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Evaluating causes of pleural effusion in cats

Domínguez Ruiz M, Vessières F, et al.  Characterization of and factors associated with causes of pleural effusion in cats. J Am Vet Med Assoc.  2018; 253:181-187.

The cat in respiratory distress is a common presentation in primary care and emergency clinical practice.  While there are a number of etiologies for feline respiratory distress, pleural effusion is a frequent cause. In this retrospective case series, the medical records of 380 cats diagnosed with pleural effusion at a European referral veterinary hospital between 2009 to 2014 were reviewed.

Normally a very small amount of serous fluid (0.1-0.3 mL/kg of body weight) is present in a cat’s pleural cavity and performs the important physiological function of lubricating intrathoracic organs, allowing them to glide over or around each other and the parietal pleura during respiration.  When there is a pleural effusion, an abnormally large volume of fluid fills the pleural space due to underlying disease.  This excessive fluid accumulation inhibits normal expansion of the lungs, causing respiratory dysfunction and distress.  Pleural effusions may have a variety of etiologies and compositions, and are characterized as transudates, modified transudates, exudates, chylous, hemorrhagic, or neoplastic, depending on their chemistry, specific gravity, protein content, included cell types, and etiologic agents present.  Most cases of pleural effusions in cats are due to congestive heart failure (CHF), neoplasia, pyothorax, feline infectious peritonitis (FIP), or idiopathic chylothorax.

These researchers used medical record data to investigate and characterize potential associations between causes of pleural effusion and clinical signs in cats.  Of the 380 cats included in the study, half (50.3%; 191/380) were castrated males; intact males comprised 9.7% (37/380), 124/380 (32.6%) were spayed females, and 27/380 (7.4%) were intact females.  Most cats were domestic shorthairs (321/380; 84.5%); additional breeds represented were Birman, Maine Coon, Siamese, Chartreux, Persian, British Shorthair, Norwegian Forest Cat, among others.  Lifestyle history was recorded for 233 of the 380 cats; half (51.1%; 119/233) were indoor only, and half (48.9%; 114/233) were allowed outdoors.  The cause of the pleural effusion was determined in all of the cats.

Clinical signs at hospital admission were recorded for all patients.  Difficulty breathing was reported for 172/380 (45.3%), anorexia or hyporexia for 76/380 (20.0%), lethargy for 41/380 (10.8%), coughing for 17/380 (4.5%), vomiting for 16/380 (4.2%), hind limb paresis or paralysis for 16/380 (4.2%), lateral recumbency for 7/380 (1.7%), trauma (road traffic accident or fall from height) in 11/380 (2.9%), as well as other miscellaneous clinical signs or reasons.  Although owners reported difficulty breathing in half of the study cats, respiratory distress was found at admission in 277/380 (72.9%) of the patients.

Of the 380 cats, 254 (66.8%) of the cats had a rectal temperature recorded when admitted to the hospital, with a mean value of 99.9 +2.9⁰F.  Hypothermia was present in 124 of these cats, and over half of the 124 (67; 54.0%) had CHF.

Most of the cats (304/380; 80.0%) had thoracic radiographs.  A pleural effusion was identified in virtually all (299/304) of the animals on thoracic radiographs; 275/299 (92.0%) had bilateral effusion, and 24/299 (8.0%) had unilateral effusion.  Thoracic radiographs demonstrated abnormalities of the lungs in 38/304 (12.5%), cardiomegaly in 16/304 (5.3%), diaphragmatic hernia in 9/304 (3.0%), thoracic mass in 6/304 (2.0%), and pneumothorax in 1/304 (0.3%).  Pleural effusion was confirmed in all of the cats who had thoracic ultrasonography  (128/380; 33.7%), including the 5 who had no radiographic evidence of pleural effusion.

Approximately half of the cats (183/380; 48.2%) had echocardiographic studies; of these, two-thirds (127/183; 69.4%) had obvious pleural effusion and 21/183 (11.5%) had pericardial effusion.  Of those cats with pericardial effusion, CHF was considered the cause of the pericardial effusion in most (18/21; 86%). In over ¾ of the cats who had echocardiography (141/183;77.0%), heart disease was identified as the cause of the pleural effusion.  Over half of the 70 pleural effusion patients who also had abdominal ultrasonography (41/70; 59%) were found to have abdominal effusion as well.

Chemical and cytological analysis of pleural fluid was performed in 199/380 cats (52.4%). Septic exudates were found in 55/199 (27.6%), neoplastic effusions in 50/199 (25.1%), chylous effusions in 30/199 (15.1%), sterile exudate in 29/199 (14.6%), transudate in 23/199 (11.6%), and modified transudate in 12/199 (6.0%).  Serologic testing for coronavirus was performed in 6/380 cats, but only 3 had high coronavirus titers.  Sixteen of the 380 cats were tested for FIP status with viral RT-PCR testing on pleural fluid, and 9 had a positive result.

Screening for hyperthyroidism  using total serum thyroxine was performed in 155/380 (40.8%); hyperthyroidism was identified in 20/155 (12.9%) with CHF, and 20/73 that were found to have left ventricular hypertrophy specifically.  Of the 66 cats tested for retroviruses, five were positive for the feline immunodeficiency virus, and five tested feline leukemia virus positive; no cats were positive for both retroviruses. Four of the five cats who were feline leukemia positive had mediastinal neoplasia.

Results of the study revealed that the two most common causes of pleural effusion in the patients were CHF (155/380; 40.8%) and neoplasia (98/380; 25.8%).  Pyothorax was identified in 55/380 (14.5%), idiopathic chylothorax in 24/380 (6.3%), and trauma in 16/380 (4.2%).  Other causes included FIP (12/380; 3.2%), nontraumatic diaphragmatic hernia (8/380; 2.1%), probable vasculopathy (5/380; 1.3%), probable uremic pleuritis (3/380; 0.8%), hypoproteinemia (2/380; 0.5%), and vitamin K antagonist toxicosis (2/380; 0.5%).  Those cats with pleural effusion of traumatic origin or due to FIP were significantly younger (mean age 2-3 years) than those with CHF (9.6 +4.7 years) or neoplasia (10.2 +4.4  years).

The prognosis of cats with pleural effusion is overall poor.  In this study 87/380 cats (22.9%) did not survive to hospital discharge; they either died or were euthanized while still in the hospital. Based on the results of this study, the most likely presenting historical signs in cats with pleural effusion are difficulty breathing and anorexia or hyporexia. Coughing is uncommon. In young cats trauma and FIP are common etiologies for pleural effusion; when a cat over 5-6 years old has a pleural effusion, the top-ranked differential diagnoses are CHF and neoplasia.  CHF is most likely if the cat is hypothermic, with a rectal temperature of 36.9 +1.2⁰C (98.4 +2.2⁰F), in contrast to those cats with other causes of pleural effusion (37.9 +1.2⁰C; 100.2 +2.2⁰F).  Additional studies are required to learn more about the apparent majority of cats who do survive past hospitalization, their survival times, treatments employed in this population, and whether low rectal temperature at admission is associated with a poorer prognosis for cats with CHF as it is in humans with CHF. [PJS]

See also:

Beatty J, Barrs V. Pleural effusion in the cat: a practical approach to determining aetiology.  J Feline Med Surg.  2010;12:693-707.