16th January 2024

Silicosis

Silicosis is caused by inhalation of crystalline silica (silicon dioxide), the most abundant mineral on earth. The main clinical subtypes of silicosis are acute, accelerated, and chronic silicosis. 

Pathophysiology

Following inhalation, silica particles deposit in the airways. This produces macrophage activation, which exerts toxic effects on the lung parenchyma, provoking an inflammatory cytokine response. This process promotes fibrosis of the lung and formation of upper lobe silicotic nodules. 

Silicosis can be further categorised based on radiological findings- simple silicosis and progressive massive fibrosis (PMF). Simple silicosis is characterised by rounded nodules, less than 10mm in size, found predominantly in upper lung zones. In PMF, these nodules coalesce, causing upper lobe lung parenchyma, which causes fibrotic retraction of upper lobes, which contributes to air trapping and emphysema in the lower lobes. 

Presentation

Broad symptoms of chronic and accelerated silicosis include cough and dyspnoea, and examination may reveal a range of findings, including wheeze and crackles. In cases of PMF, respiratory impairment and symptoms are more severe. PMF is not associated with clubbing. 

Unfortunately, once silicosis occurs, the damage is irreversible, and despite removal of the causative agent, it may continue to progress. Inhalation of silica is linked to other diseases such as lung cancer, tuberculosis, connective tissue diseases and renal diseases.

Chronic silicosis features

• Patients are initially asymptomatic until an interval of 10-20 post-exposure.
• Normally associated with simple radiological pattern

Acute silicosis features (silicoproteinosis)

• This occurs following short duration of high-concentration exposure
• Presentation within one year of exposure
• Symptoms include non-productive cough, dyspnoea, weight loss, fatigue, fever, and pleuritic pain.

Accelerated silicosis features

• Onset of symptoms following 5-10 years of exposure
• Accelerated progression of disease
• At higher risk of PMF

Diagnosis

Imaging – CXR/ HRCT

• Simple silicosis features
  • Well-defined upper zone opacities less than 10mm in size
• Progressive Massive Fibrosis features
  • Larger, coalescent nodules
  • Upper lobe fibrosis
  • Hyperinflation of lower lobes 
  • Hilar lymphadenopathy with eggshell calcification

Bronchoalveolar lavage 

• This is used to exclude other differentials
• Opaque effluent consistency
• Foamy macrophages and periodic acid-Schiff (pAS) staining