Silicotuberculosis: a critical narrative review

Abstract

Silicotuberculosis, the combination of silicosis and pulmonary tuberculosis (TB), remains a substantial clinical and public health problem in high TB burden countries with silica-exposed workforces. The objectives of this narrative review are to propose a definition of silicotuberculosis which includes post-tuberculous lung disease, to emphasise the importance of understanding how the two diseases modify each other, and to identify as yet unanswered questions relevant to clinical practice and disease control and mitigation. The unique aetiological relationship between silica exposure and TB is now firmly established, as is the accelerated impairment and mortality imposed by TB on individuals with silicosis. However, the rich clinical, pathology and laboratory literature on combined disease from the pre-TB treatment era appears to have been largely forgotten. The close clinical and pathological appearance of the two diseases continues to pose a challenge to imaging, diagnosis and pathological description, while inconsistent evidence regarding TB treatment and TB preventive treatment prevails. Many other topics raise questions to be answered, inter alia: the range of phenotypes of combined disease; the rates and determinants of disease progression; the role of computed tomography in identifying and characterising combined disease; appropriate screening practice; acceptable policies of management of workers that combine risk reduction with social security; and the workplace respirable silica concentration that protects against the excess TB attributable to inhaled silica.

FIGURE 1

Serial chest radiographs demonstrating co-occurring silicosis and tuberculosis (TB) in a silica-exposed miner. History: former gold miner, diagnosed silicosis in 2000, culture-positive Mycobacterium tuberculosis and M. kansasii in 2005. a) Chest radiograph dated 16 October 2012, on repeat TB treatment (empirical). b) Chest radiograph dated 26 February 2013, post-TB treatment. The left upper zone opacification seen in a) (red arrows) is attributed to active TB infection with adjacent left apical pleural reaction. Following completion of treatment, this has resolved in b), leaving a residual thin-walled cavity (black arrows) as evidenced by a lucent region which has replaced the region of opacification. The dense nodular infiltrate noted in a) is unchanged in b) and mainly composed of small nodules (<10 mm; International Labour Organization (ILO) classification: r/q 3/2). Some large opacities (>10 mm) are noted in the periphery of the right upper zone (black circle) in b). These are consistent with silicosis (ILO classification: A) but post-TB fibrosis cannot be excluded.

FIGURE 2

Chest computed tomography post-tuberculosis treatment dated 15 October 2013 (same patient as in figure 1). Selected axial images at the a) mid-thoracic tracheal level and b) tracheal carina show residual thin-walled cavities bilaterally, the left larger and visible on the chest radiographs. The background numerous small (<10 mm) nodular opacities are clustered and predominantly posterior (black ovals) and typical in location for silicotic nodules. Additionally there are multiple nodules studded along the pleural surface, also a typical location for silicotic nodules which are mainly perilymphatic in contrast to TB nodules which are bronchocentric. The larger (>10 mm) opacities are indicated by red arrows, image b) showing the peripheral para-cicatricial emphysema indicated by white arrows in image a) which is also a typical finding in patients with progressive massive fibrosis. b) Reproduced from [33] with permission.

FIGURE 3

Histology images of silicosis and/or tuberculosis (TB) in lung tissue from silica-exposed ex-miners. Array of pulmonary lesions from two former gold miners who had autopsy examination of their lungs to determine the presence of occupational disease for compensation purposes: a, d) 60 years of age, 15 years of gold mining, with multiple nodules in both upper lobes; and b, c) 68 years of age, 17 years of gold mining, with bilateral upper lobe lesions. a) Classical rounded, well-circumscribed silicotic nodule, 5 mm in diameter, characterised by acellular collagen fibres. b) Active tuberculous granuloma with central necrosis and a marginal zone of inflammatory cells including multinucleate Langhans giant cells; Ziehl–Neelsen stain positive for numerous acid-fast bacteria (not shown). c) Inactive TB in which the necrotic zone is surrounded by an acellular rim of collagen; Ziehl–Neelsen stain negative. d) Nodule randomly sampled from the upper lobe; central necrosis with a margin of collagenous fibrosis; acid-fast bacilli absent; consistent with silicosis with ischaemic necrosis or with inactive TB. Haematoxylin/eosin stain. Scale bars: 400 μm. Courtesy of the National Institute for Occupational Health, National Health Laboratory Service, Johannesburg, South Africa.