Page 1270 - Small Animal Internal Medicine, 6th Edition

1242 PART XI Immune-Mediated Disorders

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A B

FIG 73.4
Photographs of three dogs with
immune-mediated thrombocytopenia
and ecchymotic hemorrhage. (A)
Note the ecchymotic hemorrhages in
the skin of the abdomen. (B) Note
the hemorrhage into the anterior
chamber of the eye. (C) Note
petechial hemorrhage in the oral
C
mucous membranes.

examination of a bone marrow aspirate is the most reli- bone marrow abnormalities or result in a definitive diagno-
able method for differentiating lack of platelet production sis, so this procedure has a relatively low diagnostic yield in
from increased platelet consumption or destruction. A bone this subset of patients.
marrow evaluation should ideally be performed early in the The presence of a positive assay for platelet-bound anti-
diagnostic workup of cases with severe thrombocytopenia body (see Chapter 71) is highly sensitive but not specific
(<20,000/µL) and evidence of other hematologic abnor- for a diagnosis of ITP. A diagnosis of ITP is unlikely if the
malities, especially other cytopenias. In these cases, evalu- test result is negative. Prior immunosuppressive therapy may
ation of bone marrow is helpful to rule out disorders such result in a negative test. A positive test result is not specific
as myelophthisis, neoplasia, megakaryocytic aplasia, and for ITP because immune-mediated mechanisms are respon-
aplastic anemia (see Chapter 86). Megakaryocytic aplasia sible for many causes of thrombocytopenia in dogs, includ-
is a rare disorder in which aplasia of the megakaryocytic ing thrombocytopenia due to neoplasia, inflammation, drug
cell line results in severe thrombocytopenia. This disease reactions, and infectious causes. In summary the diagnostic
may be a primary immune-mediated disease or secondary approach to a dog with suspected ITP includes a thorough
to infections such as Ehrlichia canis and Borrelia burgdorferi. history and physical examination; a minimum database
Immune-mediated megakaryocytic aplasia will look similar (CBC, serum biochemical profile, urinalysis); evaluation
to primary immune-mediated thrombocytopenia on presen- of coagulation status (platelet count, aPTT, partial throm-
tation but has a more severe clinical course and a poorer boplastin time [PTT], FDPs); diagnostic imaging (thoracic
prognosis. Bone marrow aspiration and biopsy can be safely radiographs, abdominal ultrasound); infectious disease titers
performed even in severely thrombocytopenic dogs because (depending on geographic location); plus or minus bone
hemorrhage can be controlled with local pressure. In most marrow cytology; and histopathology. Although the major-
dogs with ITP, normal to increased numbers of megakary- ity of cases of ITP will have megakaryocytic hyperplasia,
ocytes are present on a bone marrow aspirate. Decreased the rare case of megakaryocytic aplasia or hypoplasia can
numbers of megakaryocytes in the bone marrow have been only be diagnosed by examination of the bone marrow; if a
associated with a poorer prognosis in dogs with ITP. In bone marrow examination is not performed at the time of
dogs with severe thrombocytopenia (<20,000/µL) and no diagnosis, it should definitely be performed in any dog with
evidence of other hematologic abnormalities, the results of a suspected ITP that does not respond to routine immunosup-
bone marrow examination rarely reveal nonmegakaryocytic pressive treatment.