Overcoming the diagnostic and therapeutic challenges of canine immune-mediated thrombocytopenia - Veterinary Medicine
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Overcoming the diagnostic and therapeutic challenges of canine immune-mediated thrombocytopenia
Determining that the immune system is the cause of a dog's decreased platelet count can be difficult. These clinicians walk you through the diagnostic and treatment process so you can help patients with this life-threatening bleeding disorder.


VETERINARY MEDICINE


Transfusions

In some cases, transfusions are needed to address anemia due to hemorrhage. In other instances, transfusions are given in an attempt to increase platelet numbers. Whole blood, platelet-rich plasma, or platelet concentrates are sources of platelets for transfusion. Transfusions with these products transiently boost platelet numbers, but thrombocytopenia rapidly recurs as the circulating life span of donor platelets is markedly reduced in patients with immune-mediated thrombocytopenia, often lasting only a few hours.6,8

Transfusions are also used to stop immediate bleeding in critically ill patients and may be administered before surgery (e.g. splenectomy) or other potentially invasive procedures. Fresh platelet-rich plasma is given at a dose of 10 ml/kg, which is expected to raise the platelet count by about 10,000/μl.8,18 If platelet numbers do not increase or if the increase is not sustained for at least two hours, additional transfusions are not indicated.8

Preparation of platelet-rich plasma and platelet concentrate requires special centrifugation, but these products may be purchased through commercial blood banks.18,21 Although whole blood increases the risk of volume overload and transfusion reaction, it may be given.8 Whole blood transfusion is indicated for patients with severe thrombocytopenic hemorrhage.21 Refrigeration has deleterious effects on platelet viability, so store whole blood at room temperature and use it within four hours of collection.18,21

Immunosuppressive therapy

Immunosuppressive therapy is needed to achieve the primary goal in treating immune-mediated thrombocytopenia, which is to restore normal hemostasis. Initially, it is usually adequate to achieve a platelet count > 50,000/μl, as bleeding would no longer be expected. Normalization of platelet numbers may take considerably longer.8

Glucocorticoids. Glucocorticoids are the mainstay of immunosuppressive therapy and have the most immediate effects. They suppress the mononuclear phagocytic system, decrease the affinity of antibody binding to platelets, and are the initial treatment of choice.2,6,8,19 Give prednisone or prednisolone at a dosage of 2 to 4 mg/kg/day orally. Alternatively, dexamethasone may be given at a dose of 0.2 mg/kg intravenously every 24 hours. Platelet numbers usually increase within one week of initial therapy.8

Administer the prednisone until the platelet count normalizes, and then gradually taper it. Evaluate the platelet count before any reduction in the prednisone dose, and do not taper the drug if the platelet count has decreased or has failed to increase. In some cases, normal platelet counts cannot be achieved; however, if the platelet count is adequate to prevent bleeding, tapering can be initiated. Tapering too gradually is associated with increased side effects, and tapering too rapidly may result in relapse of the immune-mediated thrombocytopenia.2

Generally, prednisone is continued at the initial immunosuppressive dose for two to four weeks. If the platelet count is adequate, the dose is reduced by 50% and continued for another two to four weeks. The dose is adjusted in this manner until the lowest effective dose that maintains an adequate platelet count is achieved. A maintenance dose of 0.5 to 1 mg/kg every 48 hours is usually well-tolerated if needed.

Side effects of prednisone, particularly at high doses, include polyuria, polydipsia, hyperventilation, and other signs of hyperadrenocorticism such as hair loss, muscle wasting, weakness, and predisposition to infection. If the platelet count decreases during the tapering of the prednisone, increase the prednisone to the last dose that effectively maintained normal platelet numbers. If the required dose of prednisone is not acceptable for long-term administration, additional immunosuppressive medications should be added.

Vincristine. Vincristine is a vinca alkaloid that transiently increases platelet numbers by stimulating transient megakaryocyte platelet release.8 It may be used to facilitate increasing the initial platelet count to a degree that the patient can be discharged earlier from the hospital.14 In addition to stimulating increased platelet release from bone marrow, vincristine may also have mild immunosuppressive effects.


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Source: VETERINARY MEDICINE,
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