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Q1: What is the differential diagnosis?
A history of recent fall in a patient on anticoagulation therapy, particularly when linked with a bloody knee joint aspirate, should prompt the suspicion of traumatic haemarthrosis. Against this diagnosis goes the chronicity of symptoms preceding the injury, thex ray findings, the fistula, etc. Blood during aspiration might merely be the consequence of a traumatic tap.
The other likely diagnosis is tuberculous monoarthritis, suggested with the abnormal knee and chest radiographs, draining popliteal fossa sinus, and fact that the diagnosis was established with synovial tissue biopsy and culture. Additionally, the patient had emigrated from an area endemic for tuberculosis.
Q2: What is the diagnosis?
The patient had tuberculous arthritis. The synovial biopsy showed multiple granulomas and multinucleated giant cells. Rare acid fast bacilli were seen as well. Synovial tissue cultures subsequently grew mycobacterium tuberculosis.
Q3: What is the pathophysiology of these abnormalities?
Tuberculous monoarthritis, once common, is now rare because of the availability of antitubercular treatment. Articular involvement is seen in less then 1% of tuberculous infections. If untreated it may lead to complete joint destruction.
Only a minority of patients will have concurrent active tuberculosis that might be pulmonary or extrapulmonary. Other patients will usually have evidence suggesting previous exposure to tuberculosis (for example, see the chest radiograph, fig 1, on p 600). The vast majority will typically have a tuberculin positive skin test, but not all. In one report, up to 40% of patients with tubercular monoarthritis had a negative skin test.1
Articular tuberculosis, like other forms of extrapulmonary tuberculosis, is usually due to reactivation of a haematogenously seeded focus and need not be associated with active tuberculosis elsewhere.2
The most frequently affected joints are the knee, followed by the hip, wrist, and other small joints.3 Weight bearing joints are more frequently affected. It is possible that daily microtrauma of weight bearing predisposes these joints to infection.4
After local trauma experimental animals were predisposed to tuberculous joint infection. Tuberculous monoarthritis presents insidiously with joint pain and joint swelling. Sinuses and fistulas are not a rare finding in tuberculous arthritis.5
Figure 1 (see p 600) shows the characteristic radiographic appearance of tuberculous arthritis. Changes seen classically include joint space narrowing, metaphyseal and subchondral erosions (all seen on fig 1), and subchondral cysts. The pattern of vascular anatomy predisposes to this typical radiographic appearance. Metaphyseal capillaries loop sharply in the area of the metaphyseal plate. They anastomose extensively and become dilated, causing turbulence and slowing of blood flow. This in turn predisposes to bacterial localisation and growth.
The gold standard for diagnosing tuberculous arthritis remains synovial biopsy and tissue cultures. The chemotherapy for articular tuberculosis is the same as for other forms of tuberculosis.
In summary, tuberculous arthritis should be suspected typically in tuberculin skin test positive patients with chronic monoarticular pain and abnormal chest and joint radiographs. Excisional synovial biopsy and tissue culture give the best diagnostic yield. Treatment is the same as for other forms of tuberculosis.
Tuberculous monoarthritis of the knee joint.
Learning points: tuberculous monoarthritis
Seen in 1% of tuberculous infections.
Seen in active or dormant tuberculosis.
Patients have evidence of exposure to tuberculosis like a positive skin test (typically but not always) or suggestive chest radiography findings.
Weight bearing joints preferably affected: knee > hip > wrist > other small joints.
Typical findings on the knee radiograph: joint space narrowing, metaphyseal and subchondral erosions, and subchondral cysts.
Gold standard of diagnosis: synovial tissue biopsy and tissue cultures
Treatment is similar to other forms of tuberculosis.