Lung parenchyma has no power of regeneration. Hence, destructive lesions lead on to fibrosis. Fibrosis of the lung parenchyma may take three forms-replacement fibrosis focal fibrosis, and interstitial fibrosis.
In this form fibrous tissue is laid down over areas of lung destruction. The fibrosis is often localized and its extent depends on the extent of parenchymal destruction. Common causes include advanced pulmonary tuberculosis, bronchiectasis, lung abscess, pulmonary infarcts, pneumonias, atelectasis, fungal infections, pleural diseases such as chronic pleural effusion and empyema, response to foreign materials such as lipoid pneumonia, and irradiation of the lung.
This is seen in pneumoconiosis such as silicosis. The extent of fibrosis may vary from small nodular lesions to extensive areas (progressive massive fibrosis).
This is the end result of interstitial lung disease. Interstitial fibrosis may result from chronic pulmonary edema (occurring in mitral stenosis), allergic alveolitis, connective tissue disorders such as progressive systemic sclerosis and rheumatoid disease, cryptogenic fibrosing alveolitis, radiation injury to the lung, sarcoidosis, asbestosis, and idiopathic pulmonary hemosiderosis. In this form, interstitial fibrosis and emphysematous changes coexist.
Replacement fibrosis: The most common cause in is Chronic pulmonary tuberculosis. The upper lobes are affected more frequently. The chest is asymmetrical with flattening of the affected side, drooping of the shoulder and diminution of movement. Trachea and mediastinal structures are pulled towards the same side, unless they were already fixed by pre-existing disease. The percussion note is diminished. Vocal fremitus and vocal resonance depend upon the severity of fibrosis. In extensive fibrosis they are reduced. If a major bronchus lies subjacent to the fibrotic area the vocal fremitus and resonance are increased and breath sounds become bronchial. In extensive fibrosis, especially fobrothorax, the breath sounds are considerably diminished. Adventitious sounds may be heard and these are variable. Symptoms and hemodynamic disturbances depend upon the extent of pulmonary lesions and the cause.
Replacement fibrosis has to be distinguished from pulmonary collapse in which there may be ipsilateral shift of midline structures. Pulmonary collapse is usually of shorter duration and the underlying cause may be evident.
Other forms of fibrosis: Since these are generalized, affecting both lungs, there is no marked shift of midline structures. The symptoms are those of ventilatory and diffusive defects predominantly characterized by dyspnea, cyanosis, frequwent respiratory infections, and chronic cor pulmonale. Physical examination may reveal gross clubbing, tachypnea, cyanosis, and diminished respiratory movements. Breath sounds are diminished. Diffuse rales (Crackles) which persist after coughing are characteristic. X-ray of the chest may show generalized loss of translucency and increased reticulation. Cardiac findings include right ventricular enlargement and hypertrophy, and lound pulmonic second sound indicating pulmonary hypertension.
Causes and prognosis
Replacement fibrosis does not usually progress further. The course of the disease and longieivity depend on the extent of the lesion, occurrence of secondary infections and the development of cor pulmonale. In general, with reasonable care, localized fibrosis is compatible with prolonged survival. Diffuse interstitial fibrosis is progressive in most cases and life is considerable shortened. Death is due to respiratory failure or cardiac failure.
General measures include the avoidance of smoking, treatment of intercurrent infections, reduction of weight and respiratory exercises. Specific treatment for the underlying cause should be given, if there are signs of activity of the disease. Diffuse interstitial fibrosis may respond to corticosteroids or immunosuppressant drugs. But the results are not encouraging. Cardiac complications are treated suitably.