Interstitial lung disease


Interstitial lung disease, or diffuse parenchymal lung disease, is a group of respiratory diseases affecting the interstitium and space around the alveoli of the lungs. It concerns alveolar epithelium, pulmonary capillary endothelium, basement membrane, and perivascular and perilymphatic tissues. It may occur when an injury to the lungs triggers an abnormal healing response. Ordinarily, the body generates just the right amount of tissue to repair damage, but in interstitial lung disease, the repair process is disrupted, and the tissue around the air sacs becomes scarred and thickened. This makes it more difficult for oxygen to pass into the bloodstream. The disease presents itself with the following symptoms: shortness of breath, nonproductive coughing, fatigue, and weight loss, which tend to develop slowly, over several months. While many forms are progressive and serious, some types of ILD remain mild or stable for extended periods, especially with early detection and appropriate treatment. The average rate of survival for someone with this disease is between three and five years. The term ILD is used to distinguish these diseases from obstructive airways diseases.
There are specific types in children, known as children's interstitial lung diseases. The acronym ChILD is sometimes used for this group of diseases. In children, the pathophysiology involves a genetic component, exposure-related injury, autoimmune dysregulation, or all of the components.
Thirty to 40% of those with interstitial lung disease eventually develop pulmonary fibrosis which has a median survival of 2.5-3.5 years. Idiopathic pulmonary fibrosis is interstitial lung disease for which no obvious cause can be identified and is associated with typical findings both radiographic and pathologic.
In 2015, interstitial lung disease, together with pulmonary sarcoidosis, affected 1.9 million people. They resulted in 122,000 deaths.

Causes

ILD may be classified as to whether its cause is not known or known.

Idiopathic

is the term given to ILDs with an unknown cause. They represent the majority of cases of interstitial lung diseases. They were subclassified by the American Thoracic Society in 2002 into 7 subgroups:
Secondary ILDs are those diseases with a known etiology, including:

Connective tissue and autoimmune diseases

Connective tissue-related disease represents approximately 25% of all cases of ILD.
Diagnosis of ILD involves assessing the signs and symptoms as well as a detailed history investigating occupational exposures. ILD usually presents with dyspnea, worsening exercise tolerance and 30-50% of those with ILD have a chronic cough. On examination, velcro crackles, in which the crackles compare to the sound of velcro being unfastened, are common in ILD. Pulmonary function tests usually show a restrictive defect with decreased diffusion capacity of carbon monoxide indicating reduced alveolar to blood capillary transport. Pulmonary function testing is indicated for all people with ILD and the FVC loss and DLCO is prognostic, with an FVC loss of greater than 5% per year associated with a poor prognosis in fibrosis subtypes of ILD.
A chest x-ray is 63% sensitive and 93% specific for ILD. With advances in computed tomography, CT scans of the chest have supplanted lung biopsy as the preferred diagnostic test for ILD. A thoracic CT scan is 91% sensitive and 71% specific for ILD. In higher income countries, less than 10% of people with ILD undergo a lung biopsy as part of the diagnostic evaluation.
A lung biopsy may be required if the clinical history and imaging are not clearly suggestive of a specific diagnosis or malignancy cannot otherwise be ruled out. Surgical lung biopsy or via a video-assisted thoracoscopic surgery biopsy is associated with a mortality rate up to 1-2%. A bronchoscopic transbronchial cryobiopsy, in which a camera is introduced into the airways followed by rapid freezing of an area of lung tissue prior to biopsy is associated with a lower complication rate and a much lower mortality rate compared to VATS or surgical biopsy with near comparable diagnostic accuracy. There are four types of histopathologic patterns seen in ILD: usual interstitial pneumonia, non-specific interstitial pneumonia, organizing pneumonia, and diffuse alveolar damage. There is significant overlap of the histopathological and radiologic features of each ILD type making diagnosis challenging; even with lung biopsy, 15% of cases of ILD cannot be classified.

Pulmonary function testing

Most patients with suspected ILD are likely to undergo complete pulmonary function testing. These tests are useful in diagnosis and determining severity of the disease.
Although there is large diversity in interstitial lung disease, most follow a restrictive pattern. Restrictive defects are defined by decreased TLC, RV, FVC and FEV1. As both FVC and FEV1 are reduced, the FVC to FEV1 ratio remains normal or is increased.
As disease progression increases and the lungs become stiffer lung volumes will continue to decrease; lower TLC, RV, FVC and FEV1 scores are associated with a more severe disease progression and poorer prognosis.

X-ray and CT (computed tomography)

is usually the first test to detect interstitial lung diseases, but the chest radiograph can be normal in up to 10% of patients, especially early in the disease process.
High-resolution CT of the chest is the preferred modality and differs from routine CT of the chest. Conventional CT chest examines 7–10 mm slices obtained at 10 mm intervals; high resolution CT examines 1–1.5 mm slices at 10 mm intervals using a high-spatial-frequency reconstruction algorithm. The HRCT therefore provides approximately 10 times more resolution than the conventional CT chest, allowing the HRCT to elicit details that cannot otherwise be visualized.
Radiologic appearance alone, however, is not adequate and should be interpreted in the clinical context, keeping in mind the temporal profile of the disease process.
Interstitial lung diseases can be classified according to radiologic patterns.

Pattern of opacities

Consolidation
  • Acute:
  • * Alveolar hemorrhage syndromes
  • * Acute eosinophilic pneumonia
  • * Acute interstitial pneumonia
  • * Cryptogenic organizing pneumonia
  • Chronic:
  • * Chronic eosinophilic pneumonia
  • * Cryptogenic organizing pneumonia
  • * Lymphoproliferative disorders
  • * Pulmonary alveolar proteinosis
  • * Sarcoidosis
    Linear or reticular opacities
  • Acute:
  • * Pulmonary edema
  • Chronic:
  • * Idiopathic pulmonary fibrosis
  • * Connective tissue-associated interstitial lung diseases
  • * Asbestosis
  • * Sarcoidosis
  • * Hypersensitivity pneumonitis
  • * Drug-induced lung disease
    Small nodules
  • Acute:
  • * Hypersensitivity pneumonitis
  • Chronic:
  • * Hypersensitivity pneumonitis
  • * Sarcoidosis
  • * Silicosis
  • * Coal workers pneumoconiosis
  • * Respiratory bronchiolitis
  • * Alveolar microlithiasis
    Cystic airspaces
  • Chronic:
  • * Pulmonary Langerhans cell histiocytosis
  • * Pulmonary lymphangioleiomyomatosis
  • * Honeycomb lung caused by idiopathic pulmonary fibrosis or other diseases
    Ground glass opacities
  • Acute:
  • * Alveolar hemorrhage syndromes
  • * Pulmonary edema
  • * Hypersensitivity pneumonitis
  • * Acute inhalational exposures
  • * Drug-induced lung diseases
  • * Acute interstitial pneumonia
  • Chronic:
  • * Nonspecific interstitial pneumonia
  • * Respiratory bronchiolitis-associated interstitial lung disease
  • * Desquamative interstitial pneumonia
  • * Drug-induced lung diseases
  • * Pulmonary alveolar proteinosis
    Thickened alveolar septa
  • Acute:
  • * Pulmonary edema
  • Chronic:
  • * Lymphangitic carcinomatosis
  • * Pulmonary alveolar proteinosis
  • * Sarcoidosis
  • * Pulmonary veno-occlusive disease

    Distribution

Upper lung predominance
  • Pulmonary Langerhans cell histiocytosis
  • Silicosis
  • Coal workers pneumoconiosis
  • Carmustine-related pulmonary fibrosis
  • Respiratory broncholitis associated with interstitial lung disease
    Lower lung predominance
  • Idiopathic pulmonary fibrosis
  • Pulmonary fibrosis associated with connective tissue diseases
  • Asbestosis
  • Chronic aspiration