Seronegative and septic arthritis: Pathology review

Maurice is a 28 year old male who presents with a 2 year history of gradually progressive low back pain and stiffness. He mentions that the pain wakes him up several times at night, and that the stiffness tends to be worse when he wakes up and improves as he moves. Examination shows mild deformity of the spine and hip, as well as tenderness over the buttock. Then you see Clint, a 63 year old male shows up with a red, warm and swollen left knee, which hurts so much he can barely walk. Clint tells you that symptoms started a few days ago, after he tripped and cut his knee. His body temperature is 38 celsius degrees or 100.4 degrees Fahrenheit. X-rays are ordered in both cases, showing, in Maurice's case, erosion of the sacroiliac joint. On the other hand, in Clint's case, X-rays look pretty normal, so an arthrocentesis is performed, revealing that synovial fluid is purulent.

Based on the initial presentation, both cases seem to have some form of arthritis. But first, a bit of physiology real quick. There are many types of joints, including fibrous, cartilaginous, and synovial joints. synovial joints, like those of the wrist, elbow, knees, shoulders, and hips, are mobile joints that connect two bones via a fibrous capsule that is continuous with the periosteum, which is the outer layer of bones. The fibrous capsule is lined with a synovial membrane, which has cells that remove debris and produce synovial fluid. This is a viscous fluid found inside the joint capsule which lubricates joint. Together, the synovial membrane and articular cartilage form the inner lining of the joint space.

Now, arthritis refers to a group of diseases that cause destruction of one or more joints. First, we have seronegative arthritis, which is called seronegative because there's an absence of both rheumatoid factor or RF, and anti-cyclic citrullinated peptide antibody or anti-CCP, which are commonly found in rheumatoid arthritis. Something specific to note is that seronegative spondyloarthropathies have a strong association with the gene HLA-B27, which encodes for a specific type of MHC class I molecule.

Now there are several subtypes of seronegative spondyloarthropathies and you can remember them by a mnemonic PAIR, where P stands for Psoriatic arthritis; A for Ankylosing spondylitis; I for Inflammatory bowel disease-associated arthritis; and R for Reactive arthritis.