Thoracic Sympathectomy

Anatomy of the Cervical and Thoracic Sympathetic Ganglia

Thoracic sympathetic ganglia are the most laterally placed structures in the posterior mediastinum, and each sympathetic trunk alongside the thoracic vertebrae enters the thorax by passing anterior to the neck of the corresponding rib and exits the chest by passing behind the medial arcuate ligament of the diaphragm. In the upper part of the thorax it lies on the necks of the ribs, and in its lower course, it lies on the vertebral bodies as the vertebrae become broader. The thoracic sympathetic trunk usually contains 11 or 12 ganglia. The first thoracic and inferior cervical ganglia might fuse together in front of the neck of the first rib to form the stellate ganglion ( Figure 1 ).

Each thoracic sympathetic ganglion is connected with the corresponding spinal nerve by white and gray ramus communications. The white ramus brings preganglionic fibers from the spinal nerve to the sympathetic ganglion, and the grey ramus carries postganglionic fibers from the ganglion to the spinal nerve (see Figure 1 ).

The preganglionic fibers from the T1 segment go mostly to the head and neck region; however, in about 10% of the population, T1 fibers supply the upper extremity as well. The 2nd, 3rd, and 4th ganglia give branches to the cardiac and pulmonary plexuses. The upper five ganglia give fine branches to the aorta to form an aortic plexus (see Figure 1 ).

The three lower branches of the thoracic sympathetic chain are the greater, lesser, and least splanchnic nerves. These sympathetic fibers pass to the abdominal cavity as follows: The greater splanchnic nerve arises from ganglia 5 to 9 and passes through the crus of the diaphragm to end in the celiac plexus; the lesser splanchnic nerve arises from the 9th and 10th thoracic ganglia and crosses the crus of the diaphragm and ends, joining the celiac plexus as well; and the least splanchnic nerve may arise from 11th and 12th ganglia and end in the rectal plexus. For clinical application, it is worthwhile to note that if spinal anesthesia is given high up, it can produce a temporary drop in the blood pressure by abolishing the effect of sympathetic vasoconstrictor fibers that arise from T5 downward and pass to the abdominal viscera by way of the splanchnic nerves.

Destruction of the white ramus of the 1st thoracic nerve produces Horner’s syndrome, because this nerve carries fibers to the dilator pupillae muscle, Muller’s muscle of the upper lid, and vasoconstrictor and secretory fibers to the sweat glands. This syndrome involves four clinical features: miosis (constricted pupil), ptosis (drooping upper lid), anhidrosis (loss of perspiration on the affected side), and enophthalmos (sunken eye). Therefore, sympathectomy for vascular diseases of the upper limb should be done below the level of the 1st ganglion. Most clinicians recommend excision of the 2nd and 3rd thoracic ganglia with their rami and their intervening part of the trunk to avoid Horner’s syndrome.

Physiology of Sympathetic Nervous System

All sympathetic preganglionic fibers exit the spinal cord in the anterior nerve roots of the thoracic and first two lumbar nerves. These fibers (white ramus) either synapse in the thoracic and lumbar ganglia or run up or down through the sympathetic chain to synapse with other ganglia (see Figure 1 ). Some of these fibers might pass through ganglia to synapse in a more peripheral ganglion. The term white ramus is used because all of the preganglionic fibers are myelinated. The fibers leaving the ganglia are called efferent postganglionic cells; these are unmyelinated and gray.

These fibers are distributed via branches of the spinal cord to the target blood vessel (adrenergic fibers), sweat glands (cholinergic fibers), and arrector pili muscles (adrenergic fibers). These fibers are vasomotor, sudomotor, and pilomotor. These blood vessels have no other antagonistic fibers; therefore, vasoconstriction is secondary to stimulation of sympathetic fibers, and absence of sympathetic stimulation contributes to vasodilation. Every spinal nerve except the cervical and lower lumbar receives a gray ramus; therefore the thoracic and first two lumbar spinal nerves have a preganglionic white ramus, ganglia, and gray ramus, which constitutes the somatic branches of the sympathetic ganglia. Distribution of the autonomic nervous system is more prominent in the arteriolar bed. Therefore, skin hyperemia can result from stimulation of the autonomic nervous system.

The highest concentration of sweat glands is on the palms and the soles. Cholinergic stimulation of the sympathetic nervous system will activate postganglionic gray matter and nonmyelinated C fibers, consequently producing sweating from sudomotor effect. This understanding supports the concept of sympathetic block that has been advocated by some before any permanent sympathectomy is done. Also, for complete sympathectomy, excision of the ganglia as well as postganglionic fibers is crucial.

Sweating is not based completely on the above mechanism. There are other mechanisms: Emotional sweating is cortically mediated, thermal sweating is mediated via the hypothalamus, gustatory sweating is controlled via medullary nuclei, and spinal sweating is controlled by spinal cord.