Ultrasonographic technique and appearance of presumptively normal clinically relevant lymph nodes in horses

Background: Lymph nodes are routinely assessed ultrasonographically in humans and small animals. The ultrasonographic assessment of equine lymph nodes has only been sporadically reported. Objectives: The aims of this study were to establish anatomical landmarks, ultrasonographic technique and normal ultrasonographic appearance of clinically relevant equine lymph nodes difficult to assess by palpation and to provide a useful reference for equine practitioners. Study design: This was prospective observational study. Methods: Medial retropharyngeal, cranial and caudal deep cervical, and caecal lymph nodes were visualised using transcutaneous ultrasonography in 19 clinically healthy horses, and lymph nodes of the iliosacral lymphocentre were visualised using transrectal ultrasonography in nine horses with no metabolic or infectious disorders. Anatomical landmarks, size and echogenicity were recorded. Results: A total of 268 lymph nodes were visualised and measured in 25 horses. Lymph nodes were ovoid with regular contour and homogeneous parenchyma, hypoechoic to isoechoic to the surrounding tissue. The visualised lymph nodes of the sacroiliac lymphocentre were the smallest (mean ± standard deviation: 2.4 ± 0.9 × 5.9 ± 1.8 mm), and the largest were the caudal deep cervical lymph nodes (mean ± standard deviation: 5.1 ± 1.9 × 10.9 ± 5.6 mm). The short-to-long-axis ratio varied from 0.44 to 0.60 among the lymph nodes examined. Main limitations: Normality of lymph nodes was not confirmed by histopathology. Conclusions: This study describes the ultrasonographic technique, anatomical landmarks and appearance of presumptively normal medial retropharyngeal, cranial and caudal deep cervical, caecal lymph nodes and caudal lymph nodes of the iliosacral lymphocentre.