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Small animal thoracic radiology: Beyond pulmonary patterns (Proceedings)

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Aug 01, 2010

Objectives of the Presentation

      1. Provide practitioners with a basic interpretation paradigm for the evaluation of the small animal thorax.
      2. Provide practitioners with a basic overview and a simplified approach for describing pulmonary abnormalities.
      3. Evaluate anatomic distribution of disease processes as a valuable method for assessing pulmonary abnormalities.
      4. Think in terms of next step and how to get a cytologic diagnosis in as non-invasive a fashion as possible.

Key Etiologic and Pathophysiologic Points

      1. Technical factors including technique, phase of respiration and the positioning of the patient have to be taken into account when interpreting thoracic radiographs.
      2. Different disease processes will go through various pulmonary patterns depending on the stage and severity of the disease.
      3. One must think in terms of recheck thoracic radiographic evaluation on a daily basis if necessary to determine the response of the patient to therapy or the progression of the disease process.
      4. The pathophysiology of many pulmonary diseases do not equate with a specific pulmonary pattern.

Key Clinical Diagnostic Points

      1. One should try to compartmentalize radiographic abnormalities into extrathoracic, pleural, pulmonary and mediastinal (including cardiac), recognizing that any disease can have multicompartmental components.
      2. One should try to determine the anatomic location of pathology within the lung first and foremost and then worry about the pulmonary pattern. Even though there may be several pulmonary patterns, one must identify the dominant pattern in order to evaluate for differentials.
      3. Interstitial lung patterns will not exfoliate using transtracheal washes or bronchoalveolar lavage. Sometimes US guided lung aspirates may be beneficial for cytology, but this is not 100%.

Key Therapeutic Points

      1. Follow-up radiographs should be obtained in rapid succession if warranted (within the same day or within 24 to 48 hours).
      2. Edema from cardiogenic causes responds rapidly to diuretics unless the dog or cat has been in chronic failure and is resistant to standard diuretic therapy.
      3. Edema from non-cardiogenic causes will not clear rapidly, but may take several days to see significant resolution of pulmonary abnormalities.
      4. Key Prognostic Points
      5. Progressive alveolar disease associated with respiratory distress, DIC, pancreatitis and oxygen toxicity carries a grave prognosis.

Pulmonary patterns have the bane of radiology since the beginning of time (1896 when x-rays were discovered). Pulmonary pattern recognition is the most difficult concept to teach and the most difficult and frustrating to learn, yet, the pattern itself is only part of the puzzle. The recognition that the disease is actually within the pulmonary parenchyma and not in the pleural space, extra-thoracic structures or the mediastinum is the first step key step. To this end we will build on the basic interpretation paradigm of the thorax and look at the steps for evaluating the thoracic radiographs in dogs and cats. Technique and positioning are critical in this process and should be the cornerstone for high quality thoracic radiographs. Finally, we will tackle pulmonary patterns with an emphasis on describing what is on the thoracic radiographs and equally important, recognizing anything that is missing from the thoracic radiograph (a structure that I normally see that is not apparent on the images that one is interpreting).

In spite of these rather negative statements, thoracic radiography is still the most common first line assessment for diseases and conditions of the pleural space, pulmonary parenchyma and mediastinum. However, pulmonary patterns are one of the most frustrating areas for veterinarians. The reality is that pulmonary patterns do not equate to pathologic processes and any given disease can progress or regress through several pulmonary patterns or even have a combination of pulmonary patterns at the same time. The purpose of this lecture will be to rethink the use of pulmonary patterns and take a fresh approach at the evaluation of the pulmonary parenchyma in the dog and cat. Differences between the two species will be stressed. The emphasis will be placed on anatomic localization, description of the pulmonary change and then formulation of a reasonable differential diagnosis list. Cases will be used to illustrate the presentation important points.