Interstitial Lung Disease

Interstitial lung disease (ILD) is a broad group of diffuse parenchymal lung disorders characterised by inflammation and fibrosis (scarring) of the lung interstitium, the tissue surrounding the alveoli. The result is a restrictive pattern of lung impairment with stiff lungs and impaired gas exchange.

Definition

Interstitial lung disease is a heterogeneous group of diffuse parenchymal lung diseases in which inflammation and/or fibrosis of the alveolar walls and surrounding interstitium reduce lung compliance and impair gas exchange, producing a restrictive ventilatory pattern.

Scope

This entry describes interstitial lung disease as a category of parenchymal pathology: the patterns of interstitial inflammation and fibrosis, the principal subgroups (including idiopathic pulmonary fibrosis and other idiopathic interstitial pneumonias), and the resulting functional impairment. It is reference-educational and does not provide antifibrotic, immunosuppressive, dosing, or individualised management guidance.

Core questions

• Which histological and radiological pattern characterises the interstitial process?
• Is the disease idiopathic or secondary to a known exposure or systemic disorder?
• How does interstitial fibrosis produce restrictive physiology and impaired diffusion?
• What distinguishes idiopathic pulmonary fibrosis from other interstitial pneumonias?

Key concepts

• Diffuse parenchymal (interstitial) involvement
• Restrictive ventilatory pattern
• Idiopathic pulmonary fibrosis and usual interstitial pneumonia pattern
• Idiopathic interstitial pneumonias
• Secondary ILD (connective-tissue disease, exposures, drugs)
• Impaired diffusing capacity
• Multidisciplinary diagnosis

Mechanisms

In interstitial lung disease, injury to the alveolar epithelium and abnormal repair lead to inflammation and deposition of extracellular matrix in the alveolar walls and surrounding interstitium. In fibrotic forms such as idiopathic pulmonary fibrosis, the dominant process is progressive scarring with the histological and radiological pattern termed usual interstitial pneumonia, in which fibroblast activity and matrix accumulation distort lung architecture, as reviewed by Lederer and Martinez. Thickened, scarred interstitium stiffens the lung (reducing compliance) and widens the barrier to gas diffusion, producing restrictive physiology and a reduced diffusing capacity. Many cases are secondary to connective-tissue diseases, inhaled or occupational exposures, or drugs, while others are idiopathic and classified by pattern.

Clinical relevance

Interstitial lung diseases are characterised and distinguished through integrated clinical, radiological, and pathological assessment, and the ATS/ERS classifications and guidelines structure how the idiopathic interstitial pneumonias and idiopathic pulmonary fibrosis are defined. Recognising fibrotic versus inflammatory patterns frames prognosis. This entry is descriptive reference material, not individualised clinical advice.

Epidemiology

Interstitial lung diseases are individually uncommon but collectively important; idiopathic pulmonary fibrosis is one of the more frequent and serious fibrotic forms, typically affecting older adults and carrying a poor prognosis, while ILD secondary to connective-tissue disease and to environmental or occupational exposures contributes a substantial additional burden.

History

The idiopathic interstitial pneumonias were progressively distinguished from one another through correlation of histological patterns with clinical course, culminating in successive ATS/ERS multidisciplinary classifications that separated idiopathic pulmonary fibrosis (with its usual interstitial pneumonia pattern) from other entities and established multidisciplinary diagnosis as the standard approach.

Debates

How should the idiopathic interstitial pneumonias be classified?

Distinguishing idiopathic pulmonary fibrosis from other interstitial pneumonias relies on integrating histological and radiological patterns, and the boundaries and terminology have been revised across successive ATS/ERS statements as understanding has advanced.

Seminal works

• lederer-2018
• travis-2013
• raghu-2018

Frequently asked questions

Why does interstitial lung disease cause a restrictive breathing pattern?

Inflammation and fibrosis thicken and stiffen the lung interstitium, reducing lung compliance and limiting expansion, which produces a restrictive pattern with low lung volumes rather than airflow obstruction.

Is interstitial lung disease a single disease?

No. It is a broad group of diffuse parenchymal disorders that share interstitial inflammation or fibrosis; some are idiopathic, such as idiopathic pulmonary fibrosis, while others are secondary to connective-tissue diseases, exposures, or drugs.