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SEPTIC ARTHRITIS
Claire Simpson and Belen Carsi
BACKGROUND
Septic arthritis (SA) usually occurs as a result of inoculation of bacteria into a
joint. The inoculation can occur as a haematogenous event, by direct penetration
of a joint, or by spread from an adjacent focus of infection. May be monoarticular
(esp. knee) or polyarticular (12%). SA is an emergency because of the damage
it causes to bone as well as cartilage, and its potential to cause septic shock.
It occurs most commonly in children as a result of haematogenous spread of
bacteria. In adults, there is often an associated arthritic condition such as
rheumatoid arthritis or a medical condition that affects the immune system
(cancer, diabetes, sickle cell anemia, SLE, IVDU, alcoholism). Patients with a
joint replacement, recent joint injuries, instrumentation or surgery are also at risk.
Gonococcal infections may also cause SA, typically affecting women and male
homosexuals.
In children, most cases involve hip. Other joints may be affected. Metaphyseal
osteomyelitis can rupture into the joint in the proximal femur, humerus and distal
fibula.
AETIOLOGY BASED ON AGE
Newborn: Staphylococcus aureus, Enterobacteriaceae, Group B Streptococcus
Children (<15y): Staph. aureus, Str. pyogenes, Str.pneumoniae, Haemophilus
influenza, gram-negative bacilli.
Young sexually active adults: Neisseria gonorrhoeae, Staph aureus,
Streptococcus.
Older adults: Staph. aureus, Streptococcus, Gram-negative bacilli.
DIFFERENTIAL DIAGNOSIS
• Acute rheumatic fever (migratory arthralgia, carditis, increased ASOT, group A
streptococcal infection).
• Juvenile Rheumatoid Arthritis (morning stiffness, usually mild joint swelling).
• Lyme disease (indolent onset, erythema migranes, cardiac and neurological
manifestations).
• Osteoarthitis
• Rheumatoid arthritis (morning stiffness, symmetric involvement, positive
rheumatoid factor, elevated ESR).
• AVN of the hip. May have severe pain, normal X-rays and bloods.
• Transient synovitis of the hip in children (limited hip motion, afebrile).
EXAMINATION
Should Include a generalized inspection to identify a source of the infection, such
as a furuncle or abscessed tooth, or site of penetrating wound near the joint.
Fever and constitutional symptoms may be present. Note any swelling and the
position of the joint. With septic arthritis, the patient holds the joint in a position
of comfort, such as flexion of the hip or knee. Passive motion of the joint causes
severe pain. Palpate the joint for increased warmth and effusion. With a chronic
infection, hypertrophy of the synovium will be present.
INVESTIGATIONS
• WBC: normal or elevated.
• ESR: frequently but not always elevated.
• CRP: useful in following response to therapy as well as detecting an acute
process in chronically affected joints.
• AP and LAT radiographs of the affected joint: normal or previous arthritis and
soft tissue swelling.
• Blood culture: Important as they may identify causative organism when joint
fluid cultures are negative. Obtain specimens of urethra, cervix, pharynx and
rectum if gonococcal infection is suspected.
• Aspiration of joint for analysis, Gram stain and culture: Always perform joint
aspiration under the most sterile conditions possible to prevent the
introduction of infection. If joint is not easily accessible or technically difficult,
involve orthopaedics/radiology. Usually WBC >50,000/mm3. Our laboratory
does not quantify cells, but it will say pus cells ++/+++. However, the WBC of
the joint fluid typically is less elevated in gonococcal arthritis. Prosthetic joint;
refer orthopaedics With chronic infections, cultures should include study for
acid-fast and fungal organisms, as well as pyogenic bacteria. A negative
culture does not necessarily exclude septic arthritis, particularly in chronic
cases. To avoid therapeutic delays, screen for crystals by polarizing
microscopy and for organisms by Gram stain. If crystals are present and
Gram findings negative, treat the patient for crystal-associated arthritis.
• Biopsy of the synovium may be necessary to diagnose one of the many
causes (ie, mycobacterial, fungal) of granulomatous synovitis.
INITIAL TREATMENT
• Admission to hospital mandated in all cases.
• Adequate and timely drainage of the infected synovial fluid.
• Immobilization of the joint to control pain.
• Administration of appropriate antimicrobial therapy.
Never start antibiotics without aspirating fluid first. Once this is done, broadspectrum coverage antibiotics can be started until specific sensitivities arrive.
S.aureus is the commonest organism, hence:
(1) Healthy Adult
Flucloxacillin 1g IV 6 hrly and Fucidic Acid 500mg PO 8 hrly.
Or cefuroxime 1.5g IV 8 hrly and Fucidic Acid 500mg PO 8 hrly
(pen allergic).
Or Vancomycin 1g IV 2 hrly and Fucidic Acid 500mg PO 8 hrly.
(2) Child (consider Haemophilus)
Cefuroxime and Flucloxacillin. See Medicines for Children for doses
Follow Hospital Antibiotic Policy (2004) for further treatment options.
Note: Erythromycin and Clarythromycin do not penetrate into synovial fluid and
should not be used.
MONITORING AND REFERRAL
Prolonged courses of antibiotics are needed (3-6 weeks). Monitor inflammatory
markers.
Refer to Orthopaedics if high index of suspicion for septic arthritis.
If low index of suspicion, refer to Rheumatology. Consider alternative diagnosis:
• Gout/pseudogout.
• Reactive arthritis.
• Sero-negative spondarthritis.
• Rheumatoid arthritis.
In children, consider alternative diagnosis only after SA is ruled out:
• Transient synovitis, refer paediatrics.
• SUFE, refer orthopaedics.
• Perthes, refer orthpoaedics.