Embryology of the Thoracic Duct [4,5,6]
- Lymphatic system develops at the end of the sixth week of life.
- Lymphatic vessels form similar to blood vessels as they are derived from hemangioblastic stem cells.
- Lymphatic clefts and sacs form around large embryologic veins and develop as evaginations from the venous system.
- Lymphatic clefts eventually form extensive plexuses between each other and fuse to form larger vessels including the embryonic right and left thoracic ducts.
- Formation of lymph nodes,
beginning predominantly in the ninth week,
results in a decrease of many of these plexuses.
- Through selective atrophy,
within the thorax and abdomen only,
a single duct remains which drains the lymph of the entire lower body and left head and arm,
emptying at the confluence of the jugular and left subclavian veins.
- Lymph of the right head and arm empties into the corresponding location on the right.
- Lower 2/3 of the thoracic duct is formed from the embryonic right thoracic duct and the upper 1/3 is formed from the embryonic left.
- Disturbances during lymph node formation as well as during the selective atrophy process may result in the extensive variations of the thoracic duct.
Typical Course of the Thoracic Duct [2,3,5,7]
Fig. 1: (A) Schematic diagram of the typical course of the thoracic duct. (B) Fluoroscopic image of a typical thoracic duct, however, the duct crosses midline earlier than is classically described. dapted from Allaham AH, Estrera AL, Miller CC, III, et al. Chylothorax complicating repairs of the descending and thoracoabdominal aorta. Chest 2006; 130(4): 1138-1142.
- Originates at the cisterna chyli in the abdomen at T12-L2 to the right of the aorta and behind the right crural pillar.
*Cisterna chyli measures up to 3 cm in length and 1 cm in width.
*Thoracic duct measures 2-5 mm in diameter.
- Courses superiorly along the right anterior aspect of the vertebral column,
between the aorta and azygous vein,
entering the thorax through the aortic hiatus.
- Intrathoracic portion of the duct courses within the posterior mediastinum,
continuing in between the aorta and azygous vein,
posterior to the esophagus and anteriorly to the right of midline,
along the vertebral column.
- At the T5-T6 vertebral levels,
the thoracic duct crosses to the left of midline and passes behind the aortic arch and to the left of the esophagus as it ascends approximately 2-3 cm above the clavicle before making an arch to curve inferiorly.
- In the superior mediastinum,
it courses behind the left internal jugular vein curving inferiorly to drain into the venous system at the junction of the left internal jugular and subclavian veins.
Clinically Significant Anatomic Variants [2,8,9,10,11,12,13]
While the thoracic duct may vary along any aspect of its course,
many overlapping classification systems have been developed to characterize the types of variation at every level.
Described are variations which may have clinically significant impacts on the decision and approach to perform thoracic duct embolization.
Left-Sided Course of the Thoracic Duct and Cysterna Chyli
Fig. 2: Schematic diagram of the cisterna chyli and the thoracic duct course along the left aspect of the vertebral column throughout its entire length (solid black arrow indicates right lumbar or intestinal lymphatic branch).
- In a thoracic duct with a left-sided cisterna chyli,
be wary of attempting to cannulate the cisterna chyli due to its close proximity to the aorta.
- Consider accessing the right lumbar or intestinal lymphatic branch instead (Figure 2,
solid black arrow),
which may be technically challenging due to its small diameter.
Right-Sided Thoracic Duct Emptying into the Right Venous Angle
Fig. 3: (A) Schematic diagram of the thoracic duct and cisterna chyli course along the right aspect of the vertebral column throughout its entire length, terminating into the confluence of the right internal jugular and subclavian veins. (B) Fluoroscopic image of the thoracic duct which crosses back to the right and terminates into the right venous system, where a leak is identified (solid yellow arrow).
- Recognize that the thoracic duct could conceivably terminate within the right venous angle and not to be alarmed if such a course is seen.
Partial Duplication of the Thoracic Duct
Fig. 4: Schematic diagrams demonstrating that the thoracic duct may be partially duplicated adjacent to its take off from the cisterna chyli or more distally.
Fig. 5: (A) and (B) Fluroscopic and digital subtraction angiography images demonstrating a duplicated thoracic duct just distal the cisterna chyli (solid yellow arrows). (C) Fluoroscopic image of the thoracic duct demonstrating proximal embolization to the cisterna chyli (solid yellow arrow). Not shown, but N-BCA glue was injected into the cisterna chyli and take off of the duplicated system to prevent filling of the left sided trunk.
- The thoracic may be disrupted in either or both trunks and care must be taken to adequately embolize proximally in the case of a more distal duplication,
or possibly both trunks in the case a more proximal duplication.
Plexiform Variation of the Thoracic Duct
Fig. 6: (A) Schematic diagram demonstrating plexiform variation of the thoracic duct. (B) Fluoroscopic image of a plexiform variant of the thoracic duct. Needle is in a large retroperitoneal duct (solid yellow arrow). Not shown, but the wire could not be advanced distally. (C) Fluoroscopic image of another patient with a plexiform variant of the thoracic duct with no discernible cisterna chyli or thoracic duct.
- May be challenging to advance a guidewire and catheter in the plexiform variant due to the small size of the multiple trunks and often results in a technical failure of the procedure.
- Consider a retrograde approach via a left upper extremity vein and accessing the thoracic duct at its termination at the confluence of the left internal jugular and subclavian veins.
Absence of the Cisterna Chyli
Fig. 7: (A) Schematic diagram demonstrating absence of a cisterna chyli. The cisterna chyli is defined by Loukas et al as a 200% dilation of the thoracic duct. (B) and (C) Fluoroscopic images demonstrating no cysterna chyli (solid yellow arrows highlight the intrathoracic portion of the thoracic duct).
- May be technically challenging to access the thoracic duct when the cisterna chyli is absent due to the small diameter of the thoracic duct and even smaller diameter of the lumbar or intestinal branches.
- Consider a retrograde approach via a left upper extremity vein and accessing the thoracic duct at its termination at the confluence of the left internal jugular and subclavian veins.