The interstitium of the lung is the space between the capillaries and the alveoli, which supports the exchange of gases between the air we breathe and our blood supply, the process which oxygenates our blood and removes carbon dioxide as a waste product. In a healthy interstitium, there is minimal tissue, but in the setting of interstitial lung disease, excess interstitium replaces the alveoli and capillaries and normal interstitium (pulmonary fibrosis), and results in a repertoire of symptoms including dyspnoea (exertional breathlessness), a non-productive cough, and dependent upon the classification of the ILD possibly sputum production, haemoptysis (coughed blood) and wheezing.
Interstitial lung disease is a very broad term which is used to characterise more than one hundred separate disease entities. What separates the different diseases is whether they have a known or unknown (idiopathic) cause, whether the cause is genetic, due to another systemic disease process, or caused by exposure to an agent such as asbestos. ILDs are further characterised by treatment and prognoses.
Once the lung has been damaged it must repair itself. Or if it is being damaged due to exposure to an agent, the exposure must cease. If exposure does not cease, and/or the repair process is imperfect, this may result in permanent lung damage. Progressive tissue damage can lead to worsening physiologic impairment and even death.
The diagnostic possibilities can be overwhelming, so investigations take place to determine the specific disease entity, and how best to treat it.
After a patient presents with symptoms a radiographic scan will be ordered, preferrably a CT high resolution computed tomography of the thorax. In most interstitial lung disease, a chest xray reveals reduced lung volumes with bilateral reticular or reticulonodular opacities. There is considerable variability among the specific diseases in the character and distribution of radiographic abnormalities. In contrast to the fibrotic type of injury described above, some diseases cause an inflammatory abnormality with a much different radiographic image.
Lung function testing (spirometry) is performed, and it is found that lung volume is decreased (restrictive), resulting in inceased work in breathing and the diffusing capacity for carbon monoxide is decreased, which reflects a pathologic disturbance of the alveolar-capillary interface. Spirometry testing uses carbon monoxide tests for obstructive or restrictive lung abnormalities.
Asbestos Exposure
Radiographically, evidence of asbestos exposure presents with pleural plaques, fibrosis, effusions, rounded atelectasis, mesothelioma, parenchymal scarring and lung cancer, although the absence of these characteristics radiographically does not mean the patient has not been exposed to asbestos which has caused their interstitial lung disease. It is the oral history of exposure that is significant in concluding a diagnosis and the basis for a compensation claim.
Asbestos related pulmonary disease is another variety of ILD, in contrast to asbestosis which shows radiographic evidence of parenchymal fibrosis.
The symptoms of asbestos related pulmonary disease are a very slow progressive dyspnoea (breathlessness) on exertion, crackles on lung examination by a physician and restrictive impairment, a lowered carbon monoxide diffusing capacity on spirometry, and a chest xray may show bilateral lower-zone reticulonodular infiltrates. Due to the lag period of disease presentation and exposure to asbestos, many patients develop a disease 30-40 years after exposure, which renders them ineligible for transplantation due to their advanced age.
Silicosis
Inhaled silica particles can cause silicosis. Radiographically, this presents with upper lung zone predominant abnormalities characterised by multiple small nodular opacities in the central lung tissue, which can develop into large masses called progressive massive fibrosis. There may also be enlargement and eggshell calcification of hilar lymph nodes. There will be mixed restrictive and obstructive impairments and a decreased diffusing capacity. Symptoms will be exertional dyspnoea and variable mucous production. There is no proven therapy.
Medications / Drugs / Radiation Therapy
There are no distinct physiologic, radiographic nor pathologic patterns. Drugs from different therapeutic classes can cause ILD.
Hypersentsitivity Pneumonitis
Subtle interstitial inflammatory reactions over months or years, or acutely sudden shortness of breath, chest pain, fever, chills, malaise, cough (purulent [pus]/productive). For example, heated exposy resin can cause epoxy resin lung.
Tobacco Exposure
Tobacco is a risk factor for development of ILD.
Systemic Disease
For example, connective tissue disease. Systemic disease has to be excluded as a cause of interstitial lung disease for compensation claims such as asbestos exposure.