How Pannus Develops With Arthritis

Pannus is a complication of late-stage rheumatoid arthritis (RA). It occurs when a layer of fibrous tissue grows over the surface of a normal structure in your body. In RA, it's an overgrowth of the synovium (joint lining) that invades and covers the space between bones and articular cartilage inside a joint.

This can cause erosions on the bone, invade the bone and bone marrow, and destroy surrounding structures—including the joint capsules and tendons. All of this can cause inflammation and pain that only adds to the effects of an already painful condition.

Why Pannus Forms

In RA, joints are infiltrated with inflammatory cells and proteins, such as cytokines. Research suggests that immune cells, including interleukin 1beta (IL-1b) and tumor necrosis factor-alpha (TNF-a), may be involved in pannus formation. Researchers also theorize that cytokines start a cycle that ends with pannus.

The abundance of cytokines causes hypervascularization in the synovium, which is an excess proliferation of blood vessels.

That leads to an increase in blood flow to the area and multiplication of synovial cells, which causes synovial thickening. The joint doesn't have enough space to accommodate the excess tissue, so the synovium infiltrates into adjacent small spaces, covering the structures that surround it.

Pannus in OA vs. RA

Pannus is associated with RA, but it can occur with osteoarthritis (OA) as well, although it is not a frequent complication of OA. According to a study published in 2019 that directly compared pannus tissues removed from people with both diseases, the samples looked nearly indistinguishable under a microscope.

However, the pannus of OA featured:

• Less overgrowth of synovial tissues
• Less fibrous tissue
• Less inflammation of cells from the immune system
• Slightly fewer extra blood vessels

These factors likely explain why pannus tends to be less destructive in OA. This may be due to autoimmunity, a complicated immune system dysfunction that characterizes RA, but not OA.

Symptoms and Complications

The symptoms caused by pannus include joint pain and inflammation. This results from the destruction of bone and cartilage in the joint.

The thickened synovial tissue also contributes to a buildup of excess synovial fluid, which can increase pain and swelling. Proteins in this fluid can cause further damage to joint structures.

If it goes untreated, pannus can lead to diminished mobility and permanent joint deformity.

Pannus can also form in places other than the joints, including over the cornea in the eye (resulting in vision loss) or on a prosthetic heart valve. As pannus grows, it may appear tumor-like.

Diagnosis

Your healthcare provider might consider a pannus as a possible cause of your pain and swelling, especially if you have RA or joint deformities.

Your medical history will include questions about:

• How long you've had that particular pain
• Whether your pain has worsened recently
• Whether your pain is occurring symmetrically (in the same joint on both sides of your body)

If your healthcare provider suspects that you may have a pannus, they might order imaging studies, such as an X-ray, ultrasound, magnetic resonance imaging (MRI) scan, or a computed tomography (CT) scan to visually examine the structures that might be involved.

Treatment

The fact that you've developed pannus may indicate that your current medication isn't adequately controlling your underlying condition, so your healthcare provider may switch you to a different drug or add a new one to your regimen.

Drug classes frequently used for cases involving pannus include:

• Disease-modifying antirheumatic drugs (DMARDs)
• Corticosteroids
• Biologics
• Non-steroidal anti-inflammatory drugs (NSAIDs)

Some research shows the involvement of Janus kinase enzymes in the formation of RA pannus, which could mean that the newer class of drugs called Janus-kinase (JAK) inhibitors could be used to treat pannus in the future.

In more serious cases, or if your pannus doesn't respond to treatment, surgical removal may be an option.

A Word From Verywell

If your arthritis is well treated, you're unlikely to ever develop this complication. Taking your medication, keeping follow-up appointments, and letting your healthcare provider know about any changes in your symptoms can help you manage your disease well, and maintain a good quality of life, in the long term.