One cause of ectopic splenic tissue in the liver may be splenosis, which is the implantation and growth of fragments of spleen
after trauma3,6,8 or elective splenectomy.8 The cells are transplanted as emboli of splenic tissue enter into the portal circulation followed by implantation and growth.6,8 In dogs, the direction of portal blood flow results in perfusion of the left liver lobes to a greater degree than the right
liver lobes.5 This streamline effect results in splenic tissue perfusing the left liver lobes more than the right lobes.5 Typically, with splenosis the splenic tissue is separated from the liver by a capsule.3 The nodules are usually small but numerous with a limited blood supply.8
Another suggested possible cause of ectopic splenic tissue in the liver in this dog is that the ectopic tissue was an accessory
spleen. Accessory spleens are characterized as congenital, few in number, and large. They are supplied by branches of the
splenic artery.4,6,8 Microscopically, the splenic tissue is structurally the same as a normal spleen: hilus, capsule, and parenchyma. In cases
of splenectomy, the ectopic tissue in the liver becomes hyperplastic once the spleen is removed.1,3,8
In this dog, distinct masses in the liver parenchyma consisted of reticuloendothelial stroma and endothelial cells that formed
sinusoids. The stroma had lymphoid cells and histiocytes. Numerous hematopoietic cells were also observed. These histopathologic
results were consistent with the other cases of ectopic splenic tissue,1-3,5,6 but the specific cause in all of these cases is unknown. Given this dog's history of a splenectomy with two previous liver
biopsies that did not reveal splenic tissue, the fact that no branches of the splenic artery supplying the nodules were identified,
and the fact that most of the nodules were in the left liver lobes, we hypothesize that the cause of this dog's intrahepatic
splenic tissue was splenosis.
Magnetic resonance imaging (MRI) currently is the modality of choice to characterize hepatic lesions in people.9,10 MRI provides superior soft tissue contrast, and the documentation of characteristics of the most common lesions has made
histologic confirmation unnecessary in people.9,10 To date, only one pilot study has been performed in dogs.11 Until further documentation of hepatic lesion characteristics with MRI is available, histologic confirmation is necessary,
and in most cases this will involve biopsy through exploratory laparotomy.
The dog previously described in the literature and this dog presented with signs attributable to hemoperitoneum. Splenic nodules
frequently develop into small hematomas as a result of marginal zone circulation failure and blood accumulation, leading to
hypoxia and necrosis.12 Life-threatening hemorrhage and hemoperitoneum occur secondary to rupture of the capsule.12 Ectopic splenic tissue should be considered as a possible cause of hemoperitoneum even in dogs that have had splenectomies.
Without histologic evaluation of the splenic tissue, it is impossible to distinguish neoplastic from nonneoplastic processes.
The prognosis for intrahepatic splenosis in veterinary patients is unknown. Only one dog had been reported before this case,
and the dog was euthanized before the diagnosis was made. More cases would need to be reported and followed to determine the
course of the disease in dogs.
Catharine A. Loughin, DVM
Dominic J. Marino, DVM, DACVS
Curtis W. Dewey, DVM, MS, DACVS, DACVIM (neurology)
Department of Surgery
Lond Island Veterinary Specialists
163 S. Service Road
Plainview, NY 11803
David A. Gamble
Antech Diagnostics
111 Marcus Ave.
Lake Success, NY 11042
REFERENCES
1. Lacerda MA, Ludwig J, Ward EM. Intrahepatic spleen presenting as a mass lesion. Am J Gastroenterol 1993;88:2116-2117.
2. al-Ahmadi M, Brundage S, Brody F, et al. Splenosis of the mesoappendix: Case report and review of the literature. J R Coll Surg Edinb 1998;43:200-202.
3. Davidson LA, Reid IN. Intrahepatic splenic tissue. J Clin Pathol 1997;50:532-533.
4. Tanimoto T, Ohtsuki Y. Heterotopic splenic tissue in the liver of a swine. J Vet Med Sci 1993;55:485-486.
5. Knostman KAB, Weisbrode SE, Marrie PA, et al. Intrahepatic splenosis in a dog.Vet Pathol 2003;40:708-710.
6. Allen J, Sands M, Baggs R. Radioisotope imaging for detection of an ectopic spleen in a macaque. J Am Vet Med Assoc 1982;181:1428-1429.
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