The red pulp filters blood and removes senescent or damaged blood cells, aids in the metabolism and subsequent recycling of iron, and serves as a site for hematopoiesis, producing leukocytes, platelets, and RBCs when demand is increased. Regions of white pulp consist of lymphoid tissue that produces and stores immune cells (eg, lymphocytes, macrophages), which provide immune surveillance of blood for “foreign” material (eg, cells harboring infectious organisms, antibody-coated cells that are targeted for destruction and removal from circulation). The spleen also acts as a reservoir of blood, storing up to 20% of the total RBC mass and up to 30% of the platelet mass in the body. The spleen releases RBCs and platelets readily to meet physiologic demand.
Splenic disorders are common in middle-aged and older dogs, with clinical signs ranging from vague signs of illness to life-threatening hemoabdomen. In these disorders, splenomegaly is often present, regardless of disease severity. Splenomegaly is typically caused by discrete nodule(s) or diffuse enlargement.1 Although splenomegaly can be present in the absence of clinical signs, patients with chronic splenic disease may exhibit lethargy, inappetence, vomiting, abdominal enlargement, and weight loss. Patients presented with hemoabdomen are often collapsed and in hypovolemic shock. Arrhythmias are common with splenic diseases, especially hemangiosarcoma.2 Splenomegaly may be detected on abdominal palpation or through imaging modalities such as radiography, ultrasonography, or advanced imaging. Imaging is useful for differentiating a splenic mass from diffuse splenomegaly and is important in narrowing the possible causes of splenomegaly.
The “two-thirds/two-thirds”2 and “fifty/fifty”3 rules are often cited regarding the incidence of splenic malignancy. In a study of 325 dogs, 66% of dogs with splenomegaly were diagnosed with splenic malignancy, and 65% of those malignancies were hemangiosarcoma.2 An earlier study of 1480 dogs found that approximately 50% of splenic samples represented malignancy, with hemangiosarcoma accounting for approximately 50% of malignancies.3 However, in a study of 105 dogs with nonruptured splenic masses, 70.5% had benign splenic lesions and 29.5% had malignant neoplasia, with hemangiosarcoma accounting for 58% of malignancies.4 Although splenic neoplasia is a common cause of splenomegaly, it is impossible to differentiate between malignant and benign lesions grossly. A definitive diagnosis should be obtained before considering euthanasia. Fine-needle aspiration of the spleen is safe, and cytologic diagnoses correspond to histologic diagnoses in at least 50% of cases.5-7 Submission of the entire spleen for histopathology is recommended to increase the likelihood of distinguishing between benign and malignant processes, particularly between hemangiosarcoma and hematoma.1
Following are the authors’ top 5 causes of splenomegaly seen most often in veterinary practice.