Haemorrhagic Disease Pathophysiology

Introduction

Haemorrhagic diseases can be categorised into two different causal mechanisms:

• Increased vessel fragility which leads to a non-thrombocytopenic purpura.
• Inadequate haemostatic response. This may be caused by platelet dysfunction or deficiency, which leads to a primary or secondary thrombocytopenic purpura, or deficiencies/derangements of clotting factors.

Syndromes caused by vascular fragility and platelet dysfunction tend to be purpuric and acquired. Syndromes caused by clotting factor defects tend to cause more severe bleeding and are usually congenital.

Vascular Fragility

There are several ways that vascular fragility may arise. Capillaries can be damaged by toxins - this occurs in severe bacterial infections in all species. Streptococcus and Pasteurella infections and anthrax would be good examples of organisms that feature capillary damage as a symtom of infection.

Drug-induced damage to the vascular system does occur, although the mechanisms of damage induced by drugs are not clearly understood. Examples include heavy metals, e.g. lead, bismuth, mercury, iodides, fluorides, chlorinated hydrocarbon pesticides and salicylates.

Vascular fragility can occur as a consequence of an underlying disease state, which is seen in allergy or anaphylactic reactions, connective tissue diseases such as Ehlers-Danlos syndrome in dogs, Diabetes mellitus and antibody-antigen complex reactions.

Platelet dysfunction

Bleeding disorders may occur if platelets are deficient (thrombocytopaenia) or if the platelets are unable to function adequately (thrombocytopathia). Immune-mediated thrombocytopaenia (ITP) is one common cause of thrombocytopaenia and this may be a primary or secondary disease. Thrombocytosis refers to an increase in the blood platelet concentration above the normal level, and has no clinical importance except as an indicator of another disease process. Thrombocytopaenia and thrombocytopathia both result in reductions in the effectiveness of primary haemostasis, giving rise to a bleeding disorder. Since heamorrhage is usually sealed by a fibrin clot, disorders of primary haemostasis tend to be less severe than those caused by deficiencies in the coagulation factors.