Septic Arthritis

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Diagnosis and reasoning

This patient has presented with an acute monoarthritis, in a background of Rheumatoid Arthritis (RA). Key diagnoses include a flare-up of RA, as well as septic arthritis, crystal arthropathies, reactive arthritis and bursitis. An important clinical point to note is that, in postmenopausal women, acute gout should be considered in the differential diagnosis of a swollen knee, due to the absence of the uricosuric effects of estrogen. Three important points should be considered immediately: - The knee joint symptoms are out of proportion to the disease activity detected in the other joints. - She is on long-term immunosuppressive therapy - Patients with long-term RA are at increased risk for joint sepsis (because of both the disease process, as well as the immunosuppressive effects of the drugs used in their treatment). Thus, there is strong clinical suspicion that this might be septic arthritis; empirical antibiotic therapy should be commenced as soon as basic investigations and arthrocentesis are complete. Note that fever is encountered in only 57% of patients with septic arthritis; thus the absence of this clinical finding in no way invalidates this diagnosis. The remaining conditions are considerably less likely; the absence of a history of preceding infection, diarrhoea, uveitis and sexual promiscuity, argues against reactive arthritis; the fact that both active and passive movements are similarly restricted is against bursitis (where passive movements are usually less painful); there is no predisposing history which would raise suspicion of a crystal arthropathy. Joint aspiration should be performed as soon as possible, as this will allow definitive diagnosis; the extremely high neutrophilia (in excess of 50,000/mm3) is almost pathognomonic of septic arthritis; the presence of gram positive cocci clinches the diagnosis. Once the diagnosis of septic arthritis is made, all patients should be screened for a potential septic focus (which may be occult); thus, a chest x-ray and urine culture should be performed. At least two sets of blood cultures to detect systemic invasion are also important - and turn out to be positive in this patient. When considered along with the absence of fever, this is concerning as it suggests a poor immune response. It is also important to determine if systemic inflammatory response syndrome (SIRS) is present; a full blood count to determine her leukocyte count is important here. However, this is within normal parameters, while her heart rate, respiratory rate and temperature are also normal. A plain x-ray should also be obtained as a baseline, to help detect juxta-articular osteoporosis and bone erosions which may occur later on in the course of the disease. Aggressive empirical antibiotic therapy is key here, both because there is evidence of systemic sepsis, and because rapid eradication of the infection will minimize joint damage. Once culture results are available, the antibiotic should be modified as appropriate - and continued for 3 to 4 weeks. The purulent material in the joint should also be evacuated via percutaneous or surgical methods. The joint should not be immobilized - instead, limited mobilization with range-of-movement exercises should be started once antibiotic therapy is commenced; isometric muscle strengthening exercises should also be performed to prevent quadriceps atrophy. Note that corticosteroid therapy should be stopped until the septic arthritis has been successfully managed.


Discussion

Septic arthritis is one of the most important considerations in patients presenting with a single acutely hot, swollen and painful joint. The incidence in the general population is around 4 to 10 per 100,000 patient years and seems to be rising due to orthopedic-procedure related infections, an ageing population and an increase in the use of immunosuppressive therapy. While the majority of cases are due to bacterial infections, viruses, and fungi are also occasionally implicated. The knee is the most frequently involved joint (50% of cases) in adults with bacterial septic arthritis; the hips (20%), shoulder (8%), wrists and ankles (7%) are also commonly affected. Septic arthritis can be broadly classified into 3 groups, based on the pathogens involved: gonococcal arthritis (GC), non-gonococcal arthritis (NGC) and other (i.e. Lyme disease, mycobacterial, and fungal). GC is the most common form, being encountered in around 75% of cases; it is typically found in younger, healthy sexually active individuals. NGC often affects older persons, is acute in nature, and is monoarticular in more than 80 percent of patients. Staphylococcus aureus is the most common (40%) causative pathogen of NGA in adults and in children older than 2 years, with Streptococcus spp being the next most common (28%). Gram-negative infections represent approximately 14% to 19% percent of NGC septic arthritis cases; anaerobic organisms are found in only 1% of cases. Important risk factors for septic arthritis include advanced age, comorbidities such as diabetes, pre-existing joint disease, joint prostheses, skin infections, intravenous (IV) drug abuse, alcoholism, and immunosuppression or a reduced immune response (i.e. HIV). The classic presentation of NGC septic arthritis is a single, acutely painful, hot and swollen joint, with a severely decreased active and passive range of motion; a polyarticular pattern may be seen in 10% to 20% of cases. The presenting symptoms in GC arthritis include migratory arthralgias, moderate fever, chills, dermatitis, and tenosynovitis. Synovial fluid cultures are positive in the majority of patients with NGC arthritis, as opposed to blood cultures, which are positive in only 50% of patients. Only 50% of synovial cultures are positive in GC arthritis, therefore, there is a high dependence on the clinical presentation, an accurate history, and positive cultures from affected sites for the diagnosis of this disease and choice of antibiotic treatment. White blood cell counts and inflammatory markers (CRP, ESR) should be measured, as they may be useful to monitor response to treatment. Radiographic images are usually uninformative in the first few days of infection, as they are often normal or show only pre-existing joint disease. However, as the infection progresses, X ray images can be useful to evaluate joint destruction. Ultrasonography is capable of showing both intra and extra-articular abnormalities not apparent by plain radiography, and is a powerful tool for early detection of joint effusions. Ultrasound may also also be of benefit in guiding joint aspiration and drainage procedures - particularly of deep joints such as the hip. The overriding principle in patients with septic arthritis is to start (empirical) IV antibiotics as soon as possible - ideally, once a clinical diagnosis is made. Treatment should not be delayed pending investigation results. The empirical antibiotic should be based on the organism found in the Gram stain of the synovial fluid, on the suspicion of a pathogen from the patient's clinical presentation, or based on the common trends and patterns in the relevant geographic area. The duration of treatment should be based upon the clinical response and microbiology results, as should adjustments to the route of administration (i.e. from IV to Oral). Any purulent material present should be evacuated via arthrocentesis or surgical techniques; the latter is more attractive if frequent drainage becomes necessary. Evacuation of purulent material with arthrocentesis or surgical methods is necessary in further management. If septic arthritis is left untreated, cartilage and bone destruction occurs in as little as 10 days. Significant morbidity (e.g. amputation, arthrodesis, prosthetic surgery, or severe functional deterioration) occurs in one third of patients with (bacterial) septic arthritis; poor outcomes are more common in older patients, those with preexisting joint disease, and those with synthetic intra-articular material. The mortality ranges from 10% to 20%, depending on the presence of comorbidities.


Take home messages

  1. Septic arthritis is a rheumatological emergency, diagnosed based on clinical ground supported by synovial fluid analysis.
  2. Prompt treatment with appropriate antibiotic and synovial drainage are mandatory once septic arthritis is suspected to prevent long term complications.
  3. It is a serious medical condition that, delayed or inadequate treatment can result in irreversible joint destruction.

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