RheumatologyTier 1Disease (DEADMAN)

Arthritis - septic

Description

Bacterial infection of a joint space - an orthopaedic/medical emergency, joint destruction can occur within 24-48 hours of untreated infection
Native joint septic arthritis - usually monoarticular, knee commonest
Prosthetic joint infection - a distinct entity with different diagnostic thresholds and management (often requires implant removal), managed separately from native joint infection

Risk factors: pre-existing joint disease (esp. rheumatoid arthritis), prosthetic joints, IV drug use, immunosuppression, diabetes, age >80, recent joint surgery/injection, skin infection/breach
Higher incidence and worse outcomes in patients with underlying inflammatory arthritis, partly due to diagnostic delay (attributed to a disease flare)

Staphylococcus aureus (including MRSA) - commonest organism overall
Streptococcus species - second commonest
Neisseria gonorrhoeae - important cause in sexually active young adults, can present as a migratory polyarthritis/tenosynovitis-dermatitis syndrome rather than classic monoarthritis
Gram-negative organisms - more common at extremes of age, IV drug use, immunosuppression
Route: haematogenous seeding (commonest), direct inoculation (trauma, surgery, injection), or contiguous spread from adjacent osteomyelitis/soft tissue infection
Bacterial proliferation in synovial fluid -> host inflammatory response (neutrophil proteases, cytokines) -> cartilage degradation begins within hours - the joint destruction is driven as much by the host response as the organism itself

Clinical

Acute hot, swollen, painful joint with markedly reduced range of motion, often with fever/systemic upset (though fever can be absent, especially in the elderly/immunosuppressed)
Any acute monoarthritis is septic arthritis until proven otherwise - the default working diagnosis, not one option among several

Investigations - joint aspiration is mandatory and should not be delayed

Synovial fluid aspiration before antibiotics wherever possible - Gram stain, culture, cell count, crystal analysis
Synovial WCC >50,000/mm^3 (often quoted higher, >100,000) with neutrophil predominance is strongly suggestive, but a lower count does not exclude septic arthritis, especially with prior antibiotic exposure or an atypical organism (gonococcal arthritis often has a lower count)
Blood cultures - positive in a significant proportion, especially with staphylococcal disease
Bloods: FBE, CRP/ESR (trend for monitoring response, not diagnostic alone)
Imaging - plain X-ray at baseline (joint effusion, baseline for comparison), MRI/ultrasound if deep joint (hip) or diagnostic uncertainty
Do not wait for aspiration results to start empirical antibiotics if the patient is systemically unwell - send the sample first, then treat

A. Source control - as urgent as the antibiotics

Joint drainage is required alongside antibiotics - arthroscopic or open washout for most joints, or repeated needle aspiration for accessible joints with a good response; the hip usually requires surgical drainage given difficulty achieving adequate needle decompression
Orthopaedic involvement early for any deep/difficult-to-access joint or failure to improve

B. Empirical antibiotics - started promptly after aspiration, before culture results

Cover Staphylococcus aureus (including MRSA per local risk/prevalence) and Streptococcus as the default; add gonococcal cover (ceftriaxone) if clinical picture suggests it; broaden for gram-negative cover in the elderly/immunosuppressed/IV drug use pending culture
IV therapy initially, narrowed once organism/sensitivities known, with transition to oral therapy once clinically improving and able to tolerate oral intake

C. Duration

Total antibiotic duration is typically 4-6 weeks for native joint septic arthritis, combining an initial IV period with oral step-down - shorter courses have been studied but standard practice remains the longer course, particularly for staphylococcal infection, given the relapse risk documented with shorter regimens
Individualise duration by organism, response, and whether adequate source control was achieved

D. Adjuncts

Analgesia, physiotherapy input once infection is controlled to preserve joint function
Address any predisposing factor (glycaemic control, review immunosuppression where feasible)

Rheumatoid arthritis and other inflammatory arthritides - both a risk factor and a diagnostic trap (attributed to a flare)
Prosthetic joints, IV drug use, diabetes, immunosuppression
Gonococcal arthritis - sexually active young adults, may present atypically (migratory, tenosynovitis, pustular skin lesions)
Recent joint injection/aspiration or surgery

Irreversible cartilage destruction can begin within 24-48 hours of untreated infection - the reason for treating suspected cases as an emergency before confirmatory results return
Delayed diagnosis/treatment - joint destruction, chronic pain, reduced function, and in severe cases sepsis/mortality (particularly in the elderly and those with delayed presentation)
Relapse risk if antibiotic duration is inadequate or source control (drainage) is incomplete
Prognosis is generally good with prompt diagnosis, adequate drainage, and a full antibiotic course - the recurring theme in poor outcomes is delay, not the disease itself

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