Medical School · Year 1 · Respiratory · includes a quiz and discussion video

Lecture 8: Restrictive Lung Diseases

Unit 1.8: Respiratory System

Learning Objectives

By the end of this lecture, students will be able to:

1. Define restrictive lung disease and its physiologic features
2. Classify restrictive diseases by etiology
3. Describe the pathophysiology of interstitial lung diseases
4. Explain the features of common ILDs including IPF and sarcoidosis
5. Describe extrapulmonary causes of restriction
6. Interpret pulmonary function tests in restrictive disease

Lecture Content

I. Overview of Restrictive Lung Disease

Restrictive lung diseases share the common feature of reduced lung expansion, limiting the volume of air that can be inhaled. Unlike obstructive diseases where airflow is impeded, restrictive diseases impair the ability of the lung or chest wall to expand fully, reducing all lung volumes.

The defining characteristic of restrictive disease is reduced total lung capacity, typically below 80 percent of predicted. This reduction may result from intrinsic lung disease that stiffens the parenchyma or from extrapulmonary factors that limit chest wall expansion. Both mechanisms produce similar spirometric patterns but differ in their effects on gas exchange.

Spirometry in restrictive disease shows reduced forced vital capacity reflecting the limited expandability. Forced expiratory volume in one second is also reduced, but proportionally to the reduced vital capacity. Consequently, the ratio of forced expiratory volume in one second to forced vital capacity remains normal or even increases above 0.70, distinguishing restrictive from obstructive patterns. This pattern, however, can only suggest restriction; definitive diagnosis requires measurement of total lung capacity, which is reduced. Residual volume typically decreases in parenchymal restrictive disease, though it may increase in neuromuscular restriction where weakness prevents forceful expiration.

Classification divides restrictive diseases into intrinsic (parenchymal) and extrinsic (extrapulmonary) categories. Intrinsic restriction results from interstitial lung diseases affecting the lung parenchyma itself. Extrinsic restriction results from abnormalities of the chest wall, pleura, or neuromuscular system that limit lung expansion despite normal lung parenchyma.

<image>Panel A: Spirometry tracings comparing normal versus restrictive with proportional reduction in both FEV1 and FVC while preserving ratio. Panel B: Flow-volume loops showing restrictive loop as miniature version of normal maintaining shape but reduced in all dimensions. Panel C: Lung volume diagrams comparing normal versus restrictive with proportionally reduced TLC, FRC, and RV for parenchymal disease. Panel D: Classification box dividing intrinsic interstitial lung diseases from extrinsic chest wall, neuromuscular, and pleural causes with note that TLC measurement confirms restriction.</image>

II. Interstitial Lung Diseases - Overview

Interstitial lung diseases comprise a heterogeneous group of disorders affecting the lung parenchyma, including the interstitium, alveoli, and small airways. Despite their diverse etiologies, these diseases share common pathophysiologic features and clinical presentations.

The classification of interstitial lung diseases organizes conditions by known versus unknown cause. Known causes include drug-induced lung disease (amiodarone, bleomycin, methotrexate), radiation pneumonitis, occupational exposures (asbestosis, silicosis), and connective tissue disease-associated interstitial lung disease. Idiopathic interstitial pneumonias, with no identifiable cause, include idiopathic pulmonary fibrosis, nonspecific interstitial pneumonia, cryptogenic organizing pneumonia, and acute interstitial pneumonia. Granulomatous diseases include sarcoidosis and hypersensitivity pneumonitis. Other entities include lymphangioleiomyomatosis and pulmonary alveolar proteinosis.

The common pathophysiology involves inflammation and fibrosis of the lung parenchyma. Inflammation thickens the interstitium and alveolar walls. Fibrosis replaces normal tissue with collagen, stiffening the lung and reducing compliance. Loss of alveolar units decreases surface area for gas exchange. Ventilation-perfusion mismatch develops as scarred regions remain perfused but poorly ventilated. These changes produce the characteristic physiologic abnormalities: reduced compliance requiring increased work to inflate the lungs, reduced diffusing capacity from thickened air-blood barrier and lost surface area, and hypoxemia from ventilation-perfusion mismatch that worsens with exercise.

The clinical presentation of interstitial lung diseases follows a recognizable pattern regardless of specific etiology. Dyspnea, initially with exertion and later at rest, represents the cardinal symptom, reflecting the increased work of breathing and impaired gas exchange. Cough, usually dry and non-productive, occurs commonly. Physical examination reveals fine, inspiratory crackles, often described as Velcro-like, heard best at the lung bases. Clubbing of the fingers develops in some patients, particularly with idiopathic pulmonary fibrosis. Cyanosis appears in advanced disease with severe hypoxemia.

<image>Panel A: ILD classification scheme showing known causes (drugs, radiation, occupational, CTD) versus unknown causes (idiopathic interstitial pneumonias, granulomatous diseases). Panel B: Pathophysiology progression from normal alveolus through inflammation with thickened interstitium to fibrosis with collagen deposition and distorted architecture. Panel C: Clinical features showing patient with dyspnea, Velcro crackles on stethoscope, finger clubbing, and chest radiograph with bilateral reticular opacities. Panel D: Physiologic consequences showing decreased compliance, decreased DLCO, and V/Q mismatch leading to hypoxemia.</image>

III. Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis represents the most common and most serious of the idiopathic interstitial pneumonias. This chronic, progressive fibrotic disease carries a poor prognosis, with median survival of only three to five years from diagnosis.

The disease typically affects older adults, with most patients presenting between ages 60 and 70. Males predominate slightly. Cigarette smoking associates with increased risk, suggesting environmental contribution to an undefined pathogenic mechanism. Familial clustering occurs in a minority of cases, and certain genetic variants in telomere-related genes and surfactant protein genes increase susceptibility.

The pathologic pattern of usual interstitial pneumonia defines idiopathic pulmonary fibrosis. This pattern features heterogeneous involvement, with areas of fibrosis adjacent to relatively normal lung. Fibroblastic foci, zones of active fibroblast proliferation representing the leading edge of fibrosis, alternate with established scarring. Honeycombing, the formation of cystic spaces lined by bronchiolar epithelium, develops in advanced disease. The distribution favors peripheral subpleural and basal lung regions.

Clinical presentation involves insidious onset of exertional dyspnea that gradually worsens over months to years. Dry, persistent cough occurs commonly. Physical examination reveals bilateral, basilar, fine inspiratory crackles with the characteristic Velcro quality. Clubbing develops in approximately half of patients and may predate the diagnosis.

Diagnosis relies on high-resolution computed tomography demonstrating the usual interstitial pneumonia pattern: basal and peripheral predominant reticular opacities, honeycombing, and traction bronchiectasis, with minimal ground-glass opacity. When imaging is definitive, surgical lung biopsy is not required. When imaging shows possible or indeterminate pattern, multidisciplinary discussion incorporating clinical, radiologic, and sometimes histopathologic information establishes the diagnosis.

Management of idiopathic pulmonary fibrosis has evolved with the availability of antifibrotic therapies. Pirfenidone and nintedanib both slow the rate of lung function decline, though neither reverses established fibrosis. Supplemental oxygen addresses hypoxemia and improves quality of life. Pulmonary rehabilitation provides symptomatic benefit. Lung transplantation offers the only potentially curative option for eligible patients. Notably, corticosteroids are not beneficial and may be harmful in idiopathic pulmonary fibrosis, distinguishing its management from many other interstitial lung diseases.

<image>Panel A: IPF epidemiology showing age histogram peaking at 60-70 years with male predominance and risk factors including smoking and genetics. Panel B: UIP pattern on CT with labeled honeycombing at periphery and bases, traction bronchiectasis, and reticular opacities plus histology showing fibroblastic foci adjacent to established fibrosis. Panel C: Management showing antifibrotics pirfenidone and nintedanib with slow decline annotation, oxygen, pulmonary rehabilitation, and lung transplant with prominent no steroids symbol. Panel D: Prognosis bar indicating median survival of 3-5 years with poor overall outcome.</image>

IV. Other Idiopathic Interstitial Pneumonias

Beyond idiopathic pulmonary fibrosis, several other idiopathic interstitial pneumonias present distinct patterns with different prognoses and treatment responses. Recognition of these entities guides appropriate management.

Nonspecific interstitial pneumonia demonstrates uniform inflammation with or without fibrosis, in contrast to the heterogeneous appearance of usual interstitial pneumonia. The distribution tends to be diffuse with characteristic subpleural sparing. High-resolution computed tomography shows ground-glass opacities and fine reticulation; honeycombing, if present, is minimal. Nonspecific interstitial pneumonia often occurs in association with connective tissue diseases or drug exposure, though an idiopathic form exists. Prognosis is substantially better than idiopathic pulmonary fibrosis, and corticosteroids often provide benefit.

Cryptogenic organizing pneumonia presents as patchy consolidation that may be migratory, appearing to resolve in one location while developing in another. The pattern reflects buds of granulation tissue (organizing pneumonia) filling distal airways and alveoli. Patients typically present subacutely with flu-like illness followed by persistent cough and dyspnea. High-resolution computed tomography shows patchy consolidation, often with air bronchograms, predominantly in a peripheral distribution. Corticosteroids produce dramatic improvement in most cases, though relapse may occur upon tapering.

Acute interstitial pneumonia represents an idiopathic form of acute respiratory distress syndrome. Rapid onset of respiratory failure develops over days to weeks without identifiable cause. The pathologic pattern shows diffuse alveolar damage identical to acute respiratory distress syndrome from known causes. Prognosis is poor, with mortality approaching 50 percent, despite supportive care and empiric corticosteroid therapy.

Comparison of these entities emphasizes the importance of accurate pattern recognition. Usual interstitial pneumonia (idiopathic pulmonary fibrosis) has insidious onset, characteristic honeycombing on imaging, poor prognosis, and no role for corticosteroids. Nonspecific interstitial pneumonia has subacute onset, ground-glass predominance with subpleural sparing, better prognosis, and often responds to corticosteroids. Cryptogenic organizing pneumonia has subacute onset with patchy consolidation, excellent prognosis, and typically responds well to corticosteroids. Acute interstitial pneumonia has acute onset, diffuse ground-glass opacities, and high mortality despite treatment.

<image>Panel A: IPF/UIP column showing CT honeycombing with insidious presentation, poor prognosis, and no steroid response. Panel B: NSIP column showing CT ground-glass with subpleural sparing, subacute presentation, better prognosis, and often steroid responsive. Panel C: COP column showing CT patchy consolidation, subacute presentation, good prognosis, and excellent steroid response. Panel D: AIP column showing CT diffuse ground-glass, acute presentation, poor prognosis, and supportive treatment only with comparison bar graph across conditions.</image>

V. Sarcoidosis

Sarcoidosis is a systemic granulomatous disease of unknown etiology characterized by non-caseating granulomas in affected organs. The lungs and thoracic lymph nodes are involved in over 90 percent of cases, making pulmonary manifestations the most common presentation.

Epidemiologic patterns show predilection for young to middle-aged adults, typically presenting between ages 20 and 40. African Americans in the United States have higher incidence and often more severe disease than Caucasians. Northern European countries, particularly Scandinavian nations, show elevated prevalence. Slight female predominance exists. The cause remains unknown despite intensive investigation; theories include aberrant immune response to unidentified antigen(s), possibly infectious or environmental.

While sarcoidosis can affect virtually any organ, certain manifestations predominate. Pulmonary involvement occurs in over 90 percent of patients, with symptoms ranging from none to progressive dyspnea. Lymph node involvement, particularly bilateral hilar and mediastinal adenopathy, is nearly universal. Cutaneous manifestations occur in about 25 percent and include erythema nodosum (painful subcutaneous nodules on the shins, often acute and self-limited) and lupus pernio (violaceous plaques on the face, indicating chronic disease). Ocular involvement occurs in about 25 percent, predominantly as anterior uveitis. Cardiac sarcoidosis, affecting about 5 percent clinically but up to 25 percent at autopsy, causes arrhythmias and cardiomyopathy. Neurologic sarcoidosis, also affecting about 5 percent, commonly presents as cranial nerve palsy (particularly facial nerve) or basilar meningitis.

Pulmonary staging by chest radiography provides prognostic information. Stage 0 shows normal chest radiograph. Stage I shows bilateral hilar lymphadenopathy without parenchymal infiltrates. Stage II shows bilateral hilar lymphadenopathy with parenchymal infiltrates. Stage III shows parenchymal infiltrates without adenopathy. Stage IV shows pulmonary fibrosis. Spontaneous remission occurs in most stage I patients, fewer stage II patients, and rarely in advanced stages.

Diagnosis typically requires demonstration of non-caseating granulomas on biopsy, with clinical and radiographic features consistent with sarcoidosis, after excluding other causes of granulomatous disease particularly tuberculosis. Serum angiotensin-converting enzyme elevation supports the diagnosis but lacks specificity. Bronchoalveolar lavage shows lymphocytosis with elevated CD4 to CD8 ratio.

Management depends on organ involvement and severity. Stage I disease often requires only observation given high spontaneous remission rates. Symptomatic pulmonary disease, progressive lung function decline, and significant extrapulmonary involvement (cardiac, neurologic, ocular) warrant treatment with corticosteroids. Steroid-sparing agents including methotrexate and azathioprine are used for chronic disease. Anti-tumor necrosis factor agents provide options for refractory cases.

<image>Panel A: Sarcoidosis epidemiology showing age peak 20-40 with African American predilection and Scandinavian prevalence plus body diagram with organ frequencies (lungs 90%+, lymph nodes 80%, skin 25%, eyes 25%, heart 5%, CNS 5%). Panel B: Pulmonary staging with chest radiograph examples for stages I through IV and corresponding remission rates. Panel C: Pathology showing non-caseating granuloma with multinucleated giant cells plus diagnostic workup with biopsy, ACE level, and BAL CD4/CD8 ratio. Panel D: Management algorithm branching from observation for stage I through corticosteroids for symptomatic disease to steroid-sparing and anti-TNF for refractory cases.</image>

VI. Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis, also termed extrinsic allergic alveolitis, represents an immune-mediated interstitial lung disease triggered by repeated inhalation of organic antigens. The condition spans a spectrum from acute, reversible disease to chronic fibrosis indistinguishable from other fibrosing interstitial pneumonias.

Numerous antigens from various sources can induce hypersensitivity pneumonitis. Farmer's lung results from exposure to thermophilic actinomycetes in moldy hay. Bird fancier's lung develops from exposure to avian proteins in droppings and feathers. Hot tub lung follows exposure to Mycobacterium avium complex in aerosolized water. Humidifier lung results from microbial contamination of air conditioning systems or humidifiers. Many other occupational and environmental exposures have been implicated.

The disease presents in three forms depending on intensity and duration of exposure. Acute hypersensitivity pneumonitis develops hours after intense exposure, with fever, chills, dyspnea, cough, and myalgias resolving within hours to days after exposure cessation. Subacute hypersensitivity pneumonitis develops gradually with ongoing moderate exposure, producing progressive dyspnea and cough over weeks to months. Chronic hypersensitivity pneumonitis results from low-level prolonged exposure, causing insidious fibrosis that may be clinically and radiographically indistinguishable from idiopathic pulmonary fibrosis.

Pathologically, hypersensitivity pneumonitis shows poorly formed non-caseating granulomas, lymphocytic interstitial infiltrate, and bronchiolocentric inflammation. Giant cells may be present. Chronic disease adds fibrosis that may progress to honeycombing.

Diagnosis requires recognizing the exposure history, which may be subtle. High-resolution computed tomography shows ground-glass opacities, mosaic attenuation reflecting air trapping from small airway involvement, and centrilobular nodules in acute and subacute disease, with fibrotic changes in chronic disease. Bronchoalveolar lavage demonstrates marked lymphocytosis with reduced CD4 to CD8 ratio, contrasting with the elevated ratio in sarcoidosis. Serum precipitating antibodies (precipitins) against specific antigens support prior exposure but do not confirm active disease.

Management centers on antigen avoidance, which is both diagnostic (symptoms improve) and therapeutic. Patients may need to change occupations, eliminate pets, or remediate environmental contamination. Corticosteroids hasten recovery in acute and subacute disease but do not prevent progression if exposure continues. In chronic fibrotic disease, corticosteroids have limited efficacy.

<image>Panel A: Common antigen sources showing farmer with hay for thermophilic actinomycetes, birds for avian proteins, hot tub for MAC, and air conditioner for microbial contamination. Panel B: Disease spectrum from acute with abrupt reversible symptoms through subacute with gradual progression to chronic with insidious fibrosis and corresponding CT images. Panel C: Diagnostic features showing BAL lymphocytosis with decreased CD4/CD8 ratio compared to increased in sarcoidosis plus precipitins indicating exposure. Panel D: Management algorithm emphasizing antigen avoidance as cornerstone with corticosteroids for symptomatic disease.</image>

VII. Occupational Lung Diseases

Occupational lung diseases result from inhalation of dust, fibers, or fumes in the workplace. The pneumoconioses, caused by inorganic dust inhalation, comprise an important subset of restrictive lung diseases with distinct clinical and radiographic features.

Asbestosis results from inhalation of asbestos fibers, historically encountered in mining, construction, shipbuilding, and insulation work. The disease develops after a latency of 15 to 20 years from first exposure. Pathologically, asbestosis produces diffuse interstitial fibrosis with asbestos bodies (fibers coated with iron-containing protein) and ferruginous bodies visible on microscopy. High-resolution computed tomography shows bilateral lower lobe predominant reticular opacities and honeycombing resembling idiopathic pulmonary fibrosis. Pleural plaques, often calcified, on the parietal pleura provide a marker of asbestos exposure. Asbestos exposure also increases risk of lung cancer (synergistic with smoking) and malignant mesothelioma. No specific treatment exists; management is supportive with monitoring for malignancy.

Silicosis results from inhalation of crystalline silica, encountered in mining, sandblasting, quarrying, and construction. Simple silicosis produces small nodular opacities on imaging, typically in the upper lung zones. The nodules may calcify, and hilar lymph nodes may show "eggshell" calcification (calcification around the node periphery). Complicated silicosis, or progressive massive fibrosis, develops as nodules coalesce into large masses exceeding one centimeter. Acute silicoproteinosis follows massive exposure and produces alveolar filling resembling pulmonary alveolar proteinosis. Silicosis increases susceptibility to tuberculosis.

Coal worker's pneumoconiosis results from coal dust inhalation. Simple coal worker's pneumoconiosis produces small nodular opacities on imaging. Complicated disease with progressive massive fibrosis may develop. Caplan syndrome describes the combination of coal worker's pneumoconiosis (or other pneumoconiosis) with rheumatoid arthritis and pulmonary nodules.

Berylliosis results from beryllium exposure in electronics, aerospace, and nuclear industries. The disease mimics sarcoidosis clinically and pathologically, producing non-caseating granulomas. The beryllium lymphocyte proliferation test distinguishes berylliosis from sarcoidosis.

<image>Panel A: Asbestosis column showing construction worker source, basal fibrosis with pleural plaques imaging, asbestos bodies pathology, and mesothelioma/lung cancer associations. Panel B: Silicosis column showing miner source, upper lobe nodules with eggshell calcification imaging, silicotic nodule pathology, and tuberculosis association. Panel C: Coal worker's pneumoconiosis column showing coal mine source, small nodules or PMF imaging, coal macule pathology, and Caplan syndrome association. Panel D: Berylliosis column showing electronics source, granulomatous imaging pattern, non-caseating granuloma pathology, and sarcoid-like presentation with latency timeline.</image>

VIII. Connective Tissue Disease-Associated ILD

Interstitial lung disease commonly complicates connective tissue diseases, occurring with varying frequency and patterns depending on the underlying rheumatologic condition. Recognition of these associations enables appropriate screening, monitoring, and treatment.

Rheumatoid arthritis is frequently complicated by interstitial lung disease, with patterns including usual interstitial pneumonia and nonspecific interstitial pneumonia. The usual interstitial pneumonia pattern in rheumatoid arthritis behaves similarly to idiopathic pulmonary fibrosis, though prognosis may be slightly better. Rheumatoid nodules may occur in the lungs. Pleural disease, including effusions and thickening, also occurs.

Systemic sclerosis (scleroderma) has the highest prevalence of interstitial lung disease among the connective tissue diseases, affecting approximately 50 to 80 percent of patients. Nonspecific interstitial pneumonia predominates over usual interstitial pneumonia. The extent of interstitial lung disease predicts mortality in systemic sclerosis. Pulmonary arterial hypertension may coexist, arising from vascular involvement independent of interstitial disease.

Inflammatory myopathies (polymyositis and dermatomyositis) frequently involve the lungs. Anti-Jo-1 antibodies and other antisynthetase antibodies identify patients at high risk for interstitial lung disease. Patterns include nonspecific interstitial pneumonia, cryptogenic organizing pneumonia, and diffuse alveolar damage. Rapid onset of interstitial lung disease may precede or accompany muscle symptoms.

Sjögren syndrome may produce nonspecific interstitial pneumonia or lymphocytic interstitial pneumonia, characterized by lymphoid infiltration of the interstitium. Systemic lupus erythematosus less commonly causes chronic interstitial lung disease but may produce acute lupus pneumonitis or diffuse alveolar hemorrhage.

Several features distinguish connective tissue disease-associated interstitial lung disease from idiopathic disease. Patients tend to be younger. Female predominance reflects the demographics of most connective tissue diseases. Prognosis is generally better than idiopathic pulmonary fibrosis, even with the same histologic pattern. Treatment typically responds to immunosuppression, in contrast to idiopathic pulmonary fibrosis where immunosuppression is harmful.

Screening patients with connective tissue disease for interstitial lung disease involves baseline pulmonary function testing and high-resolution computed tomography, with monitoring for development or progression. Early treatment of emerging interstitial lung disease may prevent progressive fibrosis.

<image>Panel A: CTD-ILD prevalence showing rheumatoid arthritis with UIP/NSIP, systemic sclerosis at 50-80% with NSIP predominant, and myositis with NSIP/COP/DAD associated with anti-Jo-1. Panel B: Additional CTDs showing Sjogren with NSIP/LIP and SLE with acute pneumonitis/DAH both less common. Panel C: Distinguishing features from idiopathic ILD including younger age, female predominance, better prognosis, and response to immunosuppression. Panel D: Screening approach with baseline PFTs and HRCT at CTD diagnosis, periodic monitoring, and early treatment of emerging ILD to prevent progression.</image>

IX. Extrapulmonary Restriction

Restriction of lung expansion may occur from factors extrinsic to the lungs themselves, including abnormalities of the chest wall, neuromuscular system, or pleura. These extrapulmonary causes produce similar spirometric patterns to parenchymal restriction but differ in their effects on gas exchange.

Chest wall disorders limit lung expansion through mechanical means. Kyphoscoliosis, severe curvature of the spine, distorts thoracic geometry and limits rib cage excursion. Severe deformity can produce restrictive physiology with chronic hypercapnic respiratory failure. Ankylosing spondylitis fuses spinal joints and costovertebral articulations, fixing the thorax in position. Flail chest, resulting from multiple rib fractures creating a segment that moves paradoxically with respiration, impairs ventilatory mechanics. Obesity reduces chest wall compliance by adding weight to the thorax and limiting diaphragmatic descent, producing a pattern sometimes called obesity-related restrictive disease or contributing to obesity hypoventilation syndrome.

Neuromuscular disorders impair respiratory muscle function, limiting the force available to inflate the lungs. Muscular dystrophies produce progressive weakness affecting respiratory muscles. Myasthenia gravis causes fatigable weakness that may fluctuate. Guillain-Barré syndrome produces acute ascending weakness that may compromise respiratory function. Amyotrophic lateral sclerosis causes progressive weakness as motor neurons degenerate. Spinal cord injuries above the level of phrenic nerve origin (C3-C5) paralyze the diaphragm, while lower injuries spare diaphragm function but impair intercostal and abdominal muscles.

Pulmonary function testing in neuromuscular disease shows characteristic findings. Vital capacity decreases as weakness prevents full inspiration and expiration. Maximal inspiratory and expiratory pressures are reduced, directly demonstrating respiratory muscle weakness. Vital capacity falls further in the supine position when diaphragm weakness is present, as abdominal contents push against the weakened diaphragm. Serial measurements track progression.

Pleural disorders limit lung expansion by occupying space or restricting pleural motion. Large pleural effusions compress the underlying lung. Pleural thickening and fibrosis, whether from prior infection, hemothorax, or malignancy, restrict expansion. Pneumothorax prevents lung inflation until air is evacuated.

Importantly, diffusing capacity remains normal in extrapulmonary restriction because the lung parenchyma is intact. This distinction helps differentiate extrapulmonary restriction from parenchymal interstitial lung disease, where diffusing capacity decreases.

<image>Panel A: Chest wall disorders showing kyphoscoliosis with curved spine limiting expansion, ankylosing spondylitis with fused bamboo spine, and obesity with weight compressing thorax. Panel B: Neuromuscular disorders including muscular dystrophy, myasthenia, GBS, ALS, and spinal cord injury with PFT findings of reduced VC, reduced MIP/MEP, and decreased supine VC indicating diaphragm weakness. Panel C: Pleural disorders showing effusion compressing lung, thickening restricting expansion, and pneumothorax. Panel D: Distinguishing feature box emphasizing DLCO remains normal in extrapulmonary restriction versus reduced in parenchymal ILD.</image>

X. Pulmonary Function in Restrictive Disease

Pulmonary function testing confirms restrictive lung disease, quantifies severity, distinguishes parenchymal from extrapulmonary causes, and monitors progression. Understanding the characteristic patterns enables accurate interpretation.

Spirometry provides the initial screening test but cannot definitively diagnose restriction. Forced vital capacity is reduced. Forced expiratory volume in one second is also reduced but proportionally to the reduced vital capacity. The ratio of forced expiratory volume in one second to forced vital capacity is normal or increased, typically above 0.70 and often above 0.80. This pattern, sometimes called a "proportional reduction," suggests restriction but could also occur with poor effort or submaximal inspiration.

Measurement of total lung capacity confirms restriction. Total lung capacity below 80 percent of predicted establishes the diagnosis. This measurement, performed by body plethysmography or gas dilution techniques, is essential because spirometry alone cannot rule out restriction.

Additional lung volume measurements further characterize the restriction. Residual volume typically decreases in parenchymal restriction, reflecting reduced lung compliance at all volumes. However, residual volume may be normal or increased in neuromuscular restriction, where weakness prevents forceful expiration and respiratory muscles cannot compress the lungs to their minimal volume. Functional residual capacity decreases, reflecting the reduced equilibrium volume of the stiffer respiratory system.

Diffusing capacity for carbon monoxide provides crucial information distinguishing intrinsic from extrinsic restriction. In parenchymal interstitial lung disease, diffusing capacity decreases due to thickening of the air-blood barrier, loss of alveolar surface area, and ventilation-perfusion mismatch. In extrapulmonary restriction, the lung parenchyma remains normal; therefore, diffusing capacity is normal or may even be increased when corrected for alveolar volume.

Flow-volume loops in restrictive disease maintain normal shape but appear miniaturized, as if a normal loop were scaled down proportionally. Peak flows are reduced, but the expiratory limb maintains its normal contour without the scooping seen in obstructive disease. This "miniature normal" pattern visually distinguishes restriction from obstruction.

<image>Panel A: Spirometry showing reduced FVC, reduced FEV1, normal or increased FEV1/FVC ratio often greater than 0.80 with normal tracing overlaid on smaller restrictive tracing. Panel B: Lung volumes showing TLC reduced confirming restriction with RV reduced in parenchymal disease but potentially normal or increased in neuromuscular disease. Panel C: DLCO contrast showing reduced in parenchymal ILD with thickened air-blood barrier versus normal in extrapulmonary restriction with compressed but normal lung. Panel D: Flow-volume loop showing miniature normal pattern with preserved contour but reduced volumes plus summary table distinguishing parenchymal from extrapulmonary restriction.</image>

Summary

Restrictive lung diseases are defined by reduced total lung capacity below 80 percent of predicted, with preserved or increased FEV1/FVC ratio. Classification divides restriction into intrinsic (parenchymal) causes from interstitial lung diseases and extrinsic (extrapulmonary) causes from chest wall, neuromuscular, or pleural abnormalities.

Interstitial lung diseases share common pathophysiology of inflammation and fibrosis reducing compliance and impairing gas exchange. Idiopathic pulmonary fibrosis, the most common idiopathic interstitial pneumonia, shows the usual interstitial pneumonia pattern with honeycombing and fibroblastic foci, carries a median survival of 3-5 years, and is treated with antifibrotic agents rather than corticosteroids. Other idiopathic interstitial pneumonias (nonspecific interstitial pneumonia, cryptogenic organizing pneumonia, acute interstitial pneumonia) have distinct patterns and often better prognoses with corticosteroid responsiveness.

Sarcoidosis is a multisystem granulomatous disease with non-caseating granulomas. Pulmonary involvement occurs in over 90 percent of cases, with staging by chest radiography predicting spontaneous remission rates. Corticosteroids treat progressive or symptomatic disease.

Hypersensitivity pneumonitis results from immune reaction to inhaled antigens. Antigen avoidance is the cornerstone of management, as continued exposure leads to progressive fibrosis.

Occupational pneumoconioses (asbestosis, silicosis, coal worker's pneumoconiosis) have characteristic exposures, imaging patterns, and associated complications.

Connective tissue disease-associated interstitial lung disease occurs commonly in systemic sclerosis, rheumatoid arthritis, and inflammatory myopathies, typically with better prognosis and response to immunosuppression than idiopathic disease.

Extrapulmonary restriction from chest wall, neuromuscular, or pleural disorders produces similar spirometric patterns but preserves diffusing capacity, enabling distinction from parenchymal disease.

Key Terms

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