Cytology in evaluation of lymphoid tissue in the dog and cat (Proceedings)


Cytology in evaluation of lymphoid tissue in the dog and cat (Proceedings)

Aug 01, 2010

Lymph Nodes

Lymph node sampling and cytology is quick, easy, and usually rewarding. Cytologic samples of peripheral and/or internal lymph nodes may be collected by fine-needle aspiration biopsy or nonaspiration fine-needle biopsy techniques. Sampling can also be performed by imprints or scrapings from lymph nodes that have been surgically removed or at necropsy.

Lymph node cytology is an excellent way to evaluate a lymphadenopathy whether it is a single, multiple, or a generalized lymph node enlargement. If multiple lymph nodes are enlarged, more than one should be sampled. A lymph node away from the mouth or any site of inflammation should be aspirated as well as any lymph node close to a site of inflammation. Generally, if no lymph nodes are enlarged, lymph node cytology is generally not helpful. In addition to be unrewarding, aspiration of a nonenlarged node is difficult and usually results in aspiration of perinodal fat with little or no lymphoid tissue present. Nonetheless, normal sized lymph nodes may be aspirated on occasion to investigate the potential for metastatic disease.

Selection of a Node

The lymph nodes generally palpated in dogs and cats include the submandibular, prescapular, and popliteal lymph nodes. Popliteal and prescapular lymph nodes are preferred biopsy sites for animals with generalized lymphadenopathy. When possible, avoid submandibular lymph nodes since they are frequently reactive due to constant exposure to antigens from the oral cavity. Also, it is best to avoid extremely enlarged lymph nodes since they may yield misleading information due to the presence of necrosis or hemorrhagic tissue. A moderately enlarged lymph node is preferred.

Sample Collection and Preparation

Aspiration Procedure

For cutaneous lymph nodes, the skin over the node to be aspirated needs no special preparation. It is prepared as one would prepare the skin for giving an injection. The aspiration technique requires the use of a 22 gauge needle and a 6 or 12 cc syringe. A 22 gauge butterfly catheter may be substituted for small or hard to reach nodes. When possible, insert the needle toward the periphery of the node, avoiding necrotic centers. A slight negative pressure is applied and the needle is advanced into the lesion and then redirected, if the lymph node is large enough, in a fan-like pattern until material appears in the hub of the needle. Do not pump the plunger of the syringe as this will damage the fragile lymphoid cells. During redirection of the needle, care should be taken not to withdraw the needle from the lymph node. When material appears in the hub of the needle, the plunger is released and the needle is withdrawn from the node and skin. The needle should be removed from the syringe. Air is then drawn into the syringe, and the needle is replaced onto the syringe. The aspirated material is then gently expelled onto a clean glass slide. A second clean slide is gently laid on top of the material, parallel to the first slide. The material is allowed to diffuse out, and the slides are gently slid apart. Slides are air-dried and are then stained using Diff Quik or some comparable Romanowsky-type stain.

Cytological Interpretation of Lymph Node Aspirates

If the previously described guidelines are adhered to, there is generally good correlation between cytologic and histologic diagnosis. Any enlarged lymph node may be aspirated for the purpose of classifying the lesion into the following classifications:
     1. Normal lymph node.
     2. Reactive (lymphoid hyperplasia).
     3. Inflammation (lymphadenitis).
     4. Lymphoid neoplasia (lymphoma).
     5. Metastatic disease.
     6. Edema (lymphadema).