The lymph nodes are part of the lymphatic system. They have several important functions and are good indicators of disease. During a physical exam the external ones can be palpated to determine if they are enlarged. The medical term for this is lymphadenopathy.
Lymph nodes can enlarge due to several reasons:
A reaction to a foreign body might cause this.
Bacterial, viral, or fungal infections can cause them to enlarge. This enlargement might be a sign that they are performing their job as expected. When the infection subsides the lymph node should return to normal size.
The most common cancer that causes this problem is called lymphoma (also know as lymphosarcoma)
Internal Lymph Nodes
Numerous lymph nodes exist within the thoracic and abdominal cavities. They can not be palpated easily, and their enlargement does not show up on routine blood samples. They might be seen on radiographs or ultrasound if they are significantly enlarged. They can be found when performing an exploratory surgery.
This radiograph of the chest shows the heart and lungs clearly. The lower arrow points to the location of the sternal lymph node. It is not visible in this dog because it is normal. The top arrow points to a round, white structure. It is a tumor nodule that has spread from cancer elsewhere in body.
This radiograph of the abdomen shows the liver and stomach clearly. The lower arrow points to the general location of the mesenteric lymph node. The top arrow points to the location of the sublumbar lymph node. Neither one is visible.
The scalpel blade is pointing to the mesenteric lymph node. This node is found at the center of the small intestine loops. This one is enlarged because this cat has kidney cancer, which has spread to the mesenteric node.
Here is another enlarged mesenteric lymph node from a cat with intestinal cancer called lymphosarcoma.
External Lymph Nodes
Mammals have numerous external lymph nodes (also called peripheral lymph nodes) that can be palpated during a physical examination. This is of great advantage because we can easily feel if they are enlarged without having to rely on diagnostic tests. Reptiles and birds do not have external lymph nodes.
Five external lymph nodes are routinely palpated during a physical exam. All of them are paired, which is another advantage because we can compare one to another and notice subtle changes in size:
They are located in the neck area near the angle of the jaw. They lay right next to the salivary glands, so it is important to palpate the correct structure.
Located just in front of the shoulders. Follow the arm up to the point of the shoulder to find them.
Found on each side of the chest in the armpit area. They tend to be small, and are more forward then most people realize.
Found on the abdomen towards the inside of each rear leg. There is significant fat in this area, so they might be difficult to find, even if enlarged. Also, there is mammary tissue in the area that can easily feel like an enlarged lymph node if the mammary tissue has an infection or cancer.
Found on each rear leg on the opposite side of the knee. They are relatively superficial and easy to feel.
We will gladly teach you how to palpate these lymph nodes whenever you bring your pet in for a physical exam. These are internal organs so they are always palpated in a gentle manner. You might want to palpate them on a weekly basis at home. If you think they are enlarged bring your pet in for an exam so we can determine if there is a problem.
Fine Needle aspiration
This test is a relatively simple way to obtain a significant amount of information from an enlarged lymph node. In this test we use a small needle (similar to the one we use to give vaccines) and gently insert it into the enlarged lymph node. We aspirate some of the lymph node material and put it on a microscope slide.
This ferret has a very large right submandibular lymph node. Do you see the needle on the left side of the picture that we are using to obtain the aspirate sample?
The aspirated material is put on a microscope slide for analysis by a pathologist. They will put special stains on the material and carefully scrutinize it under the microscope.
This is a typical report obtained when the pathologist reads the sample on the microscope slide. It is from a labrador with 2 enlarged lymph nodes.
The fine needle aspirate test is not infallible at finding a problem. When the needle is inserted it is put in only by feel. It is possible for the lymph node to have a problem area that the needle missed. Also, we are giving the pathologist only a very small sample to read. It can be difficult to determine the health of the whole lymph node when only a small sample is taken.
Due to these limitations the pathologist sometimes can not say for certain what caused the lymph node to enlarge. If it comes back inconclusive then we might keep the lymph node under close observation. If there are other changes in the rest of the physical exam, or laboratory data indicates there might be a significant problem, we will remove the whole lymph node and submit it for analysis.
If the fine needle aspirate report comes back that cancer is suspected we will remove the lymph node, no matter what the other laboratory tests indicate.
The most accurate way to determine if a lymph node is seriously diseased is to remove the whole node and submit it for analysis. It gives us significantly more information than the fine needle aspirate. It requires anesthesia and an small incision in the skin. We routinely use our laser for skin incisions for its great ability to minimize bleeding, swelling, and post-operative pain.
This picture shows a popliteal lymph node in Dr. P’s hands. He is getting ready to make an incision in the skin over the lymph node with the laser.
This is the inflamed popliteal lymph node as it appears under the skin. It will be completely removed.
Here it is after complete removal. It is the size of a large pea.
The report on this dog gave us a diagnosis of valley fever, which is a fungal infection. You don’t have to read through all of the medical mumbo jumbo to get to the valley fever diagnosis at the end. In the last paragraph you can see that special stains were needed to make the final diagnosis. These special stains are not easily performed on a fine needle aspirate.
This report of cancer came back on a Labrador Retriever. This is the same dog that had the fine needle aspirate report above.
This cat has an enlarged popliteal lymph node. We used the laser in this case also.
We use the laser to make an incision in the skin because of the lasers ability to minimize bleeding.
The inflamed lymph node is easily visualized. Note the lack of blood in the surgery site.
The anatomy of this lymph node is not normal, an indication that it is diseased.
Because of the unique qualities of the laser we are able to perform this surgery with a very small incision.
Inflamed lymph nodes are treated with an anti-inflammatory like cortisone. Pets with bacterial or fungal infections are treated with antibiotics or anti-fungal medications. There is no specific treatment for a lymph node that is enlarged due to a viral infection. If cancer is the cause of the enlargement it will be treated with surgery, chemotherapy, radiation, or a combination of all of these.