Different types of arthritis ‘present very differently’ in synovium, synovial fluid
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ORLANDO — Understanding the nature and role of the synovium and synovial fluid is critical to managing various types of arthritis, according to a presenter at the 2022 Rheumatology Nurses Society Annual Conference.
“We are often draining joints, but what can you tell from the synovial fluid you have collected?” Adam J. Brown, MD, associate program director of the Cleveland Clinic, asked attendees.
Brown’s talk covered the makeup of synovial fluid, how it works and what clinicians should be looking for in a fluid analysis. However, he stressed that the answers to many questions surrounding synovial fluid have either not been answered or are still evolving.
That said, a normal synovium has an intimal layer and then a sub-lining, or fibrous layer, he said.
“It is just a few cells thick,” Brown said. “You can’t palpate it the way you can a joint.”
Although the fibrous layer is “mostly acellular,” there are blood vessels, nerves and fat cells present, he added.
Regarding the fluid itself, there is plasma present, but a number of proteins are also secreted into the joint, including hyaluronans, glycoproteins, collagen, lubricin and eicosanoids.
“They give the fluid that thicker viscosity, allowing for the protection of the cartilage,” Brown said.
In addition, there are 0.45 ccs of synovial fluid in a normal knee joint.
The talk then moved to the differences in septic, crystalline and autoimmune arthritis regarding the impact on the synovium and synovial fluid. “
They present very differently,” Brown said.
According to Brown, septic arthritis is “far and away monoarticular.” It can be caused several factors, ranging from previous rheumatoid arthritis, gout, lupus, osteoarthritis, lupus or trauma. However, regarding the infection itself, staphylococcus is the most common cause of a septic joint.
“Septic arthritis is exceedingly painful all the time,” Brown said, noting the conditions like RA can improve with movement.
Unfortunately, management of this condition is much more challenging.
“There is still no good treatment for septic arthritis,” Brown said.
Antibiotics are a good start. Draining the joint also can be effective. However, the synergy between draining the joint and pharmacotherapeutic interventions is not fully understood.
In addition, although crystalline diseases predominantly include gout and pseudogout, Brown noted that they can resemble RA or even a septic joint.
“These can be tricky conditions,” he said.
“Huge inflammatory responses” are also a hallmark of crystalline diseases, Brown added. It is for this reason that analysis of synovial fluid is essential in diagnosing these conditions. “Looking for those crystals can be very helpful,” he said.
Turning to autoimmune diseases and the synovium, Brown addressed RA, spondyloarthropathies and systemic lupus erythematosus.
RA is marked by an increase in cells in the intimal layer of the synovium, with production of metalloproteinases, cytokines and RANK ligand. Increased vascularization may occur, along with production of interleukin-1, IL-6 and TNF.
In spondyloarthritis and psoriatic arthritis, there are bone erosions that are similar to those seen in RA. However, bone also proliferates in other areas in the spondyolarthritides. “They choose different spots,” Brown said. “We do not fully understand why.”
In SLE, there is less of an impact on the bone, according to Brown.
“There is more inflammation around the tendons and ligaments,” he said. “The tendons and ligaments loosen around the joint. It is a very different pathology from RA from a synovial perspective.”
That said, the understanding of why this pathology occurs in SLE is also not fully understood, according to Brown. He concluded that, ultimately, there are still more questions than answers surrounding the role of synovial fluid in arthritis.
“It is important to differentiate septic or crystalline disease from other types of arthritis,” Brown said.
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Brown AJ. What's in a Joint? Synovial Fluid and Inflammatory Arthritis. Presented at: Rheumatology Nurses Society annual conference; August 3-6, 2022.
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