There is a recent surge in the recognition of interstitial lung disease (ILD) in India.  Although the disease was reported off and on in the later quarter of the last century, it was largely around the turn of the century when there was the widespread availability of the high resolution computed tomography (HRCT) in the country, that it became a primary diagnosis.  Most of the earlier reports on ILDs in the 1980s and 1990s generally referred to the diffuse, pulmonary interstitial involvement in other systemic diseases for example the connective tissue disorders (CTD).

Traditionally, ILD is classified as primary or secondary depending upon its occurrence as either a “lone” pulmonary parenchymal manifestation of an idiopathic cause or in association with a systemic disease in response to a known insult.  In different reports from India, between 50 to 60 percent of all patients of ILD have a secondary cause, such as a CTD, pneumoconiosis, sarcoidosis or drugs, toxins and fumes. A variety of pulmonary infections may also present with diffuse interstitial involvement and occasionally pose a problem in the differential diagnosis.

     1.1.   Connective tissue disorders

Presence of some degree of interstitial pulmonary involvement in most CTDs is almost invariable at one or the other time.  In systemic sclerosis, parenchymal lung involvement is reported in up to 60% of patients.  While interstitial involvement is essentially one of the several extra-articular manifestation of CTDs, what is more intriguing is the primary presentation with an ILD of a patient with an underlying CTD.  The pulmonary involvement may antedate the diagnosis of the CTD by several months, or sometimes years.  Pulmonary interstitial involvement is also recognized in other CTDs such as ankylosing spondylitis, systemic lupus erythematous, Sjogren syndrome and overlap syndrome.

    1.2.  Sarcoidosis

An increasing number of reports on sarcoidosis have now appeared in the Indian literature.  Some of the authors report the occurrence of interstitial involvement in up to 80 percent of cases.  Interstitial involvement is the dominant component of pulmonary sarcoidosis in stage II, III and IV disease.  Even the stage I sarcoidosis, which manifests primarily with hilar and mediastinal lymphadenopathy, interstitial involvement is demonstrable in a majority of patients on transbronchial lung biopsy.

   1.3.  Pneumoconioses – Dust induced ILD

Epidemiologically dust induced interstitial lung diseases account for an enormous burden.  Silicosis commonly observed amongst miners, quartz stone grinders and several types of factory and foundry workers is a significantly high cause of respiratory morbidity.  Another important cause of pulmonary fibrosis is the exposure to asbestos, which has emerged as a major threat to respiratory health in many developing countries such as India, Bangladesh and China.

1.4.  Miscellaneous causes

There is a long list of causes of interstitial lung involvement.  These include viral and bacterial infections, hyper-sensitivity pneumonias due to exposure to organic dusts; toxic reactions to fumes, chemicals and drug; oxygen and radiation toxicity.  Several other diseases of unknown etiologies may also involve the lung parenchyma manifesting with diffuse, interstitial involvement.  Most of these reports are anecdotal and are seen in India as else where in the world.

Idiopathic interstitial lung disease

About 40-50% of cases of ILD in which no other systemic or primary disease is identifiable are classified as idiopathic interstitial pneumonias (IIP).  But IIP is not a diagnosis of exclusion.  Most cases are recognizable by their characteristic clinical and roentgenographic features. Idiopathic pulmonary fibrosis (IPF) is the prototype of several other types of IIPs which are separately categorized based primarily on their histopathological features.  Many of these IIPs are classifiable on the basis of radiological findings seen on high resolution computed tomographic (HRCT).  A good degree of correlation between clinical, radiographic and histopathological patterns of different IIPs constitutes the basis of their classification.

Clinical features

The IPF characteristically involves the middle aged or the elderly individuals in the 5^th to 7^th decades of life. Occasionally, the disease is recognized in children.  It is slightly more common in men and presents insidiously with gradually progressive breathlessness and nonproductive cough.  General constitutional symptoms of weakness, myalgia, easy fatigability and fever are commonly seen.  Presentation in some form of IIPs such as the acute interstitial pneumonias (AIP) is of rather short duration with rapid progression, sometimes referred to as idiopathic acute respiratory distress syndrome.  The secondary types of ILDs are also characterized with clinical features of the underlying systemic disease.

Digital clubbing and exercise induced cyanosis are two important findings seen on general physical examination.  But both these findings are absent in early disease.  Tachypnea and resting hypoxemia are seen in advanced forms of IPF.  The most characteristic finding of bi-basal, ‘velcro like’, dry, end-inspiratory crackles, is considered as fairly diagnostic.  These are reported to be present in 80% to almost 100% by different Indian investigators.  Patients with advanced IPF may also demonstrate the findings of pulmonary hypotension and chronic cor pulmonale.

Acute worsening in IPF may occur due to a respiratory infection, pulmonary embolism or heart failure.  This will generally present with specific symptoms and signs of the complication such as increased tachypnoea and respiratory distress, cyanosis, fever, purulent expectoration and pedal edema (etc.).  Acute exacerbation of IPF (AE-IPF) is recognized as a specific clinical entity defined by rapid deterioration of IPF that is not due to a recognizable secondary cause, as above.  The condition indicates a poor prognosis.