The Great Imitator - Disseminated Tuberculosis Presenting as Baker’s Cyst: A Case Report

Tuberculosis is known to be a great mimicker, and it can present in a myriad of ways, which often result in an incorrect diagnosis. In a country that is endemic to tuberculosis, the presentation can take many forms ranging from tumour to trauma. We present a case of Baker’s cyst that was provisionally diagnosed as pigmented villonodular synovitis (PVNS) of the knee and eventually turned out to be tuberculous arthritis. A 46-year-old male presented with an insidious swelling on the posterior aspect of his knee for one year. Magnetic resonance imaging was suggestive of PVNS as the likely diagnosis. The patient presented 21 days later with a foot drop. On following-up with further investigations, he was found to have a lesion at the level of the L4-L5 spine. Chest radiograph changes were suggestive of tuberculosis. A synovial biopsy of the knee was done, and the tuberculosis culture report was positive. The patient was started on anti-tubercular treatment and then operated on, with arthroscopic synovectomy and posterior open cyst excision. The histology report was positive for tuberculous synovitis. The patient completed the course of antitubercular drugs and had physiotherapy. He demonstrated a clinically and radiologically healed disease at the final follow-up with a good functional outcome. Clinicians must have a high index of suspicion for tuberculosis, especially in endemic areas. Getting a chest radiograph is recommended in every case. Early diagnosis with the appropriate treatment will give a good functional outcome for the patient.


INTRODUCTION
There is a famous maxim in medicine that "uncommon presentations of common diseases are more common than common presentations of uncommon diseases," which is aptly applicable in the case of tuberculosis. Baker's cyst is a distention of the gastrocnemius-semimembranosus bursa commonly occurring secondary to a patellofemoralarthrosis 1,2 . Tuberculosis knee presenting as Baker's cyst is rare with only a few reports available and only one case of disseminated tuberculosis presenting as Baker's cyst [1][2][3][4] .
We present a case that was provisionally diagnosed as PVNS but then turned out to be tuberculosis 4 .

CASE REPORT
A 46-year-old-male presented with an insidious onset of a swelling on the posterior aspect of his right knee for one year, which progressively increased to reach a dimension of 14cm x 6cm x 5cm. There was no history of trauma, fever, or comorbidities. However, he had pain and restriction of movement and could not carry out his daily activities and bear weight on the affected limb. On general examination, the patient was malnourished and afebrile. Clinically, the swelling was warm, fluctuant, compressible with a positive patellar tap test but without transillumination, pulsatility, tenderness, erythema, and reducibility.
The range of motion was from 10° extension to 90° flexion, which was painful with a soft endpoint (Fig. 1). A provisional diagnosis of Baker's cyst was made. The radiograph was normal apart from the soft tissue shadow posteriorly. MRI was done, which was suggestive of PVNS ( Fig. 2).
Ultrasonography of the knee joint showed a hypoechoicheterogenous collection with synovial hypertrophy. A total of 225cc of fluid was found in the cyst with no joint communication, and was aspirated and analysed ( Table I).
The leucocyte count was 7510/µL (Neutrophils=78.8%, Lymphocytes=16.97%), and the renal and liver function tests were normal. Erythrocyte sedimentation rate (ESR) was 110mm at the end of one hour, and C-Reactive Protein (CRP) was 12mg/dl (>1mg/dl = significant inflammation). The test for the human immunodeficiency virus (HIV) was negative.
Three weeks later, he developed an acute onset of right extensor hallucis longus (EHL) and ankle weakness with a power of the Medical Research Council (MRC) grade 3. A lumbosacral spine radiograph showed an L4-L5 paradiscal lesion. Chest radiograph was suggestive of miliary tuberculosis. He was isolated, and all necessary precautions were taken. A magnetic resonance imaging (MRI) of the lumbosacral spine suggested tuberculosis at the L4-L5 spinal level (Fig. 2). Sputum acid-fast bacilli (AFB) staining turned out to be positive. He was started on empirical antituberculous treatment (ATT) (Isoniazid 225mg, Rifampicin 450mg, Pyrazinamide 900mg, and Ethambutol 675mg). Tuberculosis evaluations of the family members and close contacts were negative. A computed tomography (CT) guided biopsy of the L4-L5 level was done, and the sample was sent for analysis. The six-week tuberculosis culture report of both the synovial and spine biopsies turned out positive with sensitivity to all the first-line medications. After a month of ATT, sputum AFB was negative thrice. The patient's general condition improved, and he was posted for anterior arthroscopic synovectomy and posterior open cyst excision with appropriate precautions.
Under general anaesthesia, an arthroscopic synovectomy was performed in the supine position. The patient was then turned prone, and the posterior approach was taken with a lazy S-shaped incision (proximal lateral and distal medial). The sural nerve and the common peroneal nerve were identified and retracted (Fig. 3). The tissue was dirty reddishbrown. The cyst was thoroughly excised, and its communication with the posterior capsule cut and repaired using a purse-string suture. The wound was then closed in layers with a drain in situ.
Post-operatively, the lower limb was splinted in 20° of knee flexion to avoid tension on the suture line for five days, after which the range of motion of knee exercises was started. Histology report showed Langhan's giant cells and epithelioid granuloma. The intensive phase of ATT was continued for three months, during which time the EHL and ankle power improved to MRC grade 5. At the final followup, the patient had completed 18 months of ATT and was pain-free with a good range of movement.  Culture of aerobes and anaerobes Negative 3.
Mycobacterium tuberculosis staining and Culture Positive 4.
GenXpert Positive with no rifampicin resistance 6.

DISCUSSION
Skeletal tuberculosis accounts for less than 2% of all tuberculosis cases, with the spine, hip, and knee the most common sites for tuberculosis 1,5  Thus, establishing a proper protocol for investigation is a must. At our institute, we routinely investigate all cases of swelling with a Gram stain, aerobic and anaerobic cultures, AFB staining and culture, histopathology, and GeneXpert. In a case of a fluctuant swelling, the fluid is tested for a Gram stain, aerobic and anaerobic cultures, AFB staining and culture, leucocyte count, GeneXpert, and synovial fluid adenosine deaminase (ADA) levels.
In conclusion, clinicians must have a high index of suspicion for tuberculosis, especially in the endemic areas. A chest radiograph is recommended in every case of a swelling as it can render the rare causes and expensive management unnecessary. A sound clinical and diagnostic approach is necessary for getting the right diagnosis.