Pulmonary emphysema is one of the two conditions included in chronic obstructive pulmonary disease, with the other being chronic bronchitis. Pulmonary emphysema is identified by marked overdistention of alveoli along with destruction of supporting alveolar and interstitial structures. Diagnosis is with CT imaging, and treatment includes a combination of bronchodilators and anti-inflammatory medications. Because pulmonary emphysema causes permanent damage, prevention is important.
Two major forms of pulmonary emphysema are generally recognized:
Alveolar emphysema is abnormal, permanent enlargement of air spaces distal to the terminal bronchiole and destruction of alveolar septal walls without apparent fibrosis.
Interstitial emphysema is the presence of air within the supporting connective tissue stroma of the lung (interlobular, subpleural, mediastinal, subcutaneous).
Epidemiology and Pathogenesis of Pulmonary Emphysema in Animals
In animals, pulmonary emphysema typically occurs secondary to a primary obstructive pulmonary disease process. In humans, emphysema affects up to 40% of people with chronic obstructive pulmonary disease (COPD); the main risk factors are tobacco smoke exposure and alpha-1-antitrypsin deficiency.
Although the pathogenesis of pulmonary emphysema is not fully understood, at least three mechanisms have been suggested:
imbalance between endogenous protease and antiprotease activity, resulting in destruction of alveolar walls and interstitial matrix
inappropriate maintenance of lung structure and repair after injury
development secondary to obstruction of airways on expiration due to chronic bronchitis or bronchiolitis or congenital abnormality of the airway wall
This last mechanism creates a “check valve” lesion, in which air is able to enter alveoli on inspiration or through collateral ventilation but is unable to leave freely, causing air trapping.
Cattle
Pulmonary diseases associated with airway obstruction and dyspnea (eg, bovine respiratory syncytial virus infection), acute respiratory disease syndromes (eg, acute pulmonary edema and emphysema), and moldy sweet potato toxicosis are commonly associated with interstitial emphysema. Severe interstitial emphysema can cause large gas bullae in all parts of the lung and subcutaneous emphysema as air dissects along fascial planes from the lungs through the mediastinum and thoracic inlet to the subcutis of the back. Because of well-developed interlobular septa and lack of collateral ventilation, cattle are particularly susceptible to interstitial emphysema.
Horses
Recurrent airway obstruction, or “heaves,” in horses is associated with chronic bronchitis and bronchiolitis, resulting in alveolar hyperinflation by air trapping. The condition is partially reversible with bronchodilators; however, approximately 12% of horses develop alveolar emphysema, presumably from chronic overdistention of alveolar walls and protease-antiprotease imbalance associated with pulmonary inflammation.
Dogs and Cats
Congenital lobar emphysema of dogs (as observed in the Pekingese breed) occurs secondary to aplasia or hypoplasia of bronchiolar cartilage that collapses during expiration, leading to air trapping.
In cats, overdistention of the alveoli can occur during end-stage asthma conditions or as a result of COPD. Airway irritants commonly associated with this condition in cats (particularly indoor cats) are tobacco smoke, allergens, and air pollution. Obesity and dental disease are thought to exacerbate this condition in cats, and the Siamese breed may be at higher risk.
Clinical Findings and Diagnosis of Pulmonary Emphysema in Animals
Clinical signs
Thoracic radiographs and CT
Pulmonary function tests
Animals with pulmonary emphysema may present with labored breathing and coughing, and physical examination may reveal abnormal breath sounds such as wheezes and crackles. The area of thoracic auscultation is typically enlarged due to lung hyperinflation. Pulse oximetry or arterial blood gas may show nonspecific oxygen deficiency. Thoracic radiographs and CT may show hyperlucent lungs with a flattened diaphragm. Pulmonary function tests, though rarely available, can provide a definitive diagnosis.
At necropsy, the lungs stay overinflated and do not collapse. Histological examination is the only method to differentiate lung overinflation secondary to pulmonary emphysema from air trapping due to airway obstruction from chronic bronchiolitis or bronchitis. Air bubbles (bullae) of various sizes may be observed in the subpleural space and interstitium, as well as around the kidneys and pericardial sac in cattle with emphysema. Lesions may develop as subpleural bullae or emphysema localized to a lobe or diffused lesions.
Treatment of Pulmonary Emphysema in Animals
Oxygen
Anti-inflammatory drugs
Bronchodilators
For animals with pulmonary emphysema that are hypoxemic (eg, PaO2 < 80), hypoxic (eg, lactate above reference range), or struggling to breathe, acute treatment includes oxygen, injectable anti-inflammatory medications, and aerosolized bronchodilators, if available. If the primary disease process is known, treatment directed toward that is essential and may result in substantial improvement of clinical signs. Treatment for chronic disease includes administration of bronchodilator and anti-inflammatory drugs, along with air quality control measures.
Key Points
With the exception of congenital forms, the majority of pulmonary emphysema cases are secondary to an inflammatory condition, and the condition is irreversible.
Treatment includes bronchodilators, anti-inflammatory medications, and addressing the primary disease process when possible.
Keeping air quality high and eliminating or minimizing pulmonary irritants is the key to prevention.
For More Information
Also see pet health content regarding emphysema in dogs and emphysema in cats.
