A hard metal pneumoconiosis is usually classified as a type of fibrotic pneumoconiosis where the precipitating agent consists of a fine particulate form of hard metal such as:
- cobalt/cobalt-tungsten alloys 10
- tungsten/tungsten carbide alloys
- implicated alloys often contain small amounts of other metals 10:
A broader term used is hard metal lung disease (HMLD) which also takes into account the non-fibrotic stages/forms such as bronchitis/obliterative bronchitis caused by hard metal lung disease.
Hard metals are widely used for industrial purposes which require extreme hardness and high-temperature resistances, such as for cutting tools, oil well drilling, and jet engine exhaust ports 2. Hard metal utilization plants can contain enclosed chamber and with each application, large volumes of fine aerosols can be created 3.
Typical at-risk occupations include manufacturers and sharpeners of tools, machine operators (e.g. grinders and lathes), diamond polishers (the polishing discs employ hard metals).
The vast majority of individuals in these industries do not experience the consequent lung disease, supporting the hypothesis that immunogenetic factors are important 10.
Clinical presentation can be similar to that of hypersensitivity pneumonitis, with some patients having episodes of work-related subacute disease and some patients evolving, more or less rapidly, to lung fibrosis 6.
The pathogenesis of this condition remains unclear, however a leading hypothesis is that the metal dust (primarily the cobalt) elicits a hypersensitivity reaction in the lungs 10.
Histopathological manifestations of hard-metal disease can range from bronchitis to subacute fibrosing alveolitis to interstitial fibrosis 8. A combination of electron microscopy and energy-dispersive x-ray fluorescence spectrometry is required in suspected cases to identify the tiny particles of hard metals in the tissue samples. Tungsten particles are usually in high concentration; conversely cobalt is often low concentration only, due to the easier solubility of the cobalt in body fluids and its more rapid clearance in the tissues 10.
Some authors outlined a set of diagnostic criteria which includes 8:
- history of exposure to metal dust
- characteristic clinical features, including shortness of breath, cough, and dyspnea on exertion over a prolonged period
- radiologic findings of interstitial lung disease
- histologic findings of interstitial lung disease or a giant cell interstitial pneumonia pattern (a large number of giant cells filling airspaces), with thickening of the interstitium and alveolar walls by mononuclear cells
- histopathologic finding of metallic content in lung tissue
Plain chest radiographic features are non-specific. A chest radiograph may be normal or show a nodular, reticulonodular, or reticular pattern 7.
An exposure history of hard metal is essential in image interpretation. Described features are non-specific on their own and a dependent on the stage. These include 5:
- reticulation 5,7
- traction bronchiectasis 5,8,9
- large peripheral cystic spaces in a mid and upper lung distribution 5
- patchy lobular bilateral ground-glass opacities 7-9
- areas of consolidation 7-9
- centrilobular nodularity 7
- honeycombing 7 (occasionally)
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- 9. Kim KI, Kim CW, Lee MK et-al. Imaging of occupational lung disease. Radiographics. 2001;21 (6): 1371-91. Radiographics (full text) - Pubmed citation
- 10. Mizutani RF, Terra-Filho M, Lima E, Freitas CS, Chate RC, Kairalla RA, Carvalho-Oliveira R, Santos UP. Hard metal lung disease: a case series. (2016) Jornal brasileiro de pneumologia : publicacao oficial da Sociedade Brasileira de Pneumologia e Tisilogia. 42 (6): 447-452. doi:10.1590/S1806-37562016000000260 - Pubmed