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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.