By | 2012-11-11

Thoracic Ultrasonography

Thoracic ultrasonography currently is regarded as the preferred method to diagnose pleuropneumonia in the horse. Although the value of the art of thoracic auscultation and percussion should not be undermined, clinicians managing horses with thoracic disease recognize the limitations of these tools. With the widespread use of thoracic ultrasound, the equine practitioner currently has the ability to determine the presence of pleuropneumonia and the location and the extent of the disease. Although sector scanners are superior (preferably 3.5- to 5.0-MHz transducers), linear probes also can be used to evaluate the thorax in practice.

Thoracic ultrasonography in horses with pleuropneumonia allows the clinician to characterize the pleural fluid and to evaluate the severity of the underlying pulmonary disease. The appearance of the pleural fluid may range from anechoic to hypoechoic, depending on the relative cellularity (). This fluid usually is found in the most ventral portion of the thorax and causes compression of normal healthy lung parenchyma with retraction of the lung toward the pulmonary hilus. The larger the volume of the effusion is, the greater the amount of compression atelectasis and lung retraction that occurs.

The presence of adhesions, pleural thickening, pulmonary necrosis, and compression atelectasis also can be detected. Fibrin has a filmy to filamentous or frondlike appearance and is usually hypoechoic (). Fibrin deposited in layers or in weblike filamentous strands on surfaces of the lung, diaphragm, pericardium, and inner thoracic wall limits pleural fluid drainage. Dimpling of the normally smooth pleural surface results in the appearance of “comettail” artifacts, created by small accumulations of exudate, blood, mucus, or edema fluid. Pulmonary consolidation varies from dimpling of the pleural surface to large, wedge-shaped areas of sonolucent lung ().

Atelectatic lung is sonolucent and appears as a wedge of tissue floating in the pleural fluid. Necrotic lung appears gelatinous and lacks architectural integrity. Peripheral lung abscesses are identified ultrasonographically by their cavitated appearance and the absence of any normal pulmonary structures (vessels or bronchi) detected within. Although detection of a pneumothorax may be easy for the experienced ultrasonagrapher, it is not as easy for the less experienced. The gas-fluid interface can be imaged through simultaneous movement in a dorsal to ventral direction with respiration, the “curtain sign” reproducing the movements of the diaphragm. The dorsal air echo moves ventrally during inspiration, similar to the lowering of a curtain, gradually masking the underlying structures. A pneumothorax without pleural effusion is even more difficult to detect ultrasonographically. Although free bright gas echoes within the pleural fluid can occur after thoracentesis, they are more often seen with anaerobic infections or when sufficient necrosis has occurred in a segment of parenchyma to erode into an airway and form a bronchopleural fistula (). The absence of gas echoes in pleural fluid does not rule out the possibility that anaerobic infection may be present.

Ultrasonography is a valuable diagnostic aid in the evaluation of the pleura, lung, and mediastinum of horses with pleuropneumonia. The detection and further characterization of the above abnormalities improve the clinician’s ability to form a more accurate prognosis. Adhesions can be detected that ultimately may affect the horse’s return to its previous performance level.

Horses with compression atelectasis and a nonfibrinous pleuritis have an excellent prognosis for survival and return to performance. The detection of areas of consolidation, pulmonary necrosis, or abscesses increases the probable treatment and recovery time, and the prognosis for survival decreases as these areas become more extensive. Ultrasonography can be used as a guide to sample or drain the area with a large fluid accumulation or the least loculation. These patients often benefit from progressive scanning to assess response to treatment and the need for drainage.

Pleural Drainage

After selection of an appropriate antimicrobial agent, the next decision to be made is whether to drain the pleural space. Ideally the decision is based on an examination of the pleural fluid. If the pleural fluid is thick pus, drainage using a chest tube should be initiated. If the pleural fluid is not thick pus, but the Gram’s stain is positive and white blood cell (WBC) counts are elevated, pleural drainage is recommended. Another indication for therapeutic thoracocentesis is the relief of respiratory distress secondary to a pleural effusion.

Many options exist for thoracic drainage, including intermittent chest drainage, use of an indwelling chest tube, pleural lavage, pleuroscopy and debridement, open chest drainage/debridement with or without rib resection in the standing horse, open chest drainage/debridement under general anesthesia, and lung resection under general anesthesia. Drainage of a pleural effusion can be accomplished by use of a cannula, indwelling chest tubes, or a thoracostomy. Thoracostomy is reserved for severe abscessation of the pleural space. Thoracocentesis is accomplished easily in the field and may not need to be repeated unless considerable pleural effusion reaccumulates.

Indwelling chest tubes are indicated when continued pleural fluid accumulation makes intermittent thoracocentesis impractical. If properly placed and managed, indwelling chest tubes provide a method for frequent fluid removal and do not exacerbate the underlying pleuropneumonia or increase the production of pleural effusion. The chest entry site and end of the drainage tube must be maintained aseptically. A one-way flutter valve may be attached to allow for continuous drainage without leakage of air into the thorax. If a chest tube is placed aseptically and managed correctly, it can be maintained for several weeks. It should be removed as soon as it is no longer functional. Heparinization of tubing after drainage helps maintain patency. Local cellulitis may occur at the site of entry into the chest but is considered a minor complication. Bilateral pleural fluid accumulation requires bilateral drainage in most horses.

Open drainage or thoracostomy may be considered when tube drainage is inadequate. Open drainage should not begin too early in the disease. An incision is made in the intercostal space exposing the pleural cavity and causing a pneumothorax. If the inflammatory process has fused the visceral and parietal pleura adjacent to the drainage site, a pneumothorax may not develop. The wound is kept open for several weeks while the pleural space is flushed and treated as an open draining abscess.

Pleural Lavage

Pleural lavage may be helpful to dilute fluid and remove fibrin, debris, and necrotic tissue. Lavage apparently is most effective in subacute stages of pleuropneumonia before loculae develop; however, pleural lavage may help break down fibrous adhesions and establish communication between loculae. Care must be exercised that infused fluid communicates with the drainage tube. Lavage involves infusing fluid through a dorsally positioned tube and draining it through a ventrally positioned tube (). In addition, 10 L of sterile, warm lactated Ringer’s solution is infused into each affected hemithorax by gravity flow. After infusion, the ventrally placed chest tube is opened and the lavage fluid is allowed to drain. Pleural lavage probably is contraindicated in horses with bronchopleural communications because it may result in spread of septic debris up the airways. Coughing and drainage of lavage fluid from the nares during infusion suggest the presence of a bronchopleural communication.

Differentiation From Neoplasia

Although pleuropneumonia is the most common cause of pleural effusion in the horse, the second most common cause is neoplasia. Differentiating between the two conditions is a challenge for the equine clinician because similarities exist in the clinical signs and physical examination findings.

Pleuropneumonia effusions are more likely to have abnormal nucleated cell count more than 10,000/μl (usually >20,000/μl) with reater than 70% neutrophils. Bacteria frequently are seen both intra- and extracellularly. A putrid odor may be present.

Neoplastic effusions have variable nucleated cell count. If caused by lymphosarcoma, abnormal lymphocytes may predominate. However, neoplastic cell often are not readily apparent and a definitive diagnosis may be difficult. Rarely do neoplastic effusions have a putrid odor. Bacteria are seen rarely in the cytology preparations.

Once again, use of ultrasonography helps determine if neoplasia is responsible for the effusion. Fibrin most commonly is detected in association with pleuropneumonia but has been detected in horses with thoracic neoplasia. Mediastinal masses associated with neoplasia may be readily visible (). Abnormal solitary masses on the lung surface may be visible in horses with metastatic neoplastic disease.

Comprehensive Management

The primary goals in managing a horse with pleuropneumonia are to stop the underlying bacterial infection, remove the excess inflammatory exudate from the pleural cavity, and provide supportive care. Ideally an etiologic agent is identified from either the tracheobronchial aspirate or pleural fluid and antimicrobial sensitivity determined. Without bacterial culture results, broad-spectrum antibiotics should be used because many horses have mixed infections of both gram-positive and gram-negative and aerobic and anaerobic organisms. Commonly used therapy is penicillin combined with an aminoglycoside such as gentamicin, enrefloxacin, trimethoprim and sulfamethoxazole, or chloramphenicol. Because of the need for long-term therapy, initial intravenous or intramuscular antimicrobials may need to be followed by oral antimicrobials. Preferably the oral antimicrobials are not administered until the horse’s condition is stable and improving because blood levels obtained by this route are not as high as those achieved by use of intramuscular or intravenous administration.

Treatment of anaerobic pleuropneumonia is usually empiric because antimicrobial susceptibility testing of anaerobes is difficult due to their fastidious nutritive and atmospheric requirements. Thus familiarity with antimicrobial susceptibility patterns is helpful in formulating the treatment regimen when an anaerobe is suspected. The majority of anaerobic isolates are sensitive to relatively low concentrations (22,000 IU/kg IV q6h) of aqueous penicillin. Bacteroides fragilis is the only frequently encountered anaerobe that is routinely resistant to penicillin, although other members of the Bacteroides family are known to produce B lactamases and are potentially penicillin-resistant.

Chloramphenicol (50 mg/kg PO q4h) is effective against most aerobes and anaerobes that cause equine pleuropneumonia. However, because of human health concerns the availability of chloramphenicol may decrease. Metronidazole has in vitro activity against a variety of obligate anaerobes including B. fragilis. Pharmacokinetic studies indicate a dose of 15 mg/kg intravenously or orally four times a day is necessary to maintain adequate serum levels. Oral administration rapidly results in adequate serum levels and thus is an acceptable route of administration for horses with pleuropneumonia. Metronidazole is not effective against aerobes and therefore always should be used in combination therapy at a dose of 15 mg/kg every 6 to 8 hours. Side effects of metronidazole include loss of appetite and lethargy; use of the drug should be halted when these signs are observed. Aminoglycosides and enrofloxacin should not be considered for the treatment of pleuropneumonia caused by an anaerobe unless these drugs are used in combination therapy with penicillin.

Ancillary Treatment

Antiinflammatory agents help reduce pain and may decrease the production of pleural fluid. This in turn may encourage the horse to eat and maintain body weight. Flunixin meglumine (500 mg ql2-24h) or phenylbutazone (1-2 g q12h) is commonly used for this purpose. In this author’s opinion, corticosteroids are contraindicated for the treatment of bacterial pleuropneumonia. Rest and the provision of an adequate diet are important components of the treatment of pleuropneumonia. Because the disease course and period of treatment are usually prolonged, attempts should be made to encourage eating. Intravenous fluids may be indicated in the acute stages of the disease to treat dehydration resulting from anorexia and third-space losses into the thorax.