Ultrasonography of the Spleen

Ultrasonography of the Spleen

Because the spleen can be involved in a variety of different conditions, including those that may have non-specific clinical signs, assessment of the spleen should be included in every routine abdominal ultrasonographic examination. Specific conditions which would indicate a need for splenic ultrasonography include:

• Palpable abdominal mass
• Palpable splenomegaly
• Trauma
• Haemoabdomen
• Abdominal discomfort
• Tumour staging

Normal Anatomy

The spleen is an elongated organ which is situated in the left cranial abdomen, adjacent to the left body wall. It can be subdivided into 3 sections: the head (the most dorsal section), the body (mid-section) and the tail (the most ventral section) (Figure 1).

The head is located in a dorsal position between the gastric fundus and the cranial pole of the left kidney, occupying a space under the 11^th-13^th intercostal spaces and often folding medially on itself. Due to the short gastric vessels and the gastrosplenic ligament, both of which fix the head of the spleen to the greater curvature of the stomach, the position of the head will be influenced by gastric distension.

The body continues in a ventro-caudal direction, passing below the mid costal arch close to the left body wall. From this position the orientation and position of the tail is quite variable – sometimes extending to the urinary bladder on the left side, at other times the tail of the spleen may continue ventrally, curving over to the right ventral body wall.

The arterial blood supply of the spleen comes from the splenic artery, a branch of coeliac artery. The venous blood leaves the spleen via the splenic vein which joins the portal vein.

Figure 1. This image shows a schematic approximation of the position of the spleen adjacent to the body wall in the left cranial abdomen of a dog. The position of the tail portion of the spleen is highly variable.

Ultrasonographic Examination Technique

Ultrasonography of the spleen is carried out with patient in right lateral recumbency. However, in patients where the splenic tail extends ventrally towards the right abdomen, left lateral recumbency can be used to aid visualisation of the ventral spleen and tail.

It is important to consider the position of the spleen when clipping the hair coat – in large or deep chested dogs it is essential to include the caudal rib spaces in any clip to facilitate visualisation of the splenic head.

The spleen occupies a relatively superficial position adjacent to the left body wall, so a transducer capable of utilising a frequency of 7.5Mhz or higher can be used in dogs. In cats or small dogs, a 10Mhz or higher frequency transducer can be used.

To visualise the splenic head, the transducer can be fanned dorsally and ventrally under the left costal arch to visualise the head in its entirety. In larger dogs the caudal intercostal spaces can be used; by aligning the transducer parallel with the ribs, the transducer can be rocked dorsally and ventrally and fanned cranially and caudally to examine the head of spleen.

Because the position of the body and tail is more variable, the cranial border, middle one third and caudal border of the spleen should each be followed and examined separately in two orthogonal planes.

Normal Appearance

Assessment of splenic size is subjective and can be affected by a number of factors. Radiography can be of use evaluating the size and position of the spleen relative to surrounding organs.

The spleen is homogenously echoic with a fine echotexture (Figure 2). The echogenicity of the spleen can be compared to the neighbouring liver parenchyma (Figure 3) and the cortex of the left kidney. In many patients it is hyperechoic to both. However, the echogenicity of surrounding anatomy can vary especially if multiple organs are affected by a disease process. Therefore, comparisons of echogenicity should not be entirely relied upon for the determination of normality.

The spleen is encompassed by a thin capsule, which appears hyperechoic when the ultrasound beam is orientated perpendicularly to the capsule (on-incidence).

The splenic vasculature can be visualised as anechoic tubular structures. The intra-splenic branches of the splenic artery are not routinely visible, however colour doppler ultrasonography can be used to determine the location of the arterial branches. The branches of the splenic vein, which converge at the splenic hilus, are more readily identified.

Figure 2. Normal canine spleen – The typical homogenous smooth echotexture can be seen, with an anechoic area representing a branch of the splenic vein (arrow). Note the hyperechoic capsule on the far border of the spleen.

Figure 3. Splenic echogenicity in a dog – In this image the liver (L) is in the left (cranial) part of the image. The spleen (S) is situated to the right (caudal), directly bordering the liver. The relative hyperechoic nature of the spleen to the liver can be visualised.

Ultrasonographic appearance of splenic disorders

It is important to appreciate that ultrasound cannot easily distinguish between benign and malignant processes, and that the spleen can be ultrasonographically normal when disease is present. Ultrasound examination findings should always be interpreted alongside clinical examination findings and blood results. Attainment of a definitive diagnoses will require tissue sampling.

Splenic abnormalities can be categorised as focal, multifocal or diffuse.

Focal and Multifocal Abnormalities

Nodular hyperplasia is a benign cause of solitary or multiple lesions within the spleen of middle aged or older dogs, and occasionally cats. The nodules can be hypo-, hyper-, isoechoic or of mixed echogenicity. Where they appear isoechoic, the only notable change may be a bulge or smooth irregularity of the splenic border.

Extramedullary haematopoiesis can be a cause of diffuse splenic enlargement. However, it can also present as splenic nodules indistinguishable from those seen in nodular hyperplasia. For this reason, cytology is required to distinguish between the two conditions.

Splenic myelolipomas can appear as hyperechoic nodules on the mesenteric border of older dogs and cats (Figure 4). These benign lesions can also occur within the splenic parenchyma and along the splenic vasculature. The significance of myelolipomas remains unknown¹.

Figure 4. Examples of splenic myelolipomas – The image on the left is the spleen of a dog. Hyperechoic areas can be seen along the mesenteric border and close to a branch of the splenic vein (arrowheads). The image on the right is from a cat.  A hyperechoic area can be seen within the splenic parenchyma (arrow).

Splenic abscesses are uncommon² and can have a variable appearance, from hypoechoic areas to more complex lesions with cystic components. If gas-producing bacteria are present, reverberation or shadowing artefacts can occasionally be visualised.

Infarctions of the spleen can occur secondary to a variety of systemic diseases³. The appearance of the infarcted area can be variable and may resemble other focal splenic pathologies. Doppler ultrasonography can be used to document the lack of blood flow to the affected area.

Splenic neoplasia can be focal or multifocal and a range of tumour types are reported (Figure 5). Haemangiosarcoma is the most commonly diagnosed neoplasia in dogs^4,5 and can be associated with haemoabdomen. Mast cell tumours are the most common splenic neoplasia in cats⁶ followed by lymphosarcoma, myeloproliferative disease and haemangiosarcoma. In cats, the presence of splenic masses of >1cm diameter is suggestive of malignancy⁷.

Similar to other splenic pathologies, the appearance of splenic neoplasia is very variable. They can range from poorly defined hypoechoic areas through to more complex lesions with anechoic or hyperechoic areas. ’Target’ lesions have a hypoechoic rim and a hyperechoic centre, and are seen in some tumour types⁸.

Because the spleen can be a site for both primary neoplasia and secondary metastatic tumours, it is important to evaluate the other abdominal organs thoroughly when performing an ultrasound examination, as well as acquiring thoracic and abdominal radiographs. Ultrasound can be used to evaluate local lymph nodes; the splenic lymph nodes are not normally easily visualised on ultrasound examination but if enlarged can appear as rounded, hypoechoic soft tissue structures situated along the splenic vessels close to the hilus.

Figure 5. Three examples of focal splenic lesions in dogs: A) A well circumscribed, round, hypoechoic, heterogenous mass is present in the head of spleen. B) A round, hypoechoic, heterogenous mass is present in the body of the spleen. C) A round, heterogenous mass can be seen protruding from the tail of the spleen. Because the appearance of benign and malignant changes is highly variable, cytology and/or histology is essential for diagnosis.

Diffuse splenic abnormalities 

Inflammation of the spleen, or splenitis, can occur secondarily to several systemic inflammatory or infectious conditions. This can lead to splenomegaly with or without altered echogenicity.

Similarly, diffuse neoplasia of the spleen can lead to changes in splenic size, echotexture and echogenicity, though to what extent is highly variable (Figure 6). Lymphoma, mast cell tumor and histiocytic sarcoma are all examples of neoplasia that can cause diffuse splenic changes. As for other splenic conditions, tissue sampling is essential in achieving a diagnosis.

Figure 6. Canine splenic lymphoma. In this image of the spleen of a patient with lymphoma, diffuse hypoechoic nodules are present throughout the splenic parenchyma. (Image courtesy of Dr Sally Griffin, Willows Referrals. Not for reproduction).   

Splenic torsion can occur as a rare primary condition in dogs, or in association with gastric dilation and volvulus (GDV). Patients with splenic torsion can have vague clinical signs that may include abdominal pain, weakness, hypotension, tachycardia and collapse⁹. Torsion of the spleen causes marked splenomegaly and on ultrasound examination the spleen can appear hypoechoic with hyperechoic foci throughout the parenchyma, often described as a ‘starry sky’ appearance. (Figure 7). Doppler ultrasonography can be used to document the presence or absence of blood flow to the spleen, and in some cases, thrombi may be visualised within the lumen of the splenic vasculature¹⁰. In addition, the surrounding mesenteric fat will often appear hyperechoic.

Figure 7. Splenic torsion. In this image showing part of the spleen in a dog with splenic torsion, the splenic parenchyma has become diffusely hypoechoic, interspersed with linear hyperechoic areas. (Image courtesy of Dr Sally Griffin, Willows Referrals. Not for reproduction).

Splenic Tissue Sampling

As stated previously, ultrasonographic evaluation of the spleen cannot distinguish between neoplastic and non-neoplastic disease. Therefore, tissue sampling is essential in establishing most diagnoses.

Fine needle aspiration of the spleen or splenic lesions can be performed under ultrasound guidance and is a relatively safe procedure¹¹. A 22 – 26 gauge 1.5-3.5 inch needle can be used depending on the depth of the region to be sampled. A non-aspiration technique is preferred because it is likely to result in less blood dilution of the sample¹² though blood contamination can still lead to inconclusive samples especially in cavitated or fluid-filled lesions such as haematomas or haemangiosarcomas. For this reason, it is important, when possible, to take several samples from each lesion in order to maximise the chance of achieving a diagnosis.

References

1. Schwarz L.A., Penninck D.G., Gliatto J. (2001) Canine Splenic Myelolipomas. Veterinary Radiology and Ultrasound 42: 347-348.
2. Abdellatif A., Günther C., Peppler C., Kramer M. (2014) A Rare Case of Splenic Abcess with Septic Peritonitis in a German Shepherd Dog. BMC Veterinary Research 10: 201.
3. Hardie E.M., Vaden S.L., Spaulding K., Malarkey D.E. (1995) Splenic Infarction in 16 Dogs: A Retrospective Study. Journal of Veterinary Internal Medicine 9: 141-148.
4. Cleveland M.J., Casale S. (2016) Incidence of Malignancy and Outcomes for Dogs Undergoing Splenectomy for Incidentally Detected Non-Ruptured Splenic Nodules or Masses: 105 cases (2009–2013). Journal of the American Veterinary Medicine Association 11: 1267-1273.
5. Day M.J., Lucke V.M., Pearson H. (1995) A Review of Pathological Diagnoses Made from 87 Canine Splenic Biopsies. Journal of Small Animal Practice 36: 426-433.
6. Spangler W.L., Culbertson M.R. (1992) Prevalence and Type of Splenic Diseases in Cats: 455 Cases (1985-1991). Journal of the American Veterinary Medicine Association 5: 773-776.
7. Bertal M., Carmel E.N., Diana A., Desquilbet L., Specchi S., Pey P. (2017) Association Between Ultrasonographic Appearance of Splenic Parenchyma and Cytology in Cats. Journal of Feline Medicine and Surgery 20: 23-29.
8. Cuccovillo A., Lamb C.R. (2002) Cellular features of sonographic target lesions in the liver and spleen of 21 dogs and a cat. Veterinary Radiology and Ultrasound 43: 275-278.
9. DeGroot W., Guiffrida M.A., Rubin J., Runge J.J., Zide A., Mayhew P.D., Culp W.T.N., Mankin K.T., Amsellem P.M., Petrukovich B., Ringwood P.B., Case J.B., Singh A. (2016) Primary Splenic Torsion in Dogs: 102 Cases (1992–2014). Journal of the American Veterinary Medicine Association 248: 661-668.
10. Saunders H.M., Prudence J.N., Brockman D.J. (1988) B-Mode and Doppler Ultrasound Imaging of the Spleen with Canine Splenic Torsion: A Retrospective Evaluation. Veterinary Radiology and Ultrasound 39: 349-353.
11. Liffman R., Courtman N. (2017) Fine Needle Aspiration of Abdominal Organs: A Review of Current Recommendations for Achieving a Diagnostic Sample. Journal of Small Animal Practice 58: 599–609.
12. Le Blanc C.J., Head L.L., Fry M.M. (2009) Comparison of Aspiration and Non-Aspiration Techniques for Obtaining Cytologic Samples from the Canine and Feline spleen. Veterinary Clinical Pathology 38: 242-246.

Further reading

Hecht S., Mai W. (2015) Spleen, In: Atlas of Small Animal Ultrasonography 2nd edn., Eds. Penninck D., d’Anjou M.A., John Wiley & Sons, Inc. Chichester, pp 239-258.

Larson M. (2018) Liver and Spleen, In: Textbook of Veterinary Diagnostic Radiology 7th edn., Ed. Thrall D., Elsevier, St. Louis, pp 792-822.

Mahoney P. (2011) Spleen, In: BSAVA Manual of Canine and Feline Ultrasonography, Eds. Barr F., Gaschen L., BSAVA publications, Gloucester, pp 100-109.

Nyland T.G., Mattoon J.S. (2015) Spleen, In: Small Animal Diagnostic Ultrasound 3rd edn., Eds. Mattoon J.S., Nyland T.G., Elsevier, St. Louis, pp 400-437.