Septic Arthritis: Latest Facts, Causes, Diagnosis and Treatment

Septic arthritis, or acute infectious arthritis, is an infection of one or more joints with a germ (mainly bacteria, but also viruses and fungi). The risk is local, with rapid deterioration of the joint, and general, with the risk of spreading the infection to the rest of the body (heart, brain…).

Arthritis is associated with swelling and inflammation of the joints which can cause pain. This swelling is related to an inflammatory hypertrophy of the joint membrane (which covers the inside of the joint), i.e. a “synovitis” and an effusion of intra-articular fluid.

Knee Joint

Knee Joint

Sepsis comes from the Latin word “septicus” meaning “corrupt, rotten”.

What is septic arthritis?

Septic (or acute infectious) arthritis is an acute infection of a joint caused by a germ, usually a bacterium, but sometimes also a virus or fungus.

The joint damage is related to the proliferation of the germ in the joint (which is different from “reactive arthritis”, where the germ is in other parts of the body (digestive tract, genital tract).

Septic arthritis usually occurs as a result of an infection that occurs elsewhere in the body, where the germ migrates to the joint. However, an infection can erupt directly into a joint in case of trauma to the joint or after a medical (joint puncture, intra-articular infiltration, etc.) or surgical (meniscectomy, insertion of a prosthesis) procedure.

In general, only one joint is affected, usually the knee, sometimes two or three. The knee is the most affected joint in adults (30%), while the hip is most affected in children. The infection is most common in the large weight-bearing joints (hips, knees), but can also occur in smaller joints (fingers, elbows, ankles).

Septic arthritis is a diagnostic requiring emergency hospitalization because, on the one hand, the breakdown of cartilage (“chondrolysis”) associated with the bacterium is very rapid due to a lack of drainage of the joint and appropriate antibiotic treatment, with a risk of major consequences (“secondary arthrosis”) and, on the other hand, a high risk of serious secondary infections (sepsis, septic shock, endocarditis, other septic sites), which can lead to death.

What causes infectious arthritis?

In septic arthritis, a microorganism can enter the body and the joint through the skin, nose, throat or a small, almost invisible cut, but usually secondary to an infection elsewhere in the body (dental or skin abscesses). After the initial infection, the microorganism can be carried in the bloodstream to a joint, where it causes inflammation and multiplies, forming “purulent synovial fluid”.

Most cases of infectious arthritis are of bacterial origin, particularly secondary to Staphylococcus or Streptococcus, but sometimes also to Gonococcus, Pneumococcus, Hemophilus, Enterococcus or Spirochete (syphilis). In the more discrete forms it can be a Mycobacterium (tuberculosis), which is just as serious but more treacherous. Infectious arthritis can also be secondary to a viral infection (hepatitis, measles, parvovirus, infectious mononucleosis, mumps) or related to a fungus.

In the case of gonorrhea, measles, parvovirus and syphilis, these infections can be transmitted by contact with another person, which is not the case with other types of infectious arthritis.

What are the risk factors for infectious arthritis?

Septic arthritis occurs after an infection, a trauma to a wound or an infectious medical or surgical procedure. It usually affects men (70% of cases), those over 50 (average age 60 years), especially those who are immunocompromised (alcoholism, diabetes, cancer, kidney or liver failure, hemodialysis, intravenous drug use, chronic inflammatory rheumatism, HIV infection, immunosuppressive treatment, etc.).

Septic arthritis: Diagnosis

Doctors consider any inflammation of a single joint (“monoarthritis”), which starts suddenly (“acute”) and is accompanied by fever (“febrile”), as septic arthritis until proven otherwise.

Inflammatory rheumatic diseases can begin with a single joint, but in a less dramatic way. Septic arthritis, on the other hand, rarely affects more than one joint (oligoarthritis or polyarthritis).

In the case of septic arthritis associated with “pyogenic” (“pus-forming”) bacteria, a very intense pain suddenly appears in the joint, accompanied by swelling of the joint with significant intra-articular effusion and localized inflammatory symptoms, fever, sometimes high, and a change in the general condition. In most cases, the affected person is unable to use the joint.

However, the symptoms vary depending on the type of micro-organism causing the infectious arthritis. In the case of a pyogenic bacterium, the inflammation appears relatively suddenly, affecting only one joint and is accompanied by fever and chills (several joints in one in ten).

In the case of a virus, the infection does not necessarily lead to fever, but can affect several joints and cause general malaise.

When the germ in question is a mycobacterium or fungus, the inflammation can be local or widespread, but it is more insidious: it can take weeks or even months to appear and is sometimes accompanied by a moderate fever.

In the case of arthritis that is treated prematurely by a blind course of antibiotics, the inflammation of the joint is moderate, even in the case of pyogenic arthritis, the fever is discreet or even absent and the number of white blood cells in the fluid is less than 20,000/mm3 and doctors may have difficulty identifying the bacteria and need to take more samples.

The same is true after joint infiltration with corticoids: the clinical picture of septic arthritis can be frustrating in the initial phase due to the anti-inflammatory effect of the injected corticoid. Infectious symptoms may not occur until 3 or 4 days later.

How is septic arthritis diagnosed?

An accurate bacterial diagnosis is essential. If your doctor reports infectious arthritis, he will ask you several questions, perform a detailed clinical examination, order tests to identify the microorganism causing the infection, including taking a sample of the synovial fluid for analysis and a blood test.

Chills are very suggestive of sepsis and are often asked about during the doctor’s examination. Also, traditional promoting factors (trauma, immunosuppression, diabetes, cancer, immunosuppressive treatment..etc.) must be looked for.

The physical examination may show a very painful swollen joint: the active and passive movements of the joint are very painful and almost impossible. The patient usually holds the joint in the position that hurts least: “analgesic posture” of the joint, especially in flexion (“flexion”). There may be one or more large and painful nodules (“satellite adenopathies”) in the drainage area of the joint.  An important step in the clinical examination is also the careful search for a wound near the diseased joint or a distant infection site (teeth, chest, skin, etc.).

The diagnosis of septic arthritis and the germ in question is mainly based on the sampling and analysis of the synovial fluid. The puncture of the synovial fluid is the first essential gesture to confirm the septic nature of the arthritis and must be carried out before any treatment with antibiotics (even if the treatment is urgent). The puncture is performed under strict asepsis conditions. The perforated fluid is usually cloudy with “hypercellularity”: more than 2,000 white blood cells per mm3, but more often than not more than 50,000 per mm3, most of which are neutrophilic polynuclear cells (more than 90%). This sample should be sent to the bacteriology lab for direct examination and culture. The discovery of microcrystals in the synovial fluid should re-evaluate the diagnosis of microcrystalline arthritis (gout, chondrocalcinosis), but does not rule out the diagnosis of septic arthritis without confirmation.

Blood sampling usually shows a significant inflammatory syndrome with a higher (non-specific) sedimentation rate and CRP (more specifically when it exceeds 100 milligrams per mm3), sometimes associated with “neutrophilic hyperleukocytosis” in the blood. However, inflammatory syndrome may be absent in 10-15% of cases.

In all acute febrile monoarthritis cases, repeated blood samples (“blood cultures”) must be taken and cultured, which may increase the chances of finding the causing bacterium.

Samples should also be taken from the different possible points of entry: urine (ECBU), nose, throat, skin wounds, etc.

Additional radiological imaging tests have little diagnostic interest in superficial arthritis and should in no way delay the joint puncture and the collection of the different diagnostic samples needed. However in the case of deep joint involvement (e.g. coxofemoral), they are very useful both in the diagnosis and in the guiding of the puncture.

Standard X-rays have little diagnostic interest because of the delay in the appearance of the first joint lesions compared to the clinical picture (at least 2 weeks). At a later stage, septic arthritis causes diffuse pinching of the joint space with bone erosions under the cartilage (“subchondral”) and, paradoxically, no formation of osteophytes. After 3 to 4 weeks, a formation of secondary osteomyelitis might take place.

Cheap, non-invasive, repeatable osteoarticular ultrasound can detect a limited amount of spilled fluid or synovial swelling, especially in deep joints. It can also differentiate between extra-articular damage (“bursitis” for the joint) and associated complications (soft tissue abscesses). Ultrasound can also be used to guide the puncture of an effusion (“ultrasound guidance”).

A CT scan of the joint can be used to  visualizes the effusion and the deep joint injury.

What pathogens can be found in the puncture?

The identification of the germs is done mainly by obtaining a sample through a joint puncture, but also from other possible sites of infection and doing a culture to find out what the culprit organisms are.

Bacterial serologies are not interested in septic arthritis with common pathogens, but may be of interest in cases of syphilis or Lyme arthritis (real late arthritis in the context of untreated Lyme disease).

In decreasing order of frequency, the following pathogens are found: Staphylococcus aureus in 60% of cases, but also Staphylococcus epidermidis, Gram-negative bacilli (20% of cases), especially in the elderly and other streptococci (15 to 20% of cases).

Gonococcus should be systematically considered in young adults and adolescents due to the resurgence of sexually transmitted infections (STIs) in recent years. In this case, the joint infection is often associated with a severe inflammation of the tendons of the hand or foot (“tenosynovitis”) or even an infected blister on the skin in contact with the affected joint (“periarticular pustule”).

More rare pathogens may be evoked in a particular context: Yersinia, hemophilia, tuberculosis and mycobacteria.

In immunocompromised patients and intravenous drug users, fungi (candidiasis) or parasites should be sought by requesting appropriate culture media during culture.

What can septic arthritis be confused with?

Microcrystalline rheumatism (gout, chondrocalcinosis) are the joint diseases closest to septic arthritis with often hyperalergic monoarthritis from a sudden onset, sometimes feverish, with great functional weakness. With joint puncture, no germ is found on direct examination or cultivation, but a hypercellularity with crystals, fine tip in gout and square tip in chondrocalcinosis.

Other inflammatory diseases such as “reactive arthritis” (shigella, chlamydia…) or autoimmune joint diseases (rheumatoid arthritis, spondylitis, lupus) can sometimes mimic septic arthritis at an early stage if it starts abruptly.

In certain places (elbow, knee) a distinction must be made between septic arthritis and “bursitis”, which is located in front of the joint (“olecranon” in the elbow and “prerotulus” in the knee) and is caused by inflammation or infection. Ultrasound can help with this diagnosis. In this case, the puncture for diagnostic purposes should not be intra-articular in order not to infect a healthy joint.

Some soft tissue infections (erysipelas, lymphangitis, subcutaneous abscesses) may wrongly indicate joint damage due to periarticular inflammatory edema. Piercing the infected skin would inevitably lead to septic arthritis.

Is it necessary to go to the emergency room?

Septic arthritis quickly lead to complications (within the first 24 hours) by cartilage breakdown with a risk of secondary arthritis. In general, it can itself be the cause of a generalized secondary infection (“septicemia”), with the risk of migration of the germ to a vital organ (brain abscess, infection of the heart valves or “endocarditis”).

Therefore, any suspicion of septic arthritis requires specialized emergency and hospital care.

The determinants of the prognosis are the speed of the therapeutic treatment and the perfect adaptation of the antibiotic treatment, which should not be started before microbiological samples are taken by means of a joint puncture (in order not to prevent the growth of the bacteria in the culture). Taking antibiotics before taking bacterial samples is the first cause of misdiagnosis and even death in septic arthritis.

What is the treatment for septic arthritis?

The first treatment must be carried out as a matter of urgency: everything must be done so that the patient can undergo a joint puncture as soon as possible for the analysis of the synovial fluid and a joint flush: this joint flush (by means of a puncture or surgical drainage) is the most important prognostic factor for the joint.

Purulent fluid that has accumulated in the affected joint(s) must be drained and flushed. Usually, the joint is perforated with a needle into a syringe that allows a sample of the synovial fluid to be taken first for analysis (“joint puncture”) and then injected and re-sucked with saline syringes (“joint washing”), but surgical washing may be necessary. It may be necessary to pierce the same joint several times if fluid is still building up in the joint.

Antibiotic therapy should only be started after joint sampling (except in case of severe sepsis). Antibiotics must be given as soon as possible(but only after taking bacteriological samples), in high doses (“bactericide”), intravenously and adapted from the start or secondary to the bacteria identified by the samples and cultures. Antibiotic therapy must be bactericidal to the identified bacteria and in high doses if a β-lactam or a glycopeptide is used. Oral antibiotics can be taken after 2 weeks, usually after the fever has disappeared and the local inflammation has subsided, and will last at least 2 weeks if all goes well. However treatment must be continued for 4 to 6 weeks if the symptoms diminish slowly and if the patient is immunocompromised.

The first antibiotic therapy can only be based on probabilities or if the germ is directly identified. After joint infiltration or surgery, the most common germs are Staphylococcus aureus and a streptococcus or gram-negative bacilli. In the case of diabetes, the most common germs are Staphylococcus aureus or a gram-negative bacillus. In the case of a urinary tract infection there may be a gram-negative bacillus or gonococcus. In the case of drug addicts it is Staphylococcus aureus or Pseudomonas aeruginosa.

In the case of Staphylococcus aureus or Pseudomonas aeruginosa, treatment with Penicillin M or first generation cephalosporin should be initiated. If the staple is likely to be methyresistant, vancomycin or teicoplanin should be started. In the case of streptococcus, amoxicillin is chosen first and if it is an enterococcus, gentamicin is associated.

Supportive treatments

Resting the joint is necessary to protect it, but strict immobilization by means of a splint (“brace”), outside the initial phase, must be avoided because of the risk of secondary stiffness of the joint.

Gentle rehabilitation can begin as soon as the pain and local signs improve..

Does surgery have a place?

In the initial phase of arthritis, the operation is not systematic, with the exception of washing the joints under arthroscopy when the fluid is very purulent.

In case of an unfavorable evolution of the arthritis and general infection under an appropriate antibiotic therapy, the opening of the joint (“arthrotomy”), the removal of the synovial membrane (“surgical synovectomy”) and associated joint washing must be proposed from the 8th day or in case of a late treatment of osteomyelitis or an already present abscess.

In the case of unrepairable joint damage, the insertion of a prosthesis can be proposed, depending on the joint.

What are the risk factors for infectious arthritis?

While infectious arthritis can affect people of all ages, the risk is higher for some people, including those who have a condition that reduces the body’s ability to fight infection: diabetes, kidney failure, hiv facts, alcoholism, cancer, kidney or liver failure, use of injected drugs, immunosuppressive treatments, and so on.

The risk of developing septic arthritis is higher in people who already have joint damage because germs attack a sick and weakened joint rather than a healthy one.

Having a joint prosthesis (“joint replacement”) also has an increased risk of joint infection.

In addition, some of the immunosuppressive drugs used to treat inflammatory arthritis can reduce the body’s resistance to infection and may lead to the development of infectious arthritis.

Finally, the risk of infectious arthritis is higher in people who are often exposed to animals, plants, marine animals or soil in their work.




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