Increased pain in peripheral joints affected by any chronic inflammatory arthritic process, is usually attributed to flare up and progression of the disease and managed with appropriate alterations in the use of analgesics. Clinical differentiation between flare up of arthritic process and joint infection can be extremely difficult. Haematological parameters are often non-specific and plain radiographic changes are difficult to assess when sepsis occurs in an already damaged joint. Due to these reasons, the diagnosis of infection is often delayed and is possible only at a stage when joint destruction is advanced. This explains the high morbidity and mortality reported (1).
In the case presented here, the dramatic sequence of events that occurred when the patient became septicaemic led to an early diagnosis and appropriate management.
Recognised risk factors for adult septic arthritis include immunocompromised state, inflammatory diseases like rheumatoid arthritis (2), destructive joint diseases like osteoarthritis, gout etc. and joint prostheses (3). In fact any form of arthritis, by means of damaging the joint surfaces, can theoretically predispose it to sepsis. However, for unknown reasons, infection in joints with pre-existing ankylosing spondylitis is extremely rare with only two case reports so far (4, 5).
The portal of entry of pathogenic bacteria in ankylosing spondylitis has been postulated previously to be through an abnormal gastrointestinal tract (4), but this is very unlikely in this case since the infective organism was Staphylococcus aureus. In the case presented, in spite of detailed examination and investigations, no source of bacteraemia could be identified.
We conclude that the possibility of infection should be borne in mind even when the underlying disease is a sero-negative arthropathy. It can occur without an identifiable source of bacteraemia elsewhere in the body. As in children, a high degree of suspicion is necessary for early diagnosis and management of sepsis in an adult joint with pre-existing arthritis, to avoid potentially serious and fatal complications. One should have a very low threshold for aspirating the joint even when the diagnosis is a flare up of arthritis. Ultrasound-guided aspiration is of great value in joints difficult to aspirate, like the hip. Leukocyte scans, though equally informative, are expensive and not readily accessible to everyone.