Silicosis


Silicosis is an occupational lung disease caused by the inhalation of respirable crystalline silica dust. It is characterized by lung inflammation and fibrosis that most commonly affects the upper lobes and is classified as a form of pneumoconiosis. The disease occurs in chronic, accelerated, or acute forms, depending on the intensity and duration of exposure.
Common symptoms include shortness of breath, cough, fatigue, and cyanosis in severe cases. Because its clinical and radiographic features can resemble those of tuberculosis, pneumonia, or pulmonary edema, silicosis is sometimes misdiagnosed. There is no curative treatment; management focuses on symptom control and the prevention of complications.
Silicosis is largely preventable through effective workplace controls that limit airborne silica exposure, such as engineering controls, ventilation, and appropriate respiratory protection.

Signs and symptoms

Because chronic silicosis is slow to develop, signs and symptoms may not appear until years after exposure. Signs and symptoms include:
  • Dyspnea exacerbated by exertion
  • Cough, often persistent and sometimes severe
  • Fatigue
  • Tachypnea which is often labored
  • Loss of appetite and weight loss
  • Chest pain
  • Fever
  • Gradual darkening of skin
  • Gradual dark shallow rifts in nails eventually leading to cracks as protein fibers within nail beds are destroyed
In advanced cases, the following may also occur:
Patients with silicosis are particularly susceptible to tuberculosis infection—known as silicotuberculosis. The reason for the increased risk—3 fold increased incidence—is not well understood. It is thought that silica damages pulmonary macrophages, inhibiting their ability to kill mycobacteria. Even workers with prolonged silica exposure, but without silicosis, are at a similarly increased risk for TB.
Pulmonary complications of silicosis also include chronic bronchitis and airflow limitation, non-tuberculous Mycobacterium infection, fungal lung infection, compensatory emphysema, and pneumothorax. There are some data revealing an association between silicosis and certain autoimmune diseases, including nephritis, scleroderma, and systemic lupus erythematosus, especially in acute or accelerated silicosis.
In 1996, the International Agency for Research on Cancer reviewed the medical data and classified crystalline silica as "carcinogenic to humans." The risk was best seen in cases with underlying silicosis, with relative risks for lung cancer of 2–4. Numerous subsequent studies have been published confirming this risk. In 2006, Pelucchi et al. concluded, "The silicosis-cancer association is now established, in agreement with other studies and meta-analysis."

Pathophysiology

When small silica dust particles are inhaled, they can embed themselves deeply into the tiny alveolar sacs and ducts in the lungs, where oxygen and carbon dioxide gases are exchanged. There, the lungs cannot clear out the dust by mucus or coughing.
When fine particles of crystalline silica dust are deposited in the lungs, macrophages that ingest the dust particles will set off an inflammatory response by releasing tumor necrosis factors, interleukin-1, leukotriene B4 and other cytokines. In turn, these stimulate fibroblasts to proliferate and produce collagen around the silica particle, thus resulting in fibrosis and the formation of the nodular lesions. The inflammatory effects of crystalline silica are apparently mediated by the NLRP3 inflammasome.
Characteristic lung tissue pathology in nodular silicosis consists of fibrotic nodules with concentric "onion-skinned" arrangement of collagen fibers, central hyalinization, and a cellular peripheral zone, with lightly birefringent particles seen under polarized light. The silicotic nodule represents a specific tissue response to crystalline silica. In acute silicosis, microscopic pathology shows a periodic acid-Schiff positive alveolar exudate and a cellular infiltrate of the alveolar walls.

Silica

is the second most common element in the Earth's crust after oxygen. The compound silica, also known as silicon dioxide, is formed from silicon and oxygen atoms. Since oxygen and silicon make up about 75% of the Earth's crust, the compound silica is quite common. It is found in many rocks, such as granite, sandstone, gneiss and slate, and in some metallic ores. Silica can be a main component of sand. It can also be in soil, mortar, plaster, and shingles. The cutting, breaking, crushing, drilling, grinding, or abrasive blasting of these materials may produce fine to ultra fine airborne silica dust.
Silica occurs in three forms: crystalline, microcrystalline and amorphous. "Free" silica is composed of pure silicon dioxide, not combined with other elements, whereas silicates are SiO2 combined with an appreciable portion of cations.
  • Crystalline silica exists in seven different forms, depending upon the temperature of formation. The main three polymorphs are quartz, cristobalite, and tridymite. Quartz is the second most common mineral in the world.
  • Microcrystalline silica consists of minute quartz crystals bonded together with amorphous silica. Examples include flint and chert.
  • Amorphous silica consists of kieselgur, from the skeletons of diatoms, and vitreous silica, produced by heating and then rapid cooling of crystalline silica. Amorphous silica is less toxic than crystalline, but not biologically inert, and diatomite, when heated, can convert to tridymite or cristobalite.
Silica flour is nearly pure SiO2 finely ground. Silica flour has been used as a polisher or buffer, as well as paint extender, abrasive, and filler for cosmetics. Silica flour has been associated with all types of silicosis, including acute silicosis.
Silicosis is due to deposition of fine respirable dust containing crystalline silicon dioxide in the form of alpha-quartz, cristobalite, or tridymite.

Diagnosis

There are three key elements to the diagnosis of silicosis. First, the patient history should reveal exposure to sufficient silica dust to cause this illness. Second, chest imaging that reveals findings consistent with silicosis. Third, there are no underlying illnesses that are more likely to be causing the abnormalities. Physical examination is usually unremarkable unless there is complicated disease. The examination findings are not specific for silicosis.
Pulmonary function testing may reveal airflow limitation, restrictive defects, reduced diffusion capacity, mixed defects, or may be normal, especially without complicated disease. Most cases of silicosis do not require tissue biopsy for diagnosis, but this may be necessary in some cases, primarily to exclude other conditions. Assessment of alveolar crystal burden in bronchoalveolar lavage fluid may aid diagnosis.
For uncomplicated silicosis, chest x-ray will confirm the presence of small nodules in the lungs, especially in the upper lung zones. Using the ILO classification system, these are of profusion 1/0 or greater and shape/size "p", "q", or "r". Lung zone involvement and profusion increases with disease progression. In advanced cases of silicosis, large opacity occurs from coalescence of small opacities, particularly in the upper lung zones.
With retraction of the lung tissue, there is compensatory emphysema. Enlargement of the hilum is common with chronic and accelerated silicosis. In about 5–10% of cases, the nodes will calcify circumferentially, producing so-called "eggshell" calcification. This finding is not pathognomonic of silicosis. In some cases, the pulmonary nodules may also become calcified.
A computed tomography or CT scan can also provide a mode detailed analysis of the lungs, and can reveal cavitation due to concomitant mycobacterial infection.

Classification

Classification of silicosis is made according to the disease's severity, onset, and rapidity of progression. These include:
;Chronic simple silicosis: Usually resulting from long-term exposure to relatively low concentrations of silica dust and usually appearing 10–30 years after first exposure. This is the most common type of silicosis. Patients with this type of silicosis, especially early on, may not have obvious signs or symptoms of disease, but abnormalities may be detected by x-ray. Chronic cough and exertional dyspnea are common findings. Radiographically, chronic simple silicosis reveals a profusion of small opacities, typically rounded, and predominating in the upper lung zones.
;Accelerated silicosis: Silicosis that develops 5–10 years after first exposure to higher concentrations of silica dust. Symptoms and x-ray findings are similar to chronic simple silicosis, but occur earlier and tend to progress more rapidly. Patients with accelerated silicosis are at greater risk for complicated disease, including progressive massive fibrosis.
;Complicated silicosis: Silicosis can become "complicated" by the development of severe scarring, where the small nodules gradually become confluent, reaching a size of 1 cm or greater. PMF is associated with more severe symptoms and respiratory impairment than simple disease. Silicosis can also be complicated by other lung disease, such as tuberculosis, non-tuberculous mycobacterial infection, and fungal infection, certain autoimmune diseases, and lung cancer. Complicated silicosis is more common with accelerated silicosis than with the chronic variety.
;Acute silicosis: Silicosis that develops a few weeks to 5 years after exposure to high concentrations of respirable silica dust. This is also known as silicoproteinosis. Symptoms of acute silicosis include more rapid onset of severe disabling shortness of breath, cough, weakness, and weight loss, often leading to death. The x-ray usually reveals a diffuse alveolar filling with air bronchograms, described as a ground-glass appearance, and similar to pneumonia, pulmonary edema, alveolar hemorrhage, and alveolar cell lung cancer.