"Bright asteroids in the polar sky"-clinic-radio-pathological correlation in an unusual case of silicotuberculosis.

We present a case of acute silicosis presenting with severe breathlessness and respiratory failure. An unusual aspect in our case was the presence of acute silicosis with respiratory failure in backdrop of long-term silica exposure. The other striking aspect in this case was the demonstration of crystalline silica particles under polarizing light in bronchial lavage fluid sample and coexistence of tuberculosis with acute silicosis.

INTRODUCTION

Silicosis is a potentially fatal, irreversible, fibrotic pulmonary disease that may develop subsequent to the inhalation of large amounts of silica dust over time. In most circumstances, silicosis only develops subsequent to substantial occupational exposures. The disease has a long latency period and may clinically present as an acute, accelerated, or chronic disease.

The rate of disease progression appears to depend upon the rate of silica deposition in the lungs, as well as the total amount of crystalline silica that is actually retained in the lung. In some cases, silicosis may be associated with other diseases including tuberculosis, cancer, or autoimmune disease.

We present an unusual case of silicotuberculosis, confirmed dramatically on the bronchial lavage that showed birefringent crystals under the polarizing microscope-similar to bright asteroids!

Currently, no cure or effective treatment is available for silicosis and hence, there are strict regulations aimed at preventing the development of silicosis in high risk workers due to occupational exposure.[12]

CASE REPORT

A 43-year-old male, working as a sand blaster for the past 13 years, presented with fever, cough, breathlessness, and right-sided pleuritic type of chest pain of a duration of 2 months. He had been evaluated in another hospital where he was diagnosed as sputum-negative pulmonary tuberculosis based on contrast-enhanced computerized tomogram (CECT) chest and bronchoscopy findings. The computerized tomogram (CT) done 3 months prior to presentation showed nonspecific patchy parenchymal opacities [Figure 1a]. He was started on antitubercular treatment (ATT). But his symptoms worsened and repeat CECT of the chest done a month later revealed features suggestive of nonresolving pneumonia. There were larger and newer parenchymal opacities, predominantly in the lung bases. His breathlessness had worsened since 2 days and on admission his saturation (SpO2) was 70% on room air and 90% with 5 L oxygen through Hudson's mask.

Figure 1

CT axial lung window showing scattered parenchymal opacities (black arrow a), wider parenchymal consolidations in the CT axial lung window (b, black arrow) and mediastinal window (c, black arrow), and CT-guided lung biopsy (needle pointed by black arrow d)

On examination, he was emaciated and had bilateral end-expiratory crepitations. His erythrocyte sedimentation rate (ESR) was 86 mm/h with leukocytosis. His two sputum samples were negative for acid-fast bacilli (AFB) on Ziehl-Neelsen (ZN) stain. High-resolution CT (HRCT) of the chest showed bilateral middle and lower lobe ground-glass opacities with left lower lobe consolidation [Figure 1b and c]. CT-guided biopsy of the left lower lobe lesion was performed [Figure 1d] to rule out the possibility of malignancy and histopathological examination showed features of interstitial pneumonia with fibrosis.

The papanicolaou (PAP) stain on bronchial lavage revealed benign squamous cells, pigment-laden macrophages, and extensive acute inflammatory cell infiltrate [Figure 2a and b]. The lavage fluid was positive for AFB as demonstrable by ZN stain. Detection of numerous birefringent silica particles under polarizing microscope in the bronchial lavage [Figure 2c and d], otherwise not appreciable on PAP stain, clinched the primary diagnosis of silicosis with associated tuberculosis.

Figure 2

(a) Benign squamous cells, pigment laden macrophages and inflammatory cells (Pap, X100) (b) Extensive neutrophilic inflammation (Pap, X100) (c and d) Silica particles appearing as granular birefringent crystals (X200)

Initiation of ATT was done as per the patient's body weight. However, his hypoxia persisted and he was discharged on domiciliary oxygen. Repeat chest radiograph at end of 1 month did not show any radiological improvement. His respiratory failure persisted and he was continued on the domiciliary oxygen.

DISCUSSION

Silicosis refers to a spectrum of pulmonary diseases caused by inhalation of free crystalline silica (silicon dioxide). Workers in a broad range of industries are exposed to crystalline silica such as mining, quarrying, sandblasting, masonry, foundry work, and ceramics. Sandblasting generates respirable aerosols of silica, and is associated with an especially increased risk for silicosis.[34]

Several clinical presentations of this disease have been described. Acute silicosis is associated with unique pathologic and radiologic features, and occurs within weeks to a few years after overwhelming exposure. Chronic silicosis develops slowly, usually appearing 10 to 30 years after the first exposure. It is not uncommon for silicosis to become radiographically apparent many years after cessation of exposure.[5] Chronic silicosis usually has the similar radiographic pattern described below as simple silicosis. In a minority of those with chronic disease, nodules coalesce resulting in progressive massive fibrosis (PMF). Accelerated silicosis, though it develops within 10 years of initial exposure and is associated with high-level exposure to silica, but has the same radiographic appearance as chronic silicosis, and their presentations are variable.[6]

Acute silicosis is characterized by rapid onset of symptoms including cough, weight loss, fatigue, and sometimes pleuritic pain. These symptoms may precede significant radiologic findings. Patients rapidly develop cyanosis, cor pulmonale, and respiratory failure. Survival after the onset of symptoms is typically less than 4 years, with mycobacterial and fungal infections frequently complicating the clinical course.[7] Acute silicosis presents radiographically with a characteristic basilar alveolar filling pattern, without rounded opacities or lymph node calcifications. In contrast, most individuals with extremely high silica exposures, initially display radiographic features identical to those of simple silicosis, which progress to PMF over a period of as few as 4-5 years. Consequently, PMF is associated with more severe symptoms than simple silicosis. The progressive coalescence of silicotic nodules leads to respiratory impairment, including air trapping and emphysema. The reason why some individuals develop acute silicosis, while others develop accelerated silicosis after overwhelming silica exposure, is not known; genetic factors may play a role.[89] Silica exposure, even in the absence of findings on conventional chest radiograph, is associated with excessive decline in spirometric performance.[10]

The International Agency for Research on Cancer (IARC) has determined sufficient evidence for carcinogenicity of crystalline silica[11] Since then, additional evidence for such relationships has accumulated, although, this relationship may not increase linearly to the highest levels of silica exposure.[12]

Silicosis is associated with an increased risk of lung cancer, mycobacterial infection, autoimmune disorders, airflow obstruction, and chronic bronchitis. Strong evidence for associations between silicosis and both tuberculous and nontuberculous mycobacterial infection has been provided by several studies of South African gold miners.[13] Patients with silicosis are at a very high risk of active pulmonary tuberculosis and should be screened for evidence of latent infection. Tuberculosis should always be suspected when a patient with silicosis develops constitutional symptoms, worsening respiratory impairment, or changes in the chest radiograph. Cavitation in a PMF lesion is a particularly significant or ominous finding. Exposure to silica causes a renewed multiplication of bacilli in the healing tubercular lesions, as silica-iron complexes may activate dormant tubercle bacilli. Their synergistic action may then result in massive fibrosis.[14]

In our case, we were able to demonstrate silica particles under polarizing microscope from bronchial lavage sample that were not detectable on the PAP stain. The earliest histopathologic changes identifiable in workers with chronic low-level exposure to free crystalline silica is loose reticulin fibers in the peribronchial, perivascular and paraseptal, or subpleural areas. Involvement of hilar lymph nodes and its fibrosis, impairs silica clearance and contributes to the development of parenchymal fibrosis. The central zone of a silicotic nodule is hyalinized with concentrically arranged collagen fibers. The peripheral zone is whorled. The outer zone contains macrophages, lymphocytes, and loosely-formed collagen and is the site of active enlargement of the nodule and ongoing inflammation.[15]

Silica particles show birefringent granular or irregular crystals under polarizing microscopy. The closest differential diagnosis for silicosis is asbestosis and talcosis. The most important feature differentiating asbestos-induced pulmonary fibrosis is the presence of parietal pleural thickening in association with lung fibrosis and the presence of asbestos bodies in bronchoalveolar lavage fluid. Asbestos fibers appear as elongated fibers while talc particles appear as needle-shaped crystals inside or outside the macrophages.[1617] A long standing occupational history of exposure to these minerals is generally present as well.

There is no proven specific therapy for any form of silicosis. Symptomatic therapy should include treatment of airflow limitation with bronchodilators, aggressive management of respiratory tract infection with antibiotics, and the use of supplemental oxygen to prevent complications of chronic hypoxemia. Lung transplantation should be considered for people with end-stage silicosis.

Silicosis is a preventable disease. Primary prevention through exposure control is the critical component of silicosis prevention, health monitoring of workers with exposure to respirable crystalline quartz using chest radiographs, and spirometry may assist in the early identification. Polarizing microscopy examination for crystals is compulsory in all patients with relevant occupational history.

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Conflicts of interest

There are no conflicts of interest.