The Diagnosis, Evaluation and Management of von
Willebrand Disease
Opportunities and Needs in VWD Research, Training,
and Practice
Many recommendations in this guideline are based on
relatively limited evidence, thus underscoring the need for further research.
Some of these opportunities are discussed below.
Pathophysiology and Classification of VWD
Determinants of VWF level and bleeding risk.
The risk of bleeding in persons who have VWD depends on the level of functional
VWF and on many other factors that are poorly understood. The plasma level of
VWF can be influenced by mutations within or near the VWF gene. In addition,
VWF levels depend on ABO blood type,43 possibly on the Secretor
locus,109 and on hormonal
status and stress, as discussed in "The VWF Protein and Its Functions In
Vivo" section. Relatively few of the genetic and nongenetic determinants of
VWF level have been characterized, and how they interact is not known. In
addition, little quantitative information is available on the risk of specific
bleeding symptoms as a function of the level of VWF in plasma. This information
would be particularly useful for the management of patients who have VWF levels
in the range of 3050 IU/dL, for whom the risk of medically significant
bleeding is not well defined.
VWF level in plasma alone does not account for the
observed variation in bleeding symptoms, and recent studies are starting to
uncover some of the underlying reasons. For example, persons who have both low
VWF and defects in platelet aggregation have more severe bleeding.387 Increased bleeding also has been
associated with specific DNA markers for platelet membrane proteins.388 It is likely that multiple hemostatic
risk factors interact with VWF level in plasma to determine the likelihood of
bleeding or thrombosis. Understanding these interactions and incorporating them
into clinical practice will require additional basic, clinical, and
epidemiological research.
Heterogeneity of type 1 VWD. Partial
quantitative deficiency of VWF can be caused by several mechanisms, as
discussed in the section on "Classification of VWD Subtypes."
Some persons have dominant VWF mutations that either decrease the secretion of
VWF multimers or accelerate their clearance from the circulation. The
prevalence of increased clearance as a cause of type 1 VWD is not known.
Whether these different disease mechanisms correlate with distinct clinical
features, including response to specific treatments, also is not known. Because
type 1 VWD is the most common form of VWD, answers to these questions may have
important consequences for medical practice.
Heterogeneity of type 2 VWD. The
concentration of hemostatically effective large VWF multimers can be
selectively decreased by accelerated proteolysis or by a variety of defects in
multimer assembly.49 These
variants now are grouped together as type 2A VWD, but further subdivision of
this category would be justifiable if specific mechanisms of disease were
associated with different clinical symptoms or responses to therapy.
Most persons who have type 2M VWD have been identified
by finding a profound defect in ristocetin-induced binding to platelets
associated with a normal VWF multimer pattern.55,67,72 Defects in binding to collagen or other
connective tissue elements could cause a similar bleeding phenotype, but the
VWF:RCo assay is insensitive to such defects.73 Collagen-binding abnormalities can be
detected by the VWF:CB assays, but those assays are not used widely in the
United States. The prevalence and medical significance of collagen-binding
defects in type 2M VWD deserve further study.
Diagnosis and Evaluation
Assessment of bleeding signs and symptoms.
The initial evaluation of patients for a medically significant bleeding
disorder can be difficult because mild bleeding is very common in the healthy
population. Specific symptoms have been assessed for clinical relevance in
retrospective studies, and some appear to discriminate among healthy controls
and persons who have diagnosed bleeding disorders (Box 1). However, the utility of
these questions must be established prospectively for less highly selected
persons.
Quality and availability of laboratory
testing. Reliable testing for VWF:Ag and FVIII is widely available, but
VWF:RCo, RIPA, and VWF multimer analysis are much more variable in their
performance characteristics and can be difficult to obtain. Also, tests of
VWFFVIII binding (VWF:FVIIIB) are offered by very few laboratories. More
robust methods for assessing VWF function and multimer structure must be
developed for routine use in the diagnosis of VWD. In addition, the sensitivity
and specificity of test ratios such as VWF:RCo/VWF:Ag should be established for
identifying the qualitative defects that characterize type 2A and type 2M VWD.
Criteria should be established for VWF multimer analysis to distinguish a
significant decrease in large multimers (in types 2A and 2B VWD) from a
substantially normal multimer distribution (in types 1, 2M, and 2N VWD).
VWF gene sequencing. Mutations that cause
many types of VWD can be identified by sequencing the VWF gene in DNA samples
from patients.23 The locations of
mutations appear to correlate well with some disease phenotypes, suggesting
that DNA sequencing could be a useful diagnostic method in VWD. With
appropriate study and experience, DNA sequencing may become economical and
feasible for routine use. In addition, the widespread application of VWF gene
sequencing would provide invaluable information about the prevalence of VWF
mutations as a function of VWF level, the strength of the relationship between
VWF genotype and VWD phenotype, the penetrance of specific mutations, and the
biochemical mechanisms that cause VWD. This knowledge also would be an
outstanding resource for the identification and characterization of other
factors that modify bleeding symptoms in VWD.
Management of VWD
Many of the standard treatments for VWD have limited
experimental support. For example, the intensity and duration of therapy
necessary to control bleeding have not been established for many clinical
situations and often have been extrapolated from anecdotal experience in
hemophilia. The indications for prophylaxis of bleeding also are not well
defined. These issues should be addressed by appropriate clinical studies.
DDAVP. Many persons who have VWD respond to
DDAVP with a clinically useful rise in VWF and FVIII, but the likelihood of a
good outcome depends on the type of VWD and the underlying biochemical
mechanism of disease. In type 1 VWD, persons who have accelerated clearance of
plasma VWF may have a transient response to DDAVP.41,232
Whether DDAVP should be used at all in persons who have type 2B VWD is
controversial.229,237,257259,389391 In type 2N VWD, the baseline
FVIII level may be a good predictor of the magnitude and duration of the FVIII
response to DDAVP.77,224,228,392 The drug is thought to be safe for
use in pregnancy, but the published experience in this setting is
limited.273,274 Hyponatremia and thrombotic events
have occurred after DDAVP, but risk factors for these events and their
incidence have not been established. These important clinical issues should be
addressed by studies of DDAVP in specific types of VWD. In addition, the
availability of DDAVP for subcutaneous administration may improve management of
VWD.
Factor Concentrates. The available
plasmaderived products that contain VWF also contain FVIII as part of the
FVIIIVWF complex, and only two such products
(HumateP® and Alphanate SD/HT®) are
currently licensed in the United States for treatment of VWD. When administered
to patients who have VWD, the infused FVIII may add to the endogenous FVIII
production and cause markedly elevated FVIII levels that are much greater than
the VWF levels achieved with treatment; these have been associated with
thrombosis.215 High FVIII levels
can be avoided by adjusting the dose of product administered, but VWF levels
then may be relatively low. Whether FVIII or VWF levels, or both, should be
used to monitor treatment with FVIIIVWF concentrates is unknown. Use of a
pure VWF product in place of FVIIIVWF concentrates would avoid the
disproportionate increase in FVIII. A pure VWF concentrate has been used in
Europe393 but is not currently
available in the United States. Studies are needed to establish appropriate
treatment and monitoring regimens for these products. In addition, prelicensure
studies of recombinant VWF are needed to establish its safety, efficacy, and
role in the treatment of VWD. The licensing of other products, containing both
VWF and FVIII, also would enhance therapeutic options.
Platelets. Approximately 15 percent of the
total VWF in blood is found within platelets, and platelet VWF appears to
contribute to hemostasis. Although VWD patients who have abnormal or low
platelet VWF have been described, there has been only limited exploration of
the feasibility and utility of such testing, in part because of limitations of
practical methodologies. Clinically, platelet transfusions have been reported
to stop bleeding in some patients who have VWD and were not helped by
transfusion of FVIIIVWF concentrates.293,294 The efficacy and appropriate use of
platelet transfusions in persons who have VWD or AVWS need to be
established.
Antifibrinolytics. Tranexamic acid and
aminocaproic acid have been used alone or as adjunctive therapy to treat
bleeding in VWD. The safety, efficacy, and optimal dosing of these agents in
VWD should be established by suitable clinical studies. In addition, the
availability of orally administered tranexamic acid would broaden the
therapeutic options for antifibrinolytic therapy.
Gene Therapy of VWD
Severe type 3 VWD potentially can be treated with gene
therapy. The gene for VWF is larger than could easily be introduced into many
vectors, but "gutless" adenoviral vectors could easily accommodate a gene the
size of VWF (8.5 kilobases). The prevalence of type 3 VWD and its clinical
symptoms, however, does not place it in a high priority category for gene
therapy trials. Point mutation repair initially was an exciting approach for
VWD,394,395 but followup studies have not
achieved the same rate of success in vitro.396,397
Issues Specific to Women
VWF is particularly important for hemostasis during
menses and at childbirth. Consequently, women are affected disproportionately
by having VWD, especially during their childbearing years.
Menorrhagia. The incidence of menorrhagia
appears to vary inversely with VWF level, independent of whether women meet
criteria for having VWD.45 Because
menorrhagia is so common, even a small reduction in its severity could have
significant implications for women’s health. As discussed in the section
on menorrhagia, several treatments
have been used for menorrhagia associated with VWD, but their efficacy has not
been demonstrated convincingly. Therefore, clinical studies would be useful to
establish the effect of VWF level on menorrhagia and to evaluate specific
treatments for women who have VWD or low plasma levels of VWF.
Labor and delivery. Several small case series
indicate that women who have VWD and VWF levels <50 IU/dL at delivery have
an increased incidence of immediate and delayed postpartum hemorrhage. These
complications appear to be prevented by replacement therapy with FVIIIVWF
concentrate before delivery and by either concentrate or DDAVP in the
postpartum period.85,354,377 How the risk of bleeding correlates
with VWF level or FVIII level is not known, and the required intensity and
duration of therapy have not been established.
Training of Specialists in Hemostasis
In the United States, despite scientific progress in
basic and clinical research in bleeding and thrombotic disorders, including
VWD, there is a shortage of skilled clinicians and laboratorians with expertise
in hemostasis.398 Training
opportunities need to be developed and/or expanded for hemostasis specialists.
Recent clinical training opportunities include a new NHLBI initiative for
training in nonmalignant hematology (RFA HL06006; information available
at: http://grants.nih.gov/grants/guide/rfa-files/RFA-HL-06-006.html)
and a recent initiative from the U.S. National Hemophilia Foundation (NHF
Clinical Fellowship Program). Recognition of hemostasis as a bona fide clinical
and laboratory subspecialty in the United States could enhance entry into the
field.
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