Interstitial lung disease (Proceedings)

Interstitial lung disease represents a diffuse subset of pulmonary disease that includes abnormalities in the microscopic interstitium (the anatomic space between the basement membranes of the epithelial and endothelial cells) as well as the inflammatory and/or fibrotic processes that extend into the alveolar space, as well as the bronchioles and bronchiolar lumen. Interstitial lung disease (ILD)is uncommonly recognized in veterinary medicine in contrast with human medicine. However, in that past decade or so, a form of interstitial disease has been recognized in dogs, specifically West Highland White terriers and then just within the past year in cats. The goal of this hour is to review the approach to ILD in people and to describe what is known in cats and dogs.

Clinical signs of ILD include a history of gradually progressive exertional dyspnea, with ultimate severe distress. In people, most cases are reported in people greater than 60 years of age, so abnormalities in younger people should be aggressively evaluated for an more treatable underlying cause. Many times, people associate the progressive dyspnea as a "side effect" of getting older. This same phenomenon is commonly appreciated in older dogs, as clients typically expect dogs to slow down with age. Cats are not commonly athletes and elderly cats in particular are rarely identified early in the course of any cardiopulmonary disease.

Physical examination abnormalities consistent with interstitial lung disease in people include end-inspiratory crackles (termed "dry" or "Velcro"), finger clubbing (associated with chronic hypoxemia) or signs of pulmonary hypertension (loud S2 or elevated jugular venous pressure). In many cases, the initial assessment of the patient is bronchitis or pneumonia and it only after the patient fails to respond to conventional therapy.

Radiographic findings consistent with interstitial lung disease include bilateral diffuse infiltrate, without consolidation or mass. In recent years, high-resolution computed tomography (HRCT) has evolved into the standard of care for evaluation of patients with suspected interstitial disease. HRCT is a better correlate to physiologic dysfunction than thoracic radiography.

Pulmonary function testing is used to support a diagnosis of pulmonary impairment, including an increased A-a gradient, decreased diffusing capacity (DLCO) and a restrictive defect.

Further diagnosis in people is directed towards classification of the specific type of interstitial lung disease. The most common type is called usual interstitial pneumonia (UIP) or idiopathic pulmonary fibrosis. Other types, particularly immune-mediated disease appear to respond better to therapy with immunosuppressive agents. Additionally, exposure to environmental triggers is investigated in people, due to the potential for exposure to a variety of toxic products.

In dogs and cats, interstitial lung diseases are uncommonly recognized but should be considered in dogs or cats with otherwise unexplained respiratory dysfunction. The most common historical signs in dogs with ILD include cough, tachypnea, exercise intolerance or overt dyspnea. As is common in people, a lengthy course may exist before veterinary care is sought, due to the misperception that exercise intolerance is an accepted part of aging. A major differential is chronic bronchitis which is common in older small breed dogs as well. Physical examination in affected dogs is usually remarkable for pulmonary crackles. Occasionally, a loud S2 is present or other signs of pulmonary hypertension. Some dogs have concurrent mitral regurgitation although sinus arrhythmia is common rather than that sinus tachycardia associated with heart failure. Weight loss is not common.

Diagnostic testing in dogs is usually limited to thoracic radiographs, which document a heavy interstitial pattern occasionally with right sided cardiomegaly. Collection of arterial blood gases is often challenging in small dogs with respiratory distress, but if performed, will document an increased A-a gradient and moderate to severe hypoxemia. Bronchoscopy or BAL is unlikely helpful except in excluding other diseases. Open-lung biopsy is warranted to better classify the disease, but is rarely performed in veterinary medicine due to inherent risks and also due to the current inability to better treat patients based upon histopathological findings.

Treatment for affected dogs is limited to supportive care with or without prednisone therapy. The prognosis is guarded with many dogs dying from progressive pulmonary failure with 12-18 months. A recent paper in a limited number of affected dogs described histopathological changes different from those seen in UIP in people.

In contrast, in cats, a form of pulmonary fibrosis was very recently reported in cats with very similar histopathological findings to human UIP. This was exciting from the animal model standpoint as well as further identifying a new disease process in cats. Cats affected with pulmonary fibrosis commonly were co-affected with a pulmonary neoplasia. Thus, it is wise to evaluate cats with a suspected solitary pulmonary mass for IPF PRIOR to surgical resection to establish a good prognosis following surgery.

Figure 1. CT from a dog with chronic bronchitis.

Recommendations for practitioners include an understanding of the existence of this emerging diseases. Interstitial lung disease should be suspected in patients with tachypnea or extertional dyspnea (particularly terriers) or in cats with pulmonary masses.

Figure 2. CT scan from a dog affected with pulmonary fibrosis

Further Reading

Norris AJ, Naydan DK, Wilson DW. Interstitial lung disease in West Highland White Terriers. Vet Pathol. 2005 Jan;42(1):35-41..

Cohn LA, Norris CR, Hawkins EC, Dye JA, Johnson CA, Williams KJ Identification and characterization of an idiopathic pulmonary fibrosis-like condition in cats. J Vet Intern Med. 2004 Sep-Oct;18(5):632-41.

Norris CR, Griffey SM, Walsh P Use of keyhole lung biopsy for diagnosis of interstitial lung diseases in dogs and cats: 13 cases (1998-2001). J Am Vet Med Assoc. 2002 Nov 15;221(10):1453-9.

Corcoran BM, Cobb M, Martin MW, Dukes-McEwan J, French A, Fuentes VL, Boswood A, Rhind S. Chronic pulmonary disease in West Highland white terriers. Vet Rec. 1999 May 29;144(22):611-6.