Draining lymph nodes are much more complex.....

Veterinary Radiology & Ultrasound, 2018

Author

Obi Veterinary Education
August 11, 2025

Monique N. Mayer, Katherine A. Sweet, Michael N. Patsikas, Sally L. Sukut, Cheryl L. Waldner

Background
Staging and treatment planning for dogs with malignant disease in the popliteal lymph node rely on understanding lymphatic drainage patterns. The medial iliac lymph nodes are the primary drainage site, but an accessory efferent pathway leading to the sacral lymph nodes has been reported. This study aimed to determine the frequency of various efferent drainage patterns from the popliteal lymph node in dogs.

Methods
A prospective anatomic study was conducted using computed tomographic (CT) lymphography in 50 clinically normal adult dogs. Contrast medium (iohexol) was injected percutaneously into a popliteal lymph node under ultrasound guidance. CT scans were obtained pre- and post-contrast to evaluate lymphatic drainage patterns and lymph node characteristics.

Results
All 50 dogs exhibited direct drainage from the popliteal lymph node to the ipsilateral medial iliac lymph node. However, in 26% (13/50) of dogs, additional efferent lymphatics drained to the internal iliac and/or sacral lymph nodes, forming an accessory pathway that coursed dorsally over the pelvis. Lymphatic connections were observed between right and left medial iliac and internal iliac lymph nodes. The superficial inguinal lymph node contained contrast medium in 10% of dogs, though this may have been due to perinodal leakage rather than direct drainage.

Limitations
This study used contrast injection, which may have artificially increased pressure within lymphatic vessels, potentially affecting drainage patterns. Additionally, some lymph nodes, such as the external iliac lymph node, were not consistently identified, possibly due to small size. The study included only healthy dogs, limiting extrapolation to dogs with pathological conditions.

Conclusions
In addition to the expected medial iliac lymph node drainage, a significant proportion of dogs exhibit an accessory popliteal efferent lymphatic pathway to the sacral and internal iliac lymph nodes. These findings suggest that the internal iliac and sacral lymph nodes should be considered in lymph node staging and treatment planning for dogs with malignant disease involving the popliteal lymph node.

Schematic representation of the medial plane (A) and a ventrodorsal view (B) of the efferent lymphatic pathways of the popliteal lymph node on the right side of a dog. Direct lymph drainage from the popliteal lymph node (blue) to the medial iliac lymph node (red) was present in 100% (50/50) of dogs, and a second pathway in which efferent vessels drained from the popliteal lymph node directly to the internal iliac (orange) and/or sacral (pink) lymph nodes was found in 26% (13/50) of dogs. In 4% (two of 50) of dogs, the lymphatic vessels draining directly to the medial iliac lymph node first entered a distal femoral lymph node (green). In 10% (five of 50) of dogs, contrast medium was also seen in the superficial inguinal lymph node (purple). This pathway was not included as we could not confirm that efferent vessels passed from the popliteal lymph node to the superficial inguinal lymph node

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