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How not to miss.... septic arthritis

Rheumatologist Dr Christakis Christodoulou advises on the key signs and pitfalls in diagnosing septic arthritis

Rheumatologist Dr Christakis Christodoulou advises on the key signs and pitfalls in diagnosing septic arthritis

Worst outcomes if missed

• Death – septic arthritis is a life-threatening disease. The mortality for polyarticular disease is 30%, which is much higher than the inpatient mortality for myocardial infarction, which is 10%. The mortality for monoarticular septic arthritis is between 4% and 8%.

• Poor joint outcome – defined by the need for amputation, arthrodesis, prosthetic surgery or severe functional deterioration – is seen in 33% of surviving patients.

Poor prognostic factors include older age (over 60), pre-existing joint disease such as rheumatoid arthritis, staphylococcal infection, an infected joint containing synthetic material and delay in diagnosis and treatment.

Septic arthritis is a medical and surgical emergency that must be diagnosed and treated as quickly as possible.

Epidemiology

The prevalence of septic arthritis among adults presenting as an emergency with one or a few acutely painful joints is estimated to range from 8% to 27%.

Some 15% of all cases of septic arthritis are polyarticular with a mean of three affected joints.

Predisposing factors for septic arthritis include older age (over 80), recent joint surgery, hip or knee prosthesis, skin infection, diabetes mellitus, rheumatoid arthritis, SLE, HIV, malignancy, alcoholism and intravenous drug use.

In most cases of septic arthritis the spread of the infecting organism to the joint is haematogenous. The condition can also result from a bite, trauma or joint surgery and rarely it can be spread from adjacent osteomyelitis.

Any microbial pathogen can cause septic arthritis but Staphylococcus aureus is the most common pathogen, followed by streptococci.

Symptoms and signs

41226942Classically, patients present with acute pain and swelling in one or more joints. The affected joints may also be erythematous.

The knee is the most commonly affected joint, followed by the hip, ankle, shoulder, wrist and elbow.

Many patients are febrile but 20% are afebrile. The percentage of patients with rigors ranges from 20% to 60%.

Patients may have evidence of an associated skin, urinary tract or a respiratory tract infection and this should provide a clue to the organism likely to be causing the infection.

Prosthetic joint infections in the first six to 12 months after insertion typically present with pain, swelling, fever and sometimes discharge.

Late infections present with a prolonged subacute course with increasing joint pain. Fever in late infections occurs in less than 50% of patients.

Differential diagnosis

• Crystal arthritis – gout and pseudogout, which can mimic septic arthritis – is the main differential of an acute monoarthritis

• Reactive arthritis

• Rheumatoid arthritis

First-line investigations

41226943Joints suspected of harbouring infection should always be aspirated and this should be done promptly. The synovial fluid should be examined urgently for leukocyte count, gram stain and culture, and polarised-light microscopy for crystals.

In septic arthritis the leucocyte count is usually more than 50,000 cells/mm3 with more than 85% being neutrophils.

The sensitivity of gram stain for septic arthritis is 30-50%.

The culture is positive in the majority of patients with septic arthritis but it can be falsely negative in those who have received antibiotics recently.

Polarised-light microscopy will reveal whether or not urate crystals of gout or calcium pyrophosphate crystals of pseudogout are present.

Blood cultures should also be sent promptly since they are positive in 50% of patients with septic arthritis.

Second-line investigations

Routine blood tests such as FBC, ESR, CRP, renal and liver function should also be sent. Leucocytosis occurs in 60% of patients and ESR and CRP are elevated in the majority of patients.

Plain X-rays of the affected joints are usually normal at presentation but they should be obtained to ensure there is no associated osteomyelitis or underlying joint disease. They can also be compared with later films should the response to therapy be delayed or poor.

Ultrasound, CT scanning and MR imaging can detect effusions in deep joints such as the hip and the sacroiliac joints and such joints can be aspirated under ultrasound or CT guidance.

Dr Christakis Christodoulou was a specialist registrar in rheumatology at Queen Elizabeth Hospital, London, and is now a consultant in rheumatology and general (internal) medicine practising privately

Competing interests: None declared

Septic arthritis Key questions Red herrings Related Seminar: Musculoskeletal Medicine

Clinical Seminar: Musculoskeletal Medicine

What: A one day refresher to update GPs on the hot issues and trickiest dilemmas in rheumatology and orthopaedics.

When: Thursday 5 November 2009

Where: Raddison Hotel, Manchester

Next steps: Find out more and book

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