Philip R Judge BVSc MVS PG Cert Vet Stud MACVS (VECC, Medicine of Dogs)
The following are some key takeaways from the retrospective study by Severac et al. of 35 cats with confirmed chylothorax.
1. Cardiac disease was the most common underlying cause, contradicting previous literature
Cardiac disease accounted for 57% of cases (20 of 35 cats), making it the predominant aetiology in this study. This contrasts with earlier reports that identified idiopathic chylothorax as the most common form. The authors suggest this difference may reflect improved diagnostic imaging and greater use of echocardiography (performed in 97% of cats in this study versus 60% in a previous large study).
Idiopathic cases accounted for 31% (11 cats), neoplasia for 9% (3 cats), and peritoneopericardial diaphragmatic hernia for 3% (1 cat). The study highlights that an echocardiogram should be considered for all cats diagnosed with chylothorax.
2. Left atrial-to-aortic ratio >1.4 is a reliable predictor of cardiac aetiology
The median LA:Ao ratio in the cardiac disease group was 2.4 (range: 1.4–3.4), compared with 1.3 (range: 0.9–1.4) in the idiopathic group and 1.1 in the neoplasia group. All cats with an LA:Ao ratio greater than 1.4 had a cardiac aetiology. This finding was statistically significant (P < 0.001).
The LA:Ao ratio is easily measured with point-of-care ultrasound at hospital admission and should prompt rapid cardiac investigation in cats with chylothorax.
3. Heart murmur and gallop sound are suggestive but not definitive for cardiac chylothorax
A heart murmur was detected in 75% of cats in the cardiac disease group, significantly more prevalent than in the idiopathic (9%) and neoplasia (33%) groups (P = 0.007). However, 25% of cats with cardiac chylothorax did not have a murmur at admission, possibly due to muffling by pleural effusion. A gallop sound was heard in 40% of the cardiac group and was significantly associated with cardiac disease compared with the idiopathic group (P = 0.003). The presence of a heart murmur or gallop sound can suggest a cardiac cause, but echocardiography remains essential for definitive assessment in all cases.
4. Age is a strong predictor of neoplastic aetiology
Cats in the neoplasia group were significantly older (median 18 years; range: 8.3–18.3 years) than those in the cardiac disease group (median 8.8 years; range: 3–14 years; P = 0.004) and the idiopathic group (median 7.5 years; range: 3.5–13 years; P = 0.031). Importantly, all cats aged over 14 years in this study had a neoplastic aetiology.
Chylothorax in a patient aged over 14 years should heighten suspicion for neoplasia, particularly when thoracic imaging reveals a mediastinal or pulmonary mass.
5. Survival times differ substantially by aetiology, though not statistically significant
Median survival times were:
- Idiopathic: 416 days
- Cardiac disease: 172 days
- Neoplasia: 38 days
- Peritoneopericardial diaphragmatic hernia: 27 days
Although the differences between groups were not statistically significant (likely due to small sample sizes and high variability), idiopathic cases had the best prognosis, and neoplastic cases had the poorest. Regardless of aetiology, older age, higher BUN and cholesterol concentrations, and lower rectal temperature and sodium concentration at presentation were significantly associated with shorter survival. A 1-year increase in age resulted in a 23.7% increase in the risk of death.
6. Hypothermia at presentation is common in cardiac chylothorax
Twenty-six per cent of all cats (9 of 31 with recorded temperatures) were hypothermic (<37.8°C) at admission. Of these, 8 cats (89%) were in the cardiac disease group. While rectal temperature was not statistically associated with aetiology, hypothermia in a cat with chylothorax should raise suspicion for cardiac disease. The authors suggest this may reflect reduced cardiac output during heart failure, resulting in peripheral hypoperfusion.
7. Surgical management of idiopathic chylothorax may offer a survival advantage, but the difference was not statistically significant
In the idiopathic group, medically treated cats (n = 5) had a median survival of 205 days, while surgically treated cats (n = 6; thoracic duct ligation combined with subtotal pericardiectomy in 5 cats) had a median survival of 706 days. The difference was not statistically significant, which the authors attribute to small sample size and high interindividual variability. Pleuritis was more prevalent in the surgically managed group (66%) than in the medically managed group (20%), which may have worsened the prognosis in surgical cases. Despite this, surgical treatment remains the primary treatment recommendation for idiopathic chylothorax once the patient is stabilised.
8. Recurrence of pleural effusion is common regardless of aetiology
Seventy-one per cent of cats (28 of 35) experienced at least one recurrence of pleural effusion after hospital discharge. Median time to first recurrence was 19 days overall: 28 days for the cardiac group, 14 days for the idiopathic group, and 18 days for the neoplasia group. This high rate of recurrence highlights the need for regular monitoring and owner communication regarding the chronic nature of the condition, regardless of underlying cause.
Additional clinical points from the study:
- Fibrosing pleuritis was observed in 26% of cats (9 of 35), with higher prevalence in the idiopathic group (5 cats). This complication carries a poorer prognosis.
- The study’s limitations include its retrospective design, small sample sizes in each aetiologic group, incomplete diagnostic workups in some cases, and that CT was performed in only 2 cats, which may have resulted in undiagnosed abnormalities in some idiopathic cases.
Reference:
Severac L, Juette T, Conversy B. Cardiac aetiology is the leading cause of feline chylothorax: 35 cases (2006–2022). Journal of the American Veterinary Medical Association. 2026 Jun 12;1(aop):1-9.
