Pathology of Emphysema
Emphysema is defined as a condition of the lung characterized by abnormal permanent enlargement of the airspaces distal to the terminal bronchiole, accompanied by destruction of their walls without obvious fibrosis.
Emphysema is classified in anatomic terms, but the classification should not ignore the fact that the severity of emphysema is more important than the type.
As emphysema becomes more severe it becomes more difficult to classify, a situation similar to that of end stage of renal disease or cirrhosis of the liver.
Emphysema is more common, being found in about half of all autopsies, and is easily recognizable.
Proximal Acinar (Centriacinar) Emphysema:
Proximal part of the acinus (respiratory bronchioles) is selectively or predominantly involved in centriacinar emphysema, of which there are two forms.
Centrilobular emphysema is the common form of emphysema in nonindustrial populations.
Focal emphysema is recognized in dusty occupations, such as coal mining (coal workers emphysema or simple pneumoconiosis).
Centrilobular emphysema is the form of the disease most frequently encountered and the one usually associated with clinical symptoms.
It involves the central part of the lobule (the smallest portion of the lung bounded by lobular septa, or the cluster of terminal bronchioles near the end of the bronchiolar tree).
More exactly, it involves the proximal part of the acinus.
It is associated with, and probably due to, tobacco smoking.
The destroyed and enlarged respiratory bronchioles form airspaces that are separated from each other and from lobular septa by normal alveolar ducts and sacs. As the lesion progresses, these distal structures may also be involved.
The bronchioles proximal to the emphysematous spaces are inflamed and narrowed. Centrilobular emphysema is most common and most severe in the upper zones of the lung (upper lobe and superior segment of lower lobe).
It occurs much more often in men than in women and is commonly associated with chronic bronchitis .
Working with coal, whether as a miner or in other ways, results in an accumulation of coal or other dust in macrophages in and around respiratory bronchioles.
Mild dilatation of respiratory bronchioles results, probably from atrophy of muscle.
The lesion resembles centrilobular emphysema but differs in that the enlarged spaces are smaller and more regular, and inflammation of bronchioles is not apparent.
Thus, the lesion is primarily distensive rather than destructive.
The anatomic lesion is usually equated with a chest radiograph that shows small nodular densities, although the complete correlation has not been proved.
The condition, also referred to as black lung, has been considered to cause severe disability.
Contemporary evidence, however, suggests that simple coal pneumoconiosis causes only minor impairment of pulmonary function.
When coal miners have severe chronic airflow obstruction, it is usually due to other forms of emphysema, notably tobacco-related centrilobular emphysema.
Other forms of dust, including iron and urban soot, may also be associated with similar lesions.
The acinus is uniformly involved in panacinar emphysema. All parts of the acinus are destroyed and in the final stage a lacy network of supporting tissue is left behind (the "cotton candy lung" ).
Panacinar emphysema occurs in several different situations.
Familial emphysema is usually due to a defect in circulating alpha1-antiproteinase.
Originally described as alpha1-antitrypsin deficiency, it is now recognized that protein has several antiproteinase effects, notably antielastase.
The amount and type of alpha1-antitrypsin is determined by a pair of co-dominant alleles, referred to as Pi (proteinase inhibitor).
The most common phenotype is PiM.
The most serious abnormality is associated with the PiZ allele, which occurs in some 5% of the population, is commoner in those of Scandinavian origin, and is rare in Jews, blacks, and Japanese.
The phenotype PiZ is associated with very low levels of circulating antiproteinase inhibitor, which also has an abnormal electrophoretic mobility.
People with this phenotype often develop cirrhosis of the liver in infancy and emphysema as adults.
The majority of all patients with clinically diagnosed emphysema under the age of 40 have a alpha1-antitrypsin deficiency (PiZ).
Symptomatic emphysema is uncommon under the age of 50.
Cigarette smoking plays an important role in the causation of emphysema in most affected individuals, but a small number of nonsmokers with PiZ also develop emphysema.
PiZ is associated with panacinar emphysema, which is characteristically worse in the lower zones of the lung. Clinically, the patients usually have the "pink puffer syndrome", with greatly enlarged lung volumes and normal blood gases.
Other alleles are also associated with emphysema, the most important being PiS, which is found frequently among southern Europeans.
Panacinar Emphysema in association with Centrilobular Emphysema:
Panacinar emphysema is often associated with centrilobular emphysema.
In such cases, the panacinar form tends to occur in the lower zones of the lung, whereas centrilobular emphysema is seen in the upper ones.
The associations are the same as for centrilobular emphysema alone, notably cigarette smoking, chronic bronchitis, and chronic airflow obstruction.
When emphysema gets worse it is hard to classify, and experts may classify the same lung differently, as centrilobular, panacinar, or unclassified emphysema.
The severity of emphysema is more important than the type in producing chronic airflow obstruction.
People may have severe emphysema at autopsy without clinical manifestations of chronic airflow obstruction.
The frequency of chronic bronchitis increases with the severity of emphysema, and almost all patients with severe emphysema have chronic bronchitis.
Although it was once thought that chronic bronchitis leads to emphysema, it is now recognized that chronic bronchitis and emphysema share a common etiologic agent, which is tobacco smoke.
Patients usually present to their physician between 55 and 60 years of age with dyspnea of insidious onset.
Weight loss is probably due less to lack of calories than to the increased work of breathing.
It may be that caloric deficiency aggravates emphysema in man.
The "Pink Puffer" and the "Blue Bloater":
The literature in 1960s emphasized that patients with severe chronic airflow obstruction present with two distinct appearances.
One type of patient is severely hypoxic and hypercapneic, has peripheral edema because of right ventricular failure, and often has a rate of ventilation less than that suggested by the concentrations of blood gases.
Conversely, another patient is thin and has normal blood gas levels because of a sustained high ventilation rate.
The former was known as the "blue bloater", and airflow obstruction was thought to be due to chronic bronchitis, the latter was labeled the "pink puffer", and chronic airflow obstruction was attributed to emphysema.
The two types of chronic airflow obstruction are still identifiable and represent the opposite ends of a spectrum. Most patients have features of both.
The blue-bloater syndrome is likely to occur in those with diminished response to hypercapnea and perhaps hypoxia. Among the patients, those with the greatest degree of right ventricular hypertrophy have more narrowed peripheral airways.
This is a logical observation since those with the blue-bloater syndrome, have the greatest ventilation-perfusion inequalities, a situation consistent with severe bronchiolar narrowing.
One should regard chronic airflow obstruction as multifactorial.
In the central airways, the bronchioles, and the lung parenchyma, a variety of lesions may contribute to chronic airflow obstruction.
They occur together because of a common etiologic agent - tobacco smoke. In patients with mild chronic airflow obstruction, bronchiolar lesions, like inflammation, are predominant.
In patients with severe chronic airflow obstruction, emphysema is the most important.
Etiology and Pathogenesis of Emphysema:
Conclusive evidence now exists that the major cause of emphysema is tobacco smoking, especially cigarette smoking.
Moderate to severe emphysema is rarely found in nonsmokers.
The dominant hypothesis concerning the pathogenesis of emphyema is the proteolysis-antiproteolysis theory.
It is now thought that there is a balance between elastin synthesis and catabolism in the lung.
If elastolytic activity increases or antielastolytic activity decreases, emphysema results.
Increased numbers of neutrophils, which contain serine elastase, are found in the bronchoalveolar lavage fluid of smokers.
Smoking also reduces alpha1-antiproteinase activity in the lung, owing to the oxidation of methionine residues in alpha1-antiproteinase.
In this way, unopposed and increased elastolytic activity leads to emphysema.
important radiologic abnormalities include overinflation of the lung, as
evidenced by enlarged lungs, depressed diaphragm levels, an inversion of
the convexity of the diaphragm and an increased posteroanterior diameter
Relative avascularity of the peripheral lung
is also described as a sign. Using MRI clear pulmonary vascular information can be
obtained easily, and the relative pulmonary blood flow in the lung
parenchyma can be quantified.
Diagnosis of Emphysema:
The most important radiologic abnormalities include overinflation of the lung, as evidenced by enlarged lungs, depressed diaphragm levels, an inversion of the convexity of the diaphragm and an increased posteroanterior diameter ("barrel chest").
Relative avascularity of the peripheral lung is also described as a sign.
Using MRI clear pulmonary vascular information can be obtained easily, and the relative pulmonary blood flow in the lung parenchyma can be quantified.
Tests of expiratory flow are poorly related to the severity of emphysema because emphysema is only one cause of chronic airflow obstruction.
Tests that reflect the loss of elastic recoil, such as increased functional residual capacity, and particularly total lung capacity, show a better correlation.
The best predictor has been the diffusing capacity (transfer factor) for carbon monoxide, although no single test is entirely adequate.
It is appropriate to think of chronic airflow obstruction as a multifactorial syndrome, the most important of which is emphysema.
The severity of emphysema can be assessed only by an integrated approach.
Tests for expiratory flow establish the severity of chronic airflow obstruction.
If a patient is young and a nonsmoker, he is unlikely to have emphysema.
If he is a middle aged smoker with chronic bronchitis, he is likely to have emphysema.
The chest radiograph provides supplementary evidence, as do the functional tests previously mentioned.
In the final analysis, clinical common sense, a chest radiograph, and measurement of the diffusing capacity go a long way in making the assessment.
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