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Pulmonary emphysema

Dr Yuranga Weerakkody and Dr Luke A Danaher et al.

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".

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:

morphologic sub types
- centrilobular emphysema : most common
- panlobular emphysema
- paraseptal emphysema
- paracicatricial emphysema
idiopathic giant bullous emphysema (or vanishing lung syndrome)
congenital lobar emphysema
pulmonary interstitial emphysema

This article concerns itself with first three morphologic sub types, with particular emphasis on centrilobular emphysema.

Epidemiology

At the time of initial writing, approximately 210 million people are affected worldwide leading to 3 million deaths annually.¹ It is predominantly a disease of middle to late life owing to the cumulative effect of smoking and other environmental risk factors. It traditionally affected more men than women 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 include :

smoking : by far the most common
alpha-1-antitrypsin (AAT) deficiency
intravenous injection of methylphenidate (Ritalin lung)

Clinical presentation

The clinical features of emphysema should be distinguished from the signs and symptoms of chronic bronchitis. Patients with emphysema are hypocapnoeic and are often referred to as "pink puffers." This compares with the hypercapnoea 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 dyspnoea without significant sputum production.

Signs of emphysema include:

tachypnoea
absence of cyanosis
pursed-lip breathing
chest hyperinflation
reduced breath sounds
hyper-resonant to percussion
cor pulmonale (late)

Pathology

There are three morphologic sub types of emphysema, which are named according to their relationship to the secondary pulmonary lobule.

Centri-lobular emphysema is the most frequently encountered type and affects the proximal respiratory bronchioles, particularly of the upper zones. It is strongly associated with smoking in a dose dependent way ³. Rarely, severe centrilobular emphysema can be seen in the bases in patients with Salla disease ⁴.

Pan-lobular emphysema in contrast, 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 (AAT) deficiency (exacerbated by smoking) ^2-4, intravenous injection of methylphenidate (Ritalin lung) ³ or Swyer-James syndrome ⁴.

Para-septal emphysema 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 film

Except in the case of very advanced disease with bullous formation, chest radiography does not image emphysema directly, but rather infers the diagnosis due to associated features ^2-3:

hyperinflation
- flattened hemidiaphragm(s) : most reliable sign
- increased and usually irregular radiolucency of the lungs
- increased retrosternal airspace
- increased antero-posterior diameter of chest
- widely spaced ribs
- sternal bowing
- tenting of the diaphragm
- saber-sheath trachea
vascular changes
- paucity of blood vessels, often distorted
- pulmonary arterial hypertension
  - pruning of peripheral vessels
  - increased calibre of central arteries
  - right ventricular enlargement

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 cancer⁶

CT

CT is currently the modality of choice for detecting emphysema - HRCT is particularly effective. It should be noted however that there is relatively poor correlation between autopsy-proven emphysema, pulmonary function test abnormalities and CT with 20% of pathology-proven cases not evident on CT and 40% of patients with abnormal CT have normal pulmonary function tests.

CT is able to discriminate between centrilobular, panlobular, and paraseptal emphysema.

Centri-lobular emphysema

Centri-lobular 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), has a patchy distribution ⁴. It appears as focal lucencies (emphysematous spaces) which measure up to 1cm in diameter, located centrally within the secondary pulmonary lobule, often with a central or peripheral dot representing the central bronchovascular bundle ^2-4.

Pan-lobular emphysema

Pan-lobular 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.

Para-septal emphysema

Para-septal 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 sub-pleural, and demonstrate small focal lucencies up to 10mm in size.

Any larger than 10mm and they should be referred to as subpleural blebs or subpleural bullae (synonymous) ³.

In all three sub types, the emphysematous spaces are not bounded by any visible wall ³.

MRI

MRI as a modality is in the research phases of evaluation of lung parenchymal abnormalities like emhysema. Dynamic breathing MRI may have a role in future 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 remaining lung parenchyma. Interventions include:

smoking cessation
oxygen therapy (in chronic hypoxaemia)
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 normal lung compression, lung volume reduction therapy may be considered in selected patients.

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).