Septic arthritis: Clinical sciences

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Introduction0:00–0:53

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.

Unstable patient0:53–1:30

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.

Stable patient1:30–2:35

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.

Native joint2:35–3:52

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.

Synovial fluid analysis3:52–6:09

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.
Finally, if the gram stain and cultures are positive and the fluid is cloudy and purulent with a cell count over 50,000, and with or most often without positive crystals, you can diagnose septic arthritis.
Keep in mind that patients with a history of gout can develop septic arthritis in the same joint, and in those cases, there will be crystals in the synovial fluid.
Here’s a clinical pearl! To definitively diagnose septic arthritis, you will rely on synovial fluid analysis.
Sometimes additional supportive analyses, such as labs, primarily CBC, ESR, and CRP; as well as imaging, like X-ray, ultrasound, and MRI are also ordered.
These additional diagnostic methods can help you rule out other conditions while at the same time confirming the diagnosis of septic arthritis.
Lab findings will typically reveal leukocytosis, as well as elevated ESR and CRP, which are highly suggestive of ongoing infection.
Additionally, an x-ray may show narrowing of the joint space due to cartilage destruction, surrounding soft tissue swelling, and subchondral bony destruction.
Ultrasound can also be used to look for a joint effusion in a joint with clinical suspicion for septic arthritis, and potentially help guide joint aspiration.
Finally, MRI can be used to look more closely for joint damage, bone marrow edema, and further characterize any joint effusion.
This is crucial to rule out possible osteomyelitis!Ok, now that you are confident in your diagnosis, you can tailor antibiotics based on synovial culture results!

Native joint management6:09–6:27

You should also obtain urgent surgical consultation for further drainage and debridement, as untreated infections can result in loss of function of the joint.
Now, let’s go all the way back to our affected joint and discuss what we do if the patient has a prosthetic joint. In these cases, you will want to get an x-ray to help screen the prosthetic for loosening or fracture, perform an arthrocentesis for synovial fluid analysis, and get labs like CRP, ESR, and a blood culture, if the patient has a fever.

Prosthetic joint6:27–7:58

Okay, now, an x-ray may demonstrate an abnormal lucency where the prosthetic meets the bone, a change in the position of the prosthetic, as well as a periosteal reaction.
The synovial fluid analysis will be similar to the results for native joint septic arthritis; the gram stain and culture might be positive, but keep in mind that prosthetic joints are more susceptible to unusual organisms, like fungi, and so culture results may take weeks or may return negative.
In addition, the fluid will be cloudy and purulent, the cell count will be greater than 50,000, and the synovial fluid is typically negative for crystals.
These findings are consistent with a prosthetic joint infection. Here’s a clinical pearl!
Depending on how sick the patient is, sometimes surgical management, including debridement, is performed instead of the arthrocentesis, even before the diagnosis is known.
This happens in both native and prosthetic joint infections. In these cases, surgical findings of purulent material around the joint can help make the diagnosis as well, both visually, and with fluid analysis.

Prosthetic joint management7:58–8:28

Alright, after diagnosing a prosthetic joint infection, your next step is to start empiric antibiotics, and tailor them if culture results indicate doing so.
Unlike with native joint infections, it’s preferred to delay starting antibiotics until obtaining synovial fluid culture whenever possible.
You should also obtain an urgent surgical consultation for further drainage and debridement, if not already done, as untreated prosthetic joint infections can result in loss of the prosthetic, and even amputation of the limb.

Review8:28–9:19

Alright, as a quick recap… Septic arthritis occurs when a pathogen invades the joint space and causes inflammation. Your patient will primarily report monoarticular joint pain and swelling.
Physical exam findings include joint effusion, erythema, warmth, and limited range of motion. Native joint infections are characterized by synovial fluid analysis showing positive gram stain and cultures, cloudy purulent fluid, cell count over 50,000, and usually negative crystals; so treat by tailoring antibiotics and consulting surgery.
On the flip side, prosthetic joint infections often show an abnormal x-ray, while synovial fluid analysis shows positive gram stain and cultures, cloudy purulent fluid, cell count over 50,000, and negative crystals; so start empiric