Emphysema is a disease in which the walls of the alveoli of the lung are progressively destroyed, thereby reducing the area of lung available for exchange of gases. The alveoli, of which there are many millions in each lung, are groups of air sacs at the end of bronchioles (tiny air passages). Through their thin walls, inhaled oxygen is passed into the bloodstream and carbon dioxide is removed from the capillaries to be breathed out. Emphysema usually develops along with chronic bronchitis, in a condition known as chronic obstructive pulmonary disease.
In almost all cases, emphysema is a direct result of smoking. In rare cases, an inherited deficiency of a chemical called alpha 1-antitrypsin in the body results in emphysema and, in a minority of people, also affects the liver. Tobacco smoke and other air pollutants are believed to cause emphysema by provoking the release of chemicals within the alveoli that cause damage to the alveolar walls. Alpha 1 -antitrypsin is thought to protect against this chemical damage; therefore, people with a deficiency of this substance are particularly badly affected by emphysema. The damage is slight at first, but in heavy smokers it becomes progressively worse; the alveoli burst and merge to form fewer, larger sacs with less surface area, which consequently impairs oxygen and carbon dioxide exchange. Over the years the lungs become increasingly less elastic, which further reduces their efficiency.
Symptoms and signs
Initially, there are no symptoms, but as the disease progresses and the lungs suffer damage, there is an increasing shortness of breath. In some people, the chest becomes barrel-shaped as air is trapped in the lungs. There may also be a chronic cough (caused by the accompanying bronchitis) and a slight wheeze. Eventually (sometimes after many years) the level of oxygen in the blood starts to fall. In some cases, pulmonary hypertension (raised blood pressure in the pulmonary artery) develops, leading to cor pulmonale (enlargement and strain on the right side of the heart). Affected people start to turn blue due to a lack of oxygen in the blood; their legs subsequently swell as a result of oedema (accumulation of fluid in the tissues). Other sufferers are able to compensate for oxygen deficiency to some extent by breathing faster, there-by retaining their normal colouring. Many people show signs falling somewhere between these two extremes.
A diagnosis is made from the patient's symptoms and signs, from a chest examination, and from various tests. A blood sample from an artery may be analysed to measure the concentration of oxygen and carbon dioxide in the blood; blood oxygen levels may alternatively be measured using an oximeter. A blood test for alpha 1-antitrypsin deficiency may be performed if there is a family history of the disorder. Chest X-rays are taken to exclude the possibility of another lung disease being responsible for the symptoms and to determine how great an area of the lungs has been affected. Pulmonary function tests are carried out to assess breathing capacity and the efficiency of the alveoli in exchanging gases.
Treatment and prevention
Once the damage to the lungs has occurred, there is no treatment that can reverse it. However, giving up smoking will greatly reduce the rate at which the lungs deteriorate. The efficiency of the remaining lung tissue may be improved in various ways. For example, bronchodilator drugs may be given to widen the bronchioles and patients who are deficient in alpha 1-antitrypsin may be given replacement alpha 1-antitrypsin therapy. To treat oedema, diuretic drugs may help to reduce the volume of fluid in the body by promoting its output through increased urine production. If the blood oxygen level is continually low, oxygen therapy at home may be needed. People who have a localized area of emphysema in one lung may be offered surgery to remove the affected area of tissue, which will allow the remaining lung tissue to re-expand. A single lung transplant operation may be considered if respiratory failure is life-threatening.
Emphysema - technical
Emphysema is defined as enlargement of the airways distal to the terminal bronchioles, due to destruction of their walls without obvious fibrosis. Two major types are recognized, according to the distribution of enlarged air spaces within the acinar unit, the acinus being that part of the lung parenchyma supplied by a single terminal bronchiole:
- ◆ centriacinar (or centrilobular) emphysema, in which enlarged air spaces are initially clustered around the terminal bronchiole
- ◆ panacinar (or panlobular) emphysema, where the enlarged air spaces are distributed throughout the acinar unit
Centriacinar emphysema is more common in the upper zones of the upper and lower lobes and is the common type in COPD: panacinar emphysema may be found anywhere in the lungs, but is more prominent at the bases, and is associated with α1-antitrypsin deficiency. Both types of emphysema can occur alone or in combination in a patient with COPD. There is still debate over whether centriacinar and panacinar emphysema represent different disease processes, and hence have different aetiologies, or whether panacinar emphysema is a progression from centriacinar emphysema. There is a clearer association between centriacinar emphysema and cigarette smoking than with panacinar emphysema. Smokers with centriacinar emphysema have more small-airways disease than those patients with predominantly panacinar emphysema.
Periacinar (or paraseptal or distal acinar) emphysema describes enlarged air spaces along the edge of the acinar unit, but only where it abuts against a fixed structure such as the pleura or a vessel. This is less common and usually of little clinical significance, except when extensive in a subpleural position when it may be associated with pneumothorax. Scar and irregular emphysema are terms sometimes used to describe enlarged air spaces around the margins of a scar, unrelated to the structure of the acinus, but this lesion is excluded from the current definition of emphysema.
In the early stages of the disease, emphysematous lesions are microscopic (<1 mm diameter); they may progress to macroscopic lesions or bullae. A bullae is an area of emphysema that has locally overdistended; conventionally to more than 1 cm in size. Bullous disease can also occur in the absence of COPD.
Normal bronchioles and small bronchi are supported by attachments to the outer aspect of their walls of adjacent alveolar walls, an arrangement which maintains the tubular integrity of the airways. It has been suggested that loss of these attachments in emphysema may lead to distortion and irregularity of airways, which results in airflow limitation.
The inflammatory cell profile in the alveolar walls and the air spaces is similar to that described in the airways and persists throughout the course of the disease, even after smoking cessation. Absence of fibrosis is a prerequisite in the most recent definition of emphysema, but fibrosis occurs in the terminal or respiratory bronchioles as part of a respiratory bronchiolitis in COPD patients. Furthermore, there is an increase in collagen in the lung parenchyma in smokers compared with nonsmokers.
See Chronic Obstructive Pulmonary Disease for more information.