Page 1415 - Small Animal Internal Medicine, 6th Edition

CHAPTER 87
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Disorders of Hemostasis

GENERAL CONSIDERATIONS PHYSIOLOGY OF HEMOSTASIS

Spontaneous or excessive bleeding is relatively common in Under normal conditions, injury to a blood vessel leads to
dogs and rare in cats. As a general rule, a systemic hemo- immediate vascular changes (e.g., vasoconstriction) and
static abnormality is the underlying cause of excessive rapid activation of the hemostatic system. Changes in axial
bleeding in dogs and cats that have sustained trauma or blood flow lead to exposure of circulating blood to suben-
are undergoing a surgical procedure, and in dogs evalu- dothelial collagen, resulting in the rapid adhesion of platelets
ated because of spontaneous bleeding tendencies. Sponta- to the affected area. The adhesion of platelets to the suben-
neous bleeding disorders are extremely common in dogs dothelium is mediated by adhesive proteins, such as von
evaluated at our clinic but are rare in cats. Approaching Willebrand factor (vWF) and fibrinogen, among others.
these patients’ bleeding in a logical and systematic fashion After adhering to the area of endothelial damage, platelets
allows the clinician to confirm the presumptive diagnosis in aggregate and form the primary hemostatic plug, which is
most cases. short-lived (seconds) and unstable. The primary hemostatic
In addition to bleeding, abnormal hemostatic mecha- plug serves as a framework in which secondary hemostasis
nisms can also cause thrombosis and thromboembolism occurs because most of the clotting factors assemble the
(TE), potentially leading to organ failure. Thromboembolic thrombus or clot on the platelet plug.
disorders are rare in dogs and cats without underlying car- Although the intrinsic, extrinsic, and common coagu-
diovascular disorders (e.g., cats with hypertrophic cardiomy- lation pathways have been well characterized and are still
opathy and aortic TE; see Chapter 12), but they are now used to teach physiology of hemostasis, coagulation in vivo
increasingly being recognized and documented in dogs with does not necessarily follow these distinct pathways. Also,
liver disease, protein losing nephropathy, endocrinopathies, the cellular and biochemical phases of coagulation occur
and other diseases. almost simultaneously. For example, factors XII and XI do
The most common disorder leading to spontaneous not appear to be necessary for the initiation of coagulation;
bleeding in dogs seen at our clinic is thrombocytopenia, for example, dogs and cats with factor XII deficiency do not
mainly due to immune-mediated mechansims. Other have spontaneous bleeding tendencies. It is now generally
common hemostatic disorders leading to spontaneous bleed- accepted that the physiologic mechanism responsible for
ing in dogs evaluated at our hospital include disseminated clotting in vivo is primarily tissue factor (TF) activation of
intravascular coagulation (DIC) and rodenticide poisoning. factor VII. In the past 2 decades, the traditional coagulation
Congenital clotting factor deficiencies resulting in spontane- cascade has been thought of as a common pathway from
ous bleeding are rare. Although von Willebrand disease early in the process; the traditional intrinsic, extrinsic, and
(vWD) is common in certain breeds (see p. 1398), it is not a common pathways are now known to be interrelated (Furie
common cause of spontaneous bleeding in our patients. and Furie, 2008).
Abnormalities in hemostasis screens are frequently noted in In the traditional scheme, activation of the contact phase
cats with liver disease, feline infectious peritonitis (FIP), or of the coagulation cascade occurs almost simultaneously
neoplasia; however, spontaneous or intra- or postoperative with platelet adhesion and aggregation (Fig. 87.2) and leads
bleeding tendencies are extremely rare in these patients. to the formation of fibrin through the intrinsic coagulation
Decreased production of platelets (thrombocytopenia) or cascade. A good mnemonic is to refer to the intrinsic system
virus-induced thrombocytopathia resulting in spontaneous as the “dime store” coagulation cascade: “It is not $12, but
bleeding is occasionally seen in cats with retrovirus-induced $11.98” (for factors XII, XI, IX, and VIII). Factor XII is
bone marrow disorders. activated by contact with the subendothelial collagen and

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