FIBRINOLYS AND DISORDERS OF BLOOD COAGULATION

FIBRINOLYSIS

The process of removing unwanted, insoluble deposits formed as a result of coagulation is called fibrinolysis. It is a physiological process in which a fibrin clot is broken down by enzymes into soluble fragments. The fibrinolytic system involves conversion of a plasma proenzyme plasminogen into plasmin digest fibrin and breaks it into soluble fragments.

Plasmin is also capable of breaking down fibrinogen, factor V and factor VII in the presence of plasminogen activators. These activators are present in the largest quantities in the endothelial cells of capillaries. They are also found in small amounts in plasma and in lysosomes of most body cells.

The endothelial cells also secrete another important plasminogen activator called tissue-type plasminogen activator (t-PA). The t-PA does not circulate in an inactive form like other activators. It is secreted in large amounts in an active form by the endothelial cells in response to various stimuli. When released, it continuously interacts with plasminogen activator inhibitor (PAI) forming an inactive complex. High levels of t-PA are associated with bleeding disorders. PAI is present in small amounts in plasma and in much larger amounts in platelets. High PAI levels are seen in sepsis, in pregnancy and are associated with thrombotic activity.

Increased levels of plasminogen are observed in acute bacterial infections, inflammatory conditions, myocardial infarction, thrombophlebitis, pregnancy and after long term use of oral contraceptives.

DISORDERS OF BLOOD COAGULATION The common causes of coagulation disorders include deficiencies of coagulation factors, inhibitors of coagulation factors and defects in platelet function. The disorders may be hereditary or acquired. Some commonly occurring disorders are mentioned below.

I Inherited Disorders

1. Haemophilia A (classical haemophilia)

Haemophilia A is the most commonly inherited disorder of the coagulation factors. The frequency of the disease is approximately 1:25,000 and males are primarily affected. This disorder occurs due to the deficiency of a small subunit of the factor VIII molecule called factor VIII antigen (VIII:Ag). This deficiency is carried on the X chromosome.

Therefore, haemophilia A occurs almost exclusively in males and is very rare in females. Clinically, classical haemophilia occurs along with haemorrhage. Such haemorrhages can occur both internally and externally. Bleeding into joints and muscles is typical. Other symptoms include excessive bleeding after surgery or major trauma, and even after superficial abrasions. Bleeding may also occur through the nose, mouth, eyes and gastrointestinal tract. Severe bleeding may lead to a rapidly progressive normocytic anaemia. Intracranial haemorrhage is the most common cause of death.

Laboratory findings Partial thromboplastin time (PTT) is an effective screening test for classical haemophilia. It gives abnormal results when factor VIII is about 25% of the normal value. Platelet count and platelet function tests such as bleeding time and thrombin time are normal. Thromboplastin generation test (TGT) and prothrombin consumption test (PCT) are too sensitive and give abnormal results even in very mild haemophilia.

Assay of factor VIII is the diagnostic test. Inhibitors of factor VIII are detected in severely affected young patients. These are specific IgG antibodies to factor VIII.

2. Haemophilia B (Factor IX deficiency)

Haemophilia B is an inherited deficiency of factor IX and is an X-chromosome linked disorder like haemophilia A. Haemophilia B is five times less frequent than haemophilia A. Clinically, haemophilia B is indistinguishable from the classical haemophilia but is generally a milder form. The therapy for both the haemophilias being different, it is necessary to differentiate between the two disorders. 

Laboratory findings As in haemophilia A, a prolonged PTT, normal thrombin time and normal bleeding time is observed in haemophilia B patients. Prothrombin time is nor-mal in most cases. Factor VIII level is normal whereas factor IX is deficient and is diagnostic.Antibodies to factor IX may be seen only in patients undergoing long term therapy. 

3. Haemophilia C (Factor XI deficiency)

This is a rarely inherited bleeding disorder. The symptoms are similar to those of the other haemophilias. The severity of the disease is not related to the level of factor XI. 

Laboratory findings These are similar to the other haemophilias, and the diagnosis depends on the assay of factor XI.

4. von Willebrand syndrome

von Willebrand syndrome (VWS) is one of the commonly occurring inherited coagulation disorders. Like haemophilia A, VWS is also related to the abnormality or the deficiency of factor VIII. Factor VIII is a complex of two molecules: VIII:C and VIII:Ag.

(i) A small active molecule is the factor VIII coagulant-antigen (VIII: Ag). Deficiency of this fraction leads to haemophilia A

(ii) A larger fragment is called von Willebrand factor (vWF), which is synthesized by endothelial cells and megakaryocytes. The vWF circulating in the blood binds with factor VIII: Ag. It plays a carrier role for the smaller active fraction and protects it from proteolytic destruction. The main role of vWF is in the adhesion of platelets to foreign surfaces such as glass, especially after vessel injury. . 

Clinically, the patients with vWS characteristically bleed from mucous membranes and cutaneous sites. Easy bruising and excessive bleeding following dental extractions and tonsillectomy is common. Unlike classical haemophilia, bleeding in the joints and deep muscles is not observed in vWS except in severe cases. Gastrointestinal bleeding and menorrhagia are also common.

Laboratory findings The diagnosis of vWS is difficult. The routine laboratory screening tests are of limited value. Generally, bleeding time and PTT are abnormal. The PCT (prothrombin consumption test) is usually abnormal. Platelet adhesion and aggregation tests are more consistently abnormal. Factor VIII:Ag is often reduced. For more specific diagnosis, rocket immunoelectrophoresis is performed using an antibody to WF.

5. Other inherited deficiencies

Inherited deficiencies of coagulation factors other than those described above are very rare.

(a) Deficiency of factor VII may be associated with the tendency of severe bleeding. A prolonged prothrombin time with normal PTT and thrombin time is suggestive. Specific factor VII assay should confirm the diagnosis.

(b) Deficiencies of factor XII, HMWK or prekallikrein show very mild bleeding tendencies. The laboratory tests show a prolonged activated PTT, normal prothrombin time and normal thrombin time. If factors VIII and IX are suspected, then specific assays of these factors should be performed.

(c) Deficiency of factor X is not clinically distinguishable from that of factor VII. The severity of the disease may vary. In most cases activated PTT and prothrombin time are prolonged. Stypven time test should be performed to differentiate between the two deficiencies. The test involves use of venom of the snake Vipera russelli (Russell's viper). It acts directly on factor X to convert it to factor Xa in the absence of other clotting factors. A prolonged Stypven time in the presence of a prolonged prothrombin time is indicative of factor X deficiency.

(d) Deficiency of factor V is quite rare. The laboratory tests show prolonged prothrombin time, PTT and TGT. Coagulation time may be abnormal.

(e) In factor XIII deficiency, the laboratory results are almost normal. The diagnostic test is based on solubility of the unstabilised fibrin clot in a five molar urea solution or 1% mono-chloroacetic acid.

(f) Fibrinogen deficiency is a rare inherited disorder but may lead to severe bleeding. Laboratory results show abnormality in all the tests which have a fibrin clot as the end point, e.g.. prothrombin time, thrombin time and PTT. The PCT and TGT are normal except in severe cases.

(g) Prothrombin deficiency is quite rare. The laboratory findings show prolonged prothrombin time and PTT. Bleeding time, platelet count, clot retraction and thrombin time are normal. Diagnosis is made by the specific assay for prothrombin

II. Acquired Disorders 

1. Vitamin K deficiency

Deficiency of vitamin K leads to defects in the synthesis of coagulation factors VII, IX, X and prothrombin, all of which are dependent on vitamin K for their synthesis. Vitamin K is normally obtained in two ways; through the dietary intake of the vitamin produced by plants, and the vitamin synthesised by the intestinal bacteria. 

Vitamin K deficiency may result from inadequate diet, intestinal malabsorption, biliary obstruction or gut sterilisation. Haemorrhagic disease of the new-born may occur in neonates. Vitamin K deficiency may be intentionally induced by the administration of anticoagulant drugs such as coumarin. These drugs are administered orally and are absorbed in the blood through the gastrointestinal tract. The level of the anticoagulant in the blood is regulated in such a way that the prothrombin time ratio is maintained at 1.5 (For example, prothrombin time of patient's plasma is 15 seconds when that of the control is 10 seconds).

2. Heparin therapy

Heparin is a fast-acting mucopolysaccharide which interferes with coagulation by inhibiting the action of factors lla, Xla, XIla and XIII by blocking the conversion of fibrinogen to fibrin. It also inhibits platelet aggregation. Heparin therapy is used in the treatment of thrombosis so that the coagulation is extended with minimum of bleeding. The anticoagulant activity of the heparin treated plasma is best monitored with the help of activated partial thromboplastin time (APTT). The desired range of APTT is similar to that in the oral anticoagulant therapy i.e. 1.5 times that of the control. 

3. Diffuse intravascular coagulation (DIC)

DIC syndrome may result from a wide variety of pathological processes which lead to activation of the coagulation system. 

Intravascular fibrin deposits may be produced with excessive consumption of coagulation factors. Reduction in the level of coagulation factors in turn leads to imbalance in the haemostatic mechanism and uncontrolled bleeding may start. This process is often accompanied by fibrinolysis which increases the bleeding tendency. The aetiological agents of DIC include sepsis, neoplasms, autoimmune diseases, haematological diseases, obstetric problems and intravascular haemolysis.

Laboratory findings in DIC vary widely and depend on the cause and its severity. Generally, prothrombin time, thrombin time and PTT are increased. Most cases of DIC are associated with thrombocytopenia with leucocytosis. Schistocytes (fragmented red cells) may be seen in the peripheral blood smear.

4. Lupus-type anticoagulants

Some patients with systemic lupus erythematosus and other wide variety of disorders and those undergoing therapy with drugs such as phenothiazine, develop coagulation inhibitors known as lupustype anticoagulants. 

These substances inhibit the conversion of factor X to factor Xa. The lupus-type anticoagulants produced under various conditions may not be identical, but they are antibodies of either IgG or IgM type. The peculiar feature of these anticoagulants is that they rarely show a clinical bleeding tendency unless there is another pre-existing or additional haemostatic abnormality. The laboratory result show abnormal prothrombin time, thrombin time and PTT.

III. Inherited Disorders of Platelet Function

1. Glanzmann's thrombasthenia

In this disorder, the platelet count and individual platelet morphology is normal, but they show a characteristic tendency to resist clumping together. This results in inadequate platelet plug formation and is due to an abnormality in the surface membrane glycoproteins. Clinically, the bleeding can

be very severe. The laboratory tests show abnormal bleeding time, prothrombin consumption test and clot retraction.

2. Bernard-Soulier syndrome

This inherited disorder is due to the absence of some membrane glycoproteins which are essential for platelet adhesion. The laboratory findings include abnormally large (74 um) platelet forms, prolonged bleeding time and reduced platelet adhesion to glass. The clot retraction is normal.

3. Storage granule abnormalities

Some platelet disorders involve a deficiency or abnormality of one or more types of storage granules. In most cases, platelet release function after stimulation and platelet aggregation are affected. 

IV. Acquired Disorders of Platelet Function 

1. Drug Induced disorders A large number of drugs influence platelet function. Aspirin and its derivatives are the principle causes of such disorders. Other examples include antibiotics such as penicillins and cephalosporins; analgesics and anti-inflammatory agents like ibuprofen; and other miscellaneous drugs such as heparin. The platelet function usually returns to normal when administration of causative agent is discontinued.

include acute and chronic leukaemias, haemorrhagic thrombocythaemia, myelofibrosis, autoimmune diseases, systemic lupus erythematosus and anaemia.

V. Quantitative Platelet Disorders

1. Thrombocytopenia

A reduced number of circulating platelets (thrombocytopenia) can result from:

(1) Decreased production of platelets Decreased production of megakaryocytes or ineffective platelet production due to vitamin B12 deficiency can lead to thrombocytopenia. 

(ii) Disorders of distribution This can occur due to conditions such as hypothermia or transfusion of stored blood. 

(iii) Platelet destruction A wide variety of conditions can lead to platelet destruction. Infection, tissue injury, snake bite, thrombotic thrombocytopenic purpura, autoimmune diseases, liver disease and malignancies are some of the conditions that can cause platelet destruction.

2. Thrombocytosis Increased platelet counts or thrombocytosis may result from a benign, reactive process or may be associated with a myeloproliferative disorder. In most cases, platelet stimulation and aggregation is affected.