Pulmonary emphysema is defined as the "abnormal permanent enlargement of the airspaces distal to the terminal bronchioles accompanied by destruction of the alveolar wall and without obvious fibrosis" 1. Emphysema is best evaluated on CT, although indirect signs may be noticed on conventional radiography in a proportion of cases. This article focuses on panlobular emphysema, paraseptal emphysema, and especially centrilobular emphysema.
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Terminology
Emphysema is one of the entities grouped under the overarching term chronic obstructive pulmonary disease (COPD) and is best thought of primarily as a pathological rather than clinical entity. The Global Initiative for the Diagnosis, Management, and Prevention of Chronic Obstructive Lung Disease (GOLD) is explicit that patients present with COPD, rather than emphysema 11.
Epidemiology
In 2010, approximately 385 million people were affected worldwide by COPD, leading to 3 million deaths annually, with these numbers expected to increase for the foreseeable future 11. It is predominantly a disease of middle to late life owing to the cumulative effect of lifelong tobacco smoking and other environmental risk factors, e.g. air pollution 1.
Historically, it affected more men than women, due to the higher smoking rates in the former, but with increased smoking and environmental risk factor exposure among women, the incidence is now equal between the sexes. Patients with genetic risk factors such as alpha-1-antitrypsin deficiency may present earlier according to phenotype.
Risk factors
smoking: by far the most common, ~90% of all cases 1
intravenous injection of methylphenidate (Ritalin lung): rare
Clinical presentation
The clinical features of emphysema should be distinguished from the signs and symptoms of chronic bronchitis. Patients with emphysema are hypocapnic and are often referred to as "pink puffers". This compares with the hypercapnia and cyanosis of chronic bronchitis with patients referred to as "blue bloaters". In practice, features of these two syndromes coexist as chronic obstructive pulmonary disease.
Patients typically report dyspnea without significant sputum production.
Signs of emphysema include:
tachypnea
absence of cyanosis
pursed-lip breathing, tripod position
chest hyperinflation "barrel chest"
reduced breath sounds
hyper-resonant to percussion
cor pulmonale (late)
Pathology
Emphysema is one of a heterogeneous group of pathological processes forming chronic obstructive pulmonary disease and is itself a relatively vague term encompassing a number of entities and morphological patterns including:
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morphologic subtypes
idiopathic giant bullous emphysema (or vanishing lung syndrome)
The three morphologic subtypes of emphysema are named according to their relationship to the secondary pulmonary lobule.
Centrilobular or centriacinar emphysema 12 is the most common type and affects the proximal respiratory bronchioles, particularly of the upper zones. It has a strong dose-dependent association with smoking 3. Rarely, severe centrilobular emphysema can be seen in the bases in patients with Salla disease 4.
Panlobular or panacinar emphysema 12 affects the entire secondary pulmonary lobule and is more pronounced in the lower zones, matching areas of maximal blood flow. It is seen particularly in alpha-1-antitrypsin deficiency (exacerbated by smoking) 2-4, intravenous injection of methylphenidate (Ritalin lung) 3 or Swyer-James syndrome 4.
Paraseptal or distal acinar emphysema 12 affects the peripheral parts of the secondary pulmonary lobule and is usually located adjacent to the pleural surfaces (including pleural fissures) 3. It is also associated with smoking and can lead to the formation of subpleural bullae and spontaneous pneumothorax 3.
Radiographic features
Plain radiograph
Except in the case of very advanced disease with bulla formation, chest radiography does not image emphysema directly, but rather implies the diagnosis due to associated features 2-3,9:
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hyperinflation
flattened hemidiaphragm(s): the most reliable sign
increased and usually irregular radiolucency of the lungs
increased anteroposterior diameter of the chest
widely spaced ribs
sternal bowing
blunting of the lateral and posterior costophrenic angles
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vascular changes
a paucity of blood vessels which are often distorted
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pulmonary arterial hypertension
pruning of peripheral vessels
an increased caliber of central arteries
It should be remembered, however, that the most common plain film appearance of COPD is "normal" and the role of chest radiography is to eliminate other causes of lung symptoms such as infection, bronchiectasis or cancer 6.
CT
CT is the modality of choice for detecting emphysema; HRCT chest is particularly effective. It should be noted, however, that there is a relatively poor correlation between autopsy-proven emphysema, pulmonary function test abnormalities and CT with 20% of pathology-proven cases not being evident on CT and 40% of patients with abnormal CT having normal pulmonary function tests.
CT is able to discriminate between centrilobular, panlobular, and paraseptal emphysema.
Centrilobular emphysema
Centrilobular is by far the most common type encountered and is a common finding in asymptomatic elderly patients. It is predominantly located in the upper zones of each lobe (i.e. apical and posterior segments of the upper lobes, and superior segment of the lower lobes) and has a patchy distribution 4. It appears as focal lucencies (emphysematous spaces) which measure up to 1 cm in diameter, located centrally within the secondary pulmonary lobule, often with a central or peripheral dot representing the central bronchovascular bundle 2-4.
Panlobular emphysema
Panlobular emphysema is predominantly located in the lower lobes, has a uniform distribution across parts of the secondary pulmonary lobule, which are homogeneously reduced in attenuation 2-4.
Paraseptal emphysema
Paraseptal emphysema is located adjacent to the pleura and septal lines with a peripheral distribution within the secondary pulmonary lobule. The affected lobules are almost always subpleural and demonstrate small focal lucencies up to 10 mm in size.
Any lucency >10 mm should be referred to as subpleural blebs/bullae (synonymous) 3.
In all three subtypes, the emphysematous spaces are not bounded by any visible wall 3.
MRI
MRI is in the research phases for the evaluation of lung parenchymal abnormalities like emphysema. Dynamic breathing MRI may have a future role in assessing pulmonary emphysema.5
Treatment and prognosis
Unfortunately, once lung tissue is lost, no regrowth occurs. Treatment is therefore supportive and aimed at preserving the remaining lung parenchyma. Interventions include:
smoking cessation
oxygen therapy (in chronic hypoxemia)
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symptom and exacerbation control
short and long-acting beta-2 agonists
inhaled anticholinergics
inhaled glucocorticoids
antibiotics
pulmonary rehabilitation
In patients with severe bullous change with resultant compression of remaining normal lung parenchyma, lung volume reduction therapy may be considered in selected patients.
Lung transplantation is considered in cases of alpha-1-antitrypsin deficiency.
Prognosis is worse in patients who continue to smoke, are alpha-1-antitrypsin deficient, have low FEV1 at time of diagnosis, or have other comorbidities (e.g. heart failure, respiratory failure, frequent exacerbations).
Differential diagnosis
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cystic lung disease: all have visible cyst walls
pulmonary Langerhans cell histiocytosis (LCH): often co-exists with emphysema
honeycomb lung: usually reduced lung volumes
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