The popliteal lymph nodes lie in the tissues behind a person’s knee. Many conditions can cause them to swell, such as infections and autoimmune conditions. It is rare for cancer to affect the popliteal lymph nodes.

This article takes a detailed look at swollen popliteal lymph nodes. It discusses what can cause swelling, diagnostic tests, and what happens after diagnosis. It also explains the anatomy and function of popliteal lymph nodes.

A senior's knees wearing a brace and running on the beach-1.Share on Pinterest
kolderal/Getty Images

Lymphadenopathy is swelling of the lymph nodes. It arises when too many lymphocytes accumulate within the lymph nodes. Lymphocytes are special types of white blood cells, a kind of immune system cell.

According to a 2022 review, many health conditions can cause lymphocytes to accumulate. Some possible causes of swollen popliteal lymph nodes include:

  • Infections: Several types of infection can cause lymphadenopathy. These include bacterial, viral, and fungal infections.
  • Autoimmune conditions: Some autoimmune conditions can cause dysfunctions with immune cells. These conditions include sarcoidosis, amyloidosis, and rheumatoid arthritis.
  • Lymphoproliferative conditions: Lymphoproliferative conditions are when the immune system produces too many lymphocytes. For example, hemophagocytic lymphohistiocytosis can cause lymphadenopathy.

Swollen popliteal lymph nodes may also be due to cancer.

Learn more about lymph nodes here.

Could it be cancer?

Several cancers can cause lymphadenopathy. These include lymphomas, which are cancers that begin in the lymph nodes. However, other cancers can spread to the lymph nodes, from leukemia to neck cancer.

However, according to the American Cancer Society (ACS), most swollen lymph nodes are due to a condition that is not cancer. Moreover, an older study notes that it is rare for cancers to spread to the popliteal lymph nodes.

Learn about cancer in the lymph nodes here.

Swollen popliteal lymph nodes caused by infection may present the following symptoms:

  • pain, redness, and tenderness behind the knee
  • a pea-sized lump that may swell to half an inch in diameter
  • a lump that moves freely when pushed

These symptoms indicate an infection and should disappear within 2 weeks once the infection subsides.

Swollen popliteal lymph nodes that present the following symptoms should be checked out by a doctor:

  • painless swelling behind the knee
  • a swelling larger than half an inch in diameter
  • a hard lump that does not move freely when pushed
  • swelling lasts longer than 2 weeks

Scientists describe popliteal lymph nodes as located deep within the popliteal fossa.

The popliteal fossa is a diamond-shaped part of the leg. It lies behind the knee joint. There are 2–9 lymph nodes within every popliteal fossa.

The picture below shows swollen popliteal lymph nodes in the popliteal fossa.

Share on Pinterest
Anatomy of the back of the knee showing the popliteal lymph nodes. Medical Illustration by Bailey Mariner

Lymph nodes are part of the lymphatic system. The lymphatic system serves many important functions. For instance, it moves immune cells around the body and manages fluid buildup between cells.

Within the lymphatic system, lymph nodes help to filter bodily fluids. Lymph nodes connect to lymphatic vessels, which feed these fluids into them. The filtering process involves removing harmful cells, molecules, or pathogens. Lymph nodes can do this because they contain several immune cells.

Lymph nodes receive fluids from different body parts. The popliteal lymph nodes filter fluids from deep structures within the leg and foot.

Learn about the anatomy of the lower leg here.

Doctors can diagnose swollen lymph nodes based on a physical exam. An individual with swollen popliteal lymph nodes may feel a lump in their popliteal fossa.

Swollen lymph nodes can show up in imaging studies, such as computed tomography (CT) scans.

Because so many different conditions cause lymphadenopathy, doctors can find it challenging to diagnose the cause. Doctors may recommend the following tests:

  • Detailed physical exam: Doctors can gather useful information from a physical exam. This information includes a swollen lymph node’s size, firmness, and potential pain.
  • Blood tests: Doctors may use a blood sample for testing in a laboratory. Tests include complete blood counts, complete metabolic panel, and fungal serologies.
  • CT scans: Doctors may use a CT scan to check whether an individual has other swollen lymph nodes. These scans can also help with planning lymph node biopsies.
  • Lymph node biopsy: In this surgical procedure, doctors remove all or part of a swollen lymph node. They will then send this sample for laboratory testing. Although lymph node biopsies are not always necessary, doctors must perform them to diagnose conditions such as cancer.

An individual may need to see several doctors before receiving a diagnosis. The process could take several weeks.

After diagnosis, doctors can recommend treatments. This varies greatly, depending on the underlying cause of swollen popliteal lymph nodes. Possible treatments include:

  • For cancers: Surgery, radiation therapy, or chemotherapy can help. Sometimes, doctors recommend a combination of these.
  • For autoimmune conditions: Treatment may include immune therapy and systemic glucocorticosteroids.
  • For infectious conditions: Treatment might involve antibiotic, antifungal, or antiviral medications depending on the type of infection.

Some medications can cause swollen lymph nodes. When this occurs, doctors may recommend changing the medications or their dosages.

The popliteal fossa is an area of tissue behind a person’s knee joints. Deep within this structure lie 2–9 popliteal lymph nodes. As parts of the lymphatic system, these lymph nodes filter fluids from the leg and foot.

There are various causes of swollen popliteal lymph nodes, including infections, autoimmune conditions and conditions that increase lymphocyte production. Cancers can also cause swollen popliteal lymph nodes, although this is rare.

Doctors can diagnose swollen popliteal lymph nodes with a physical exam. To determine the cause, they may use imaging tests, laboratory tests, or biopsies.