Restrictive lung diseases are characterized by reduced lung volumes, either because of an alteration in lung parenchyma or because of a disease of the pleura, chest wall, or neuromuscular apparatus. Unlike obstructive lung diseases, such as asthma and chronic obstructive pulmonary disease (COPD), which show a normal or increased total lung capacity (TLC), restrictive disease are associated with a decreased TLC. Measures of expiratory airflow are preserved and airway resistance is normal and the forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) ratio is increased. 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 two 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 extra-pulmonary 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).
The mnemonic "PAINT" has been used to divide the causes of restrictive lung disease into pleural, alveolar, interstitial, neuromuscular, and thoracic cage abnormalities.
Table 1. Causes of Restrictive Lung Disease. (Open Table in a new window)
|Pleural||Trapped lung, pleural scarring, large pleural effusions, chronic empyema, asbestosis||Radiography, CT scanning, pleural manometry, pleural biopsy||Low RV, low TLC, low FVC|
|Alveolar||Edema, hemorrhage||Radiography, CT scanning, physical examination||Increased DLCO in hemorrhage (Intrapulmonary hemoglobin absorbs the carbon monoxide, thus increasing the DLCO reading.)|
|Interstitial||Interstitial lung disease including IPF, NSIP, COP||Radiography, CT scanning, physical examination, echo often shows pulmonary hypertension||Low RV, low FVC, low TLC, decreased DLCO, poor lung compliance|
|Neuromuscular||Myasthenia gravis, ALS, myopathy||Physical examination, EMGs, serology||Low RV, low TLC, low NIF, low MMV|
|Thoracic/extrathoracic||Obesity, kyphoscoliosis, ascites||Physical examination||Low ERV and FRC in obesity, low VC, TLC, FRC in kyphoscoliosis|
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 at rest. 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 disorders of pulmonary parenchyma is primarily caused by ventilation-perfusion mismatching, with further contribution from an intrapulmonary shunt. Decreased diffusion of oxygen contributes significantly to 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 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 two 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.
Obesity is becoming a major cause of restrictive lung disease in the developed world. Over 30% of American adults are classified as obese, with a BMI greater than 30. There is an inverse relationship between BMI and lung volumes. Jones et al  showed that FRC and ERV were the parameters most dramatically reduced by increasing BMI, but that VC and TLC decrease as well. FRC and ERV can even be significantly reduced in the overweight, with BMI of 25-30.
For intrinsic lung diseases, studies cite an overall prevalence of 3-6 cases per 100,000 persons. The prevalence of idiopathic pulmonary fibrosis (IPF) is 27-29 cases per 100,000 persons.(I pulled some recent refs with varying numbers. [2, 3] 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.
According to the CDC, 35.9% of Americans older than 20 years are obese, and 69% of Americans are at least overweight (BMI 25-30). 
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.
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.
Lymphangioleiomyomatosis (LAM) and lung involvement in tuberous sclerosis occur primarily in premenopausal women, although a handful of cases of LAM have been reported in men. 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.
IPF is rare in children. Some intrinsic lung diseases present in patients aged 20-40 years. These include sarcoidosis, collagen-vascular–associated diseases, and pulmonary Langerhans cell histiocytosis (formerly referred to as histiocytosis X). Most patients with IPF are older than 50 years.
The natural history of interstitial lung diseases is variable. It depends on the specific diagnosis and the extent and severity of lung involvement based on high-resolution CT scanning and lung biopsy.  IPF is typically a relentless progressive disorder, and patients have a mean survival of 3-6 years after diagnosis.  Early recognition of IPF is important for directing patient management and predicting prognosis. 
Pulmonary sarcoidosis has a relatively benign self-limiting course, with spontaneous recovery or stabilization in most cases. Approximately 15% of patients develop pulmonary fibrosis and disability.
Prognosis for collagen-vascular diseases, eosinophilic pneumonia, COP, and drug-induced lung disease is generally favorable with treatment.
Patients with chest wall diseases and neuromuscular disorders develop progressive respiratory failure and succumb during an intercurrent pulmonary infection.
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.
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