Anatomy — The Lymphatic System -- Clinical Assessment and Lymphadenopathy

The Lymphatic System -- Clinical Overview

The lymphatic system serves three primary functions: immune surveillance and response, fluid homeostasis (returning interstitial fluid to circulation), and lipid absorption from the gastrointestinal tract (via lacteals). In clinical practice, the lymph nodes are the most accessible and clinically relevant component of this system.

Peripheral Lymph Node Locations and Palpation Technique

The following peripheral lymph nodes are routinely assessed during physical examination:

• Submandibular (mandibular): Located ventral to the mandible at the angle of the jaw. Palpate bilaterally with fingertips. Normal: small, smooth, oval, 1-2cm in dogs. Drain: head, oral cavity, nasal passages.
• Prescapular (superficial cervical): Located cranial to the shoulder joint, deep to the trapezius muscle. Palpate by pressing cranially and ventrally at the shoulder. Drain: head, neck, forelimb.
• Axillary: Located in the axilla (armpit). Often not palpable in normal animals. Palpate with the forelimb abducted. Drain: thoracic wall, forelimb.
• Inguinal (superficial inguinal): Located in the inguinal region medial to the hindlimb. Palpate with the patient in lateral recumbency. Drain: caudal abdomen, perineum, hindlimb.
• Popliteal: Located in the popliteal fossa (behind the stifle). Most consistently palpable peripheral node. Palpate with the hindlimb slightly flexed. Drain: distal hindlimb, foot. Normal: 1-2cm, firm, smooth, moveable.

Characterizing Lymphadenopathy -- What to Document

When lymphadenopathy is detected, document the following for each affected node:

• Size: Estimate in centimeters or compare to familiar objects (pea, grape, golf ball)
• Consistency: Soft, firm, hard, fluctuant
• Surface: Smooth or irregular/lobulated
• Mobility: Freely moveable or fixed to surrounding tissue
• Pain: Painful or non-painful on palpation
• Temperature: Normal or warm (suggests inflammation/infection)
• Distribution: Localized (one region) or generalized (multiple regions)

Clinical Interpretation

Reactive lymphadenopathy (lymphadenitis): Typically soft to firm, painful, warm, and localized to the drainage region of an infection or inflammation. Often accompanied by fever. Common causes: skin infection, tooth abscess, wound infection.

Neoplastic lymphadenopathy: Typically firm to hard, non-painful, non-warm, and may be fixed. Can be localized (metastatic disease) or generalized (lymphoma). Lymphoma nodes are often dramatically enlarged -- sometimes 5-10x normal size -- and have a characteristic rubbery consistency.

Immune-mediated lymphadenopathy: Variable presentation. May be generalized. Associated with immune-mediated diseases such as systemic lupus erythematosus (SLE) or immune-mediated hemolytic anemia (IMHA).

When to Escalate Immediately

• Any node larger than 2cm in a cat (cats have smaller normal nodes than dogs)
• Generalized lymphadenopathy (multiple regions simultaneously)
• Rapidly enlarging nodes
• Hard, fixed, or irregular nodes
• Lymphadenopathy accompanied by systemic signs (fever, weight loss, lethargy, anorexia)
• Any lymphadenopathy in a patient with a known or suspected neoplastic history

Fine Needle Aspirate (FNA) Assistance

FNA of enlarged lymph nodes is a common in-house diagnostic procedure. Your role includes: restraining the patient appropriately, preparing the site (clip and clean if needed), preparing slides for cytology, and labeling samples correctly. The veterinarian performs the aspiration. Results can often be read in-house within minutes and may provide a rapid diagnosis of lymphoma, reactive hyperplasia, or metastatic disease.