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What is the normal and variant anatomy of lymphatic drainage in the abdomen and thorax?
By the ninth week of fetal life, the jugular and retroperitoneal lymphatic sacs connect and this sets the stage for lymphatic drainage from the abdomen to the left jugular vein that traverses the thoracic duct.
The thoracic duct originates at the cisterna chili, located at the level of the second lumbar vertebral body, posterior and to the right of the aorta and adjacent to the right crus of the diaphragm. The duct passes through the aortic hiatus into the chest, where it is found anterior to the vertebral bodies and right intercostal arteries and posterior to the esophagus. In general, in the region between the fourth and sixth thoracic vertebrae the duct passes toward the left side of the mediastinum and runs posterior to the aorta and the left subclavian artery adjacent to the ligamentum arteriosum, where it is susceptible to injury during duct ligation and other cardiac procedures.
In about one-third of individuals, the anatomy of the thoracic duct differs from the standard description and awareness of these variants is important in order to avoid inadvertent operative injury and to diagnose the etiology of a chylothorax .
What congenital conditions lead to chylothorax?
The majority of cases of chylothorax in infants and children are secondary to thoracic duct obstruction or injury from thoracic and cardiac surgical procedures. However, when a chylothorax is the result of a congenital condition associated with a lymphatic malformation of the lungs or pleura, management may be more complicated and less successful.
Congenital pulmonary lymphangectasia is a condition in which the interlobar and subpleural pulmonary lymphatic vessels are dilated and prone to leakage . This may occur due to the failure of pulmonary interstitial tissues to appropriately regress during fetal life . Lymphangectasia may occur sporadically or in association with a variety of disorders including trisomy 21 , Noonan syndrome  and Turner syndrome . Gorham-Stout disease is a condition in which there is extensive proliferation of lymphatic vessels within bone that leads to bone loss. When rib involvement extends beyond the bone a large volume chylous pleural effusion may develop  that is very difficult to treat .
In rare situations, infants are born with extensive central venous thrombosis and this can result in chylothorax that may be treated with thrombolysis .
The occurrence of chylothorax after repair of congenital diaphragmatic hernia (CDH) was first reported in 1973 . The Congenital Diaphragmatic Hernia Study Group recently found the incidence of chylothorax after CDH repair to be 4.7%  and, similar to a number of smaller series , noted that patch repair and extracorporeal life support (ECLS) were associated with a higher likelihood for chylothorax. A prevailing theory persists that chylothoraces develop in patients with CDH due to lymphatic disruption during repair based upon the finding that sac resection  and patch repair  are associated with a greater incidence of chylothorax. However, it is increasingly apparent that the embryological events that lead to CDH also predispose to lymphatic abnormalities  and chylothorax that is exacerbated by greater degrees of pulmonary hypertension.
Occasionally, a chylothorax can occur with transdiaphragmatic flow from a chylous ascites.
What is the mechanism of traumatic chylothorax?
For the purposes of this chapter, traumatic chylothorax is considered as distinct from iatrogenic injuries to the thoracic duct that result from operations or other medical interventions. It is extremely unusual for blunt trauma to the chest to result in a chylothroax  but when no other etiology for a chylothorax in a child is apparent, nonaccidental trauma should be considered . A small series of thoracic duct injuries with chylothorax due to penetrating trauma to the lower neck and chest has been reported in adults  but because this mechanism is so rare in children only a single case has been published .
What are the iatrogenic causes of chylothorax?
The incidence of a chylothorax following operations for congenital heart disease is about three percent . Younger and smaller patients are at a much greater risk and a chylothorax has been identified in as many as 9.2% of infants  and in as few as 1.3% of older children  undergoing cardiac procedures. Children with a postoperative chylothorax are more likely to have had longer cross clamp and cardiopulmonary bypass times .
Injury to the thoracic duct is probably not the causative event in most cases of postcardiac surgery chylothorax as the preceding operations often do not involve dissection near the duct’s course . A more frequent etiology is likely to be disruption of small lymphatic vessels surrounding the aorta or pulmonary artery. In addition, operations that result in increased caval pressures, such as the Fontan procedure, are associated with a greater incidence of chylothorax . Finally, central venous thrombosis is significantly more prevalent in children with chylothorax after cardiac surgery  suggesting that this may be a causative factor independent of lymphatic injury.
Chylothorax has been reported after anterior and posterior spinal dissection and stabilization due to direct injury to the thoracic duct .
What are the consequences of chronic lymph loss?
Respiratory insufficiency is the typical presenting symptom of a chylothorax but after thoracic drainage morbidity is usually due to the loss of chylous fluid. The manifestations of chronic lymph depletion include fluid and electrolyte imbalance, malnutrition, intravascular thrombosis and immunodeficiency.
Ongoing, large volume chylous drainage leads to substantial protein loss  that must be managed with supplementation of enteral or parenteral protein intake beyond typical maintenance amounts in order to avoid growth failure, infection and poor wound healing. Optimally, this is done empirically, but monitoring of prealbumin, transferrin, retinol binding protein and the physical exam can aid in the detection of a recent deterioration in nutritional status .
Selenium is particularly prone to depletion with chylous drainage and deficiency may lead to cardiomyopathy . Standard selenium concentrations in parenteral formulas are inadequate to compensate for losses that may occur with a chylothorax.
Children with chylothorax following cardiac operations have been found to have significantly lower antithrombin levels than similar children without chylous loss  suggesting that antithrombin is lost in chylous fluid. Antithrombin should be supplemented in children with a long standing chylothorax. In the context of a new thrombotic event, antithrombin deficiency should be strongly considered as the etiology.
Children with chylothorax are at a substantial risk for the development of immunodeficiency and infection is the typical cause of mortality when a chylous leak cannot be stopped or internalized. Chylothorax is associated with lymphopenia and immunoglobulin G deficiency  although levels of natural killer cell and memory T cells are maintained. Unfortunately, the administration of intravenous immunoglobulin has not been shown to reduce the incidence of infection in children with chylothorax .