Background
Restrictive lung diseases are characterized by reduced lung volume, either because of an alteration in lung parenchyma or because of a disease of the pleura, chest wall, or neuromuscular apparatus. In physiological terms, restrictive lung diseases are characterized by reduced total lung capacity (TLC), vital capacity, or resting lung volume. Accompanying characteristics are preserved airflow and normal airway resistance, which are measured as the functional residual capacity (FRC). If caused by parenchymal lung disease, restrictive lung disorders are accompanied by reduced gas transfer, which may be marked clinically by desaturation after exercise.
The many disorders that cause reduction or restriction of lung volumes may be divided into 2 groups based on anatomical structures.
The first is intrinsic lung diseases or diseases of the lung parenchyma. The diseases cause inflammation or scarring of the lung tissue (interstitial lung disease) or result in filling of the air spaces with exudate and debris (pneumonitis). These diseases can be characterized according to etiological factors. They include idiopathic fibrotic diseases, connective-tissue diseases, drug-induced lung disease, and primary diseases of the lungs (including sarcoidosis).
The second is extrinsic disorders or extraparenchymal diseases. The chest wall, pleura, and respiratory muscles are the components of the respiratory pump, and they need to function normally for effective ventilation. Diseases of these structures result in lung restriction, impaired ventilatory function, and respiratory failure (eg, nonmuscular diseases of the chest wall, neuromuscular disorders).
Pathophysiology
Air flows to and from the alveoli as lungs inflate and deflate during each respiratory cycle. Lung inflation is accomplished by a contraction of respiratory, diaphragmatic, and external intercostal muscles, whereas deflation is passive. FRC is the volume of air in the lungs when the respiratory muscles are fully relaxed and no airflow is present. The volume of FRC is determined by the balance of the inward elastic recoil of the lungs and the outward elastic recoil of the chest wall. Restrictive lung diseases are characterized by a reduction in FRC and other lung volumes because of pathology in lungs, pleura, or the structures of the thoracic cage.
The distensibility of the respiratory system is called compliance, the volume change produced by a change in the distending pressure. Lung compliance is independent of the thoracic cage, which is a semirigid container. The compliance of an intact respiratory system is an algebraic sum of the compliances of both of these structures; therefore, it is influenced by any disease of the lungs, pleura, or chest wall.
In cases of intrinsic lung disease, the physiological effects of diffuse parenchymal disorders reduce all lung volumes by the excessive elastic recoil of the lungs, in comparison to the outward recoil forces of the chest wall. Expiratory airflow is reduced in proportion to lung volume.
Arterial hypoxemia in these disorders is primarily caused by ventilation-perfusion mismatching, with further contribution from an intrapulmonary shunt. The diffusion of oxygen is impaired, which contributes a little towards hypoxemia at rest but is primarily the mechanism of exercise-induced desaturation.
Hyperventilation at rest and exercise is caused by the reflexes arising from the lungs and the need to maintain minute ventilation by reducing tidal volume and increasing respiratory frequency.
In cases of extrinsic disorders of the pleura and thoracic cage, the total compliance by the respiratory system is reduced, and, hence, lung volumes are reduced. As a result of atelectasis, gas distribution becomes nonuniform, resulting in ventilation-perfusion mismatch and hypoxemia. In kyphoscoliosis, lateral curvature, anteroposterior angulation, kyphosis, or several of these conditions are present. The Cobb angle, an angle formed by 2 limbs of a convex prime curvature of the spine, is an indication of the severity of disease. An angle greater than 100° is usually associated with respiratory failure.
Neuromuscular disorders affect an integral part of the respiratory system, a vital pump. The respiratory pump can be impaired at the level of the central nervous system, spinal cord, peripheral nervous system, neuromuscular junction, or respiratory muscle. The pattern of ventilatory impairment is highly dependent on the specific neuromuscular disease.
Epidemiology
Frequency
United States
For intrinsic lung diseases, studies cite an overall prevalence of 3-6 cases per 100,000 persons, with a prevalence of idiopathic pulmonary fibrosis (IPF) of 27-29 cases per 100,000 persons. The prevalence for adults aged 35-44 years is 2.7 cases per 100,000 persons. Prevalence exceeded 175 cases per 100,000 persons among patients older than 75 years. Exposure to dust, metals, organic solvents, and agricultural employment is associated with increased risk.
- In North America, the prevalence of sarcoidosis is 10-40 cases per 100,000 persons.
- The incidence of chronic interstitial lung diseases in persons with collagen-vascular diseases is variable, but it is increasing for most diseases.
- Kyphoscoliosis is a common extrinsic disorder. It is associated with an incidence of mild deformities amounting to 1 case per 1000 persons, with severe deformity occurring in 1 case per 10,000 persons.
- Other nonmuscular and neuromuscular disorders are rare, but their incidence and prevalence are not well known.
International
In Sweden, the prevalence rate for sarcoidosis is 64 cases per 100,000 persons. In Japan, the prevalence rate of sarcoidosis is 10-40 cases per 100,000 persons. The prevalence of sarcoidosis is difficult to determine, and tuberculosis is common.
The worldwide prevalence of fibrotic lung diseases is difficult to determine because studies have not been performed.
Mortality/Morbidity
The mortality and morbidity from various causes of restrictive lung disease is dependent on the underlying case of the disease process.
The median survival time for patients with IPF is less than 3 years. Factors that predict poor outcome include older age, male sex, severe dyspnea, history of cigarette smoking, severe loss of lung function, appearance and severity of fibrosis on radiologic studies, lack of response to therapy, and prominent fibroblastic foci on histopathologic evaluation.
See the image below.
Gross pathology of small and firm lungs due to restrictive lung disease from advanced pulmonary fibrosis. Race
Although a familial variant of IPF exists, a genetic predisposition is not documented. US prevalence of sarcoidosis is estimated to be 10-17 times higher among African Americans compared to white Americans.
Sex
Lymphangioleiomyomatosis (LAM) and lung involvement in tuberous sclerosis occur exclusively in premenopausal women. Men are more likely to have pneumoconiosis because of occupational exposure, IPF, and collagen-vascular diseases (eg, rheumatoid lung). Worldwide, sarcoidosis is slightly more common in women.
Age
IPF is rare in children. Some intrinsic lung diseases present in patients aged 20-40 years. These include sarcoidosis, collagen-vascular–associated diseases, and histiocytosis X. Most patients with IPF are older than 50 years.
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| Features | AIP | UIP | NSIP | BOOP |
| Pathologic | ||||
| Temporal appearance | Uniform | Heterogeneous | Uniform | Uniform |
| Interstitial inflammation | Scant | Scant | Usually prominent | Variable |
| Collagen fibrosis | No | Patchy | Variable, diffuse | No |
| Fibroblast proliferation | Diffuse, interstitial | Patchy (fibroblast foci) | Occasional | Patchy, airspace |
| BOOP areas | Rare | No | Rare | -- |
| Honeycomb changes | Rare | Yes | Rare | No |
| Hyaline membranes | Yes, often focal | No | No | No |

