Dust‐related diffuse fibrosis in a coal mine worker from New South Wales

Clinical record

A 77‐year‐old never‐smoker presented with an abnormal screening chest x‐ray. He was still working, having been employed in coal mining for more than 40 years. He had spent more than 30 years working on the coal face and had also worked on roof bolting, in mine development and on ventilation support. Recently, he had worked as a coal mines inspector, a job for which he had been chosen because of his broad experience. He had undergone periodic medical examinations during all his employment without a previously abnormal result. The patient’s last chest x‐ray had been 4 years before his diagnosis. Although results had been reported as normal at the time, review of this image had later shown early lower zone interstitial changes (International Labour Organization category 0/1), and the recent chest x‐ray showed category 1/1 changes (subtle abnormalities).

He reported mild exertional dyspnoea on climbing hills over the past 2 years and occasionally coughed up phlegm. He had no other respiratory history or medical history. Clinical examination showed an oxygen saturation of 97% on room air, and bilateral lower zone scanty late inspiratory crackles, but was otherwise normal.

The patient’s spirometry had also been measured 4 years previously for his periodic examination and had been believed to be at the lower limits of normal. He underwent full lung function testing, which is more sensitive for early respiratory disease, and typically shows a loss of diffusion capacity of the lungs for carbon monoxide and fall in total lung capacity before spirometric abnormality is detected. Full lung function showed a restrictive ventilatory defect (forced expiratory volume in 1 second/forced vital capacity, 1.81/2.31 L (55/54% predicted) and a total lung capacity of 3.48 L (55% predicted). Diffusion capacity of the lungs for carbon monoxide was reduced at 53% predicted.

A computed tomography scan revealed interstitial pulmonary fibrosis in a usual interstitial pneumonia pattern (Box 1 and Box 2), with scanty small intraparenchymal nodules in both upper lobes. Auto‐antibody test results were all negative. He was diagnosed with dust‐related diffuse fibrosis.

Discussion

Dust‐related diffuse fibrosis (DDF) is an interstitial lung disease which occurs in dust‐exposed workers. It is probably under‐recognised in Australia, although good prevalence data are lacking.1 DDF represents a significant proportion of coal mine lung dust diseases and is classified as a type of pneumoconiosis, or pulmonary fibrosis secondary to dust inhalation (pneumon = lung; konis = dust [Greek]).2 However, the clinical picture differs from classical coal workers’ pneumoconiosis and clinicians therefore may not make the link with dust exposure, leading to a potential misdiagnosis of idiopathic pulmonary fibrosis (IPF).

The existence of lower zone interstitial pulmonary fibrosis is very well documented in coal mine workers,2,3,4 occurring in autopsy studies in up to 16% of miners, where it can occur alongside classical coal workers’ pneumoconiosis.3,4,5 In Australia, a recent review of coal mine lung disorders in Queensland confirmed DDF in 6%.1,6 A study of miners in the United States Coal Workers’ Health Surveillance Program found that 38% of coal miners had radiographic findings of interstitial lung disease, with irregular opacities of mainly lower zone predominance (upper, 21%; middle, 38%; lower, 41%).5

In contrast to classical coal workers’ pneumoconiosis, DDF has radiographic appearances of irregular (rather than rounded) opacities, predominantly in the lower zones. Clinical examination reveals lower zone late inspiratory crackles, unlike in classical coal workers’ pneumoconiosis, and clubbing is unusual. Radiologically, the classical nodular changes of coal workers’ pneumoconiosis do not always occur and are not required for making the diagnosis.2 Unless a thorough occupational history is obtained going back many years, DDF can easily be missed. Misdiagnosis can also occur because the average age of presentation is in workers aged in their 50s or 60s, similar to the age of presentation for IPF, and at a time when the patient may no longer be working. However, IPF implies the absence of a relevant occupational exposure and can only be diagnosed after exclusion of known causes of pulmonary fibrosis.7 Pulmonary function testing reveals a low diffusion capacity and a restrictive impairment identical to IPF, or a mixed pattern identical to combined pulmonary fibrosis and emphysema.

Pathological findings have been described for many years, and include interlobular septal thickening, often accompanied by emphysema, with numerous silica or silicate particles in the fibrotic septa and intra‐alveolar macrophages containing silica or silicate particles.2,3,4,8 Honeycombing may occur. Surprisingly, diffuse black pigmentation is not always present, with up to half of cases failing to show any pigmentation.2,3,5 Polarised light microscopy is needed to identify silica or silicate particles in the lungs, and then demonstrate alveolar macrophages laden with anthracotic pigment and birefringent silica or silicate particles. The histological precursor to the development of coal mine DDF is probably desquamative chronic interstitial pneumonia.8 The contribution of smoking compared with coal mine dusts is now believed to be negligible.2,5,8

DDF is more frequent in coal miners with many years of dust exposure and in older workers.2,3,4,5 Its prognosis appears to be better than that of IPF, but is yet poorly described.3 The efficacy of modern treatments including antifibrotic treatments (eg, nintedanib, perfenidone) has not yet been studied, but a new treatment trial using nintedanib (https://clinicaltrials.gov/ct2/show/NCT04161014) has recently commenced in New South Wales.

We present this case in order to highlight DDF as an under‐recognised disorder and to point out the ease with which it can be mistaken for other interstitial lung disorders. Enhanced respiratory surveillance using computed tomography and complex lung function testing, as recommended by the Thoracic Society of Australia and New Zealand, is likely to discover many more such cases.9 We encourage clinicians to take a careful occupational history such as that recommended by the US Centers for Disease Control and Prevention (https://www.atsdr.cdc.gov/csem/exphistory/docs/CSEMExposHist‐26‐29.pdf) and to remember that a history of coal mine or other dust exposure, even from many years before, may have considerable implications both for diagnosis and treatment in such patients.

Box 1 – Axial computed tomography scan through lung apices showing reticular opacity in the subpleural and peribronchovascular lung (arrows)

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Box 2 – Prone 1 mm high resolution computed tomography scan of the lungs showing peripheral lung fibrosis: there is subtle peripheral opacity, including a reticular component and a subpleural line

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