Aetiopathogenesis

Pyothorax (empyema) is an accumulation of purulent exudate within the pleural space, usually caused by bacterial infection. Sources of bacteria include penetrating chest trauma (especially bites), migrating foreign bodies (particularly plant awn material), oesophageal perforation and extension of infectious foci (both locally within the thorax and by haematogenous spread from remote sites); the aetiology is not identified in most cases. It is considered in hunting/retrieving breeds of dog that migrating inhaled foreign bodies may be the most common cause despite these being isolated from very few dogs subsequently undergoing surgery. In cats most cases have been associated with infection in other sites, especially the respiratory tract, which may affect the pleural space by direct extension or by haematogenous spread.

Clinical Signs

The most common clinical signs are dyspnoea/tachypnoea. Presence of fever and or cough is an inconsistent finding and pyothorax should not be excluded as a possibility based on absence of these. Lethargy, anorexia and weight loss are common. Occasionally animals will present as emergencies but the majority of dogs and cats presenting with pyothorax have a history of gradually worsening dyspnoea.

Diagnosis

Physical examination findings are variable and may include tachypnoea, dyspnoea, tachycardia, poor body condition, muffled heart sounds, pleural rub sounds and dehydration. Patients with pyothorax may be extremely fragile, and diagnostic evaluation should be delayed until administration of oxygen in a low-stress environment (e.g., an incubator or oxygen cage) has been carried out. Therapeutic thoracocentesis is appropriately performed in some animals prior to diagnostic imaging studies. Thoracic radiography commonly shows bilateral pleural effusion. Fluid pockets may be seen in some areas if encapsulated by fibrinous adhesions. Thoracic ultrasonography may demonstrate encapsulated fluid pockets, restrictive pleuritis or, rarely, presence of foreign material. Clinical laboratory findings are nonspecific. Mild anaemia, hypoalbuminaemia and leucocytosis consistent with an inflammatory response are the most common findings. A normal neutrophil count should not be used to exclude pyothorax.

Fluid removed by thoracocentesis should be evaluated cytologically for cell count/morphology, protein assessment and microscopic examination, aerobic and anaerobic culture. Gross appearance is highly variable from dark brown to frankly purulent fluid. Infections with filamentous Actinomyces and Nocardia spp. are frequently associated with the presence of pale yellow 'sulphur' granules. The fluid is an inflammatory exudate with a total protein >30 g/l, specific gravity (SG) >1.018 and a total nucleated cell count >3-7 x 10⁹/l. Microscopically the exudate comprises degenerate neutrophils, and bacterial organisms are seen with Gram stain or modified Wright Giemsa in approximately 60-80% of cases. Filamentous organisms are found in 10-20% of cases; they are more commonly isolated from dogs than cats. The principal differential diagnosis in the cat is effusive feline infectious peritonitis (non-septic exudate). Culture is positive in 69-96% of cases; negative cultures may be seen in cases previously treated with antibiotics. Many cases of pyothorax involve more than one bacterial species (see management). Presence of Actinomyces spp. is often taken to indicate presence of migrating plant awn foreign bodies, though this is based on circumstantial evidence. Culture and sensitivity can help guide treatment choices, but rational treatment should be undertaken prior to receiving these results if the fluid cytology is supportive.

Emergency/Acute Management

Dyspnoeic dogs and cats should receive a 'hands-off' approach with provision of oxygen therapy which should precede investigation. If clinical examination indicates the presence of pleural fluid ('choppy' dyspnoea, absent lung sounds ventrally, percussive dullness ventrally), then therapeutic thoracocentesis using a butterfly needle, extension set and syringe should be performed. If exudate is so thick that this is insufficient to permit drainage, a larger over-the-needle catheter or specific thoracocentesis set may be used or it may be necessary to immediately place a thoracostomy tube. It is usually possible to drain a substantial amount of exudate from the pleural space and alleviate dyspnoea using needle drainage prior to elective placement of a longer-term thoracostomy tube.

Short-Term Management

Many patients with pyothorax have been anorexic and unwell for some time and have a systemic inflammatory response syndrome (SIRS) and intravenous fluid therapy should be given.

Antimicrobial therapy is given based on expected bacterial organisms involved and is subsequently guided by culture and sensitivity. Obligate anaerobes are isolated from 60% of dogs and 89% of cats with positive cultures and suitable antibiotic choices for these include potentiated amoxicillin and metronidazole. Some antibiotics (e.g., fluoroquinolones, aminoglycosides) have limited or no efficacy against anaerobes. Additionally gram-negative aerobes of enteric origin such as Escherichia coli are frequently isolated from dogs and Pasteurella multocida is frequently isolated from both cats and, less commonly, dogs. Isolates of Pasteurella multocida are sensitive to ampicillin, potentiated amoxicillin, enrofloxacin, tetracycline and trimethoprim-sulphonamide. Gram-negative aerobes of enteric origin are frequently susceptible to fluoroquinolones, though resistance to these appears to be increasing and aminoglycosides and third-generation cephalosporins may be considered where resistance is seen on culture and sensitivity. The intravenous route of administration is preferred in severely ill cats and dogs. Strict dosage intervals in antibiotics whose effects are time-dependent (e.g., potentiated amoxicillin) should be used.

Most authorities agree that regular drainage of septic effusions via indwelling thoracostomy tubes is desirable in hastening recovery, and preventing fibrinous pleural adhesions and restrictive pleuritis. However, there are some reports that this may not be necessary in all cases which warrant further examination. The authors' practice is to relieve as much fluid as possible by needle thoracocentesis, then, once the patient is stabilised and hydrated, to place, under short general anaesthesia, a wide-bore indwelling thoracostomy tube for continued drainage. A unilateral tube is usually placed after clipping and sterile preparation of one hemithorax. Additional local anaesthesia via infiltration within the route of the tube placement or an intercostal nerve block should be considered. The tube is clamped, and positioning is confirmed radiographically. The tube is fixed in position with a Chinese finger-trap suture, clamped and a spigot with adaptor firmly attached. This can be further secured to the tube with glue, cable ties or orthopaedic wire. It is then bandaged securely in place. Great care must be taken to secure the tube to prevent patient dislodgement, accidental entry of air into the tube and subsequent development of pneumothorax. Tube security must be regularly monitored.

The effusion can be drained continuously by an underwater drainage system or, more practically, by intermittent manual drainage. If continuous drainage is used a suction unit with low-pressure settings (10-15 cmH₂O) must be used. With intermittent manual drainage the effusion is initially gently drained completely every 2-4 hours, the volume recorded and the cytology of fluid examined microscopically once daily; progressive improvement in bacterial numbers should be appreciated over time. Frequency of drainage is decreased as appropriate. Failure to remove expected quantities of fluid usually result from tube blockage due to fibrin deposits or tube kinking. Gentle flushing of warmed sterile saline down the tube may help correct this. Some authorities recommend lavaging the pleural space twice daily with 5-10 ml/kg warmed sterile saline infused slowly via the tube over several minutes. The patient is gently rolled and the fluid retrieved. It is not clear to what extent this may accelerate recovery but there is limited evidence that modest reduction in the duration of tube drainage may result.

Pyothorax is painful and thoracostomy tubes are uncomfortable, especially for cats. Provision of analgesia should continue for the duration of tube placement and administration of intrapleural analgesia with bupivacaine via the thoracostomy tube should be considered. There is no good rationale for administering antibiotics via the chest tube as absorption and distribution are unpredictable. Drainage is continued until the amount of fluid retrieved decreases substantially and radiography fails to show re-accumulation. Chest drains usually 'irritate' the production of about 2 ml/kg/24hr of fluid and reduction in amount drained per day to below or near this amount should prompt removal. Thoracostomy tubes usually stay in place for 4-10 days depending on productiveness. Failure of fluid production to substantially reduce beyond this time should prompt consideration of surgery.

Surgery (thoracotomy, evaluation for presence of foreign material, resection of abscessated/ fibrinous material and debridement) may also be considered where there is evidence (e.g., radiographic, ultrasonographic) of a foreign body, where clinical improvement within 7-10 days is not seen, where tube drainage is inadequate due to focal pocketing of fluid or in cases where relapse is seen. Surgery has been considered in one report to offer a better outcome to medical therapy. Foreign bodies are retrieved from less than 40% of cases undergoing surgery.

Chronic Management

Once fluid production has diminished, the thoracostomy tube is removed. When good hydration, oral food and water intake is established, antibiosis is continued with oral medications in the home environment. Antibiotics are continued for at least 6 weeks, though if Actinomyces or Nocardia are isolated up to 16 weeks' therapy is recommended. Respiratory pattern and auscultation characteristics should be regularly assessed during convalescence. Signs of deterioration should be evaluated by repeat thoracic radiography.

Prognosis

Between 66 and 78% of cats can be expected to survive; the prognosis appears worst in cats younger than 1 year and which have lower heart rates on admission. In one study, cases that survived beyond the first 48 hours of diagnosis, had a good outcome. Survival of dogs with pyothorax is currently reported to be about 80-90%.

References

1.  Barrs VR, Allan GS, et al. Feline pyothorax: a retrospective study of 27 cases in Australia. Journal of Feline Medicine and Surgey 2005; 7: 211-222.

2.  Demetriou JL, Foale RD, et al. Canine and feline pyothorax: a retrospective study of 50 cases in the UK and Ireland. Journal of Small Animal Practice 2002; 43: 388-394.

3.  Scott JA, Macintire DK. Canine pyothorax: pleural anatomy and pathophysiology. Pyothorax: clinical presentation, diagnosis and treatment. Compendium of Continuing Education for the Practising Veterinarian 2003; 25: 172-193.

4.  Waddell LS, Brady CA, Drobatz KL. Risk factors, prognostic indicators, and outcome of pyothorax in cats: 80 cases (1986-1999). Journal of the American Veterinary Medical Association 2002; 221: 819-824.

5.  Walker AL, Jang SS, Hirsh DC. Bacteria associated with pyothorax of dogs and cats: 98 cases (1989-1998). Journal of the American Veterinary Medical Association 2000; 216: 359-363.