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How not to miss: septic bacterial arthritis

Dr Rod Hughes advises on the key signs to look out for

Worst outcomes if missed

The following outcomes are all related to

duration of untreated infection, bacterial virulence, host defences and age.

•Mortality (especially in the elderly or

immuno-suppressed) – appropriately

treated early septic arthritis has a good prognosis; non-gonococcal bacterial

arthritis has a 10-15 per cent mortality

•Septicaemia – up to 50 per cent have

positive blood cultures on presentation

•Rapid joint destruction (especially in childhood) if treatment is not rapid

• Chronic infection of prosthetic joints (mortality of 5-15 per cent dependent on site)

•Chronic joint damage and impaired

function – estimated at 25-50 per cent

Epidemiolgy and incidence

Age at diagnosis

•Two peaks – under-fives and over-64s

•50 per cent of prosthetic joint infections occur in adults over 70


•Prior joint damage and joint prostheses are predisposing factors for occurrence

•Staphylococcus aureus preferentially localises to joints

•Proteolytic enzymes promote acute inflammation and destruction of cartilage and underlying bone

Microbial isolates

•Staphylococcus aureus is the most

common at 40 per cent

•Streptococci 30 per cent

•Gram negative bacteria 20 per cent

•Gonococcus 2 per cent

•Others 8 per cent


•Over 50 per cent of cases occur in the knee

•10-20 per cent are polyarticular, especially if there is an associated systemic illness such as RA

•The lower extremities are most often affected in adulthood

Dissemination – five possible routes

•Haematogenous from a remote bacterial focus (teeth, wounds, lungs)

•Dissemination from an acute osteomyelitic focus (children)

•Lymphogenic from an adjacent infection such as a wound (post-operative)

•Iatrogenic from joint injection

•Penetrating trauma and dirt inoculation


•Annual estimated incidence in northern Europe of 5-10/100,000

•It is more common in warm, humid and socio-economically deprived areas

•30-70/100,000 incidence in patients with rheumatoid arthritis or joint prostheses

•30 per cent of cases occur in previously healthy children

•Approximately 50 per cent due to haematogenous infection of 'native' joints

•IV drug use, HIV infection or diabetes are risk factors

•Recent improvements led to fall in surgical prosthetic infection rate to <1 per="">

Symptoms and signs

Presentation varies with age:

•Up to age one presentation may be systemic rather than localised to affected joint

•Small children develop high fevers, are ill and immobilise the joint

•Older children are systemically ill but present with local joint inflammation as in adults

Classic signs

•Abrupt onset of a hot, painful and swollen joint

•Joint inflammation and synovial effusion

•Restriction of active and passive

movement and reluctance to move or put weight on the affected joint


•Rigors if septicaemic

•Hip, if affected, held in flexion

Differential diagnosis

The following diagnoses should be considered with a hot swollen joint:

•Septic bursitis (ie pre-patellar bursitis)

•Crystal-induced arthritis (consider

if there is a history of previous


•Haemarthrosis (trauma or joint injury)

•Reactive arthritis (knee or ankle with

preceding infective history – may have


•Lyme arthritis (typical rash and endemic area)

•Psoriatic arthritis (skin or scalp rash)

•Flare of rheumatoid joint (existing RA)

•Osteomyelitis complicating septic

arthritis (early radiological abnormalities)

First-line investigations

•Joint aspiration and synovial fluid

examination by Gram stain and culture – should not be delayed until antibiotics are started (culture positive in 70-90 per cent of cases)

•Culture of respiratory, cutaneous and genitourinary sites

•Blood culture – positive in 50 per cent

•Full blood count – leucocytosis in 60 per cent

•ESR and CRP – elevation occurs but is non-specific

•X-ray of affected joint – to assess baseline state of the joint and to exclude

osteomyelitis; periarticular osteoporosis may be evident early or gas appearance

within the joint may indicate infection with E. coli or anaerobes

Second-line investigations

•CT or MRI may be useful when joints are difficult to evaluate clinically, such as

shoulder, hip, sternoclavicular or sacroiliac joints

•Bone scans using labelled white cells (Indium111), nanocolloid (Technitium99) or labelled immunoglobulin may help in the detection of infection but are not specific

•Polymerase chain reaction may help

identify infection in synovial fluid


Five key questions

When patient has hot swollen joint

1 Have you got a fever? This may alert you to infection and is more likely in septic arthritis than other inflammatory joint conditions – see also red herrings.

2 Have you had other swollen joints previously?

This may indicate another joint pathology such as gout, pseudogout or rheumatoid arthritis, all of which can mimic septic arthritis.

3 Have you had a recent acute infection?

Acute reactive arthritis can resemble septic arthritis and may be preceded by food poisoning or

non-specific urethritis. Current urethral

discharge may alert to gonococcal arthritis.

Dental sepsis may indicate risk of septic arthritis. Prior viral infection may indicate viral reactive monoarthritis.

4 Have you got psoriasis of the skin or scalp?

Acute psoriatic arthritis can affect single joints and resemble septic arthritis.

5 Have you ever taken recreational drugs IV or are you at risk of HIV infection? Both have higher risk for immunosuppression and sepsis.

Five red herrings

1 Fever – can be prominent in non-septic conditions such as gout or pseudogout

2 X-ray changes – radiological change in acute sepsis usually takes weeks to become obvious

3 Swollen joints – in young children, septic arthritis may not present with a swollen painful joint. HIV-infected adults with AIDS may have minimal joint inflammation in the presence of active joint sepsis

4 Lack of a recent history of infection – septic arthritis may be the first manifestation of 'hidden' infection such as gonorrhoea

5 Existing rheumatoid arthritis – a single hot swollen joint in RA and systemically ill patient suggests sepsis

Rod Hughes is a consultant rheumatologist at Ashford and St Peter's NHS trust, Chertsey, Surrey – he has a research interest in bacterial causation of inflammatory arthritis and has written and taught on the management of acute septic arthritis

Competing interests None declared

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