India is the tuberculosis (TB) capital of the world, and therefore, the disease is a considerable public health problem ^[1]. Several reasons are attributable to this: Dense population, extensive immigration, and malnutrition in the lower socioeconomic class ^[2]. Osteoarticular TB (OATB) is a 15% burden of all forms of extra-pulmonary TB (EPTB) ^[3]. Primary TB of a rib is a very uncommon presentation and accounts for <1% of OATB ^[4]. TB of rib is seen in both immunocompetent and immunocompromised individuals ^[5]. As its diagnosis is difficult, especially in its early stages, a high index of suspicion is mandatory, along with imaging and other relevant studies to differentiate it from other diagnoses. Hence, we report an uncommon case of TB of the rib in an immunocompetent individual, its presentation, diagnosis, and treatment.

A 34-year-old male laborer by profession (belonging to lower socioeconomic status) presented with chief complaints of dull-aching pain associated with a swelling over the right-side lower chest wall, low-grade evening fever, and unintentional weight loss for 2 months. He denied any other joint pain or any history of trauma. His past history was insignificant, though his father had pulmonary TB 20 years back, for which he had taken anti-tubercular therapy (ATT) and was relieved. Upon physical examination, there was no lymphadenopathy or abnormality in the spine or other joints. However, there was tender swelling of 5 × 3 cm over the lateral side of the right lower chest wall, which was soft, non-compressible, and did not trigger a cough impulse (Fig. 1a and b). His complete blood count was within normal limits, but the erythrocyte sedimentation rate (ESR) was 90 mm/h, and C-reactive protein was reactive. Liver and renal function tests were normal, and he was negative for HIV. Radiological investigations such as X-ray chest were normal (Fig. 1c and d). A magnetic resonance imaging (MRI) was ordered given soft-tissue swelling, which showed a lytic lesion in the right 9^th rib with a superficial cold abscess that was peripherally enhancing with thin walls. The spine, lung, and pleura were normal (Fig. 2a, b, c, d, e, f). An ultrasound (USG)-guided aspiration was done and was sent for routine microscopy, culture sensitivity, and geneXpert. The results showed positivity for Mycobacterium tuberculosis, which was rifampicin sensitive.

The patient was advised 1 year of ATT as per index TB guidelines for OATB. He started to show improvement. By 2 weeks, his fever had subsided, and his appetite improved; he gradually regained weight. The pain disappeared, and the cold abscess diminished in size progressively and disappeared within 3 months without sinuses, scars, or the need for surgery. The patient was then followed up regularly and was symptom-free at 12 months on completion of ATT. (Fig. 3).

OATB is the most common form of EPTB, accounting for 10–15% of all the TB burden in developing countries, unlike the Western world, where it accounts for only 1–2% of cases ^[3]. Among OATB, the vertebral column encompasses 50%, followed by the hip (15%) and the knees (5%). However, the involvement of ribs in TB is relatively infrequent. TB localization in the thoracic cage is rare [6, 7]. Among the 980 patients suffering from OATB, 19 (2%) had lesions in the ribs and 14 (1.5%) in the sternum ^[8]. Nearly one-third of these patients had detectable TB lesions in other parts of the skeleton or the lungs ^[8]. TB is also considered the second most common cause of rib destruction after metastasis. TB rib usually forms a contagious spread from nearby pleuro-pulmonary foci or sometimes hematogenous from a distant focus ^[7]. About 50% of patients with rib TB have associated pulmonary presentation, but this was not seen in our patients ^[9]. TB destroys bone, producing caseous necrosis by granulation tissue, which may be pathognomonic ^[7]. The cold abscess can track along the chest wall to form a visible swelling and further can progress to form a sinus tract. However, overall, the symptoms are slow, indicated by local pain, unexplained low-grade fever, and reduced range of movement, especially if extensive involvement of the thoracic cage can herald even systemic complications. This makes the diagnosis delayed, complex, and dependent on radiological and microbiological/pathological investigations. Positive X-ray findings appear much later than usual clinical presentation, like swelling (due to cold abscess) and sinuses. The involved rib may show expansion with punched-out erosions, sometimes thickening, and rarely, there may be sequestrum formation [9, 10]. Possible differentials of a lytic lesion in the rib are primary malignancy, TB (second most common), primary and secondary bone tumors, multiple myeloma, lymphoma, granulomatous diseases such as sarcoidosis, eosinophilic granulomas, and infections with fungi or amoeba [11-13]. In our patient, the contact history, socioeconomic status, constitutional symptoms, presence of a cold abscess, very high ESR, and positive geneXpert favored TB. M. tuberculosis geneXpert test has become an invariable and speedy tool if the biopsy is unsuccessful or unavailable ^[14]. In most cases, X-rays are normal in the early stages of the disease, as in our patients. Therefore, a high degree of suspicion is warranted. Computed tomography is sensitive and enables diagnosis and management planning ^[15]. Further MRI is helpful in soft-tissue extension. The classical TB cold abscesses had low-attenuated centers with a rim-enhancing periphery. Newer tests for the diagnosis of EPTB are also applicable to rib TB, which include the T-SPOT^™ (Oxford Immunotec, Abingdon, Oxford, UK) TB test, a T cell-based assay, and a quantitative polymerase chain reaction of formalin-fixed paraffin-embedded skeletal specimens, and 18-fluorodeoxyglucose positron emission tomography imaging [16-18]. An MRI was done given soft-tissue swelling in our patient, and it was diagnosed as rib tubercular cold abscess. It was confirmed by aspiration and treated. USG-guided aspiration can be a valuable technique for diagnosing and starting ATT ^[15]. TB rib is essentially a medicinal disease. Singh et al. and similarly Nakiyingi et al. reported case of rib TB involving who had excellent response to medical therapy similar to our patient [5, 7]. There was a need for more clarity regarding the duration of treatment, but recent recommendations from the index TB guidelines clarify that treatment for all forms of OATB should be one full year [19, 20]. Surgical treatment may be rarely justified for doubtful diagnosis, a non-responsive case, or removal of large sequestrum, as in our patient [10, 12, 20].

Isolated TB rib is rare. Hence, early diagnosis and ATT will improve outcomes and prevent complications and recurrences. This case report suggests that TB is to be considered a differential even for atypical skeletal presentations. As TB rib cases are so few, randomized studies are impossible. Hence, we report this case as it can be applied to a more extensive study and will help better manage this rare condition of a common disease.

Isolated TB rib is rare and therefore it requires a high degree of suspicion, timely diagnosis after ruling out differentials and proper medical treatment with occasional surgical intervention.