Thromboembolism in children

ADAMTS12, a new candidate gene for pediatric stroke

We recently reported a family-based genome wide association study (GWAS) for pediatric stroke pointing our attention to two significantly associated genes of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) gene family ADAMTS2 (rs469568, p = 8x10-6) and ADAMTS12 (rs1364044, p = 2.9x10-6). To further investigate these candidate genes, we applied a targeted resequencing approach on 48 discordant sib-pairs for pediatric stroke followed by genotyping of the detected non-synonymous variants in the full cohort of 270 offspring trios and subsequent fine mapping analysis. We identified eight non-synonymous SNPs in ADAMTS2 and six in ADAMTS12 potentially influencing the respective protein function. These variants were genotyped within a cohort of 270 affected offspring trios, association analysis revealed the ADAMTS12 variant rs77581578 to be significantly under-transmitted (p = 6.26x10-3) to pediatric stroke patients. The finding was validated in a pediatric venous thromboembolism (VTE) cohort of 189 affected trios. Subsequent haplotype analysis of ADAMTS12 detected a significantly associated haplotype comprising the originally identified GWAS variant. Several ADAMTS genes such as ADAMTS13 are involved in thromboembolic disease process. Here, we provide further evidence for ADAMTS12 to likely play a role in pediatric stroke. Further functional studies are warranted to assess the functional role of ADAMTS12 in the pathogenesis of stroke.

Magnetic resonance imaging of cardiovascular thrombi in children

Cardiovascular thrombosis is rare in children and usually occurs in the presence of predisposing conditions, such as indwelling vascular catheters, tumors, aneurysms, ventricular dysfunction, or after surgery. Clots can occur in the cardiac chambers, arteries or veins, or inside conduits. Detection of thrombi is feasible with a variety of magnetic resonance imaging (MRI) techniques, including unenhanced methods but also contrast-enhanced MR angiography. In this essay we illustrate the MRI appearance of cardiovascular thrombosis in children and suggest an imaging protocol based on our clinical experience.

Fatal Pulmonary Embolism in a Teenager: Case Report

Massive pulmonary embolism is unusual in young people. We present a case of massive pulmonary embolism in a teenage Chinese patient. Initially he presented to the emergency department with dyspnoea and fever. Influenza B was diagnosed during his four-day stay in the paediatric ward. One week later he suddenly collapsed and presented to the emergency department with chest pain, syncope and dyspnoea. Despite undergoing rapid assessment, along with prompt admission to intensive care and cardiopulmonary resuscitation, he died. Autopsy revealed massive pulmonary embolism, which is rare at this age. Computed tomography pulmonary angiography and early thrombolytic therapy may improve the outcome in similar cases.

Two children with both arm ischemia and arterial ischemic stroke during the perinatal period

It is rare for both limb ischemia and arterial ischemic stroke to occur in the same child during the perinatal period. Two children who appear to have had perinatal emboli to both an arm and a middle cerebral artery territory are presented here. One child required amputation of the ischemic limb below the shoulder, and the other required skin grafts to the distal ischemic fingers. Each of these children later received cerebral magnetic resonance imaging for evaluation of developmental delay and was found to have what appeared to be old perinatal arterial ischemic stroke. Both children were homozygous for the methylenetetrahydrofolate reductase C677T gene variant. Eight other children with perinatal limb ischemia and stroke were found on literature review; several also had delayed diagnosis of perinatal stroke. This report examines the approach to diagnosis and treatment in each of these and makes suggestions for the similar cases in the future.

Risk, clinical features, and outcomes of thrombosis associated with pediatric cardiac surgery

BACKGROUND

Thrombosis, usually considered a serious but rare complication of pediatric cardiac surgery, has not been a major clinical and/or research focus in the past.

METHODS AND RESULTS

We noted 444 thrombi (66% occlusive, 60% symptomatic) in 171 of 1542 surgeries (11%). Factors associated with increased odds of thrombosis were age <31 days (odds ratio [OR], 2.0; P=0.002), baseline oxygen saturation <85% (OR, 2.0; P=0.001), previous thrombosis (OR, 2.6; P=0.001), heart transplantation (OR, 4.1; P<0.001), use of deep hypothermic circulatory arrest (OR, 1.9 P=0.01), longer cumulative time with central lines (OR, 1.2 per 5-day equivalent; P<0.001), and postoperative use of extracorporeal support (OR, 5.2; P<0.001). Serious complications of thrombosis occurred with 64 of 444 thrombi (14%) in 47 of 171 patients (28%), and were associated with thrombus location (intrathoracic, 45%; extrathoracic arterial, 19%; extrathoracic venous, 8%; P<0.001), symptomatic thrombi (OR, 8.0; P=0.02), and partially/fully occluding thrombi (OR, 14.3; P=0.001); indwelling access line in vessel (versus no access line) was associated with lower risk of serious complications (OR, 0.4; P=0.05). Thrombosis was associated with longer intensive care unit (+10.0 days; P<0.001) and hospital stay (+15.2 days; P<0.001); higher odds of cardiac arrest (OR, 4.9; P<0.001), catheter reintervention (OR, 3.3; P=0.002), and reoperation (OR, 2.5; P=0.003); and increased mortality (OR, 5.1; P<0.001). Long-term outcome assessment was possible for 316 thrombi in 129 patients. Of those, 197 (62%) had resolved at the last follow-up. Factors associated with increased odds of thrombus resolution were location (intrathoracic, 75%; extrathoracic arterial, 89%; extrathoracic venous, 60%; P<0.001), nonocclusive thrombi (OR, 2.2; P=0.01), older age at surgery (OR, 1.2 per year; P=0.04), higher white blood cell count (OR, 1.1/10(9) cells per 1 mL; P=0.002), and lower fibrinogen (OR, 1.4/g/L; P=0.02) after surgery.

CONCLUSIONS

Thrombosis affects a high proportion of children undergoing cardiac surgery and is associated with suboptimal outcomes. Increased awareness and effective prevention and detection strategies are needed.

Activated protein C resistance in cord blood from healthy and complicated newborns

OBJECTIVES

Newborns are susceptible to thrombosis secondary to the immature hemostatic system and maternal and fetal complications. The contribution of activated protein C resistance (APCR) to thrombosis tendency has not yet been established. This study was conducted to investigate the effects of maternal and fetal complications on APCR levels.

METHODS

APCR levels were determined in cord blood from healthy term infants and compared with those in healthy preterm and complicated neonates as well as that in adult venous blood.

RESULTS

The mean value of APCR in cord blood from healthy term infants (166 +/- 40 s) was not significantly different from that in adult venous blood (173 +/- 40 s). No significant differences in the mean cord blood APCR values were observed between healthy term and preterm infants, infants with vaginal and cesarean delivery, infants from preeclamptic and non-eclamptic mothers, and infants with or without perinatal asphyxia. The activity levels of protein C, protein S, and antithrombin III were not significantly different between these groups except for lower levels in preterm babies.

CONCLUSIONS

The level of APCR in cord blood is comparable to that in adults and not influenced by maternal and fetal complications. It appears that APCR does not contribute to the thrombotic tendency in newborns.

Incidence of and risk factors for childhood thrombosis: a single-center experience in Ankara, Turkey

This study was conducted to analyze the incidence of and risk for thrombosis in thrombotic children monitored in the Department of Pediatric Hematology of our hospital at the time of diagnosis, in addition to the clinical characteristics of those patients. The clinical and laboratory findings of 122 patients diagnosed with thrombosis from 1997 to 2006 were retrospectively analyzed. The incidence of thrombosis was 88.6/10,000 hospital admissions. The authors found that 31.1% of the patients studied had a thrombosis in more than 1 region. The incidence of thrombosis by anatomic site was as follows: 42 thromboses in the peripheral arterial system, 39 in an intracardiac region, 38 in the abdominal venous system, 36 in the deep peripheral venous system, and 28 in the cerebral vascular system. The mean age of the patients at the time of diagnosis was 4.9 years. Of the patients studied, 10.7% were neonates, 35.3% were infants younger than 1 year, and 48.4% were younger than 2 years. Most of the patients had a congenital cardiac disease and spontaneous thrombosis, and 66.1% had at least 1 acquired risk factor, the most common of which were having undergone surgery (42%) or wearing a central venous catheter (39%). A hereditary factor for the development of thrombosis was present in 54% of the patients. The most frequently observed hereditary risk factor was the MTHFR 677C-T mutation, and the second most common was the factor V Leiden mutation. Thrombosis should be considered a systemic disorder, and thrombotic patients should be evaluated with appropriate methods. Acquired and hereditary risk factors should be analyzed systematically in thrombotic patients.

Inherited Thrombophilia

Inherited thrombophilia can be defined as a genetically determined predisposition to the development of thromboembolic complications. Since the discovery of activated protein C resistance in 1993, several additional disorders have been described and, at present, it is possible to identify an inherited predisposition in about 60 to 70% of patients with such complications. These inherited prothrombotic risk factors include qualitative or quantitative defects of coagulation factor inhibitors, increased levels or function of coagulation factors, defects of the fibrinolytic system, altered platelet function, and hyperhomocysteinemia. In this review, the main inherited prothrombotic risk factors are analyzed from epidemiological, laboratory, clinical, and therapeutic points of view. Finally, we discuss the synergism between genetic and acquired prothrombotic risk factors in particular conditions such as childhood and pregnancy.

Case of adolescent with Paget-Schroetter syndrome and underlying thrombophilia due to an elevated lipoprotein (A)

Upper extremity deep vein thrombosis (UEDVT) is a rare disorder in children and is most often associated with a central venous catheter (CVC), cancer, or an underlying thrombophilia. In adults, repetitive or strenuous upper extremity activity has been linked to mechanical compression of the thoracic outlet resulting in subclavian vein thrombosis, also known as Paget-Schroetter Syndrome (PSS). We present the case of a 14 year old female who was found to have an UEDVT consistent with PSS. On subsequent thrombophila screening, she was noted to have a significantly elevated lipoprotein (a) level of 83 (normal: <30 mg/dL). She underwent 3 months of anticoagulation therapy prior to her first rib resection and in follow-up was noted to have complete patency of her subclavian vein. This case illustrates the necessity for thrombophilia screening in the pediatric patient with UEDVT despite evidence of anatomic abnormality or mechanical venous compression.

Kidney gene expression analysis in a rat model of intrauterine growth restriction reveals massive alterations of coagulation genes

In this study, low birth weight was induced in rats by feeding the dams with a low-protein diet during pregnancy. Kidneys from the fetuses at the end of gestation were collected and showed a reduction in overall and relative weight, in parallel with other tissues (heart and liver). This reduction was associated with a reduction in nephrons number. To better understand the molecular basis of this observation, a transcriptome analysis contrasting kidneys from control and protein-deprived rats was performed, using a platform based upon long isothermic oligonucleotides, strengthening the robustness of the results. We could identify over 1800 transcripts modified more than twice (772 induced and 1040 repressed). Genes of either category were automatically classified according to functional criteria, making it possible to bring to light a large cluster of genes involved in coagulation and complement cascades. The promoters of the most induced and most repressed genes were contrasted for their composition in putative transcription factor binding sites, suggesting an overrepresentation of the AP1R binding site, together with the transcription induction of factors actually binding to this site in the set of induced genes. The induction of coagulation cascades in the kidney of low-birth-weight rats provides a putative rationale for explaining thrombo-endothelial disorders also observed in intrauterine growth-restricted human newborns. These alterations in the kidneys have been reported as a probable cause for cardiovascular diseases in the adult.

Long-term production of major coagulation factors and inhibitors by primary human hepatocytes in vitro: perspectives for clinical application

BACKGROUND/AIMS

Patients with coagulation factor disorders require lifelong symptomatic treatment. This is associated with limited efficacy and transmission risks. From a clinical point of view, hepatocyte transplantation offers a rational alternative but is currently being hampered by lack of functional stability of engrafted cells. It was the aim of our study to devise culture conditions providing stable cell polarity, attachment and growth factor stimulation to improve longevity and coagulation factor production.

METHODS

Human hepatocytes (HC) were plated on different extracellular matrices, inside collagen gel or Matrigel. HC were grown inside growth factor-enriched serum-free medium (SFM) or exposed to media switching from differentiation (DM) to dedifferentiation (DeDM).

RESULTS

Over more than 30 days in vitro human HC synthesized coagulation factors (factors VII, VIII, IX, fibrinogen) and coagulation inhibitors (antithrombin III, protein C). Protein synthesis was augmented when HC were grown inside a 3D collagen type I matrix, while Matrigel showed no additional benefit. Soluble growth factors improved coagulation factor production when applied in SFM or in sequential DM/DeDM. Coagulation factor levels ranged from 3% to 12% in the first week to 2.5-5% after 4 weeks, reaching biologically relevant levels.

CONCLUSION

Preserved synthesis and secretion of coagulation factors in balanced proportion by human HC in this model may offer new perspectives for HC transplantation in coagulation defects of patients.

Mutations and Polymorphisms in Genes Affecting Haemostasis Components in Children with Thromboembolic Events

The distribution of mutations/polymorphisms in genes affecting haemostasis [factor V Leiden (FVL), FV H1298R (FVR(2)), FII 20210A, b-Fib 455G-->A, FXIII V34L, PAI-1 4G, HPA-1b] among 141 children with thrombosis at various sites and 103 controls was compared. Additionally, the carriage of these mutations/polymorphisms was associated with the levels of their corresponding proteins in thrombosed children. Thrombosis was more frequent in boys (p = 0.021). No studied mutation/polymorphism was found to be a risk factor for thrombosis, except for FVL (odds ratio 3.8, 95% CI 1.4-10.6). The risk of thrombosis for FVL carriers was twice as high in children with an idiopathic thrombosis (odds ratio 5.4) than in thrombosed children with an underlying disease or a triggering event (odds ratio 2.7). FVL carriers had an odds ratio of 5.9 (95% CI 1.8-19.6) when FVR(2) was absent. In thrombosed children, the activated protein C resistance ratio was significantly lower in the presence of FVL (p < 0.001). Prothrombin and fibrinogen levels, although higher in FII 20210A and b-Fib 455G-->A carriers, did not reach statistical significance.

Intra-atrial thrombus in a neonate with coarctation of the aorta

It is uncommon for thrombus to form within the heart of neonates with congenital cardiac disease. We describe a newborn with coarctation of the aorta, in whom a left atrial thrombus was discovered on the second day of life, and was thought to have been present before birth.

Continuous infusion of factor concentrates in children with haemophilia A in comparison with bolus injections

Although the concept of continuous infusion (CI) of factor concentrates is well known, prospective paediatric data are rare. We present a prospective open-labelled non-randomized study focusing on safety, efficacy and factor VIII (FVIII) usage compared with bolus injections (BI) in children. In 43 consecutive patients (0.5-17 years; median: 9.6) undergoing different operations, CI was started with an initial FVIII-bolus of 70 IU kg(-1) bodyweight, followed by a median infusion rate of 4.4 IU kg(-1) h(-1) (range: 2.8-9.5) dose adjusted for daily FVIII levels (target: 60-80%). No direct serious adverse events occurred; however, two out of 43 patients, both from the group of four patients with less than 20 exposure days (ED) before starting CI, developed a high-responding inhibitor. Two CI patients showed mild thrombophlebitis or rash. Infusion rates needed to achieve adequate FVIII levels were highly predictable and could be reduced because of decreasing FVIII clearance. Bleeding, requiring additional boli, was observed in eight out of 43 patients. Therapy duration and factor usage of CI were influenced by the procedure, but not by the product used or thrombophilia. Twelve of these CI patients were compared with 12 contemporary consecutive age- and procedure-matched BI patients. Compared with BI patients, CI patients saved 30% FVIII (812.9 vs. 563.2 IU kg(-1), P < 0.006). We conclude that CI forms a safe and effective method for perioperative care in children and reduces factor usage. Because of the unknown risk of inhibitor development, we will use CI only in patients beyond 20 ED.

3' end processing of the prothrombin mRNA in thrombophilia

Clinical and genetic studies have led to the discovery of specific genotypes that predispose to thromboembolism in adults and children. The exploration of the underlying pathologies has revealed a broad variety of affected molecular mechanisms. Most recently, the functional analysis of the prothrombin (F2) 20210*A variant revealed increased efficiency of 3' end processing as a novel genetic mechanism predisposing to human disease. Here, we review the 3' end processing of the human F2 mRNA and demonstrate how clinically relevant mutations in the F2 gene contribute to thrombophilia by interfering with a tightly balanced architecture of noncanonical 3' end formation signals.

Long-term treatment of thrombosis with enoxaparin in pediatric and adolescent patients

Thrombosis is a rare event in childhood and adolescence. Nevertheless, increasing numbers of invasive diagnostic and therapeutic procedures also result in increasing numbers of thromboses in pediatric cases, necessitating effective antithrombotic treatment regimens. In recent years, low-molecular-weight heparins (LMWH) in particular have been proved to be a safe and effective alternative to unfractioned heparins. However, the application of LMWH in pediatric patients has not been supported by a single controlled study so far. Furthermore, there is no official approval of these drugs for children. In this pilot study 27 children with deep venous thromboses (DVT) were treated with the LMWH enoxaparin at a dosage of 1.5 mg/kg body weight b.i.d. in neonates and infants and 1 mg/kg body weight b.i.d. in children. This dosage was lowered for prophylaxis if therapeutic success was achieved. The aim of the study was to investigate both, efficacy with respect to patency rates and safety during acute and long-term follow-up. Sufficient therapeutic success required a rapid production of anti-Xa target activity and was reached in 85% of the treated patients, who showed patency of the affected vessel at last follow-up. The mean duration of treatment with full dosage was 16.5 days, followed by prophylaxis over a mean duration of 9.8 months. Rethrombosis or adverse events including heparin-induced thrombocytopenia were not observed in any patient. In conclusion, enoxaparin provides an effective and safe alternative to unfractioned heparins in the treatment of thrombosis in infancy, childhood and adolescence.

Clinically important venous thromboembolism in pediatric critical care: a Canadian survey

PURPOSE

Pediatric venous thromboembolism (VTE) is becoming an increasingly recognized morbidity associated with critical illness. The objective of this survey is to identify the patient factors and radiological features that pediatric intensivists consider more or less likely to make a venous thrombosis (VTE) clinically important in their patients.

MATERIALS AND METHODS

Our definition of clinically important VTE was a VTE likely to result in short- or long-term morbidity or mortality if left untreated. We asked respondents to rate the likelihood that patient factors and radiological features make a venous thrombosis clinically important using a 5-point scale (1 = much less likely to 5 = much more likely).

RESULTS

The 38 (58.5%) of 65 pediatric intensivists responding rated 4 patient factors as most likely to make a VTE clinically important: clinical suspicion of pulmonary embolism (mean score, 4.8), symptoms (mean, 4.5), detection by physical exam (mean, 4.4), and the presence of an acute or chronic cardiopulmonary comorbidity that might limit a patient's ability to tolerate pulmonary embolism (mean, 4.3). Of the radiological features, the 2 considered most important were VTE involving the vena cava extending into the right atrium (mean, 5) and central veins (mean, 4.5).

CONCLUSIONS

When labeling a VTE as clinically important, pediatric intensivists rely on several specific patient factors and thrombus characteristics.

Evaluation of anticoagulant system in Turkish children with Perthes disease

BACKGROUND

Perthes Disease (PD) is generally a self-limiting disease of childhood but it causes severe pain and may lead to deformity of the femoral head. Intravascular thrombosis seems to form the main mechanism in the pathogenesis of the disease. The aim of this study was to determine hereditary thrombotic risk factors in Turkish children with PD.

METHODS

In 46 Perthes patients (35 male, 11 female), family history of thrombotic events was investigated, Protein C (PC), free-Protein S (f-PS), antithrombin (AT) activities, fibrinogen level, and resistance to activated Protein C (APC) were measured. The results were compared with a healthy control group of 79 children matched by age and sex. The relationship between the severity of disease and coagulation system abnormalities was evaluated.

RESULTS

While the mean PC and AT activities were significantly lower in the patients than those of the controls, the proportions of patients with low AT activity, resistance to APC, and a history of hereditary thrombophilia were significantly higher than those of the controls. No difference was observed in coagulation system disorders relative to severity of the disease and bilateral or unilateral disease involvement.

CONCLUSIONS

This study shows that a possible association between PD and inherited hypercoagulability. Determination of thrombotic risk factors in these patients may bring a new approach to the treatment. Most importantly, this may be a stimulant to take precautions for other thrombotic events, which patients may face later in life.

Genetic thrombophilia

The genetic thrombophilias are an important cause of venous thrombotic events. Much has been learned about the natural history of these disorders, their genetics, and, to a lesser degree, their treatment. This article provides an overview of the genetics of thrombophilia. Specific information on the factor V Leiden mutation;the prothrombin G20210A mutation; and protein C, proteinS, and antithrombin deficiency is reviewed. Current testing and treatment options for the genetic thrombophilias also are discussed.

Cerebral venous thrombosis in a patient with Evans syndrome: a rare association

In this report we describe a case of extensive cerebral venous thrombosis in a patient with Evans syndrome. A 19-year-old male patient with Evans syndrome was admitted to the hospital with the complaints of headache, convulsive seizure, and vomiting. The cerebral venous thrombosis including left lateral, left sigmoid, straight sinus, and vena jugularis interna was diagnosed by cerebral magnetic resonance angiography. When the thrombosis developed, he was in hematological remission and he was not receiving any medications except lamivudine for chronic hepatitis B infection. As a genetic prothrombotic risk factor, he had heterozygous prothrombin G20210A gene mutation. His clinical and radiologic findings improved after unfractionated heparin and subsequently with coumadin therapy. On follow-up, cerebral venous thrombosis reoccurred in different localizations, but complete recanalization could be obtained with antithrombotic therapy. We present the case since the association of cerebral venous thrombosis and Evans syndrome is very rare.

Analysis of pediatric thrombotic patients in Turkey

This study analyzes the data of thrombotic children who were followed up in different pediatric referral centers of Turkey, to obtain more general data on the diagnosis, risk factors, management, and outcome of thrombosis in Turkish children. A simple two-page questionnaire was distributed among contact people from each center to standardize data collection. Thirteen pediatric referral centers responded to the invitation and the total number of cases was 271. All children were diagnosed with thromboembolic disease between January 1995 and October 2001. Median age at time of first thrombotic event was 7.0 years. Of the children 4% of the cases were neonates, 12% were infants less than 1 year old, and 17% were adolescents. Thromboembolic event was mostly located in the cerebral vascular system (32%), deep venous system of the limbs, femoral and iliac veins (24%), portal veins (10%), and intracardiac region (9%). Acquired risk factors were present in 86% of the children. Infection was the most common underlying risk factor. Inherited risk factors were present in 30% of the children. FVL was the most common inherited risk factor. Acquired and inherited risk factors were present simultaneously in 19% of the patients. Eleven children had a history of familial thrombosis. Due to the local treatment preferences, the treatment of the children varied greatly. Outcome of the 142 patients (52%) was reported: 88 (62%) patients had complete resolution, 47 (33%) had complications, 12 (9%) had recurrent thrombosis, and 34 (24%) died. Three children (2.1%) died as a direct consequence of their thromboembolic disease. The significant morbidity and mortality found in this study supports the need for multicentric prospective clinical trials to obtain more generalizable data on management and outcome of thrombosis in Turkish children.

The factor V G1691A mutation is a risk for porencephaly: A case-control study

This study was initiated to investigate prothrombotic risk factors in children with porencephaly. 76 porencephalic and 76 healthy infants were investigated for factor V (FV) G1691A mutation, factor II G20210A variant, methylenetetrahydrofolate reductase (MTHFR) C677T genotype, lipoprotein (a), protein C, protein S, and antithrombin. Only the FV mutation (p = 0.005) and combinations of two or three different risk factors (p = 0.003) were significantly associated with porencephaly. These data give evidence that the FV G1691A mutation and a combination of prothromboic factors play a major role in the development of childhood porencephaly.

Anticoagulation therapy in children

Thromboembolic disease (TED) is increasingly recognized as a major cause of morbidity and mortality in tertiary pediatrics. Children younger than 1 year of age and teenage girls are at greatest risk of thromboembolism. Although anticoagulation therapy is the treatment of choice for TED, the treatment strategy is often difficult, especially in children. Treatment relies largely on anticoagulation with heparin and warfarin. Recommendations for antithrombotic therapy in children have been loosely extrapolated from recommendations for adults; however, it is likely that optimal treatment of children with TED differs from adults because of important ontogenic features of hemostasis that affect both the pathophysiology of the thrombotic processes and the response to antithrombotic agents. Until recently, the primary treatment for TED has been unfractionated heparin (UFH) in conjunction with warfarin. Warfarin, the most commonly used oral anticoagulant, acts through inhibition of the vitamin K-dependent transcarboxylation reactions that convert precursors of clotting factors into their active form. Appropriate use of UFH and warfarin requires close patient monitoring and dosage adjustments to ensure tolerability and efficacy. In recent years, low molecular weight heparins (LMWH) have become available as alternatives to UFH and warfarin, for both the prevention and treatment of TED. Potentially, LMWH have significant advantages. They have superior pharmacokinetics, which results in minimal laboratory monitoring, offering important benefits to children with poor venous access. Based on available data, LMWHs are at least as effective and well tolerated as UFH, and are more convenient. Although LMWHs are more expensive than UFH, the expense is likely to be offset by savings from a reduced hospital stay.

Evaluation of lipoprotein(a) as a prothrombotic factor: progress from bench to bedside

PURPOSE OF REVIEW

Since the homology between apolipoprotein(a) (apo(a)) and plasminogen was discovered in 1987, the role of lipoprotein(a) (Lp(a)) as an inhibitor of the normal fibrinolytic role of plasmin(ogen) has been a major research focus. In this review we summarize recent basic research aimed at identifying mechanisms by which Lp(a) can either inhibit fibrinolysis or promote coagulation, as well as recent clinical studies of Lp(a) as a risk factor for thrombosis either in the presence or absence of atherosclerosis.

RECENT FINDINGS

It has recently been reported that the inhibition of plasminogen activation by apo(a) results from the interaction of apo(a) with the ternary complex of tissue-type plasminogen activator, plasminogen and fibrin, rather than competition of apo(a) and plasminogen for binding sites on fibrin. Lp(a) species containing smaller apo(a) isoforms bind more avidly to fibrin and are better inhibitors of plasminogen activation. Recent clinical studies have provided strong evidence that Lp(a), either alone or in synergy with other thrombotic risk factors, significantly increases the risk of venous thromboembolism and ischemic stroke.

SUMMARY

Lp(a) both attenuates fibrinolysis, through inhibition of plasminogen activation, and promotes coagulation, through alleviation of extrinsic pathway inhibition. Further basic and clinical studies are required to more clearly define the role of Lp(a) in thrombotic disorders, and to determine the extent to which thrombotic risk is dependent on apo(a) isoform size.