Pleural lymphocyte-rich transudates in cats

Abstract

Objectives

Non-chylous lymphorrhagic pleural effusions are transudative effusions with a predominance of lymphocytes; however, they do not contain chylomicrons and therefore do not have the classical milky aspect of true chylous effusion. This type of effusion has been anecdotally associated with cardiac diseases in cats, but studies are lacking. The aim of this study was to investigate the association between this type of effusion and the primary disease.

Methods

In this study, feline non-chylous lymphorrhagic pleural effusions were retrospectively selected from the database of the authors’ institutions over a 3 year period. All cases underwent thoracic imaging, including echocardiography. Effusions classified as transudates with a predominance of lymphocytes on cytology were included.

Results

Thirty-three cases fulfilled the inclusion criteria: 23 (69.7%) had a concurrent cardiac disease, eight (24.2%) cases were associated with the presence of a mediastinal lymphoma or carcinoma or a thoracic mass, one case (3.0%) was a thymoma and one case (3.0%) was a sequela of a pyothorax.

Conclusions and relevance

Since a clear lymphatic origin of the fluid could not be demonstrated, lymphocyte-rich transudate might be considered a better designation for these kinds of effusions rather than non-chylous lymphorrhagic effusions. Although the number of cases in this preliminary study is low, the presence of a pleural lymphocyte-rich transudate in a cat should prompt the search for cardiac disease or intrathoracic neoplasia.

Introduction

Effusion is an abnormal accumulation of fluid within a body cavity and it represents a non-specific finding that can result from a variety of diseases.^1,2 Cats are frequently admitted to veterinary practices with signs referable to thoracic effusions, often as emergencies. A rapid and accurate diagnosis is important for effective treatment, but differentiation of the underlying cause can be challenging.

The classification of effusions as transudates, modified transudates and exudates is traditionally based on measurement of protein concentration, specific gravity and total nucleated cell counts, in order to characterise them as transudates, modified transudates and exudates. ² Nevertheless, the large number of disorders associated with modified transudates and the fact that this category has overlapping protein content and cellularity with pure transudates and exudates, limits this current veterinary classification scheme. ³ Therefore, the newest approach to pleural effusion classification follows the Light criteria used in human medicine, where effusions are identified only as transudates or exudates. ³ Another type of classification is the aetiological classification based on total protein concentration, cell count and cytological criteria.^4,5 This classification divides effusions into four groups: transudates, exudates, effusions from cell exfoliation, and effusions from a ruptured vessel or viscus.

Non-chylous lymphorrhagic effusions are transudate forms due to leakage of lymph, but do not contain chylomicrons, and therefore do not show the classical milky aspect of true chylous effusion. ⁴ This kind of effusion should contain <10 × 10³/µl total nucleated cells, with a predominance of small lymphocytes; the triglyceride content is usually <100 mg/dl (or lower than the serum triglyceride concentration), and in most of the cases the triglyceride to cholesterol ratio of the effusion is lower than 1. ⁶

The absence of chylomicrons can be due to involvement of lymphatic vessels that are not in the drainage path from intestine, to a dietary lack of lipids or to a consumption of the chylomicrons in the effusion. ⁴ Regardless of the reason, since a clear lymphatic origin cannot be demonstrated, lymphocyte-rich transudate (LRT) might represent an alternative and more appropriate designation for these kind of effusions.

One of the most common causes of pleural effusion in cats is an increased hydrostatic pressure of the vascular or lymphatic system, ⁷ often caused by congestive heart failure (CHF). Almost all cats with pleural effusions due to CHF present with bilateral pleural accumulation of fluid. ⁷ Recent studies have focused on novel techniques to differentiate cardiac from non-cardiac causes of pleural effusions in cats,^8,9 but with no distinction regarding the classification of the effusions. LRT are frequent findings in practice, but current guidelines regarding specific characteristics and aetiology of these effusions are lacking.

The aim of this study was to retrospectively investigate the clinical features and aetiology of feline pleural LRTs, and to assess the association between this condition and heart diseases in the cat.

Materials and methods

Pleural effusion samples collected from cats referred to the authors’ institutions between 2013 and 2016 were retrospectively considered. Only cats that received a complete clinical examination, thoracic radiography and echocardiography were included in the study. Pleural fluid samples were obtained through thoracocentesis and collected in EDTA tubes to inhibit fibrin clot formation. Smears were immediately prepared to limit artefactual changes in cellular morphology secondary to delays in fluid processing, stained with a modified Romanowsky stain and examined by two board-certified clinical pathologists. A 100 nucleated cell differential count was performed on the effusion smear to establish the percentage of each cell population.

Total nucleated cell count, total protein measurement by refractometry and chemistry analysis of the supernatant of the effusions (obtained by centrifugation at 1000 × g per 8 mins), including albumin, triglyceride and cholesterol concentration, was performed, when possible. Biochemistry was performed using the automated spectrophotometer Cobas Mira (Roche Diagnostic) and the multilayer dry-slide analyser Vitros 350 Chemistry System (Ortho-Clinical Diagnostics).

All transudative effusions with a predominance of lymphocytes on cytology that were considered non-chylous based on visual appearance were included. Pleural effusions classified as neoplastic, exudates or chylous were excluded from this study. The diagnosis and classification of cardiac disease was primarily based on echocardiography criteria, as described by Ferasin. ¹⁰

Results

Results recorded in the cats included in the study are reported in Table 1.

Table 1

Signalment, gross fluid aspects, clinical diagnosis, total cell count and main cytological and biochemical features of the 33 effusions

Age(years)	Sex	Breed	Gross aspects	Clinical diagnosis	NCC/µl	Lymphocytes (%)	TP (g/l)	Cholesterol (mg/dl)	TG (mg/dl)
18	F	Siamese	Yellow, turbid	RCM	1700	100	23	5 of 65	6 of 96
16	F	Persian	Yellow, turbid	RCM	4300	94	32	–	–
14	F	DSH	Yellow, turbid	RCM	500	99	23	48	24
10	F	DSH	Pink, turbid	RCM	25,680	89	30	95	60
8	M	Persian	Colourless, clear	RCM	300	53	16	26	28
9	M	DSH	Colourless, clear	RCM	1900	95	24	48	31
–	–	–	Pink, turbid	RCM	2200	89	15	37	47
15	M	DSH	Colourless, clear	RCM	500	60	21	35	33
12	M	DSH	Pink, turbid	RCM	5400	89	20	65	9
14	M	DSH	Clear	RCM	7120	85	36	–	–
13	M	Chartreux	Clear	RCM	3690	87	24	–	–
4	M	DSH	Yellow, clear	HCM	4200	90	20	–	–
20	F	DSH	Pink, turbid	HCM	2200	92	31	–	–
12	M	DSH	Yellow, clear	HCM	4100	73	45	–	–
14	M	DSH	Colourless, turbid	HCM	2400	90	24	40	68
14	M	DSH	Yellow, fibrin	HCM	1720	65	32	60	30
0.5	M	DSH	Yellow, turbid	DCM	200	49	28	53	114
4	M	DSH	Colourless, turbid	DCM	1600	69	24	61	36
12	F	DSH	Pink, turbid	DCM	750	73	21	–	–
13	F	DSH	Yellow, clear	DCM	3400	71	21	–	–
16	M	Devon Rex	Yellow, clear	ARVC	600	71	28	–	–
0.5	F	Siberian	Pink, turbid	IA septal defect	12,000	89	46	48	157
2	F	DSH	Pink, turbid	Systolic defect	3800	93	31	71	54
9	F	DSH	Pink, turbid	Mediastinal lymphoma	1000	93	33	102	19
3.5	M	DSH	Yellow–pink	Mediastinal lymphoma	5460	85	60	75	45
4	F	DSH	Yellow, turbid	Mediastinal lymphoma	9050	94	40	–	–
3	F	DSH	Pink, turbid	Mediastinal lymphoma	4100	45	51	–	–
14	M	DSH	Yellow, clear	Mediastinal carcinoma	1080	42	32	–	–
5	M	Siberian	Pink, turbid	Thymoma	21,000	89	40	116	43
9	F	DSH	Yellow, clear	Pulmonary mass	1600	59	48	–	–
13	F	DSH	Yellow, clear	Pulmonary mass	8220	80	36	–	–
12	F	DSH	Yellow, clear	Thoracic mass	1400	42	48	181	33
10	M	DSH	Pink, turbid	Pyothorax	22,000	81	22	–	–

NCC = nucleated cell count; TP = total protein; TG = triglycerides; F = female; RCM = restrictive cardiomyopathy; DSH = domestic shorthair; M = male; HCM = hypertrophic cardiomyopathy; DCM = dilated cardiomyopathy; ARVC = arrhythmogenic right ventricular cardiomyopathy; IA = interatrial

Thirty-three thoracic effusion samples fulfilled the inclusion criteria. Most of the patients were domestic shorthair cats (25/33; 75.7%); other breeds included Siberian (n = 2), Persian (n = 2), Siamese (n = 1), Devon Rex (n = 1) and Chartreux (n = 1); in one case signalment was not reported. Age ranged from 6 months to 20 years (mean 10.32 years; median 12 years). Seventeen cats were male (51.5%) and 15 cats were females (45.4%). Overall, 23/33 cases (69.7%) had a concurrent cardiac disease; among these, 11 were restrictive cardiomyopathy (47.8% of the cardiomyopathies) and five were hypertrophic (21.7%). The other seven cardiomyopathies were four dilated (17.4%), one arrythmogenic (4.4%), one due to a congenital interatrial septal defect (4.4%) and one due to a systolic defect (4.4%). One cat with dilated cardiomyopathy also had an intestinal mast cell tumour. Among the remaining 10 cases of LRT (30.3%), four (12.1%) had a mediastinal lymphoma (confirmed by cytological and/or histological examination), one (3.0%) had an intra thoracic carcinoma of undetermined origin (diagnosed by ultrasound and fine-needle aspirate), one had a thymoma (3.0%) (confirmed by cytology and histology), one (3.0%) was a sequel to a previous pyothorax, whereas the other 3 (9.1%) presented with an intra thoracic space-occupying lesion, probably neoplastic, but without a definite diagnosis since cytology, histology or necropsy were not performed.

All effusion samples had a serous or serous–haemorrhagic appearance. Eleven samples (33.3%) were colourless to yellow and clear, eight (24.2%) were colourless to yellow and turbid, two (6.1%) were colourless or yellow clear to turbid with fibrin clot and 12 (36.4%) were described as slightly haemorrhagic (pink) and turbid.

A complete nucleated cell count was available for all cats: the mean and median cell counts were, respectively, 5051 cells/µl and 2400 cells/µl (interquartile range 1600–5400 cells/µl), ranging from 200–25,680 cells/µl. The mean and median percentage of lymphocytes were 79% and 85%, respectively, ranging from 42–100%. Lymphocytes comprised >80% of all nucleated cells in 20/33 cases, 60–79% in 8/34 cases and <60% in five cases. Total lymphocyte counts ranged from 98–22,855/µl (mean 4255/µl; median 2024/µl). An example of the cytological appearance of the LRT is shown in Figure 1. Erythrocyte counts in the effusion were available for 23 cases and ranged from 10,000–480,000 red blood cells (RBCs)/μl, with a mean and median count, respectively, of 112,375 RBC/µl and 45,000 RBC/µl. Effusion packed cell volume was always <3%.

Figure 1

Predominance of small lymphocytes in a feline lymphocyte-rich transudate. Cyto spin preparation, × 400

Total fluid protein concentration was available for all cats: mean and median concentration were 31.9 and 31.0, respectively, ranging from 15–51 g/dl. Albumin concentration was measured in only 10 cases and ranged from 9–25 g/dl, with a mean and median of 16.1 g/dl and 15.5 g/dl, respectively.

Triglyceride and cholesterol concentration were available for 19 cases. Triglycerides ranged from 9–157 mg/dl, with a mean and median concentration of 50.78 mg/dl and 39.5 mg/dl, respectively. Cholesterol ranged from 35–181 mg/dl, with a mean and median concentration of 69.78 mg/dl and 60.5 mg/dl, respectively. Triglyceride to cholesterol ratio was <1 in 13 cases. In two cases the triglyceride concentration was higher than 100 mg/dl, but in both cases it was lower than plasma concentration.

Discussion

Effusions in domestic animals are usually classified according to gross findings, cell count, protein content and cytological examination, but the relationship between fluid characteristics and the underlying pathophysiology is not always well defined. In some cases, a clear distinction between transudates and exudates is not easy because of an overlapping of some laboratory findings.

Colour and other gross findings of the feline effusions in this study were consistent with data reported elsewhere. ⁴ Reference data concerning the absolute number of lymphocytes in LRTs are not available; therefore, we decided to include in our study those transudative fluids where lymphocytes predominated over other leukocytes on a manual differential cell count. In most cases the predominance of this cell type was marked. Lymphocyte appearance was unremarkable, since the majority of these cells were small- to medium-sized lymphocytes. A limitation of this study was the lack of a complete fluid biochemistry in some cases; the possibility that some cases could have been true chylous effusions could not be excluded, despite the macroscopic appearance being inconsistent with chyle. Chyle is a particular kind of lymph draining the gastrointestinal tract and it is rich in dietary fat. After digestion, dietary lipids are repackaged by intestinal epithelium into triglyceride-rich chylomicrons; their large size prevents local uptake by capillaries, and these lipid-rich particles enter the lymphatic system. Chylomicron-enriched lymph eventually enters the thoracic duct, which anastomoses with the venous system via lymphaticovenous junction cranial to the heart. The high chylomicron content imparts the characteristic opaque, milky-white appearance to chylous effusion, although concurrent haemorrhage or blood contamination can result in a pink-tinged effusion. Prolonged hyporexia or anorexia, as often exhibited by sick cats, may result in a much less opaque fluid with low triglyceride content.

Triglyceride concentrations <100 mg/dl, or a cholesterol to triglyceride ratio <1, are highly supportive of a non-chylous effusion, ⁴ which is frequently found as a result of a lymphorrhage; this term is often used as a synonymous of lymphorrhoea, but it can also indicate a focal accumulation of lymphocytes in tissues. ⁴ Cytologically, this kind of effusion should have a predominance of small/medium lymphocytes with a lower number of other inflammatory cells (granulocytes, macrophages). If a consistent proportion of lymphoid cells are large with evident nucleoli, a neoplastic effusion due to intra-thoracic lymphoma should be considered.

Data from cats included this study demonstrated that the majority of feline pleural LRTs are associated with several types of cardiac diseases. Right-sided heart failure in dogs frequently causes ascites, regardless of the underlying cardiac disease, whereas ascites is much less common in cats. ¹¹ Heart failure in cats is commonly associated with pleural effusion, but the exact pathophysiological mechanism responsible for pleural effusion secondary to heart failure is unknown in this species. ¹² It has been established that increased ventricular diastolic pressure results in increased capillary hydrostatic pressure in the pulmonary circulation; ¹³ the most likely possibility is therefore that feline visceral pleural veins and lymphatics drain into the pulmonary veins such that elevated pulmonary venous pressure causes the formation of pleural effusion. ¹² Most of the feline thoracic effusions associated with cardiomyopathy have been classified as modified transudates, true chylous or pseudochylous effusions. ¹² Moreover, similar aetiologies are recognised for both chylous and pseudochylous pleural effusions. However, pseudochyle is a confusing description, as the features of pseudochylous are similar to those of a transudate. We suggested, therefore, to simply use the name LRT, which summarises the features of these common types of fluids: appearance of a transudate on macroscopic inspection, biochemistry and cell count similar to other kind of transudates and a predominance of small lymphocytes on cytology.

Regardless, in the classification of lymphorrhagic effusions, either chylous or non-chylous, diagnostic imaging remains a critical step for the identification of the underlying cause; nevertheless, the results of this study indicate that cardiomyopathy or intra thoracic mass are the most likely causes.

Conclusions

In this study we focused on pleural transudate effusions commonly observed in cats, in which lymphocytes are the predominant cell type. This predominance of lymphocytes could be related to a true lymphatic leakage from serosal vessels; however, since a clear pathophysiological mechanism is not easy to demonstrate in practice, we suggest defining this type of effusion as ‘lymphocyte-rich transudate’. The results of this study show that cardiac disease is a common cause of pleural LRT in the feline species. Although the number of cases in this preliminary study is low, the presence of a LRT in cats should prompt the search for a cardiomyopathy or intra-thoracic neoplasia.