Silicosis

Overview

Silicosis is caused by inhalation of respirable crystalline silica, leading to progressive pulmonary fibrosis. It is the most prevalent occupational lung disease worldwide and remains relevant in the UK - particularly in stone masonry, quarrying, and engineered stone worktop manufacturing.

Classification

Silicosis subtypes

Feature	Simple (chronic)	Accelerated	Acute (silicoproteinosis)
Exposure intensity	Low-moderate, >10 years	Higher, 5-10 years	Very high, <5 years
Onset	Slow, often asymptomatic	Faster progression	Rapid - months
Prognosis	Stable if exposure ceases	Progressive	Fatal within 2-5 years

💡

Simple silicosis can progress - and even develop - after exposure ceases. Always ask about past occupational history, not just current work.

Presentation

•Simple silicosis - often asymptomatic for years; found incidentally on CXR

•Progressive disease - dyspnoea on exertion, dry cough, reduced exercise tolerance

•Advanced disease - fine inspiratory crackles (upper zones), finger clubbing (especially PMF), cor pulmonale (raised JVP, peripheral oedema)

•Acute silicosis - rapid breathlessness, hypoxaemia, resembles pulmonary alveolar proteinosis; very poor prognosis

Investigations

•First-line - CXR: small round upper-zone nodules (1-10 mm); eggshell calcification of hilar lymph nodes is pathognomonic; PMF = large conglomerate upper-zone opacities >1 cm

•First-line - spirometry: restrictive pattern (reduced FVC, normal/raised FEV1/FVC, reduced TLC); reduced TLCO; mixed pattern can occur

•First-line - TB screening: IGRA or Mantoux in all patients (markedly elevated TB risk)

•First-line - autoimmune screen: ANA, rheumatoid factor (association with RA, systemic sclerosis)

•Gold standard - HRCT chest: confirms nodular pattern, characterises PMF, upper-lobe predominance; distinguishes from other ILD

🎯

Upper-zone nodular shadowing + eggshell calcification of hilar lymph nodes + appropriate occupational history = silicosis until proven otherwise. Eggshell calcification can rarely occur in sarcoidosis but is otherwise virtually pathognomonic.

Differential diagnosis

•Pulmonary TB - upper-zone, cavitation; coexistence common (silicotuberculosis)

•Sarcoidosis - bilateral hilar lymphadenopathy, upper-zone nodules; serum ACE may help

•Coal worker's pneumoconiosis - also upper-zone nodules and PMF; distinguished by exposure (coal vs silica)

•IPF - lower-zone predominant, honeycombing on HRCT

Management

•First-line - remove from exposure: halting silica inhalation is the single most important intervention; disease can progress even after cessation

•First-line - smoking cessation: offer varenicline, bupropion, or NRT plus behavioural support

•First-line - TB treatment: latent TB - isoniazid 6 months (or rifampicin-isoniazid per NICE NG33); active TB - standard four-drug therapy

•First-line - pulmonary rehabilitation: improves exercise tolerance and quality of life

•First-line - vaccination: annual influenza + pneumococcal

•Second-line - LTOT: if PaO2 <7.3 kPa at rest (or <8 kPa with complications); at least 15 hours/day

•Third-line - lung transplantation: end-stage disease, no contraindications

•Occupational/legal: prescribed disease under Industrial Injuries Disablement Benefit; reportable under RIDDOR 2013; refer to occupational health physician

Complications

•Progressive massive fibrosis (PMF) - conglomerate masses >1 cm; severe restrictive disease, rapid decline

•Silicotuberculosis - TB risk 2-4x higher than general population; silica impairs macrophage killing of Mycobacterium tuberculosis

•Lung cancer - independent IARC Group 1 carcinogen risk

•Pulmonary hypertension and cor pulmonale - from fibrosis and hypoxic vasoconstriction

•Autoimmune diseases - systemic sclerosis, RA, SLE, Caplan syndrome (rheumatoid nodules in pneumoconiosis)

🚨

Any patient with silicosis who develops fever, night sweats, haemoptysis, or accelerating weight loss must be urgently investigated for active tuberculosis. Silicotuberculosis can be rapidly fatal and is easily attributed to disease progression alone.

Prognosis

•Simple silicosis detected early with exposure ceased - can be stable for years

•PMF - poor prognosis; progressive decline even after complete cessation

•Acute silicosis - often fatal within 2-5 years of diagnosis

•Silicotuberculosis or lung cancer significantly worsens outcomes

Pathophysiology (key mechanism)

•Silica → macrophage phagocytosis → lysosomal rupture → macrophage death → pro-inflammatory cytokines (IL-1, TNF-alpha) → fibroblast recruitment → collagen deposition → hyalinised silicotic nodules

•Nodules coalesce (upper lobes) → progressive massive fibrosis (PMF) if >1 cm conglomerate masses

•Crystalline silica = IARC Group 1 carcinogen - independent lung cancer risk beyond smoking