CHAPTER 23

Thoracic Duct, Azygos and Hemiazygos Veins, and Thoracic Sympathetic Trunks

THORACIC DUCT

The thoracic duct is the largest lymphatic vessel (trunk or great lymph channel) which drains lymph from most of the body into the bloodstream. The lymph in the thoracic duct is milky-white in appearance because it contains a product of fat digestion (chyle) from the intestine. The duct appears beaded due to the presence of numerous valves in its lumen.

Area of drainage: The thoracic duct drains the lymph from all the parts of the body except the (a) right side of the head and neck, (b) right side of the chest wall, (c) right lung, (d) right side of the heart, and (e) right surface of the liver.

N.B. Thoracic duct drains lymph from whole of the body except the right upper quadrant of the body which is drained by the right lymphatic duct (Fig. 23.1).

Fig. 23.1 Lymphatic drainage of the body. Note that the lymph from right upper quadrant of the body is drained by the right lymphatic duct. The lymph from remaining area of the body is drained by the thoracic duct.

Extent: The thoracic duct extends to the upper end of cisterna chyli on the posterior abdominal wall at the lower border of T12 vertebra to the junction of left internal jugular and left subclavian veins at the root of the neck.

Measurements: The measurements of the thoracic duct are as follows:

Length: 45 cm (18 inches).

Width of lumen: 5 mm (at the ends but narrow in the middle).

FORMATION, COURSE, AND TERMINATION (Fig. 23.2)

The duct begins in the abdomen at the lower border of T12 vertebra, as a continuation of cisterna chyli (lying in front of the bodies of L1 and L2 vertebrae) and enters the thorax through the aortic opening of the diaphragm. It then ascends in the posterior mediastinum to the right of midline on the front of vertebral bodies. On reaching the T5 vertebra, it crosses the midline from right to left side and enters the superior mediastinum to run along the left border of the esophagus and reaches the root of the neck.

Fig. 23.2 Formation, course, and termination of the thoracic duct.

At the root of neck it arches laterally at the level of C7 vertebra—in front of vertebral system (e.g., vertebral artery and vertebral vein) and left cervical sympathetic trunk and behind the carotid system (e.g., left common carotid artery, left internal jugular vein, and left vagus nerve). The summit of arch lies 3–4 cm above the clavicle. Finally, the duct descends in front of the first part of left subclavian artery and finally terminates by opening into the junction of left subclavian and left internal jugular veins.

N.B. The thoracic duct begins in abdomen, courses through thorax and terminates in the neck.

RELATIONS (Fig. 23.3)

The relations of thoracic duct are as follows:

Fig. 23.3 Relations of thoracic duct in the root of the neck.

A At the aortic orifice of the diaphragm

Anterior:	Median arcuate ligament of diaphragm.
Posterior:	T12 vertebra.
To the right:	Azygos vein.
To the left:	Aorta.

B In the posterior mediastinum

Anterior:	1. Diaphragm. 2. Descending aorta (lower part). 3. Esophagus (upper part).
Posterior:	1. Vertebral column. 2. Anterior longitudinal ligament. 3. Terminal parts of hemiazygos and accessory hemiazygos veins. 4. Right posterior intercostal arteries.
To the right:	Azygos vein.
To the left:	Descending thoracic aorta.

C In the superior mediastinum

Anterior:	1. Arch of aorta. 2. Commencement of left subclavian artery.
Posterior:	Vertebral column.
To the right:	Edge of esophagus.
To the left:	Left lung and pleura.

D In the root of the neck (Fig. 23.3)

Anterior:	Carotid sheath containing left common carotid artery, left internal jugular vein, and left vagus nerve.
Posterior:	1. Vertebral artery and vein. 2. Scalenus anterior muscle (medial border). 3. Phrenic nerve. 4. Thyrocervical trunk and its branches (e.g., suprascapular, transverse cervical, and inferior thyroid arteries).

TRIBUTARIES

The tributaries of the thoracic duct are as follows (Fig. 23.4):

A In the abdomen

Efferent from lower six intercostal lymph nodes of both sides.

B In the thorax

1. A pair of the ascending lymph trunks which drains lymph from the upper lumbar lymph nodes (para-aortic lymph nodes).

2. A pair of the descending lymph trunks which drain lymph from the posterior intercostal lymph nodes of upper six spaces.

3. Lymph vessels from the posterior mediastinal lymph nodes.

C In the neck

1. Left jugular lymph trunk, draining lymph from the neck.

2. Left subclavian lymph trunk, draining lymph from the left upper limb.

3. Left bronchomediastinal trunk.

Clinical correlation

• Injury of thoracic duct: The thoracic duct is thin walled and may be colorless, therefore, it is sometimes injured inadvertently during surgical procedures in the posterior mediastinum. Laceration of the thoracic duct during lung surgery results in chyle entering into the pleural cavity producing a clinical condition called chylothorax.

The cervical part of thoracic duct may be damaged during block dissection of the neck. It should be ligated immediately. If ligated, the lymph returns by anastomotic channels. But if the injury is not detected at the time of operation, and hence not ligated, it may cause an unpleasant chylus fistula and leakage of lymph. Immediate ligation of duct is required to stop the leakage.

• Obstruction of thoracic duct: Sometimes in filarial infection, the thoracic duct is obstructed by microfilarial parasites (Wuchereria bancrofti) leading to widespread effects, such as chylothorax, chyloperitoneum, chyluria, and even the accumulation of chyle in the tunica vaginalis (chylocele).

Development

There are three stages in the development of the thoracic duct (Fig. 23.5).

Stage I: In this stage, network of lymph channels is seen in front of the thoracic part of the vertebral column.

Stage II: In this stage, two longitudinal lymph channels appear, in the network of lymph channels, one on the left and another on the right with a number of cross communications.

Stage III: In this stage, the cross communication appears opposite the T5 vertebra, right longitudinal channel below this cross communication and left longitudinal channel above this cross communication persists and form the thoracic duct. All the other parts disappear.

AZYGOS AND HEMIAZYGOS VEINS

The term azygos means single i.e. without a companion. Azygos system of veins consists of azygos, hemiazygos, and accessory hemiazygos veins. These veins lie in front of thoracic part of vertebral column and play an important role in the venous drainage of the thorax.

AZYGOS VEIN (Fig. 23.6)

The azygos vein is present only on the right side in the upper part of the posterior abdominal wall and the posterior mediastinum. It connects the inferior vena cava with the superior vena cava. It is provided with valves and may appear tortuous.

Fig. 23.6 Azygos, hemiazygos, and accessory hemiazygos veins.

The functions of azygos vein are as follows:

1. It drains venous blood from the thoracic wall and upper lumbar region.

2. It forms an important collateral channel connecting the superior vena cava and inferior vena cava.

FORMATION

The formation of azygos vein is variable. It is formed in one of the following ways:

1. Formed by the union of right subcostal and right ascending lumbar vein at the level of T12 vertebra (common).

2. Arises from the posterior aspect of the inferior vena cava (IVC) near the renal veins.

3. As a continuation of right subcostal vein.

4. Occasionally, it may arise from right renal or right first lumbar vein.

COURSE AND TERMINATION

The azygos vein after formation ascends up and leaves the abdomen by passing through the aortic opening of the diaphragm and enters the posterior mediastinum. There it ascends vertically lying in front of vertebral column up to the level of T4 vertebra, where it arches forwards above the hilum of the right lung to terminate in the superior vena cava at the level of the 2nd costal cartilage.

RELATIONS

Anterior:	Esophagus (right edge).
Posterior:	1. Lower 8th thoracic vertebrae. 2. Right posterior intercostal arteries.
To the right:	1. Right lung and pleura. 2. Greater splanchnic nerves.
To the left:	1. Thoracic duct. 2. Descending thoracic aorta. 3. Esophagus (right border).

The arch of azygos vein is related below to the root of right lung, on right side to the right lung and pleura, and left side to the right border of esophagus, trachea, and right vagus nerve.

TRIBUTARIES

The tributaries of the azygos vein are as follows:

1. Lower 7th right posterior intercostal veins except first.

2. Right superior intercostal vein (formed by union of 2nd, 3rd, and 4th right posterior intercostal veins).

3. Hemiazygos vein (at the level of T7 or T8 vertebra).

4. Accessory hemiazygos vein (at the level of T8 or T9 vertebra).

5. Right subcostal vein.

6. Right bronchial vein.

7. Right ascending lumbar vein.

8. Esophageal veins with the exception of those at its lower end.

9. Mediastinal veins.

10. Pericardial veins.

Clinical correlation

In case of obstruction of SVC, it serves as the main collateral channel to shunt the blood from the upper half of the body to IVC (for details see Clinical correlation on p. 285.

HEMIAZYGOS VEIN (Fig. 23.6)

The hemiazygos vein (syn. inferior hemiazygos vein) lies on the left side only and corresponds to the lower part of the azygos vein (i.e., mirror image of the lower part of the azygos vein).

FORMATION

The hemiazygos vein formed on the left, similar to the azygos vein, by the union of left ascending lumbar vein and left subcostal vein. It may arise from the posterior surface of the left renal vein.

COURSE AND TERMINATION

It pierces the left crus of the diaphragm and ascends vertically in front of the left side of the vertebral column up to the level of T8 vertebra. At T8 vertebra it turns to the right and crosses in front of the vertebral column posterior to the aorta, esophagus and thoracic duct to terminate in the azygos vein.

TRIBUTARIES

The tributaries of the hemiazygos veins are as follows:

1. Lower three (9th–11th) left posterior intercostal veins.

2. Left subcostal vein.

3. Left ascending lumbar vein.

4. Small esophageal and mediastinal veins.

ACCESSORY HEMIAZYGOS VEIN

The accessory hemiazygos vein (syn. superior hemiazygos vein; Fig. 23.6) lies on the left side only and corresponds to the upper part of the azygos vein (i.e., mirror image of the upper part of the azygos vein).

COURSE AND TERMINATION

The accessory hemiazygos vein begins at the medial end of left 4th or 5th intercostal space and descends to the left side of the vertebral column. At the level of T8 vertebra, it turns to the right passes in front of the vertebral column posterior to the aorta, esophagus and thoracic duct to terminate in the azygos vein.

N.B. Sometimes the terminal parts of hemiazygos and accessory hemiazygos veins join together to form a common trunk which crosses across the vertebral column to open into the azygos vein.

TRIBUTARIES

The following are the tributaries of accessory hemiazygos vein:

1. Fifth to eighth (5th–8th) left posterior intercostal veins.

2. Left bronchial veins (sometimes).

THORACIC SYMPATHETIC TRUNKS (Fig. 23.7)

The thoracic sympathetic trunk is a ganglionated chain situated on either side of the vertebral column. Superiorly it is continuous with the cervical sympathetic chain at the thoracic inlet and inferiorly with the lumbar sympathetic chain after passing behind the medial arcuate ligament of the diaphragm.

Fig. 23.7 Thoracic sympathetic trunks and splanchnic nerves.

COURSE AND RELATIONS

The sympathetic chain descends in front of the neck of the 1st rib, head of 2nd–10th ribs and along the bodies of T11 and T12 vertebrae, in front of posterior intercostal nerve and vessels, passes behind the medial arcuate ligament to become continuous with the lumbar sympathetic trunk.

GANGLIA

Initially, each thoracic sympathetic trunk has 12 ganglia corresponding to the 12 thoracic spinal nerves. The first ganglion commonly fuses with the inferior cervical sympathetic ganglia to form the cervico-thoracic/stellate ganglion. The second ganglion also may occasionally fuse with the first ganglion. Thus there are usually 11 ganglia in the thoracic sympathetic trunk; sometimes there may be only 10 ganglia (vide supra). Each ganglion lies at the level of the corresponding intervertebral disc and is connected to the corresponding spinal nerve by white and grey ramus communicans.

BRANCHES

The branches of thoracic sympathetic trunks are divided into two groups: medial and lateral.

A Medial branches

The medial branches supply the viscera. They are as follows:

1. The medial branches from 1st to 5th ganglia consist of postganglionic fibres and are distributed to the heart, great vessels, lungs, and esophagus through the following plexuses:

(a) Pulmonary plexus.

(b) Cardiac plexus.

(d) Esophageal plexus.

2. Medial branches from 5th to 12th thoracic ganglia consist of preganglionic fibres and from three splanchnic nerves as follows:

(a) Greater splanchnic nerve: It is formed by the preganglionic fibres arising from 5th to 9th ganglia. It descends obliquely on the vertebral bodies, pierces the corresponding crus of diaphragm and terminates mainly in the celiac ganglion. Partly it also terminates in the aorticorenal ganglion and the suprarenal gland.

(b) Lesser splanchnic nerve: It is formed by the preganglionic fibres from 10th and 11th ganglia. It course is obliquely similar to the greater splanchnic nerve, pierces the corresponding crus of diaphragm and terminates in the celiac ganglion.

(c) Least (lowest) splanchnic nerve: It is also called renal nerve. This tiny nerve arises from the 12th thoracic ganglion and may even be absent. It descends obliquely as greater and lesser splanchnic nerves, pierces either the crus or passes behind the medial arcuate ligament of diaphragm and terminates in the renal plexus.

B Lateral branches

The lateral branches supply limbs and body wall. Their supply is pilomotor, sudomotor, and vasomotor to the skin of these regions.

The preganglionic fibres arise from the lateral horns of spinal segments and enter the sympathetic ganglion via white rami communicantes of the spinal nerve. The preganglionic fibres from the ganglion re-enter the spinal nerve via grey rami communicantes and supply the corresponding dermatome of the upper limb and the body wall.

Clinical correlation

• Thoraco-abdominal sympathectomy: The bilateral thoraco-abdominal sympathectomy is done to relieve severe hypertension. The surgical procedure involves removal of sympathetic trunk from T5 to L2 ganglia and excision of the splanchnic nerves. As a result, there occurs splanchnic vasodilatation and consequent fall in the blood pressure.

The upper limb sympathectomy is used to treat the Raynaud’s disease. In this, part of thoracic sympathetic chain is excised below the level of stellate ganglion.

N.B. Injury to stellate ganglion may cause ipsilateral Horner’s syndrome.

• Hypotension during spinal anesthesia: Sometimes hypotension occurs during high spinal anesthesia due to paralysis of sympathetic outflow to the splanchnic nerves.

• Referred pain of diaphragm: The irritation of diaphragm secondary to peritonitis causes pain due to stimulation of the phrenic nerve (root value C3, C4, and C5). The pain is referred to the corresponding tip of shoulder, being supplied by the supraclavicular nerve (root value C3, C4, and C5).

Golden Facts to Remembe

Largest lymphatic channel in the body	Thoracic duct
Chylothorax	Accumulation of chyle in the pleural cavity
Chylocele	Accumulation of chyle in the tunica vaginalis
Renal nerve	Least splanchnic nerve
Commonest cause of damage of thoracic duct at the root of the neck	Block dissection of the neck
Largest collateral channel connecting superior and inferior vena cavae	Azygos vein
Largest splanchnic nerve	Greater splanchnic nerve
Most posterior intercostal veins drain into	Azygos venous system

Clinical Case Study

A 50-year-old male visited the hospital and complained of swelling of the scrotum associated with periodic fever and passage of milky urine. On examination, the doctor found that scrotum as a whole was enlarged. The scrotal skin was thickened mainly at the bottom. On aspiration of scrotum a milky fluid came out. He was diagnosed as a case of chylocele and chyluria.

Questions

1. What is chylocele?

2. What is the cause of chylocele?

3. What is the cause of thickening of scrotal skin?

Answers

1. Accumulation of chyle in the tunica vaginalis around the testis.

2. When the thoracic duct is obstructed by the microfilaria (Wuchereria bancrofti). It leads to widespread effects such as chylothorax, chyloperitoneum, chyluria.

3. Lymphostasis.