Septic arthritis: Clinical sciences

Septic arthritis refers to joint inflammation that occurs when a pathogen invades the joint space. Once within the synovial cavity, pathogens trigger the immune system and stimulate the production of cytokines that can eventually result in joint damage.

Now, there are various ways for bacteria to get into a joint. For example, it can spread directly from an infection in the adjacent bone, such as osteomyelitis. It can also reach the joint through hematogenous spread from a distant infectious site in the body; or by direct inoculation, which can occur as a complication of orthopedic surgery. Septic arthritis is usually monoarticular, affecting one large joint, and the diagnosis typically relies on synovial fluid aspiration and analysis.

Now, if you have a patient presenting with signs and symptoms of septic arthritis, you should first perform an ABCDE assessment to determine if your patient is unstable or stable. If the patient is unstable, stabilize their airway, breathing, and circulation. Next, obtain IV access, start IV fluids, and begin continuous vital sign monitoring including blood pressure, heart rate, and oxygen saturation. Provide supplemental oxygen, if needed, and don’t forget to start broad-spectrum IV antibiotics.

Okay, now let’s go back to the ABCDE assessment and look at stable patients. First, let’s start by taking a focused history and physical exam. Your patient will typically report monoarticular joint pain and swelling, commonly of the hips, knees, shoulders, or ankles. These joints are the most vulnerable to infection because they have a richer blood supply than small joints, and that enables pathogens to more easily reach the larger joints. Your patient might also report systemic symptoms, such as fever, malaise, and decreased appetite. They may also have a history of risk factors, including overlying skin infection or ulceration, history of a prosthetic joint or recent joint surgery, immunosuppression, IV drug use, and smoking.

Additionally, a physical exam may reveal an ill-appearing patient with a joint effusion, or erythema and warmth of the skin overlying the affected joint. Also, local edema and pain typically results in a limited range of motion of the affected joint.

At this point, you should suspect septic arthritis, so the next step is to assess whether the affected joint is a native or prosthetic joint.

Let’s say your patient has their native joint. Your next step is to do an arthrocentesis and order synovial fluid analysis, including gram stain and cultures, cell count with differential, and crystal analysis. While awaiting results, you should start empiric antibiotics!

Now, here’s a high-yield fact to keep in mind! Septic arthritis is most commonly caused by bacteria, and based on the pathogen that’s causing it, septic arthritis can be further subdivided into gonococcal and nongonococcal. Gonococcal arthritis is caused by Neisseria gonorrhoeae, which typically spreads hematogenously from the initial infection of the urethra, cervix, or even pharynx. Remember that this is more common in sexually active adolescents and young adults. On the other hand, nongonococcal arthritis is caused by all other pathogens, especially Staphylococcus aureus, but also Streptococcus species, as well as Mycobacterium tuberculosis and Borrelia species. Another less common cause of septic arthritis in sexually active individuals is Chlamydia!

Ok, now that you’ve obtained synovial fluid and started empiric antibiotics, you should next assess the results of the synovial fluid.

If the gram stain and cultures are negative and the synovial fluid is clear and yellow with a white blood cell count less than 2000 and is negative for crystals, you should consider non-inflammatory arthritis, such as osteoarthritis.

On the other hand, if gram stain and culture are negative and the synovial fluid is cloudy and yellow containing a cell count up to 50,000, and with or without positive crystals, you should think of inflammatory arthritis. In this case, consider either gout or calcium pyrophosphate deposition disease, which are associated with positive synovial fluid crystals, or rheumatoid arthritis, in which there are no crystals present.