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Von Willebrand disease
Von Willebrand disease (vWD) is the most common hereditary coagulation abnormality described in humans, although it can also be acquired as a result of other medical conditions. It arises from a qualitative or quantitative deficiency of von Willebrand factor (vWF), a multimeric protein that is required for platelet adhesion. It is known to affect humans and in veterinary medicine, dogs. There are four types of hereditary vWD. Other factors including ABO blood groups may also play a part in the severity of the condition.
The various types of vWD present with varying degrees of bleeding tendency. Severe internal or joint bleeding is rare (only in type 3 vWD); bruising, nosebleeds, heavy menstrual periods (in women) and blood loss during childbirth (rare) may occur. Death may occur.
When suspected, blood plasma of a patient needs to be investigated for quantitative and qualitative deficiencies of vWF. This is achieved by measuring the amount of vWF in a vWF antigen assay and the functionallity of vWF with a glycoprotein (GP)Ib binding assay, a collagen binding assay or, a ristocetin cofactor activity (RiCof) or ristocetin induced platelet agglutination (RIPA) assays. Factor VIII levels are also performed as factor VIII is bound to vWF which protects the factor VIII from rapid breakdown within the blood. Deficiency of vWF can therefore lead to a reduction in factor VIII levels. Normal levels do not exclude all forms of vWD: particularly type 2 which may only be revealed by investigating platelet interaction with subendothelium under flow (PAF), a highly specialized coagulation study not routinely performed in most medical laboratories. A platelet aggregation assay will show an abnormal response to ristocetin with normal responses to the other agonists used. A platelet function assay (PFA) will give an abnormal collagen/adrenaline closure time but a normal collagen/ADP time. Type 2N can only be diagnosed by performing a "factor VIII binding" assay. Detection of vWD is complicated by vWF being an acute phase reactant with levels rising in infection, pregnancy and stress.
Other tests performed in any patient with bleeding problems are a full blood count (especially platelet counts), APTT (activated partial thromboplastin time), prothrombin time, thrombin time and fibrinogen level. Testing for factor IX may also be performed if hemophilia B is suspected. Other coagulation factor assays may be performed depending on the results of a coagulation screen.
Classification and types
There are four hereditary types of vWD described - type 1, type 2, type 3, and platelet-type. There are inherited and acquired forms of vWD. Most cases are hereditary, but acquired forms of vWD have been described. The International Society on Thrombosis and Haemostasis's (ISTH) classification depends on the definition of qualitative and quantitative defects.
Type 1 vWD (60-80% of all vWD cases) is a quantitative defect (heterozygous for the defective gene) but may not have clearly impaired clotting, most patients usually end up leading a nearly normal life. Trouble may arise in the form of bleeding following surgery (including dental procedures), noticeable easy bruising, or menorrhagia (heavy periods). Decreased levels of vWF are detected (10-45% of normal, i.e. 10-45 IU).
Type 2 vWD (20-30%) is a qualitative defect and the bleeding tendency can vary between individuals. There are normal levels of vWF, but the multimers are structurally abnormal, or subgroups of large or small multimers are absent. Four subtypes exist: 2A, 2B, 2M and 2N.
This is an abnormality of the synthesis or protelysis of the vWF multimers resulting in the presence of small multimer units in circulation. Factor VIII binding is normal. It has a disproportionately low ristocetin co-factor activity compared to the von Willebrand's antigen.
This is a "gain of function" defect leading to spontaneous binding to platelets and subsequent rapid clearance of the platelets and the large vWF multimers. A mild thrombocytopaenia may occur. The large vWF multimers are absent in the circulation and the factor VIII binding is normal. Like type 2A, the RiCof:vWF antigen assay is low when the patient's platelet-poor plasma is assayed against formalin-fixed, normal donor platelets. However, when the assay is performed with the patient's own platelets ("platelet-rich plasma"), a lower-than-normal amount of ristocetin causes aggregation to occur. This is due to the large vWF multimers remaining bound to the patient's platelets. Patients with this sub-type are unable to use desmopressin as a treatment for bleeding, because it can lead to unwanted platelet aggregation.
This is caused by decreased or absent binding to GPIb on the platelets. Factor VIII binding is normal.
Type 2N (Normandy)
This is a deficiency of the binding of vWF to factor VIII. This type gives a normal vWF antigen level and normal functional test results but has a low factor VIII. This has probably led to some 2N patients being misdiagnosed in the past as having hemophilia A, and should be suspected if the patient has the clinical findings of haemophilia A but a pedigree suggesting autosomal, rather than X-linked, inheritance.
Type 3 is the most severe form of vWD (homozygous for the defective gene) and may have severe mucosal bleeding, no detectable vWF antigen, and may have sufficiently low factor VIII that they have occasional hemarthoses (joint bleeding), as in cases of mild hemophilia.
Platelet-type vWD is an autosomal dominant type of vWD caused by gain of function mutations of the vWF receptor on platelets; specifically, the alpha chain of the glycoprotein Ib receptor (GPIb). This protein is part of the larger complex (GPIb/V/IX) which forms the full vWF receptor on platelets. The ristocetin activity and loss of large vWF multimers is similar to type 2B, but genetic testing of VWF will reveal no mutations.
Acquired von Willebrand disease
Acquired vWD can occur in patients with autoantibodies. In this case the function of vWF is not inhibited but the vWF-antibody complex is rapidly cleared from the circulation.
vWF is mainly active in conditions of high blood flow and shear stress. Deficiency of vWF therefore shows primarily in organs with extensive small vessels, such as the skin, the gastrointestinal tract and the uterus. In angiodysplasia, a form of telangiectasia of the colon, shear stress is much higher than in average capillaries, and the risk of bleeding is increased concomitantly.
In more severe cases of type 1 vWD, genetic changes are common within the VWF gene and are highly penetrant. In milder cases of type 1 vWD there may be a complex spectrum of molecular pathology in addition to polymorphisms of the vWF gene alone. The individual's ABO blood group can influence presentation and pathology of vWD. Those individuals with blood group O have a lower mean level than individuals with other blood groups. Unless ABO group–specific vWF:antigen reference ranges are used, normal group O individuals can be diagnosed as type I vWD, and some individuals of blood group AB with a genetic defect of vWF may have the diagnosis overlooked because vWF levels are elevated due to blood group.
The vWF gene is located on chromosome twelve (12p13.2). It has 52 exons spanning 178kbp. Types 1 and 2 are inherited as autosomal dominant traits and type 3 is inherited as autosomal recessive. Occasionally type 2 also inherits recessively.
In humans, the incidence of vWD is roughly about 1 in 100 individuals. Because most forms are rather mild, they are detected more often in women, whose bleeding tendency shows during menstruation. The actual abnormality (which does not necessarily lead to disease) occurs in 0.9-3% of the population. It may be more severe or apparent in people with blood type O.
Patients with vWD normally require no regular treatment, although they are always at increased risk for bleeding. Prophylactic treatment is sometimes given for patients with vWD who are scheduled for surgery. They can be treated with human derived medium purity factor VIII concentrates complexed to vWF(antihemophilic factor, more commonly known as Humate-P®) Mild cases of vWD can be trialled on desmopressin (1-desamino-8-D-arginine vasopressin, DDAVP) (desmopressin acetate, Stimate®), which works by raising the patient's own plasma levels of vWF by inducing release of vWF stored in the Weibel-Palade bodies in the endothelial cells.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Von_Willebrand_disease". A list of authors is available in Wikipedia.|