Vasculature
Splenic veins exit the spleen at the centrally located hilus and merge into the single large splenic vein deep to the spleen, which can be traced transversely across the abdomen to its junction with the portal vein. Color flow Doppler evaluation, if available, allows for additional evaluation of the splenic vein and its branches (see Figures 9.1AB, 9.2 and 39.8).
Pearl: The splenic capsule is hyperechoic (bright white) and its vessels cross the splenic capsule. In contrast, the hepatic capsule is not sonographically apparent and its vessels do not cross the hepatic capsule. This anatomical difference can be used to distinguish spleen from liver.
Clinical Significance and Implications of Abnormal Findings
Abnormal findings that may be appreciated during the POCUS spleen examination include the four features mentioned above: subjective evaluation of size (splenomegaly), changes in echogenicity (evaluation for diffuse parenchyma disorders), presence of mass lesions (solitary and multifocal mass lesions), and assessment of vasculature (thrombosis and torsion).
Figure 9.4. Normal appearance of spleen. Spleen by itself showing its homogeneous nature with normal echogenicity. The anechoic areas (circular) are normal vasculature and readily appreciated in real time evidenced by their branching. The splenic capsule is recognized as a hyperechoic (bright white) line.
Pearl: The four key features to evaluate with the focused spleen examination are size, echogenicity, presence of nodules or masses and assessment of the vasculature.
Splenic Size
Splenomegaly is usually a subjective assessment except in cases of severe enlargement. Ultrasonographic clues that help in discriminating between normal size and splenomegaly include the following.
Marked enlargement may result in the distal tip of the spleen folding back on itself (Figure 9.5,B) and therefore the distal tip or tail is visualized medial to the left kidney (see Figure 9.5C,D). In addition, the markedly enlarged spleen may extend caudally and come in contact with a small to medium‐sized urinary bladder (Figure 9.6). In cats, a folded spleen supports splenomegaly (Hecht 2008), as does a spleen thicker than 10 mm (Reese et al. 2013) (see Figure 9.9B).
Pearl: In cats a folded spleen invariably indicates splenomegaly which is always abnormal in felines; splenic thickness is normally <10 mm in cats.
Marked splenomegaly most commonly occurs with neoplasia (lymphosarcoma), other infiltrative processes (fungal infection), acute inflammation, or in some cases of immune‐mediated hemolytic anemia. Extramedullary hematopoiesis can cause marked splenomegaly. Marked splenomegaly may also occur in animals with splenic torsion (see additional description below) (see Figure 9.15).
Pearl: With the exception of splenic torsion (often urgent surgical disease), fine needle biopsy with cytological evaluation is indicated in cases of moderate to marked splenomegaly.
When splenomegaly is confidently ascertained, correlation between size and echogencity can be clinically helpful as follows.
Mild to moderate splenomegaly with normal echogenicity and architecture is most commonly associated with sedation, extramedullary hematopoiesis, antigenic stimulation (acute inflammatory and infectious diseases), and passive congestion (Figures 9.7 and 9.8A,B). However, diffuse infiltrative processes including lymphoma and mast cell tumor can also have normal echogenicity.
Splenomegaly with mild to moderate hypoechogenicity may be associated with nodular hyperplasia or extramedullary hematopoiesis, passive congestion, inflammation, infection (fungal, bacterial, rickettsial), immune‐mediated diseases, and lymphosarcoma. Comparison with adjacent liver and the renal cortex can help when assessing echogenicity (Figure 9.9; see also Figure 9.8; see normal relative echogenicity in Figure 9.3).
Pearl: With the exception of splenic torsion, fine needle biopsy with cytological evaluation is indicated in cases of moderate to marked splenomegaly.
Figure 9.5. Splenomegaly. (A) Splenomegaly or splenic enlargement as evidenced by its folding. The “Y”‐shaped splenic veins departing from its hilus are helpful for identification and distinguishing it from the liver. The spleen here has a homogeneous normal echotexture. (B) Another example of an enlarged folded spleen enveloping the left kidney (Lt Kidney). (C) Splenic enlargement similar to (B) being folded and medial to the left kidney with comparative normal echogenicity. The spleen is hyperechoic (brighter) when compared to the cortex of the left kidney. (D) Similar to (C), the spleen extends caudal to the left kidney.
Splenic Mass Lesions
Masses and Nodules
The identification of nodules or mass lesions within the splenic parenchyma is the most basic of the four features of the POCUS spleen (size, echogenicity, nodules, vasculature). It is important to recognize, however, that ultrasound does not allow differentiation between benign and malignant processes, unless advanced contrast‐enhanced ultrasound studies are performed (limited availability) that still may not prove helpful (Fife et al. 2004; Rossi et al. 2008). The sonographer should keep in mind that according to one study, benign splenic masses are more common than malignant splenic masses in dogs (Fife et al. 2004).
Nodular hyperplasia. Nodules associated with nodular hyperplasia or extramedullary hematopoiesis may be variable in echogenicity (hyperechoic, hypoechoic, isoechoic or mixed echogenicity) when compared to normal splenic parenchyma (Figure 9.10A,B). In addition, nodular hyperplasia may be associated with a mildly irregular splenic capsule without associated nodular lesions. This change is commonly seen in older dogs.
