granules, and the assembly of the cytoskeleton. Genetic polymorphisms contributing to decreased or increased platelet adhesion and activation may translate into increased clinical risks for bleeding or thrombosis. More recently, genome wide association studies have identified new genes contributing to the variation in normal platelet function.

[Glycoproteins, inherited diseases of platelets, and the role of platelets in wound healing]. / Glycoprotéines, maladies héréditaires des plaquettes, rôle des plaquettes dans la réparation tissulaire.

Recognition that platelets have a glycocalyx rich in membrane glycoproteins prompted the discovery in France that inherited bleeding syndromes due to defects of platelet adhesion and aggregation were caused by deficiencies in major receptors at the platelet surface. Identification of the alpha IIb beta3 integrin prompted the development of powerful anti-thrombotic drugs that have gained worldwide use. Since these discoveries, the genetic causes of many other defects of platelet function and production have been elucidated, with the identification of an ADP receptor, P2 Y12, another widespread target for anti-thrombotic drugs. Discovery of the molecular basis of a rare disease of storage of biologically active proteins in platelet alpha-granules has been accompanied by the recognition of the roles of platelets in inflammation, the innate immune system and tissue repair, opening new avenues for therapeutic advances.

Mouse models of diseases of megakaryocyte and platelet homeostasis.

Platelets are the small anuclear blood cells that are the product of megakaryocytopoiesis, the process of hematopoietic stem cell commitment to megakaryocyte production and the differentiation and maturation of these cells for platelet release. Deregulation or disruption of megakaryocytopoiesis can result in platelet deficiencies, the thrombocytopenias, with attendant risk of hemorrhage or thrombocytosis, a pathological excess of platelet numbers. Mouse models, particularly those engineered to carry genetic alterations modeling mutations associated with human disease, have provided important insights into megakaryocytopoiesis and deregulation of this process in disease. This review focuses on mouse models of diseases of altered megakaryocyte and platelet number, illustrating the profound contribution of these models in validating suspected roles of disease-associated genetic alterations, promoting discovery of new links between genetic mutations and specific diseases, and providing unique tools for better understanding of disease pathophysiology and progression, as well as resources to define drug action or develop new therapeutic strategies.

[Pathogenesis of thrombotic and hemorrhagic complications in myeloproliferative and myelodysplastic syndromes]. / Patogeneza complicatiilor trombotice si hemoragice în sindroamele mieloproliferative si mielodisplazice.

Chronic myeloproliferative disorders (CMD) and Myelodisplastic Syndromes (MDS) represents a group of clonal pluripotent stem-cell pathologies. During their natural history, the clinical picture reveals both thrombosis and hemorrhage. The thrombosis could affect the microvessels, and also the large vessels, including even less usual territories (suprahepatic veins, porta vein, pulmonary vein). There are many factors contributing to thrombosis in myeloproliferative chronic disorders--the associated comorbidities, the numeric alterations of blood elements and also the disorders of the platelet's function. Thus, there were described quantitative and qualitative anomalies of platelet's receptors: GP Ib, GP IIb/IIIa, GP IV, GP VI, thrombopoietin receptor of the platelet cMPL, the increase of platelet activation; the increase of P selectin and thrombospondin and the increase on GP IIb/IIIa expression--they were all correlated with thrombosis. An important role has been attributed to JAK2 mutation, which affects the platelet receptor for thrombopoietin cMPL. Regarding the hemorrhage in chronic myeloproliferative syndrome, it is favored by many disorders in platelet's function, such as: the decrease of von Willebrand factor's receptor of the platelet, which leads to acquired Bernard Soulier syndrome; quantitative and qualitative disorders of dense granules of the platelet, decrease of the secretion and platelet aggregation after epinephrine, ADP and collagen stimulation. It was also described the acquired von Willebrand syndrome, most frequently type 2.