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), amultimeric protein that is required for platelet adhesion. It is known to affect humans and dogs (notably Doberman Pinschers), and rarely swine, cattle, horses, and cats. There are three forms of vWD: inherited, acquired and pseudo or platelet type. There are three types ofhereditary vWD: vWD Type I, vWD Type II and vWD III. Within the three inherited types of vWD there are various subtypes. Platelet type vWD is also an inherited condition.
vWD Type I is the most common type of the disorder and those that have it are typicallyasymptomatic or may experience mild symptoms such as nosebleeds although there may be severe symptoms in some cases. There are various factors that affect the presentation and severity of symptoms of vWD such as blood type.
The various types of vWD present with varying degrees of bleeding tendency, usually in the form of easy bruising, nosebleeds and bleeding gums. Women may experience heavy menstrual periods and blood loss during childbirth.
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 functionality of vWF with a glycoprotein (GP)Ib binding assay, acollagen binding assay, or a ristocetin cofactor activity (RiCof) or ristocetin induced platelet agglutination (RIPA) assays. Factor VIIIlevels are also performed because 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 and in most cases (but not all) 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 complete 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 ifhemophilia B is suspected. Other coagulation factor assays may be performed depending on the results of a coagulation screen. Patients with von Willebrand disease will typically display a normal prothrombin time and a variable prolongation of partial thromboplastin time.