Thursday, March 12, 2015

Lymph Node Enlargement in the Cat

Lymph Node Enlargement in the Cat

We’ve all heard of lymph nodes, but what exactly are they?  What do they do?  And why be concerned when they become enlarged?

As a feline veterinarian, I perform dozens of physical examinations every week.  Every veterinarian performs the physical exam in his or her own style, making sure to evaluate all body systems thoroughly.  Assessment of the lymph nodes is unquestionably a part of every veterinarian’s physical exam. 

The lymphatic system is an arm of the immune system that plays a role in the development of the body’s immune response. Lymph is the fluid that flows through the lymphatic system.  It is rich in protein and white blood cells.   Cells of the immune system circulate throughout the lymphatic vessels in the body. Lymph nodes are small, oval-shaped organs that make up part of the lymphatic system.  As lymph flows through the lymphatic vessels, it passes through at least one set of lymph nodes, and often several sets, before ultimately emptying into the general circulation where it mixes with blood.   The lymph nodes are the major sites in the lymphatic system where the immune cells gather.   

Lymph nodes have a distinct anatomical structure, consisting of an outer portion called the cortex, and an inner portion called the medulla.  The cortex contains large numbers of lymphocytes, often arranged in clusters (follicles). There are two distinct types of lymphocytes found in lymph nodes: T lymphocytes and B lymphocytes. The B lymphocytes are responsible for producing antibodies, a process called “humoral immunity”.  T lymphocytes, once stimulated, are involved in a process called “cellular immunity”, the details of which are probably beyond the scope of this discussion.  The medulla contains lymphocytes as well as many other cells of the immune system, including macrophages and dendritic cells.  The entire lymph node is surrounded by a capsule comprised of connective tissue. 

The immune system’s job is to protect and defend the body against harm by mounting an attack against invading substances (antigens).  When an antigen invades the body and makes its way to the lymphatic system and eventually to the lymph node, it stimulates a discrete population of lymphocytes in the node. This causes those lymphocytes to proliferate and transform, or become “activated”.  As noted above, activated lymphocytes may produce antibodies that neutralize the antigen, or the lymphocytes may become involved in more complex “cellular” immune responses.  When these lymphocytes proliferate, lymphadenopathy develops. Lymphadenopathy is the medical term for an increase in lymph node size.   Stimulation of the immune system is a common cause of lymph node enlargement.  Cancer is an equally common cause.

Lymphadenopathy is sometimes noted as an incidental finding during physical examination of the cat.  Occasionally a cat owner will bring a cat to the veterinarian because of clinical signs related to the lymph node enlargement, for example, difficulty swallowing due to enlarged retropharyngeal lymph nodes (located at the back of the throat.  See sidebar.)  Sometimes a cat owner will bring a cat to the veterinarian because they’ve detected a lump, which turns out to be an enlarged lymph node, while patting or stroking the cat. 

The age of the cat is important when considering the significance of lymph node enlargement.  Kittens and young cats are exposed to a variety of antigens early in their lives, and an increase in lymph node size is an expected response by the immune system.  As cats mature, lymph node size usually decreases and the nodes often become more difficult to palpate in older animals.  However, in geriatric cats or cats that have lost weight due to illness, the loss of body fat around the lymph nodes may make the lymph nodes appear more prominent than expected.

Cancer, unfortunately, is a common reason for lymph node enlargement.  The size and texture of the lymph node may give clues in this regard.  Enlarged, firm, painless lymph nodes that are freely moveable is usually associated with primary lymph node cancer, i.e. lymphoma.  Very hard lymph nodes are more suggestive of metastatic cancer, i.e. a cancer arising in another site in the body that has spread to that lymph node.   The degree of enlargement may offer some clues as to the cause. Marked lymphadenopathy (the lymph node is 5 to 10 times the normal size) most often occurs with a lymph node abscess or with lymphoma; metastatic cancer rarely causes lymph node enlargement of this magnitude. 

The location of the lymph node enlargement should also be considered in the patient evaluation.  If one lymph node is enlarged, or one regional group of lymph nodes is enlarged, one should carefully evaluate that area of the body for evidence of inflammation, infection, or cancer.  For example, if the right submandibular lymph node (the node just below the jaw) is enlarged, the right side of the mouth should be examined carefully.  A tooth root abscess involving a tooth on the right side, for example, could cause this nearby lymph node to be enlarged.   If, however, all of the lymph nodes involving the head region are enlarged, a more diffuse illness involving the head, such as an upper respiratory infection, would be something to consider.

Obtaining a diagnosis in cases of lymph node enlargement may require a variety of tests, including blood and urine analysis, x-rays, and ultrasound.  Ultimately, a definitive diagnosis often requires obtaining a sample of cells from the node itself.  This is usually achieved either by aspiration cytology, or by surgical biopsy.

Aspiration cytology is a procedure in which a needle, attached to a syringe, is inserted into the lymph node.  Suction is applied to the syringe so that cells from the lymph node are aspirated or sucked into the hub of the needle.  The contents are then sprayed onto a microscope slide and are sent to the laboratory for interpretation.  The advantage of this procedure is that it fairly non-invasive and inexpensive.  A disadvantage is that the aspirate may not yield enough cells to make a diagnosis.  Only positive findings are diagnostically useful.  The absence of abnormal findings in an enlarged lymph node means that the diagnosis is still unresolved. 

Biopsy of the lymph node is the definitive diagnostic test in evaluating lymph node enlargement.  Depending on the size and location of the node, either the entire node is removed (this is called “excisional biopsy”), or a small piece of the lymph node is removed (this is “incisional biopsy”).  The biopsy specimen is then evaluated by a pathologist.  Treatment is based on the biopsy results: infections are usually treated with antimicrobials; cancer is treated by chemotherapy, surgery, radiation, or a combination of these.

Lymph nodes are encapsulated structures containing aggregates of cells of the immune system that are capable of responding to a wide variety of invading antigens.  Enlargement of the lymph nodes occurs either due to reactive enlargement (stimulation of the immune system), primary cancer of the lymph nodes (lymphoma), or metastatic cancer.  Infectious causes of lymph node enlargement are more common in younger animals, while cancerous causes tend to be seen in older animals, although there is considerable overlap.  The size, location, and texture of the enlarged node(s) may offer clues as to the cause, and diagnostic tests such as x-rays, ultrasound, and blood and urine tests give valuable diagnostic information, however, obtaining a sample of the enlarged node is usually required for a definitive diagnosis to be achieved.

Sidebar: names and locations of some of the feline lymph nodes

Submandibular – just below the mandible (lower jaw)
Retropharyngeal – in the back of the throat
Cervical – in the neck, along the trachea
Axillary – in the armpit
Sternal – inside the chest along the sternum (breastbone)
Tracheobronchial – associated with the bronchi (airways) in the lungs
Sublumbar – above the colon, just below the spine
Inguinal – in the groin area
Popliteal – along the back of the thigh
Epigastric – adjacent to the stomach
Mesenteric – in the abdomen, associated with the intestines

Inflammation and enlargement of the lymph nodes is a common finding in companion animals.  Infection is a common cause of lymph node enlargement.  Cancer is also a common reason for lymph node enlargement.

More info and photos can be found on the Long Beach Animal Hospital website.
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