Clinical presentation and diagnostics
The disease is characterized by a gradual onset and an inexorably progressive course. The main clinical sign is progressive dyspnea upon physical exertion.
On physical examination, physicians auscultate fine bilateral crackles or crepitations in the basal lung regions. Most patients also present with fever, general weakness, and weight loss. Radiography reveals bilateral enhancement of the pulmonary pattern and the «ground-glass» opacity sign, predominantly localized in the lower zones.
Pathological anatomy: early stages
Initial macroscopic changes may be mild: the lungs become unevenly aerated, congested, and dense.
Microscopically, the early stages show:
- Exudative or exudative-proliferative nature of inflammation.
- Damage and sclerosis of the interstitium of the respiratory zones.
- Formation of myofibroblast foci in the interstitium.
- Patchiness of lesions: pathological foci alternate with completely preserved tissue, with healthy areas decreasing over time.
Serous exudate accumulates in the alveolar lumina, accompanied by alveolar proteinosis, epithelial desquamation, and fibrin deposition extending to the formation of hyaline membranes. The cellular infiltrate of the interstitium is represented predominantly by lymphohistiocytic elements.
Pathogenesis and the role of macrophages
Molecular factors and alveolar macrophages play a key role in the development of early changes. The expression of c-fos and c-sis oncoproteins stimulates cell proliferation. Activated macrophages intensively produce reactive oxygen species and tumor necrosis factor-alpha (TNF-α), which damage the bronchial and alveolar epithelium.
A distinctive feature of macrophages in acute interstitial pneumonia is the predominance of the monocytoid type: they lack phagocytic activity but enhance protein synthesis and secrete large amounts of fibronectin.
Late stage and «honeycomb lung»
The prolonged course of the disease leads to the development of the terminal stage — the «honeycomb lung».
Macroscopically, the lung tissue becomes firm, acquires a rubbery consistency, completely losing its elasticity and aeration. The organ's structure is remodeled into cystic cavities.
The microscopic picture is characterized by marked interstitial sclerosis and the replacement of normal parenchyma with connective tissue. Air spaces transform into cystic spaces («honeycombs») surrounded by fibrosis. The epithelial lining undergoes dysregenerative changes: type I pneumocytes are replaced by hyperplastic type II pneumocytes, with foci of adenomatous hyperplasia, as well as squamous metaplasia and dysplasia.
Pronounced septal fibrosis and epithelial changes completely block the blood-air barrier.
Briefly: answers to topical questions
What etiological factors are hypothesized in the development of idiopathic fibrosing alveolitis?
Idiopathic fibrosing alveolitis belongs to the group of diffuse inflammatory lung diseases of unknown etiology. The term «idiopathic fibrosing alveolitis» reflects the primary and unclear nature of the disease. For respiratory bronchiolitis-associated interstitial lung disease, it is noted that it is usually found in smokers.
What pulmonary and extrapulmonary complications are characteristic of the terminal stage of idiopathic fibrosing alveolitis?
The terminal stage of the disease is characterized by the development of progressive cor pulmonale and heart failure. The main complications and outcomes include:
- «Honeycomb lung» — a combination of interstitial fibrosis and cystic transformation of terminal and respiratory bronchioles and alveoli.
- Pneumosclerosis (pulmonary cirrhosis).
- Secondary pulmonary hypertension.
- Right ventricular hypertrophy.
Additionally, the blood-air barrier is blocked and ceases to function due to pronounced interstitial fibrosis of the alveolar septa and dysregenerative changes in the epithelial lining.
What histological patterns are distinguished in the modern classification of idiopathic interstitial pneumonias?
The modern morphological classification distinguishes several main histological patterns (variants). These include:
- Usual interstitial pneumonia (usual interstitial pneumonia) — the most frequent morphological substrate.
- Nonspecific interstitial pneumonia.
- Organizing pneumonia.
- Diffuse alveolar damage.
- Respiratory bronchiolitis.
- Desquamative interstitial pneumonia.
- Lymphocytic interstitial pneumonia.
What pathophysiological mechanisms lead to respiratory failure in idiopathic fibrosing alveolitis?
Respiratory failure in the disease is caused by restrictive pulmonary function disorders and a decrease in oxygen diffusion capacity. These pathophysiological mechanisms are based on the blockade of the blood-air barrier, which ceases to function. The causes of this blockade include:
- Pronounced interstitial fibrosis of the alveolar septa.
- Dysregenerative changes in the epithelial lining (replacement of type I pneumocytes by hyperplastic type II pneumocytes, focal adenomatous hyperplasia).
The consequence of these processes is a sharp progressive decrease in vital capacity.
IFA: Mosaic of inflammation at the onset $\rightarrow$ Myofibroblasts and fibronectin $\rightarrow$ Cystic «honeycomb lung» at the end.
Class and exam questions
What is the leading clinical symptom of idiopathic fibrosing alveolitis?
The main symptom of the disease is worsening exertional dyspnea.
What is the specific distribution pattern of the pathological process in the lungs?
The process is patchy in nature: widespread lesion foci alternate with preserved lung parenchyma.
What is the macroscopic appearance of the lung in the late stage?
The lung tissue becomes dense, acquires a «rubbery» consistency, loses elasticity, and forms honeycomb-like structures («honeycomb lung»).
What else to study on this topic
- Morphoclinical classification of IFA variants
- Cytological composition of the exudate and interstitial infiltrate
- Molecular mechanisms of injury: the role of ROS and TNF-α
- Features of the monocytoid type of alveolar macrophages
- Dysregenerative epithelial changes and cellular atypia
- Causes and consequences of the blood-air barrier blockade
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