Ultrasound examination of abdominal lymph nodes

Ultrasonography is one of the most sensitive methods for studying the abdominal lymph nodes and provides information on their shape, size, echostructure and echogenicity. Table 1 lists the lymph nodes that drain the visceral organs and parietal structures_¹.

Table 1

The lymph nodes which can be most easily seen by ultrasound examination are the jejunal lymph nodes, followed by the hepatic, splenic, colic and caudal mesenteric lymph nodes; the gastric, pancreatic-duodenal and renal lymph nodes are more difficult to identify. In a study of 10 cats_², the medial iliac lymph nodes could be detected by ultrasonograpy in 100% of cases, the jejunal lymph nodes in 90%, the hepatic lymph nodes in 70%, the lumbar aortic, splenic and pancreatic-duodenal nodes in 60%, the ileo-caecal and colic nodes in 50%, the renal nodes in 40%, the gastric nodes in 30%, the sacral nodes in 20% and the mesenteric lymph nodes in 10%.

ANATOMY OF THE VISCERAL AND PARIETAL LYMPH NODES_^1,3-5

Visceral lymph nodes

Cranial mesenteric lymph nodes

• Jejunal lymph nodes:these are located around the root of the jejunal and ileal mesentery; they range from 0.5 to 6 cm in size in the dog and drain the jejunum, ileus and pancreas.
• Colic lymph nodes:these may be absent; the right colic lymph node is situated close to the origin of the ascending colon, near to the ileo-caecal valve and is about 1-2.5 cm long; the middle colic lymph node may be single or double and is located in the mesentery of the transverse colon. The right colic lymph node drains the ileum, caecum and colon; the middle colic lymph node drains the transverse and descending parts of the colon.

Caudal mesenteric lymph nodes

• Caudal mesenteric lymph nodes (left colic lymph nodes):there are two to five left colic lymph nodes which are located in the descending mesocolon close to the pelvis; they vary from 0.5 to 2 cm in length and drain the descending colon and rectum.

Coeliac lymph nodes

• Gastric lymph nodes:these may be absent or duplicated; when present they are found in the lesser curvature of the stomach, close to the pylorus; they vary from 0.5 to 2.5 cm in length and drain the oesophagus, stomach, peritoneum and diaphragm.
• Pancreatic-duodenal lymph nodes: these may not be present; they are found close to the lesser curvature of the duodenum, 2-5 cm from the pylorus in the omentum, ventrally to the pancreas; when present, they are small (not usually exceeding 1.5 cm in length). They drain the duodenum, pancreas and omentum.
• Portal lymph nodes:these are distinguished into right and left portal lymph nodes and may be single or multiple. They are about 3 cm long and are located 1-2 cm from the hilum of the liver along the portal vein; they drain the stomach, duodenum, pancreas and liver.
• Splenic lymph nodes:there are three to five of these nodes in the dog and one to three in the cat. They range from 0.5 to 4 cm in length and are found along the splenic vein. They drain the spleen, oesophagus, stomach, pancreas, liver, omentum and diaphragm.

Parietal lymph nodes

Lumbar lymph nodes

• Lumbar-aortic lymph nodes:these are distributed along the course of the abdominal aorta and extend from the diaphragm to the deep circumflex artery. There may be from 12 to 17 and their length ranges from 1 to 2 cm; they are almost never visible by ultrasound examination unless they are enlarged; they drain the lumbar, intercostal and abdominal muscles, mediastinum, pleura, diaphragm, peritoneum, adrenal glands and urogenital apparatus.
• Renal lymph nodes: these are located close to the renal blood vessels and can reach 1.5 cm in length; they drain the peritoneum, adrenal glands and ovaries.

Iliosacral lymph nodes

• Medial iliac lymph nodes (right and left):these are situated between the deep circumflex iliac arteries and the external iliac arteries, laterally to the abdominal aorta and the caudal vena cava. The right iliac lymph node is situated between the ventro-lateral surface of the caudal vena cava and the common iliac vein, more ventrally than the left node which is located between the aorta and the left external iliac artery; these nodes can be as much as 4-6 cm long; they drain the skin, subcutaneous layer and fascia (caudally to the last rib), skin of the pelvic region, including the tail, muscles, tendons and joints of the lower limbs, urogenital organs, peritoneum, colon, rectum, anus and urogenital apparatus.
• Hypogastric lymph nodes:these are small lymph nodes that vary in number from one to three; they are found at the angle formed by the median sacral artery and the internal iliac vessels, ventrally to the sixth and seventh lumbar vertebrae; they drain the bladder, uterus, prostate, genital organs and pelvis.
• Sacral lymph nodes: these are small lymph nodes (usually less than 2 cm), present in 50% of dogs; when present they are found near to the median sacral artery, ventrally to the body of the sacrum; they drain the tail and muscles next to the sacral lymph nodes.

ULTRASOUND EXAMINATION OF THE ABDOMINAL LYMPH NODES

Generally speaking the abdominal lymph nodes are not easily seen by ultrasound examination, mainly because they are small but also because they are surrounded by mesenteric adipose tissue. A careful study of lymph nodes requires instruments that guarantee high spatial resolution, a certain degree of experience by the operator and, above all, good knowledge of ultrasound anatomy. These factors facilitate the finding of the correct landmarks that guide the identification of the various lymph nodes.

By ultrasound examination, the lymph nodes (Video 1):

• are variably sized structures (from a few millimetres to some centimetres)
• are elongated or oval
• have a clear-cut outline
• are isoechoic
• have a homogeneous structure
• have a hyperechoic hilum (lymph node medulla).

In some cases, particularly those involving small nodes with a hypoechoic-anechoic appearance, colour Doppler can be helpful to differentiate these lymph nodes from nearby vessels_⁴ (Fig. 1).

Changes in the size, shape, outline, echogenicity and echostructure of the lymph nodes, evaluated by ultrasound examination, indicate lymphadenopathy_⁶. The distinction between a benign reactive process and a malignant neoplasm may be guided by a series of ultrasonographic findings (summarised in Table 2)_⁷ (Videos 2 and 3). However, a physical sample of lymph node is needed in order to make a definitive diagnosis, particularly in the early stages of a disease. Such a specimen can be taken safely under ultrasound guidance.

Table 2 (Nyman, 2004)

• The size of lymph nodes depends, in part, on the age of the animal (the nodes tend to be physiologically larger in puppies) and on its weight_⁸.
• Some authors_^9-11 affirm that an increase in the ratio between the short and long axes indicates a neoplastic pathology of the lymph node rather than a benign condition.
• One study_¹² demonstrated that, in dogs, a heterogeneous appearance of lymph nodes is an indicator of malignancy; this finding was not significant in the cat. A subsequent study_¹¹ did not, however, find substantial differences with regards to changes in perihilar fat, shape, borders and parenchymal echogenicity.
• There may be cystic lesions (Fig. 4) or necrotic lesions within the parenchyma of the lymph node; the former are usually associated with benign conditions, the latter with both benign and malignant conditions. An enlarged lymph node may sometimes have a “bull’s eye” appearance (hyperechoic at the centre, with a peripheral hypoechoic surrounding), due to the presence of adipose tissue in the lymph node hilum_⁴ (Fig. 5).
• Various studies have evaluated the use of colour Doppler_^7,8,10-14 and contrast enhanced ultrasonography (CEUS)_^7,13,14 to differentiate malignant from benign forms of lymph node involvement. Blood flow is predominantly distributed in the hilum in benign conditions and in the periphery in malignant conditions (Figs. 2 and 3). Likewise, several studies have found substantial differences in the pulsatility index (PI) and resistive index (RI) between malignant and benign lymphadenopathy_^7,10,15.
• Regardless of the foregoing, a cytological and/or histological examination of the lymph nodes_^16,17is indispensable in order to make the final differentiation between benign or malignant lymphadenopathy. An ultrasound-guided lymph node biopsy is a simple method, with a low procedure-related risk rate, which yields reliable results and a definitive diagnosis.

Systematic ultrasound evaluation of the abdomen enables generalised lymphadenopathy to be distinguished from involvement of a single lymph node.

• Subjects with lymphosarcoma often have extralymphatic alterations, besides having generalised enlargement of lymph nodes,. Cats with digestive tract lymphoma, in addition to involvement of lymph nodes (in particular mesenteric and colic lymph nodes), commonly have single or, more frequently, multiple lesions of one or more loops of bowel, the liver and the spleen.
• The clinical pictures described above must be distinguished from inflammatory bowel disorders with lymphadenopathy of satellite lymph nodes and from infectious diseases (parvovirus infection or bacterial gastroenteritis in puppies and infectious peritonitis in cats).
• Less frequently, particularly in the cat, enlargement of abdominal lymph nodes is associated with other types of neoplasms_⁴.
• When an abnormality is found in an abdominal organ, the satellite lymph nodes must always be examined (for example, the portal lymph nodes in a case of suspected liver disease and the splenic lymph nodes in the case of lesions in the spleen) (Figs. 3 and 6).
• In cases of mammary cancer involving caudal and inguinal mammary glands and tumours or infections of the prostate or bladder, it is very important to evaluate the medial iliac, hypogastric and sacral lymph nodes. The lumbar-aortic lymph nodes are involved less frequently. The same lymphocytes may also be involved in the case of cutaneous and subcutaneous neoplasms of the posterior limbs (for example, mast cell disease or sarcoma) (Video 4).

Suggested readings

1. Pugh CR. Ultrasonographic examination of abdominal lymph nodes in the dog. Vet Radiol Ultrasound. 35: 110-115, 1994.
2. Schreurs E, Vermote K, Barberet V, et al. Ultrasonographic anatomy of abdominal lymph nodes in the normal cat. Vet Radiol Ultrasound, 49(1): 68–72; 2008.
3. Bezuidenhout AJ. The lymphathic system. In Evans HE. Miller’s anatomy of the dog; ­3rd ed. Saunders, 1993.
4. MattoonJS, Nyland TG. Abdominal fluid, lymph nodes, masses, peritoneal cavity, and great vessel thrombosis. In Nyland TG, Mattoon JS. Small animal diagnostic ultrasound; 2rd ed. Saunders, 2002.
5. RogersKS, Landis M, Barton CL. I linfonodi nel cane e nel gatto. Parte I. Caratteri anatomici e funzioni. Veterinaria, 2: 39-48, 1995.
6. D’Anjou MA. Abdominal cavity, lymph nodes, and great vessels. In Penninck DG. Atlas of Small Animal Ultrasonography. Blackwell, 2008.
7. Nyman HT, Kristensen AT, Flagstad A, et al. A review of the sonographic assessment of tumor metastases in liver and superficial lymph nodes. Vet Radiol Ultrasound, 45: 438–448, 2004.
8. Agthe P, Caine AR, Posch B. Ultrasonographic appearance of jejunal lymph nodes in dogs without clinical signs of gastrointestinal disease. Vet Rad Ultrasound, 50: 195-200, 2009.
9. Llabres-Diaz FJ. Ultrasonography of the medial iliac lymph nodes in the dog. Vet Radiol Ultrasound. 45: 156-165, 2004.
10. Nyman HT, Kristensen AT, Skovgaard IM, et al. Characterization of normal and abnormal canine superficial lymph nodes using gray-scale b-mode, color flow mapping, power, and spectral Doppler ultrasonography: a multivariate study. Vet Radiol Ultrasound, 46: 404–410; 2005.
11. De Swarte M, Alexander K, Rannou B, et al. Comparison of sonographic features of benign and neoplastic deep lymph nodes in dogs. Vet Radiol Ultrasound, 2011.
12. Kinns J, Mai W. Association between malignancy and sonographic heterogeneity in canine and feline abdominal lymph nodes. Vet Radiol Ultrasound. 48: 565–569, 2007.
13. Gaschen L, Angelette N, Stout R. Contrast-enhanced harmonic ultrasonography of medial iliac lymph nodes in healthy dogs. Vet Radiol Ultrasound. 51: 634–637, 2010.
14. Salwei RM, O’Brien RT, Matheson JS. Characterization of lymphomatous lymph nodes in dogs using contrast harmonic and power doppler ultrasound. Vet Radiol Ultrasound. 46: 411–416, 2005.
15. Prieto S, Gomez-Ochoa P, De Blas I, et al. Pathologic correlation of resistive and pulsatility indices in canine abdominal lymph nodes. Vet Radiol Ultrasound. (50): 525–529, 2009.
16. Cowell RL, Dorsey KE, Meinkoth JH. Lymph node cytology. Vet Clin Small Anim, 33: 47–67; 2003.
17. Gelb HR, Freeman LJ, Rohleder JJ, et al.  Feasibility of contrast-enhanced ultrasound-guided biopsy of sentinel lymph nodes in dogs. Vet Radiol Ultrasound. 51: 628–633, 2010.