Unfolding the Mysteries of Pleural Space Disease

The pleural space can be a huge caus­ative factor in respiratory distress, according to Elizabeth Rozanski, DVM, DACVIM, DACVECC, associate professor of respiratory disease, hematology, and emergency and critical care at the Cummings School of Veterinary Medicine at Tufts University in North Grafton, Massachusetts.

Speaking at the 2018 New York State Spring Veterinary Medical Association Conference in Tarrytown, New York, Dr. Rozanski stressed the impor­tance of clinicians’ comfort in dealing with pleural space disease. “You are going to make your patients feel so much better, so much more quickly, by getting rid of pleural effusion,” she said. “When discussing the respiratory system, we often focus on the upper airway, lower airway, and parenchymal disease, and we may forget about pleural space disease.”

The pleural fluid lubricates and helps move the lungs around, Dr. Rozanski noted. In a healthy animal, just a drop or 2 of the fluid is present in the pleural cavity. Although the normal pleural space is nearly invisible on radiographs and ultrasound, it is a surprisingly active environment. The arteries and veins—in particu­lar, their endothelia—are vital for pleural fluid homeo­stasis. The homeostatic functions can be compromised by changes in oncotic and hydrostatic pressures, as well as by physical changes such as impediments to lymphatic drainage or trauma.

Causes and Diagnosis of Pleural Effusion

The presence of pleural space disease is just a sign­post pointing in the direction of the true disease state needing treatment, Dr. Rozanski said. As in all areas of medicine, the constellation of clinical signs can suggest both more and less common disease states. Clinical signs of pleural space disease seen in conjunc­tion with a heart murmur, a gallop rhythm, or jugular venous distention will likely point to heart disease. Signs of trauma would lead to suspicion of pneumo­thorax or hemothorax. A fever could indicate an infec­tious etiology causing a pyothorax. Cats with pyothorax may be drooling due to nausea, Dr. Rozanski said.

Pleural effusion due to heart failure is caused by an increase in hydrostatic pressure. For a patient with congestive heart failure and significant effusion, Dr. Rozanski recommended draining as much fluid as possible before administering diuretics. “Once you begin medications for heart failure [enalapril, furose­mide pimobendan, and/or spironolactone-hydrochlo­rothiazide], a decrease in hydrostatic pressure will result in the reabsorption of any fluid that was not removed prior to treatment,” she said.

In some cases, it may be unclear whether the primary disease process is respiratory or cardiac in nature. For these situations, Dr. Rozanski recommended using the SNAP Feline proBNP Test (Idexx) to differentiate between the 2. The test, currently available just for cats, can be performed on the pleural effusion itself and takes only a few minutes.

Neoplasia is another potential cause of pleural effu­sion. Cats with pleural effusion should be tested for feline leukemia virus (FeLV). Cats that are positive for FeLV can develop mediastinal lymphoma, which can obstruct lymphatic drainage, resulting in pleural effusion. It may be possible to palpate mediastinal lymphoma during the physical examination.

Neoplastic effusion can also develop from diffuse carcinomas or masses located in other areas of the thorax. One tip Dr. Rozanski offered: Examine all aspects of the thoracic radiographs, especially every rib. Any signs of asymmetry, bone lysis, or masses on the radiographs could indicate neoplasia as the cause of the effusion. Malignant effusion typically accumu­lates rapidly, and most pets with a neoplastic effu­sion do not survive longer than 1 to 2 months beyond their diagnosis.

The following clinical signs can indicate pleural space disease:

• A restrictive breathing pattern— short, shallow breaths and an increased respiratory rate and abdominal effort
• Head and neck extension
• Dull lung sounds

The role of diagnostics for pleural space disease is evolv­ing as more clinics integrate ultrasonography into their daily general and emergency prac­tices. Dr. Rozanski reminisced that before the advent of widely available ultrasonography, thoracentesis was performed on patients with as much a diagnostic goal as a therapeutic one. “Clinicians suspicious of the presence of fluid or air based on the physical exam or radiographs would often proceed rapidly to thoracen­tesis,” she said. “Today, ultrasound, if available, makes it much easier to assess where and how much fluid or air might be present before performing a thoracente­sis.” She was quick to add that ultrasonography is not necessary and that a diagnostic approach using thora­centesis and radiography is still appropriate.

Thoracic radiography allows for evaluation of the lung fields, heart shape and size, and other thoracic structures. On a dorsoventral thoracic radiograph, small amounts of pleural effusion can be seen most easily in the small triangle formed where the ribs meet the diaphragm. “Rounded lung borders could indicate scarring of the lung and pleura,” Dr. Rozanski said, cautioning that scarring could be caused by a chronic effusion, putting these patients at higher risk of pneu­mothorax if thoracentesis is performed.

Managing Pleural Space Disease

When excessive fluid or air is confirmed in the pleural space, the immediate treatment goal is to remove as much of it as possible. This allows the lungs to re-expand and improves ventilation. Although adverse effects are rare following thoracentesis, it is always wise to keep a few pointers in mind:

• Insert the needle cranial to the rib, because the intercostal vessels lie caudal to the rib.
• Avoid sampling too close to the heart—for obvious reasons.
• Consider and prepare for the possibility of collecting samples of the pleural fluid for cytology or culture, depending on the differential diagnosis.

“Most pleural space disease can be managed with thoracentesis,” Dr. Rozanski said. However, chest tubes also have their place and are commonly used after thoracic surgery or diaphragmatic hernia repair. A large pneumothorax that cannot keep up with the amount of air that must be removed is an indication for chest tube placement. Chest tubes are best placed under general anesthesia, Dr. Rozanski said, but she acknowledged that certain emergency situations necessitate using local anesthesia.

Impaired drainage of the lymphatics or rupture of the thoracic duct can cause chylothorax, which can be frustrating to treat. Surgical correction of chylothorax has met with variable success. If an owner is willing to take that route, Dr. Rozanski recommended doing the surgery right away. Chyle irritates the tissues and may result in a restrictive pleuritis. Low-fat diets and therapy with rutin may or may not help but likely will cause no harm, she said.

Dr. Rozanski emphasized the importance of thor­ough communication, so the owner understands the potential seriousness of any disease that may be pres­ent. “Nothing good causes pleural effusion,” she noted. Except for trauma, most of the conditions are chronic, with no quick fix.

Dr. Ambrose received her DVM from The Ohio State University. She has since worked in small animal clinical practice as well as in the veterinary pharmaceutical industry.