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Abstract

Practical relevance Diverse disease processes result in sufficient fluid accumulation within the pleural space to cause respiratory compromise. Determining the underlying aetiology is key to appropriate management. This review outlines a practical approach to cases of pleural effusion, focusing on early recognition and confirmation of pleural space disease, stabilisation of the patient and logical diagnostic investigation. It emphasises the importance of a holistic approach, incorporating fluid analysis with other clinical data to determine the underlying aetiology.

Clinical challenges Cats with pleural effusion often have severe respiratory compromise at presentation. Careful handling and prompt and adequate stabilisation, incorporating supplemental oxygen and therapeutic thoracocentesis, is essential to avoid respiratory failure. The typical, stepwise approach to the case must be adapted and the clinician may have to proceed, at least initially, without the luxury of information gained from a full history and physical examination. The challenge is to juggle stabilisation, localisation and confirmation of pleural effusion, owner communication and minimally invasive examination while remaining vigilant for clues that allow ranking of the differentials to formulate a diagnostic plan.

Evidence base Appropriately designed studies to determine the utility of diagnostic techniques in cases with confirmed aetiology are limited. The evidence supporting this review is grade II, III and IV, comprising a small number of prospective studies, several case series, reviews, extrapolation from other species, pathophysiological justification and combined clinical experience of those working in the field.

Pathophysiology — and its diagnostic relevance

The pleural space is a potential space surrounding the lungs. It is formed by the visceral and parietal pleura, which are serous membranes that line the outer surface of the lungs and inner surface of the thoracic cavity, respectively. A small volume of fluid within the pleural space lubricates movement of intrathoracic structures during respiration. Pleural fluid formation and drainage is governed by Starling's forces. Any process that disrupts capillary or interstitial hydrostatic or oncotic pressures, lymphatic drainage or vessel integrity can result in net fluid accumulation, as can extension of pathological fluid accumulation from adjacent structures (eg, parapneumonic spread of infection). Because the mediastinum is fenestrated in cats (and dogs), fluid usually accumulates bilaterally unless it becomes loculated due to inflammation.¹ Ongoing fluid accumulation eventually results in pulmonary atelectasis and hypoventilation. It is stated that 30 ml/kg of pleural effusion causes subtle dyspnoea and 60 ml/kg results in obvious dyspnoea,² although evidence for these figures is not presented.

In human medicine, pleural effusions formed by an imbalance between oncotic and hydrostatic forces are defined as transudates and those resulting from increased capillary permeability as exudates. There is a good correlation between biochemical characteristics of the fluid and serum, principally protein and lactate dehydrogenase (LDH) measurements, and pathophysiology, such that these values can be relied on to predict fairly accurately whether fluid has accumulated due to a transudative or an exudative process.³

MULTIMEDIA

Four video recordings to aid recognition of pleural effusion (ie, showing respiratory patterns in cats with and without pleural effusion and jugular pulsation) are included in the online version of this article at doi:10.1016/j.jfms.2010.07.013

This is not the case in veterinary medicine where controlled studies correlating fluid characteristics with aetiology are limited.^4–9 Effusions in animals are typically classified according to their cell count and protein content as transudates (low protein, low cell count), exudates (high protein, high cell count) or modified transudates (intermediate) (Table 1).¹⁰ The relationship between fluid characteristics and the underlying pathophysiological process is less well defined and there is often overlap between fluid type and aetiology (Table 2). Chylous pleural effusions (chylothorax) provide a good example. They result from leakage of chyle from the lymphatics and can occur with several underlying disease processes including congestive heart failure (CHF), neoplasia and trauma; in cats, however, they are commonly idiopathic.¹¹ Using total protein (TP) and total nucleated cell count (TNCC), chylous effusions may be classified as modified transudates or as exudates.¹²

TABLE 1

Classification of pleural fluid based on total protein (TP) concentration and total nucleated cell count (TNCC)¹⁰

Classification	TP g/l	TNCC/μl	Comments
Transudate	<25	<1000	Uncommon in cats. Rule out: CHF Hypoalbuminaemia Fluid overload
Modified transudate	25–35	500–10,000	Least specific. Differentials ranked according to clinical criteria and gross characteristics of fluid to guide investigation
Exudate	>30	>5000	May be subclassified as septic, non-septic, chylous∗, neoplastic. Rule out: FIP Infection Neoplasia

∗

Chylous effusions may have TP/TNCC of a modified transudate or an exudate. They are defined in cats (and dogs) as effusions with a triglyceride concentration of >100 mg/dl (>1.12 mmol/l)⁶. CHF = congestive heart failure

TABLE 2

Clinical features of common causes of pleural effusion in cats

Underlying aetiology or disease process	Signalment	History	Physical findings not related to pleural effusion	Fluid characteristics
Pyothorax/infectious pleuritis	Usually young (mean 4–6 years) Any age or breed can be affected	Variable Mean duration of signs 1–2 weeks May present acutely Dyspnoea, cough Lethargy, reduced appetite, weight loss Previous upper respiratory tract infection (25%)	Pyrexia, dehydration, poor body condition If sepsis: hypoglycaemia, hypothermia, jaundice, bradycardia	Exudate
Right and/or left congestive heart failure	Any age or breed, depending on the underlying cardiac disease	Variable, depends on aetiology of heart disease. With secondary cardiomyopathies (eg, thyrotoxic) may see signs of primary disease	Congenital: may have small stature Tachycardia, murmur, gallop, arrhythmia, jugular distension/ pulse, cyanosis, ascites/hepatomegaly May have no other signs of cardiovascular dysfunction	Variable: transudate, modified transudate, chylous
Effusive FIP	70% <1 year old. Increased risk in some breeding lines and in entire cats Common in mixed-breed cats	Recent multicat environment and/or stressor (neutering, rehoming). Non-specific lethargy, reduced appetite, weight loss. Abdominal distension	Pyrexia, poor body condition, peritoneal effusion, ocular changes, neurological signs	High protein (>35 g/l), low cellular (<5000/μl)
Neoplasia	Bronchopulmonary and thymoma: usually >10 years Mediastinal lymphoma: young Siamese breed cats	Variable, non-specific, cough, dyspnoea	Distant metastases (digits, eyes, skeletal muscle) Poor body condition Decreased thoracic compressibility Paraneoplastic syndromes	Variable: modified transudate, exudate, chylous
Idiopathic chylothorax	Any age, any breed, Siamese may have increased risk	Cough, dyspnoea	None	Fluid triglyceride >100 mg/dl (1.12 mmol/l) TP and TNCC of modified transudate or exudate

FIP = feline infectious peritonitis, TP = total protein, TNCC = total nucleated cell count

It is important not to focus on fluid analysis in isolation. While properties of the fluid may be diagnostic in cases of pyothorax, neoplasia and FIP, in other cases they must be interpreted together with other clinical information.

More useful clinically are the gross characteristics, triglyceride and cholesterol concentrations of fluid and serum, and the presence of chylomicrons. On cytology small lymphocytes predominate initially but neutrophil and monocyte counts increase over time.¹¹ The results of fluid analysis are useful only in as much as they elucidate the underlying disease process. It is important to appreciate their value and limitations (discussed later). In cases of pyothorax, neoplasia and feline infectious peritonitis (FIP), fluid characteristics may be diagnostic, whereas determining that the fluid is a modified transudate is non-specific and must be interpreted in the light of other clinical data.

Aetiology

Fortunately, the list of common causes of a moderate to large volume pleural effusion in the cat is relatively short. While incidence data are not available, FIP, CHF, pyothorax and neoplasia together accounted for 88–100% of 265 cases with a confirmed aetiology (Table 3).^4–9 In two studies, 9/19 cats (12.5% of total) and 3/7 cats (15% of total) were defined as having idiopathic chylothorax where an aetiology could not be determined despite investigation.^6,7

TABLE 3

Summary of published case series of feline pleural effusion demonstrating the major underlying aetiologies and disease processes

Date collected (and published source)	Number of cases with definitive diagnosis	FIP (% of total)	CHF (% of total)	Pyothorax (% of total)	Neoplasia (% of total)	% of total due to FIP, CHF, pyothorax or neoplasia combined
2002–03⁹	19	5	47	11	26	100
1989–92⁸	65	25	23	12	34	94
1987–95⁷	63	13	14	24	37	88
1975–77⁵	54	4	17	17	61	99
1972–74⁴	64	10	44	4	34	92

FIP = feline infectious peritonitis, CHF = congestive heart failure

CHF

Pleural effusion can result from left and/or right CHF causing a transudate, a modified transudate or a chylous effusion. Increased ventricular diastolic pressure results in increased capillary hydrostatic pressure in the systemic and/or pulmonary circulation. The visceral pleura drain into the pulmonary veins in both cats and dogs but pleural effusion from left CHF (LHF) seems to be more common in cats than dogs.^1,13 As parietal pleural veins drain into the systemic venous circuit, right CHF (RHF) may also cause pleural effusion. RHF is a fairly common cause of chylothorax in the cat.^6,14,15 Chylothorax in this setting presumably results from increased pressure in the major lymphatics as the thoracic duct terminates in the left external jugular veins or jugulosubclavian angle.

Diverse underlying cardiac problems can result in heart failure. In cats heart failure is often due to diastolic dysfunction.¹³ Common cardiomyopathies (hypertrophic, restrictive, unclassified, thyrotoxic) should be ruled out first. While cardiac signs are common in hyperthyroid cats (eg, tachycardia, murmur, gallop), earlier diagnosis has resulted in a decrease in the frequency of CHF in cats with thyrotoxicosis.^16,17 One study reported a significant decline in the prevalence of CHF in hyperthyroid cats from 12% in 1983 to 2% in 1993.¹⁶

Fortunately, the list of common causes of a moderate to large volume pleural effusion in the cat is relatively short.

Neoplasia

Mediastinal, bronchopulmonary or primary pleural neoplasia may cause pleural effusion with the characteristics of a modified transudate, an exudate or a chylous effusion. In published case series, 26–61% of feline pleural effusions were associated with neoplasia (Table 3). Mediastinal lymphoma accounted for the majority of neoplasia-associated pleural effusions (60–79%) reported in cats between 1972 and 1995.^4,5,7,8 Although the prevalence of feline leukaemia virus (FeLV) associated mediastinal lymphoma has declined with that of FeLV, an increased risk of developing mediastinal lymphoma has been documented in young cats of the Siamese breed group, independent of their FeLV status (Table 2).¹⁸ Other neoplastic and non-neoplastic mediastinal masses such as thymoma, thymolipoma, thymic hyperplasia and developmental anomalies should be considered where mediastinal lesions are identified.^19–21

Definitive diagnosis of mediastinal lesions is essential to determine prognosis and guide treatment options. In addition to fluid production, mediastinal lesions may cause dyspnoea due to a space-occupying mass effect. Regardless, they are included here because they often occur concurrently, presentation and diagnostics are similar and it can be difficult to differentiate a mass from fluid using radiology alone. Primary and, less commonly, secondary bronchopulmonary carcinoma makes up the bulk of the other neoplasms. Primary pleural neoplasia is rarely reported in the cat.²²

Infectious pleuritis

Infectious pleuritis is caused by obligate and facultative anaerobes of oropharyngeal origin in more than 80% of cases, resulting in accumulation of a purulent exudate (pyothorax). Parapneumonic spread of infection following colonisation of lung tissue by oropharyngeal flora seems to be the most frequent cause of feline pyothorax.²³

FIP

The effusive form of FIP is associated with widespread vasculitis. Subsequent exudation of high protein effusion can occur in peritoneal, pleural or pericardial cavities.

Other

Other documented causes of pleural fluid accumulation in the cat include trauma, coagulopathy, peritoneopericardial diaphragmatic hernia, uraemia, pulmonary thromboembolism, lung lobe torsion, possible extension from a perinephric pseudocyst, pancreatitis, glomerulonephropathies and Aelurostongylus abstrusus infection.^{2,7,8,24–29} Trauma, which can result in effusion secondarily to haemorrhage, diaphragmatic hernia, thoracic duct rupture or, rarely, urinothorax,⁷ was the underlying cause in 8% of cases in one study.³⁰ Although heartworm disease is often listed as a possible aetiology in the cat, evidence that pleural effusion occurs as a consequence of natural infection in this species is limited.^31,32

Undetermined

In some cases the aetiology may remain elusive because the mechanism is recognised as being idiopathic (for chylothorax) or because of lack of access to advanced diagnostic techniques. Pulmonary thromboembolism is a common cause of pleural effusion in humans but has been documented infrequently as a cause in the cat where there is limited availability of computed tomography (CT) angiography.² Even so, 15–20% of pleural effusions in humans remain undiagnosed even after pleural biopsy.³³

A practical classification to direct the clinical investigation

A clinically useful classification for pleural effusion in the cat is presented in Table 2. Based on this, the quality of data collection from a minimally invasive examination can be maximised and differential diagnoses ranked for the patient. This directs the clinical investigation and facilitates expedient identification of the underlying aetiology so that owners can be informed regarding management options and the prognosis for their pet.

A restrictive (rapid, shallow) respiratory pattern with increased inspiratory effort is typical of pleural space disease — but not pathognomonic.

Recognising pleural space disease

Observation

Observation of the respiratory pattern in a dyspnoeic patient is the first step in localising the problem. The respiratory rate and depth, the phase of respiration that is laboured (inspiratory, expiratory or both), the presence or absence of audible respiratory noise (stridor, stertor, wheeze) and the presence or absence of respiratory noises on auscultation (referred large airway sounds, breath sounds, wheezes, crackles) should be noted (Table 4).

TABLE 4

Assessment of respiratory patterns in dyspnoeic cats

	Pleural space disease	Extrathoracic airway disease	Intrathoracic large airway disease (trachea/mainstem bronchi)	Bronchial disease	Pulmonary parenchymal disease
Dyspnoea	Inspiratory	Inspiratory	Expiratory	Expiratory	Inspiratory ± expiratory
Audible respiratory noise	No	Stridor or stertor	± Expiratory wheeze/cough	± Expiratory wheeze/cough	No
Thoracic auscultatory noise	No ↓ breath sounds	↑ Breath sounds (referred upper airway noise)	↑ Breath sounds (referred upper airway noise)	Expiratory wheeze/↑ breath sounds ± crackles	↑ Breath sounds; ± crackles ± wheezes
Respiratory rate	Rapid	Normal to mildly increased	Normal to mildly increased	Rapid	Rapid
Respiratory pattern	Restrictive	Obstructive	Obstructive	Mixed	Restrictive or mixed

Cats with significant pleural space disease adopt a sternal position with abducted elbows. A restrictive (rapid, shallow) respiratory pattern with increased inspiratory effort is typical (see video 1, doi:10.1016/j.jfms.2010. 07.013). Once air is inhaled, it can be exhaled without any impediment or obstruction. A subtle increase in inspiratory excursions may be easier to discern when the cat is viewed from above. An increased respiratory rate and decreased inspiratory volume minimises respiratory effort in non-compliant lungs. Therefore, restrictive respiratory patterns can be seen in cats with pulmonary parenchymal pathology (eg, pulmonary oedema, pneumonia) and with disorders of the chest wall, diaphragm, peritoneal cavity or peripheral nerves (Table 4).

Upper respiratory tract (URT) obstruction is the other major cause of inspiratory dyspnoea. In this case respiration is usually slow and deep (obstructive) and accompanied by other URT signs such as stertor or stridor (see video 2, doi:10.1016/j.jfms.2010.07.013).

Breath sounds are decreased or absent with pleural space disease. Where there is effusion, the reduction in breath sounds is often more pronounced ventrally, and a fluid line may be appreciated.

Auscultation

Auscultation helps to distinguish pleural space disease from pulmonary parenchymal disease when there is a restrictive pattern. Breath sounds are decreased or absent with pleural space disease. Differential diagnoses then include pleural effusion, pneumothorax, intrathoracic mass or diaphragmatic hernia. Where there is effusion, the reduction in breath sounds is often more pronounced ventrally and a fluid line may be appreciated on auscultation or percussion. Concurrent pulmonary oedema may contribute to the dyspnoea in LHF and result in auscultable crackles dorsally. Pleural effusion or pericardial effusion can cause muffled heart sounds. An intrathoracic mass or focal accumulation of fluid can displace the cardiac apex beat. With diaphragmatic hernia, borborygmi may be auscultated in the thorax.

In contrast, pulmonary parenchymal diseases severe enough to cause restrictive respiration are characterised by increased lung sounds, crackles or wheezes on auscultation.

Auscultation over the trachea and larynx should be performed routinely to differentiate sounds referable to URT obstruction from sounds emanating from the LRT.

Percussion

Thoracic percussion is more difficult to perform in cats than dogs because of their small stature. If tolerated, it can be a useful diagnostic tool. Percussion is performed by firmly tapping one or two fingers placed against an intercostal space and comparing the resonance of the sound produced at several different locations over the thorax bilaterally. A more resonant sound dorsally (‘drum-like’) provides evidence for pneumothorax. A less resonant or dull sound ventrally provides evidence for loss of pulmonary aeration.

Confirming pleural space disease

The presence of pleural effusion can be rapidly and noninvasively confirmed with either a single dorsoventral (DV) radiographic view or thoracic ultrasonography. Maintaining the patient in sternal recumbency avoids positional atelectasis and requires minimal restraint. A single DV view will confirm pleural space disease and differentiate between pleural effusion, pneumothorax or diaphragmatic hernia. The increased oxygen demand associated with additional restraint for symmetric positioning is not usually justified prior to thoracocentesis as significant pleural space disease will be apparent, even from slightly rotated views. Horizontal beam (lateral) views with the cat in a standing position similarly require minimal restraint.

The radiographic signs of pleural effusion are listed below.³⁴ Unilateral effusion should raise the index of suspicion for pyothorax or chylothorax.^11,23

Thoracic ultrasonography is similarly useful for confirming a moderate to large volume pleural effusion, diaphragmatic hernia and, in the hands of a skilled operator, pneumothorax.³⁵

Radiographic signs of pleural effusion

Interlobar fissure lines

Rounding of the lung margins at the costophrenic angles

Retraction of the lobar borders from the thoracic wall

Widening of the mediastinum

Scalloping of the lung margins at the sternal border

Effacing of cardiac silhouette (silhouette sign)

Dorsal displacement of the trachea

A dorsoventral radiographic view rapidly confirms moderate to large volume pleural effusion and requires minimal restraint

Approach to the dyspnoeic cat with pleural effusion

The priorities for the clinician managing a cat with pleural effusion are discussed in the following sections beginning, first and foremost, with stabilisation of the patient. Thereafter, data obtained from signalment, history and physical examination is used to rank the list of common differentials. This ranking is further informed by the gross characteristics of fluid obtained at thoracocentesis.

Stabilisation

It is not unusual for dyspnoea to be subtle or absent prior to arrival at the clinic. As pleural fluid accumulates the haemoglobin saturation falls gradually (Fig 1). Combined with reduced activity, this allows the cat to compensate initially. When the arterial partial pressure of oxygen (PaO₂) falls below 60 mmHg, haemoglobin saturtion falls precipitously.³⁶ This will be reached gradually as underlying disease progresses — however, an acute increase in oxygen demand, such as that associated with handling, can precipitate decompensation. By the time patients with pleural effusion show signs of respiratory distress (open-mouth breathing, agitation, vocalisation, extension of the neck, failure to maintain sternal positioning) little reserve remains.

FIG 1

Oxygen dissociation curve (see text for explanation)³⁶

Cats showing clinical signs of pleural space disease have significantly compromised respiration and are at risk of respiratory failure.

Stabilisation techniques

Reduce oxygen requirements Placing the patient in a cool, quiet environment and minimising handling will reduce oxygen demand.

Supplemental oxygen Options for short term oxygen delivery to the dyspnoeic cat include oxygen chamber, mask and flow-by (see below). The method that is best tolerated by the patient should be used. Struggling must be avoided. An oxygen chamber is a useful way to deliver oxygen without the need for restraint. These chambers are available commercially (Buster ICU Cage, DLC Australia) or can be constructed from perspex. Some cats will tolerate a mask held against the face, especially if the rubber gasket has been removed. Flow-by oxygen is best tolerated but least effective.³⁷ Use of intranasal catheters should be avoided in conscious dyspnoeic cats as, even with local anaesthesia, placement is usually resented. Intranasal catheters are, however, useful for delivery of humidified oxygen after stabilisation (eg, anaesthetic recovery following indwelling thoracic drain placement).

Intravenous access This should be achieved at the earliest opportunity.

Light sedation Dyspnoeic cats may benefit from light sedation (eg, acepromazine with one or morphine, methadone or butorphanol) to reduce anxiety. Cats in respiratory distress may panic when handled.

Therapeutic thoracocentesis Usually this is carried out after imaging. However, where the suspicion for pleural effusion is high and the dyspnoea is severe, unguided needle thoracocentesis can be life-saving and the risk of causing significant harm is minimal.

Monitoring Respiratory rate and depth should be monitored so that changes can be readily appreciated. The utility of pulse oximetry is limited in conscious cats by movement and failure to tolerate the probe. Pigmented skin, severe anaemia and reduced peripheral perfusion are other potential sources of error.³⁸

Ventilation Where hypoventilation cannot be controlled by other means, the clinician should be prepared to anaesthetise, intubate and ventilate the patient.

Supplemental oxygen delivery techniques

Oxygen chamber

Mask delivery

Flow-by

Thus cats showing clinical signs of pleural space disease have significantly compromised respiration and are at risk of respiratory failure. This reinforces the importance of triage at reception. All cats should be observed on arrival. Where respiratory distress is noted or suspected, immediate stabilisation is indicated. The choice of the various techniques (see box on page 698), and order in which they are carried out, depends on assessment of the individual patient.

Dependent on the underlying disease process, other considerations for patient stabilisation include identification and correction of hypothermia, hypotension, hypoglycaemia, and fluid and electrolyte imbalances.

History

Historical abnormalities may relate to the presence of the pleural effusion and/or the underlying disease. Non-specific signs such as reduced appetite, weight loss or lethargy are common.^7,11,23 Dyspnoea is noted in 60–80% of cases and coughing in up to 30% of cats with chylothorax and pyothorax.^7,11,47 Coughing may be associated with the presence of effusion or it may be related to the aetiology (eg, concurrent bronchopulmonary infection or neoplasia, or airway compression from a cranial mediastinal mass). Coughing due to cardiac disease is not as commonly recognised in cats as dogs, but it can occur.⁴⁸ It is the authors' experience that owners may not recognise coughing in cats, but rather attribute this sign to ‘hairballs’ or dry retching. Careful questioning is required.

Signalment — an aid to ranking common differentials

Signalment data cannot be used to exclude an aetiology but may assist in ranking the common differentials (Table 2).

In young cats, effusive FIP, mediastinal lymphoma and pyothorax are likely, whereas the median age for developing bronchopulmonary neoplasia, thymoma and decompensated thyrotoxic cardiomyopathy is 11–13 years.^39–41 Approximately 70% of FIP cases occur in cats less than 1 year of age.⁴²

An increased risk of developing FIP has been demonstrated in purebred and in entire cats and there is evidence of heritability in some lines.^42,43 However, FIP is often diagnosed in domestic crossbreeds, and older cats may sometimes be affected.

Cats of the Siamese breed are overrepresented for both mediastinal lymphoma (young cats) and chylothorax.^11,18,44

Familial cardiomyopathies have been identified in many breeds, including the Maine Coon, Ragdoll, British and American Shorthair, Bengal, Sphynx, Norwegian Forest and Siberian.⁴⁵ Less information is available regarding breed predispositions for other types of cardiac disease. The Chartreux breed has been identified as being overrepresented for tricuspid dysplasia.⁴⁶

While congenital aetiologies are more likely in young cats, cats with congenital, heritable and acquired cardiac disease can develop heart failure at any age.

Some acquired cardiac problems are age-related (eg, thyrotoxic cardiomyopathy).

A previous history of upper respiratory tract infection in a young cat would raise suspicion for pyothorax.²³ Recent history of a multicat environment and/or a stressor, such as neutering or rehoming, in a young cat would increase suspicion for FIP.⁴² Owners of cats >8 years old should be questioned about signs related to hyperthyroidism such as polyphagia, weight loss, polyuria, polydipsia and gastrointestinal signs. Oesophageal signs including dysphagia, regurgitation and ptyalism may reflect compression of the oesophagus from an intrathoracic mass. Haemothorax would be less likely in cats without outdoor access or exposure to anticoagulant rodenticides. There is conflicting data regarding whether pyothorax is more common in outdoor or indoor cats.^7,49

Clinical examination

The importance of gentle handling of cats in respiratory distress cannot be overemphasised. Struggling must be avoided. Some tests, such as rectal temperature, may need to wait until the patient is stable. Nonetheless, useful information can be gained non-invasively to help rank the common aetiologies.

Examination of the cardiovascular system

Mucous membranes should be examined for colour and capillary refill time. Jugular veins can be evaluated for distension and pulsation, with the cat sitting or standing. The neck should be moderately extended and the overlying hair wetted or clipped. The jugular veins are not distended in healthy cats. Distension occurs due to increased systemic venous pressure or venous occlusion between the jugular vein and the right heart.

Normal jugular venous pulsation does not ascend higher than a third of the way up the neck in healthy cats. Jugular pulses must be differentiated from carotid artery pulsations, which may be transmitted through adjacent soft tissues in thin or agitated cats. A true jugular pulse will disappear if the jugular vein is occluded below the level of the visible pulse; pulsation will continue if it is being transmitted from the carotid artery (see video 3, doi:10.1016/j.jfms.2010.07.013). Jugular distension and pulsation occur with RHF.^48,50 Large volume, non-cardiogenic pleural effusions can raise central venous pressure and cause jugular distension but not pulsation.⁵¹ Similarly occlusion of right heart inflow from a mediastinal mass lesion can cause jugular venous distension that may be accompanied by facial, neck and forelimb oedema.⁵²

Normal jugular venous pulsation does not ascend higher than a third of the way up the neck in healthy cats.

Clinical findings that make cardiogenic causes of pleural effusion more likely include a murmur, gallop, arrhythmia, crackles dorsally, jugular distension and jugular pulsation.

Changes in cardiac rate, rhythm and heart sounds should be assessed carefully. In one study, the majority of cats with dyspnoea from cardiac disease had an abnormality on cardiac auscultation.⁵³ Increased heart rate and the presence of a murmur or gallop sound can be useful indicators of cardiogenic causes of pleural effusion. However, the possibility that the cat has structural cardiac disease but is not in heart failure — that is, the pleural effusion is non-cardiogenic — should also be considered. It is important to note that the absence of a murmur does not rule out cardiac disease. In one study, the sensitivity and specificity of the presence of a heart murmur for diagnosing cardiomyopathy were 31% and 87%, respectively.⁵⁴

Clinical examination findings that make cardiogenic causes of pleural effusion more likely include a murmur, gallop, arrhythmia, crackles dorsally, jugular distension and jugular pulsation.

Examination of other body systems

Poor body condition score is a non-specific indicator of underlying debilitating disease in cases that are not trauma or toxin-associated. Pyrexia may be a feature of FIP, pyothorax or neoplasia. Hypothermia may indicate sepsis in cases of pyothorax, especially if there is also bradycardia and hypoglycaemia. Hypothermia in conjunction with tachycardia and pale mucous membranes may indicate hypovolaemia. The cat should be examined for signs of trauma and haemorrhage at other sites. The thyroid lobes should be palpated routinely. Cranial mediastinal mass lesions may cause reduced compressibility of the anterior rib cage and may be palpable at the thoracic inlet.^52,55 Paraneoplastic syndromes associated with intrathoracic mass lesions include myasthenia gravis, exfoliative dermatitis and polymyositis.¹⁹ Pulmonary carcinoma in cats has a propensity to metastasise widely to unusual locations including the eyes, digits and skeletal muscles.⁵⁶ Cats with intrathoracic lesions may present with lameness due to digital metastases or hypertrophic osteopathy. In young cats, FIP is a major differential and signs potentially referable to this problem should be noted including abdominal effusion, ocular changes (eg, uveitis, keratic precipitates) and neurological signs.

Gentle abdominal palpation and assessment for a fluid wave may be possible. Simultaneous peritoneal and pleural effusion can occur with systemic disease processes or following extension from one cavity to the other. In the cat, double effusion increases suspicion for FIP, RHF, neoplasia, haemorrhage, systemic inflammation or infection, nephrotic syndrome and ruptured diaphragm. Of cats with effusive FIP, 62% have peritoneal effusion, 17% have pleural effusion and 21% have double effusions.⁴² Extension of pyothorax into the abdominal cavity is rare.⁵⁷ Glomerulonephropathies are a rare cause of pleural effusion and affected cats typically show other signs of nephrotic syndrome including ascites and subcutaneous oedema.

Data obtained from signalment, history and physical examination can be used to rank the list of common differentials. This ranking is further informed by the gross characteristics of fluid obtained at thoracocentesis.

Thoracocentesis

Thoracocentesis is both a diagnostic and therapeutic procedure and is often tolerated without sedation or local anaesthesia. Ultrasound guidance is useful but not essential. A 21 or 23 gauge butterfly needle, extension tubing and a three-way tap are attached to a 20 ml syringe. The cat is positioned in sternal recumbency and the chest wall is clipped and prepared aseptically. With the bevel facing the chest, the needle is passed slowly into the thoracic cavity, cranial to the costochondral junction at the seventh or eighth intercostal space. Negative pressure is applied to the syringe and sufficient pleural fluid as required for diagnostic sampling and/or therapeutic purposes is collected.

As much pleural fluid as can easily be obtained to improve respiration should be removed.

Gross characteristics of pleural effusion

Pyothorax fluid before (left) and after (right) centrifugation

Chylous pleural effusion

Serosanguineous effusion

Transudate

FIP effusion. (inset) Note the viscosity

Fluid analysis — value and limitations

Fluid analysis is variably useful in determining aetiology and the results must be interpreted in the light of clinical data. Samples collected in EDTA and plain tubes, and direct smears should be submitted for TNCC, TP, differential cell count and cytology. Additional testing will be directed by clinical data and gross evaluation of the fluid.

Malodorous effusions

A foul smell is a very useful indicator of anaerobic infection, which is typical in over 80% of pyothorax cases.²³ The fluid is usually opaque and creamy but can be pink, green-tinged or sanguineous, and flocculent material is often present. Aerobic and anaerobic culture and cytology, including Gram staining, is indicated. Fluid associated with infection typically has a high protein content (>30 g/l) and TNCC (>7000/μl), with neutrophils predominating (>85% of TNCC), and is classified as an exudate. Commercial anaerobic specimen collectors are available (eg, Vacutainer Anaerobic Specimen Collector, BD Biosciences). Where frank pus is aspirated, other tests are not necessary. Lack of odour does not rule out an infectious cause; 20% of cases of infectious pleuritis, particularly in kittens, are caused by unusual bacterial, fungal or protozoal pathogens.²³ Cytology and culture will assist in identifying these causes. Systemic toxoplasmosis can be associated with a large volume pleural effusion in the cat.⁵⁸

Milky effusions

Opaque and milky or pink fluids should be centrifuged. Chylous fluids form a cream layer upon standing but do not become clear after centrifugation because the opacity is due to lipid rather than cells or debris. The TP and TNCC lead to classification as a modified transudate or an exudate. Lipid in chylous effusions artefactually increases protein estimation when determined by refractometry. In addition to small lymphocytes or neutrophils and lipid-laden macrophages, refractile lipid droplets (chylomicrons) are often seen on cytology. In chylous effusions the pleural fluid triglyceride content is greater than the serum triglyceride concentration, while the pleural fluid cholesterol level is less than or equal to serum cholesterol. If only pleural fluid is available then triglyceride >100 mg/dl (>1.12 mmol/l) confirms chylothorax.⁶ The investigation should be directed to rule out known causes of chylothorax in the cat — principally CHF, neoplasia and trauma.¹¹ It should be expected that many cases of chylothorax in the cat will be considered idiopathic despite investigation.

Pseudochylous effusions resemble chylous effusions grossly and may not clear after centrifugation, but do not contain chyle. They are associated with chronic pleural disease and have a higher cholesterol content than serum, while the pleural fluid triglyceride level is less than or equal to the serum triglyceride. It is thought that cholesterol enters the pleural space during acute pleural inflammation and accumulates within pleural fluid over time because of a change in lipoprotein binding characteristics that impedes transfer out of the pleural space.³ Pseudochylous effusions seem to be rare in cats.⁴⁴

Serosanguineous effusions

Where fluids appear bloody, the clinician must differentiate between frank haemorrhage into the pleural space (haemothorax), and blood contamination of an effusion with a different aetiology (haemorrhagic effusion). Fluid can appear bloody with a haematocrit of <5%,³ whereas haemothorax is variably defined as fluid with a haematocrit of at least 25%, or 50% of the peripheral blood haematocrit.^3,59 Where haemorrhage has been present for more than 1 h no platelets will be seen on pleural fluid smears and the blood will be defibrinated so it will not clot in a plain tube. Causes of haemothorax include trauma, anticoagulant rodenticide intoxication and neoplasia. History and physical examination data may support one of these aetiologies. Cats with acute haemorrhage into the pleural space sufficient to restrict respiration should present with obvious signs of hypovolaemia (pale mucous membranes, weak pulses, tachycardia, hypothermia).

Distinguishing cardiogenic, neoplastic and atypical infectious effusions

Fluid cytology is useful for diagnosing neoplasia. Hirschberger and others demonstrated that cytology had a sensitivity of 61% and specificity of 100% for detecting neoplasia in body cavity effusions, including 65 feline pleural effusions, where diagnosis was confirmed.⁸ These authors report a NPV of 90% and a PPV of 100% for cytology for diagnosing malignant feline effusions. A caveat to this is that reactive mesothelial cells may be confused with neoplastic cells by those less experienced in cytology.

There is limited information on the use of other pleural fluid markers to determine aetiology in feline cases. One study showed elevated fibronectin levels to be useful for discriminating between cardiogenic and neoplastic effusions and as a negative predictor of neoplasia in the cat.⁶² LDH is a marker of pleural inflammation and elevations in this enzyme are used to differentiate exudates from transudates in human medicine, but not to further subdivide exudates.³ In a study by Zoia and others, an absolute cut-off of 226 IU/l for pleural fluid LDH could be used to differentiate neoplastic from cardiogenic pleural effusions in 14/15 feline cases, with cardiogenic effusions having lower values.⁹ LDH concentrations are usually >300 IU/l in FIP effusions.⁴² The measurement of LDH, glucose and pH has been advocated to help identify septic effusions; LDH is typically >200 IU/l, pH is <6.9 and glucose is usually <1.7 mmol/l (30 mg/dl, 0.3 g/l) and lower than a concurrent blood glucose measurement.² This is not usually necessary for diagnosing pyothorax but may have utility for unusual infections. The same source suggests that canine and feline neoplastic effusions typically have a normal or high pH (>7.4), low neutrophil count (<30%) and low glucose (0.5–4.5 mmol/l).² Accurate pH measurement is often not available as it requires anaerobic collection into a heparinised syringe and immediate analysis using a blood gas machine or hand-held analyser.⁶³ Pleural fluid pH is most useful as a prognostic marker in human parapneumonic effusions.³

Studies that validate these and other potential markers for determining aetiology and prognosis in veterinary medicine would be welcomed.

Translucent effusions

Where the fluid is clear or yellow, a TP estimation should be obtained cage-side. The protein scale on handheld refractometers is inaccurate at <25 g/l; tables are available to convert the urine specific gravity (USG) reading for the sample to the protein concentration between 10 and 25 g/l.¹² Low protein fluids are uncommon in cats and have few differentials.^9,29 If the protein concentration falls into the transudate range (<25 g/l) then serum albumin should be measured. If serum albumin is >15 g/l then pleural effusion due to decreased plasma oncotic pressure can be ruled out. Fluid overload can easily be ruled out from the history. CHF remains the major differential diagnosis and echocardiography is the most useful test here.

For translucent, yellow effusions with a protein content >30 g/l, FIP is a major differential. The effusion associated with FIP is typically viscous (evaluated by expelling the fluid from a syringe, see page 701), froths on agitation due to its high protein content, and clots on standing. While the protein content is consistent with an exudate, the TNCC, comprising neutrophils and macrophages, is low, consistent with a modified (<5000 cells/μl) or pure (<1000/μl) transudate.⁶⁰ The single most useful test for ruling in FIP as a cause of pleural effusion is the use of immunofluorescence to detect feline coronavirus (FeCoV) antigen in macrophages.⁶¹ This test has a positive predictive value (PPV) of 100%. It is less useful for ruling out infection, with a negative predictive value (NPV) of 57%. A negative result may occur in a cat with FIP if there are low numbers of macrophages or if epitope masking by patient antibody occurs. Rivalta's test can be used (PPV 86%, NPV 97%) where immunofluorescence is not available, or to support a negative finding on immunofluorescence.⁶¹ This test identifies exudates based on their ability to retain their shape in a dilute acetic acid solution. Where less invasive tests are inconclusive, and a definitive diagnosis is required, immunohistochemistry of tissue biopsies can be used to confirm FIP.

Post-drainage chest radiographs, including lateral views, should be obtained to detect abnormalities that may have been effaced by fluid or obscured by atelectasis.

Additional tests

Imaging

Post-drainage chest radiographs, including lateral views, should be obtained to detect abnormalities that may have been effaced by fluid or obscured by atelectasis. The pulmonary parenchyma and vasculature, cardiac silhouette and mediastinum should be evaluated. The presence of restrictive pleural disease should be noted. The vertebral heart score should be determined.⁶⁴ Enlargement of the cardiac silhouette usually indicates cardiomegaly but pericardial effusion or rare disorders such as pericardioperitoneal diaphragmatic hernia should be considered. Echocardiographic examination will rapidly distinguish between these possibilities and is the diagnostic modality of choice to rule out decompensated cardiac disease. Ascribing significance to subtle echocardiographic changes can be challenging. The atria dilate in response to both pressure and volume overload. Although there are exceptions, if the atria are not enlarged, then CHF as a cause of pleural effusion is ruled out. Where RHF is suspected a cursory abdominal ultrasound examination is useful to detect ascites and enlargement of the caudal vena cava and hepatic veins. Where pericardial effusion is detected concurrently with pleural effusion it is likely to be a consequence of the same underlying disease process, such as CHF, FIP or neoplasia, rather than a cause of the pleural effusion, since pericardial effusion rarely causes tamponade in cats.⁶⁵

The lung fields should be inspected for evidence of underlying disease consistent with CHF, neoplasia and abscessation. Radiographic signs of pulmonary oedema in the cat are variable. They include interstitial or alveolar opacities in the perihilar area, dorsocaudal lung fields, more focally caudal to the heart or unevenly distributed throughout the lung fields. Patchy infiltrates from pulmonary oedema can be difficult to differentiate from other lung diseases including neoplasia (primary and metastatic) and infection.^66–68 Rounding of the borders of the lung lobes is consistent with pleural fibrosis seen with long-standing or irritant effusions. Fibrosing pleuritis is a cause of persistent dyspnoea after pleural fluid drainage.

Diagnosing mediastinal mass lesions, which are often accompanied by pleural effusion, can be challenging using radiography alone since mass lesions may be effaced by fluid. On a lateral view, dorsal displacement of the trachea and caudal displacement of the carina (which normally sits at intercostal space 6 in the cat) can occur with large volume effusion, cardiomegaly or a cranial mediastinal mass. On a DV or ventrodorsal (VD) view in healthy cats the width of the mediastinum is usually less than twice the width of the spine.⁵² DV or VD projections are more useful than lateral views to differentiate mediastinal mass lesions from pulmonary masses. Cranial mediastinal masses displace the carina and lungs caudally and can cause lateral deviation of mediastinal structures.

Investigative approach to pleural effusion

Ultrasonographic evaluation of non-cardiac intrathoracic structures and guided fine-needle aspiration (FNA) biopsy or core biopsy are useful for identifying neoplasia, pulmonary abscessation, benign mass lesions and diaphragmatic hernia.⁶⁹ Pleural fluid provides an acoustic window, enhancing visualisation. The presence of concurrent peritoneal effusion should be investigated. Peripheral, focal, non-aerated pulmonary lesions are suitable for ultrasound-guided FNA biopsy. In one study of 56 feline and canine patients in which this procedure was performed no complications were noted and the sensitivity of this technique for diagnosis was 91%.⁶⁹ Although more limited in availability, thoracic CT is an excellent modality to detect, characterise and evaluate the extent of intrathoracic nodules or masses.⁷⁰ Similarly, CT-guided FNA biopsy or core biopsy, thoracoscopic biopsy or exploratory thoracotomy may be indicated.⁷¹ The most common complications associated with CT-guided or ultrasound-guided core biopsy are pneumothorax and haemorrhage.

Serum testing

Routine haematology, biochemistry, urinalysis and retrovirus testing are likely to be relatively low yield in determining the underlying disease process compared with analysis of the effusion, but form part of the minimum database to guide patient management and should be carried out once the cat is stable. Other tests, including total serum T₄ and Dirofilaria immitis antigen and antibody detection, may be indicated depending on the case. An exciting recent development is the validation of circulating natriuretic peptide levels to differentiate between cardiac and non-cardiac causes of respiratory distress in cats. N-terminal probrain natriuretic peptide (NT-proBNP) has been shown to have sensitivity and specificity of around 90% for detecting myocardial disease in this setting.^72,73

Case notes

A 14-year-old female neutered domestic shorthair was presented with acute-onset dyspnoea. The cat had become progressively inappetent over the previous 2 months and had lost weight. On the morning of presentation she refused all food and the owner noticed that she was ‘breathing up’.

Physical examination Major abnormalities were poor body condition (body condition score 1/5), tachypnoea (respiratory rate 60/min) with increased inspiratory effort and increased respiratory excursions. On thoracic auscultation heart sounds were muffled but harsh lung sounds were heard over the dorsal lung fields bilaterally. The heart rate and pulse rate were 175 bpm. Rectal temperature was 38.2°C. The femoral pulse was of normal amplitude. There was no jugular venous distension or pulsation. The thyroid lobes were not palpable. Abdominal palpation was unremarkable.

CASE WORK-UP

Preliminary assessment Tachypnoea with increased inspiratory effort and muffled heart sounds is suggestive of pleural space disease. The harsh lung sounds dorsally suggest concurrent pulmonary parenchymal disease. Pericardial effusion was considered unlikely because of the lack of jugular venous distension/pulsation and normal femoral pulse amplitude. A restrictive respiratory pattern, as seen with pleural space disease, is characterised by rapid, shallow respiration. The respiratory excursions in this patient were quite deep and more suggestive of a mixed respiratory pattern.

Thoracic ultrasonographic screening was planned after stabilisation with supplemental oxygen. The cat was placed in an oxygen chamber on admission and monitoring of respiratory rate and depth at 5 min intervals was commenced. Pleural effusion was confirmed on thoracic ultrasound.

Thoracocentesis was performed and yielded 150 ml of non-odorous, moderately cloudy, straw-coloured fluid. Respiratory rate and effort were reduced, but the patient remained dyspnoeic (see video 4, doi:10.1016/j.jfms.2010.07.013, post-thoracocentesis, respiratory rate 44 bpm).

Ranking of differential diagnoses The information gained so far can be used to rank the common causes of feline pleural effusion (see Table 2, page 694). A neoplastic or cardiogenic effusion is most likely. Effusive FIP is possible because of the gross characteristics of the fluid, but is uncommon in cats of this age. Atypical infection remains a possibility. Although, theoretically, prolonged fasting may reduce the lipid content of a chylous effusion such as to alter the gross characteristics, this is very unlikely. Given the age of the cat, primary bronchopulmonary neoplasia, metastatic neoplasia and thymoma should be considered. Possible cardiogenic causes include primary or secondary cardiomyopathies or decompensated congenital heart disease. Cardiogenic causes were ranked after neoplastic causes because of the absence of signs such as tachycardia, murmur, gallop and jugular venous distension, together with the poor body condition.

What next? Post-drainage thoracic radiographs are essential. Depending on the findings, echocardiography may be indicated to rule out CHF.

Fluid analysis should be carried out including cell count and differential, protein content and cytological analysis. Other parameters that would be useful to measure to differentiate CHF from neoplasia include LDH, pH and NT-proBNP.

A complete blood count, feline immunodeficiency (FIV)/FeLV tests, serum biochemistry profile and urinalysis are also indicated to look for markers of systemic disease.

RESULTS

Radiographic findings and interpretation Three-view post-drainage thoracic radiographs from this cat are shown on the right. Hyperinflated lung lobes with obvious pleural margins suggest emphysema or air-trapping, possibly associated with chronic pleuritis. There are three homogeneous 2–3 cm diameter pulmonary nodules. A diffuse bronchointerstitial pattern throughout the lungs indicates more extensive pulmonary infiltrative disease. Small pockets of residual pleural fluid are also evident. The sternal abnormality may be congenital, traumatic or possibly secondary to chronic dyspnoea.

These findings are most supportive of a neoplastic aetiology. Differential diagnoses include primary or metastatic pulmonary neoplasia, although the size of the pulmonary nodules is more suggestive of primary pulmonary neoplasia. An inflammatory process (eg, mycotic granuloma, FIP, toxoplasmosis, atypical bacteria) is possible, but less likely.

Haematology and biochemistry Results unremarkable. The total plasma protein was 75 g/l (reference intervals 59–78 g/l) and serum albumin was 31 (reference intervals 19–38 g/l).

Smear/cytospin There are numerous epithelial-like clusters of rounded mononuclear cells that display marked anisocytosis and feature a high nuclear:cytoplasmic ratio. There is marked anisocytosis and anisokaryosis. Nuclei are round, oval or indented. Nucleoli are large. Cytoplasm varies from scant to moderate in volume, is deep grey and sometimes vacuolated. Mitotic figures, binucleate and multinucleate cells are common. Abnormally large nuclei/cells are scattered throughout. Neutrophils and macrophages are present in the background.

DIAGNOSIS AND OUTCOME

The findings are consistent with a neoplastic effusion of epithelial origin. Exfoliating carcinoma cells appear to be present, the main differential being a pulmonary carcinoma.

Because of the poor prognosis the cat was euthanased. Post-mortem examination was permitted and the resultant histopathological diagnosis was pulmonary carcinoma.

Fluid analysis results

Fluid type	Thoracic fluid
Description	Moderately cloudy, straw-coloured
Total protein	42 g/l
Nucleated cells	4000 ×10⁶/l
Erythrocytes	10,000 ×10⁶/l
Viscosity	Not apparent

KEY POINTS

Observation and auscultation of the dyspnoeic patient with pleural space disease is usually sufficient to localise the problem. Imaging confirms pleural effusion.

Assessment for respiratory compromise, stabilisation and monitoring are crucial to a good outcome.

Congestive heart failure (CHF), feline infectious peritonitis (FIP), neoplasia, pyothorax and idiopathic chylothorax account for most cases. Other causes are reported uncommonly.

Signalment, history and data obtained from a minimally invasive physical examination are useful to rank differential diagnoses.

Appropriate laboratory testing of fluid obtained from diagnostic thoracocentesis is guided by clinical data and the gross characteristics of the fluid.

Results of fluid analysis may be diagnostic for pyothorax, neoplasia and FIP. In other cases they inform the diagnostic investigation of the patient.

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0.00 MB

Pleural Effusion in the Cat: A Practical Approach to Determining Aetiology

Abstract

Pathophysiology — and its diagnostic relevance

MULTIMEDIA

Aetiology

CHF

Neoplasia

Infectious pleuritis

FIP

Other

Undetermined

A practical classification to direct the clinical investigation

Recognising pleural space disease

Observation

Auscultation

Percussion

Confirming pleural space disease

Approach to the dyspnoeic cat with pleural effusion

Stabilisation

Stabilisation techniques

History

Signalment — an aid to ranking common differentials

Clinical examination

Examination of the cardiovascular system

Examination of other body systems

Thoracocentesis

Gross characteristics of pleural effusion

Fluid analysis — value and limitations

Malodorous effusions

Milky effusions

Serosanguineous effusions

Distinguishing cardiogenic, neoplastic and atypical infectious effusions

Translucent effusions

Additional tests

Imaging

Investigative approach to pleural effusion

Serum testing

Case notes

KEY POINTS

References

Supplementary Material