Point-of-Care Ultrasound Techniques for the Small Animal Practitioner. Группа авторов
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assessing the spleen, systematic evaluation should include the following four features: estimation of size (subjective), evaluation of changes in parenchymal echogenicity, interrogation for mass lesions and nodules, and assessment of splenic vasculature.
Size and Shape
The general contour of the spleen is tongue‐shaped and when viewed adjacent to the left kidney, it appears more triangular (than flat) in cross‐sectional views. Since the size of the spleen can vary in normal animals, estimation of size is generally subjective in dogs except in cases of marked splenomegaly. In cats, splenomegaly is almost always due to pathology and should prompt further evaluation. Spleen thickness greater than 10 mm or a folded spleen invariably indicates splenomegaly in the feline species (Hecht 2008; Reese et al. 2013) (Figure 9.2; see also Figure 39.9). Benign splenomegaly is uncommon in cats and if present, additional evaluation is warranted.
Figure 9.1. Normal splenic hilus. (A) The “Y” shape of the splenic vein, also referred to as the whale tail (mimics tail fluke of the whale), as it branches at the hilus is helpful for identifying the spleen and distinguishing it from the liver. (B) Color flow Doppler may be used to assess blood flow and rule out splenic venous thrombosis and splenic torsion when suspected.
Figure 9.2. Subjective size evaluation of the spleen. 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.
Source: Courtesy of Dr Gregory Lisciandro, DVM, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX.
Pearl: Moderate splenomegaly occurs in dogs with many forms of sedation or anesthesia (Hecht 2008; Reese et al. 2013); thus, the effects of sedation and anesthesia must be considered.
Echogenicity
The echogenicity of the splenic parenchyma should be evaluated for diffuse and focal changes. Normal splenic parenchyma is homogeneous in appearance with fine echotexture, and its thin hyperechoic (bright white) capsule may be apparent on long‐axis (sagittal) views. Spleen parenchyma is generally mildly to moderately hyperechoic (brighter) to normal liver parenchyma (Figure 9.3A,B), and significantly hyperechoic (brighter) to normal renal cortex (Hecht 2008) (see Figure 9.3C,D).
Pearl: Remember the mnemonic “SLiCK” regarding relative normal echogenicity of spleen, liver and kidney cortex. Spleen (S) is hyperechoic (brighter) to liver (Li) which is slightly hyperechoic (brighter) or isoechoic (same shades of gray) to the cortex of the kidney (CK). For the feline, hepatic echogenicity is often compared to the adjacent falciform fat and the feline liver should be isoechoic to slightly hypoechoic to the falciform fat.
Figure 9.3. Echogenicity of spleen relative to liver and cortex of kidney. (A) Comparison of the echogenicity of spleen to the liver, unlabeled. (B) Same as (A) but labeled as liver (LIV) and spleen (SP). (C) Comparison of echogenicity of the spleen to the cortex of the left kidney, unlabeled. (D) Same as (C) but labeled as spleen (SP) and left kidney (LK).
Presence of Masses and Nodules
Normal splenic parenchyma is homogeneous with fine echotexture and free of focal lesions (Figure 9.4).
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