A frustrating and expensive disease! Most cases in dogs are 'idiopathic' although occasionally there is a history of external trauma, iatrogenic injury or underlying medical cause. In cats, the incidence of underlying medical disease is significantly more important, notably congestive heart failure.
Diagnosis is often straightforward, with the appearance of a milk-like (lactescent) pleural effusion on thoracocentesis. Biochemical and cytological analysis can be used to confirm the nature of this fluid and is most important in cases where prior interventions have taken place (for example, repeated thoracocentesis may result in blood staining or secondary bacterial infection). Cardiac evaluation is an important early step in investigation of these cases because of the possibility of underlying heart disease, notably in cats. Echocardiography is most rewarding in this respect, and also allows for the investigation of intrathoracic masses, particularly since these may be obscured on radiography by the presence of effusion.
If an underlying cause is apparent, such as congestive heart failure or a cranial mediastinal mass, treatment of this should be the priority. For idiopathic cases, medical management is occasionally successful. In a few cases, spontaneous resolution after repeated drainage has been observed; however, if prolonged conservative management is chosen, significant possible complications are secondary infection, protein-energy malnutrition and visceral pleural fibrosis (pulmonary cortication). Historically, use of a low-fat diet with energy supplementation using medium-chain fatty acids was recommended as part of medical management. Most clinicians now agree that this is not helpful: it may reduce the fat content of the effusion but does not seem to reduce the rate of accumulation and contributes to the patient's cachexic state. Drug therapy has been described, using a benzopyrone such as 'Rutin' (50-100 mg/kg orally q8h). This is an unlicensed agent, sold as a food supplement through health food stores. It is believed to increase macrophage activity and therefore promote absorption of chylous effusions. Although no large veterinary studies of its use have been published, there are limited clinical publications and anecdotal experience to support its use, not least because of the significant cost and difficulties of surgical management. The majority of animals do not respond usefully to medical management however.
Various surgical procedures have been described but none is considered entirely successful. Some authors report high success rates in both species with pericardectomy as a sole procedure. This can be achieved at open surgery (lateral thoracotomy or sternotomy) or thoracoscopically. Despite the reported success rates, many surgeons have not achieved the same outcomes with pericardectomy as a sole procedure. The rationale for pericardectomy is either that the effusion may result from a low-grade right-sided overload or that chronic effusion may have resulted in restrictive pericardial disease due to fibrosis.
Alternative surgical therapies include thoracic duct ligation, thoracic duct embolisation and thoracic omentalisation. Thoracic duct ligation or embolisation is intended to prevent passage of chyle into the pleural space and encourage alternative routes of drainage via the abdominal lymphatics or venous return. A difficulty of ligation in particular is identification and definitive occlusion of the thoracic duct, which is often branching and multiple. Techniques to increase its visibility include feeding high fat prior to exploratory thoracotomy or injection of methylene blue into afferent lymphatic tissue. Lymphangiography theoretically allows precise quantification of thoracic duct anatomy and confirmation of occlusion, but is practically challenging and requires a concurrent laparotomy. One recent approach to thoracic duct occlusion which shows promise is thoracoscopic occlusion: this enhances visualisation through magnification of the mediastinal structures and allows occlusion in the costophrenic angle, which may be advantageous compared to the more cranial occlusion achieved at thoracotomy. Thoracic duct embolisation can be achieved by injection of cyanoacrylate glue into an efferent lymphatic of the ileocaecal lymph node complex at laparotomy, whilst temporarily occluding the thoracic duct in the caudal mediastinum. Embolisation offers the theoretical advantage of achieving blockage of all thoracic duct branches concurrently. Other surgeons use mass ligation of the caudal mediastinal structures in an attempt to achieve the same outcome.
A further surgical strategy is to place the omentum into the thorax through a diaphragmatic incision. The omentum is thought to act as a physiological drain and perhaps to seal leaking lymphatics within the caudal mediastinum. No large series reporting the success of this approach is available, although many surgeons chose to include omentalisation as part of a combined surgical treatment at thoracotomy, together with thoracic duct ligation and often pericardectomy.
The present commonly employed approach is to recommend thoracic duct ligation through a lateral thoracotomy, together with omentalisation through the same thoracotomy, and a diaphragmatic incision and pericardectomy, again through the same or a more cranial intercostal approach. At thoracoscopy, thoracic duct occlusion and pericardectomy can be achieved similarly, although omentalisation is impractical. Lymphangiography can be used as part of the thoracic duct ligation but is difficult and less commonly used.
Alternatives (for cases where the above have failed) include pleurodesis and pleuroperitoneal (Denver) shunt implantation. Pleurodesis is rarely used presently, although the author has had some success with the technique, using tetracycline as the agent. It may be a suitable choice for cases where alternatives have failed or where owners will not consider a surgical approach. Denver shunt placement is similarly reserved as a salvage therapy and is associated with significant technical difficulties, notably drain blockage and pulmonary cortication. Experience suggests that it fails within weeks in most cases.
Chylothorax remains a frustrating condition. Unfortunately the dog breeds in which it is often seen (sight hounds and Bull Mastiffs) do not always tolerate the intensive treatment approaches required. Although usually described as idiopathic, in reality it probably represents the outcome of several disease processes, reflecting the disappointing success rates of the various reported treatments in the hands of different clinicians.