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Anatomy of the thoracic outlet2019-04-20T02:29:40+00:00

Anatomy of TOS

Understanding anatomy is the key to understanding TOS

Abnormal anatomy causes or contributes to compression of the vital structures in the thoracic outlet. As a result, patients experience the symptoms of TOS. While the anatomy of the thoracic outlet is complex, understanding the anatomy enables understanding of TOS. We can strive to understand the structures that cause compression as well as the structures that become compressed.

We can categorize the essential anatomy into these categories:

Anatomy of the spine

The spine supports our body weight while allowing flexibility in multiple directions.

The vertebral body is the basic building block of the spine. Vertebral bodies (also known as vertebrae) are stacked one upon the other from the skull to the pelvis. The vertebral bodies support our body weight.

Between each vertebral body lies an intervertebral disc. Each disc is firmly attached to the vertebral body above it and to the body below it. The disc has a fibrous outer layer that stabilizes each vertebral body to the next. The center of each disc contains a jelly-like substance that absorbs impacts from walking, jumping and other movement. The flexible discs allow movement in multiple directions:

  • Flexion and extension: Forward and backward bending

  • Lateral bending: Side to side bending

  • Rotation: Side to side twisting

Vertebral bodies and intervertebral discs

The spine comprises five anatomic regions, from top to bottom:

  • Cervical spine: The neck

  • Thoracic spine: The mid-back; twelve paired ribs arise from the thoracic spine, forming the rib cage

  • Lumbar spine: The low back

  • Sacral spine: The base of the spine; joins the pelvis to the spine

  • Coccyx: The tailbone; in humans, the coccyx represents a vestigial tail

Spine anatomy regions

Each region of the spine has a different number of vertebral bodies:

  • Cervical spine-seven segments, numbered C1 (‘Cervical’ 1) through C7

  • Thoracic spine-twelve segments, numbered T1 (‘Thoracic’ 1) through T12; each body has a rib on each side

  • Lumbar spine-five segments, numbered L1 (‘Lumbar’ 1) through L5

  • Sacral spine-five segments, numbered S1 (‘Sacral’ 1) through S5; these segments are fused together into one bone in adults, which articulates with the pelvis on each side.

  • Coccyx-three to five segments, with variable degrees of fusion in the adult; usually referred to as a single unit rather than separate vertebrae.

A bony arch arises from the back of each vertebral body. These arches line up to form a flexible bony tunnel, the spinal canal. The spinal canal runs from the base of the skull to the pelvis, and it contains and protects the spinal cord. At each spinal level, a nerve leaves the spinal cord on each side. Each nerve exits the spinal canal through a tunnel between the bony arches to reach the body or extremities. This tunnel is called a neural foramen.

On the back and sides of each arch, bony protrusions arise, which allow for attachments of the numerous muscles that stabilize and move the spine.

Detailed anatomy of spine

A fluid filled sac (the thecal sac) fills the skull and central spinal canal, and contains the brain and spinal cord. Similar to the bony vertebral levels, the spinal cord comprises cervical, thoracic, lumbar, sacral and coccygeal segments. Thus, cervical nerve roots exit the upper spinal cord, and the coccygeal nerve roots exit the distal spinal cord. It should be noted that the levels in the spinal cord do not align with the levels in the bony spine. Therefore, exiting nerve roots travel a variable distance in the thecal sac before exiting at their respective neural foramina.

5 spinal regions: cervical, thoracic, lumbar, sacral, and coccygeal

Nerve fibers are vulnerable to compression or tension as they travel in narrow spaces throughout the body. Within the spinal canal, a bulging disc or herniated disc can press on the spinal cord. A narrowed neural foramen can compress an exiting nerve root. These are examples of nerve entrapment, where compression or tension on a nerve causes symptoms in the territory of that nerve. Thoracic outlet syndrome is another, more complex, form of nerve entrapment.

Herniation nerve entrapment

Once a nerve exits its neural foramen, it enters the thoracic outlet. Read more about the anatomy of the thoracic outlet.

Anatomy of a nerve

Nerves connect the brain and the body. At its simplest, a nerve can be thought of as a simple wire that transmits information. At a deeper level, however, nerves are complex and varied. A basic understanding of the anatomy of a nerve will help us understand the varied symptoms of TOS.

In general, the human nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system comprises the brain, the brainstem (the lowest and most primitive part of the brain) and the spinal cord. These structures are contained within the fluid-filled thecal sac, which sits inside the skull and spinal canal. The peripheral nervous system comprises all other nerves and nervous system structures.

The peripheral nervous system is divided into two parallel systems: the somatic nervous system and the autonomic nervous system (read more). The somatic nervous system controls voluntary bodily functions, while the autonomic nervous system controls involuntary bodily functions. Voluntary functions include motor functions, such as walking, and conscious sensations, such as feeling light touch against the skin. Involuntary functions include unconscious motor functions like blood pressure regulation, and unconscious sensory regulation, such as pupils constricting in bright light.

The basic building block of a nerve is the nerve cell, the neuron. A neuron is a specialized, electrically excitable cell that receives, processes, and transmits information throughout the body. Each neuron includes a cell body (control center), dendrites (which receive signals), and an axon (which transmits signals). The axon, or nerve fiber, can be extremely long relative to the size of the body. There are many different types of neurons, resulting in a large variety of nerve fibers. The body of each neuron may be located in the brain, the spinal cord, or in a ganglion. A ganglion is a cluster of neuron cell bodies arising outside the central nervous system.

Anatomy of a neuron

Classification of the different types of peripheral nerve fibers is based on the diameter of the nerve fiber and whether the nerve fiber is myelinated. Larger diameter nerve fibers conduct signals at a higher velocity than smaller diameter nerve fibers. In addition, some nerve fibers are insulated with a myelin sheath. This is a sheath of fatty tissue produced by a support cell adjacent to the nerve fiber. The myelin sheath helps to increase nerve conduction velocity. Somatic motor fibers are usually myelinated. Certain types of somatic sensory fibers and autonomic fibers are myelinated, while others are unmyelinated. All nerve fibers in the central nervous system are myelinated.

Nerve fibers myelin sheath

Whether the nerve fibers arise in the brain, spinal cord, or peripheral ganglion, they eventually form a peripheral nerve with hundreds or thousands of other nerve fibers. Peripheral nerves almost always include a mix of motor and sensory fibers, somatic and autonomic fibers, large and small, myelinated and unmyelinated fibers. The nerve fibers cluster together in fascicles, and multiple fascicles cluster together within a peripheral nerve.

Nerve anatomy detail
  • The somatic nervous system

  • The autonomic nervous system

The somatic nervous system controls voluntary bodily functions, while the autonomic nervous system controls involuntary bodily functions. Voluntary functions include motor functions, such as walking, as well as conscious sensations, such as touching a hot surface or feeling touch against your skin. Involuntary functions include motor functions, such as regulating blood vessels and blood pressure, as well as unconscious sensory regulation, such as the pupils constricting in bright light.

Somatic motor nerves control the voluntary muscles of the body, and allow us to move our bodies at will. Bear in mind that we have both voluntary muscles and involuntary muscles. Voluntary muscles are those we control with conscious thoughts, such as flexing one’s biceps or picking up a cup of coffee. Involuntary muscles are muscles that function on a subconscious level, usually to regulate functions of the body, such as moving food through the gut, or narrowing the pupil when bright light hits it.

Somatic sensory nerves transmit information from different types of receptors throughout the body back to the brain. Examples of sensory nerves include a nerve that senses a pinprick, or one that senses a hot stove. At the end of each sensory nerve is a receptor that is specialized for a specific function. Receptors are specialized for heat, pain, pressure, position sense, and other stimuli.

Autonomic nerves function as a subconscious, primitive nervous system that controls many involuntary functions of the human body. The autonomic nerve fibers control such functions as sweating, regulation of blood pressure, dilation or contraction of the pupils, and the function of the gut. Autonomic nerve fibers include both motor and sensory nerve fibers. Autonomic motor fibers control involuntary muscles. For example, autonomic motor fibers control the muscles in the walls of our blood vessels or bowel, regulate sweating, and control the adrenal glands. We have no conscious control of these functions. Autonomic sensory fibers transmit information about heart rate, body temperature, blood oxygen, or blood pressure back to the central nervous system. We have no direct. conscious sensation of these functions

Somatic and autonomic nervous systems

To sum things up so far, hundreds or thousands of different nerve fibers compose a typical peripheral nerve. Each of these fibers possesses different qualities, and each has a unique function. Compression or tension on the peripheral nerve may affect many different types of nerve fibers, resulting in a variety of symptoms.

Anatomy of TOS

Anatomy of the thoracic outlet leads to TOS

Is my anatomy normal?

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Anatomy of the brachial plexus

A plexus is a network or cluster of nerves. The brachial plexus is the second largest plexus in the human body. On each side, the brachial plexus passes through the thoracic outlet to reach the arm. The brachial plexus provides the nerve supply to both upper extremities, including portions of the chest wall and shoulders.

Medical students spend a lot of time memorizing the detailed anatomy of the brachial plexus. Few doctors use such detailed information when diagnosing and treating patients. While it is important to understand the basic structure of the brachial plexus, it is also important to understand its relationship to the surrounding structures of the thoracic outlet.

Segmental anatomy of the brachial plexus

Each brachial plexus comprises 5 nerve roots: C5, C6, C7, C8 and T1. Once these roots leave their respective neural foramina on each side of the cervical spine, they enter the thoracic outlet. The roots from three trunks. Each of the three trunks splits into two divisions. The divisions rejoin to form cords. Finally, the cords produce five terminal nerves:

Five nerve roots exit the spine on each side to form the brachial plexus: C5, C6, C7, C8 and T1.

The five nerve roots form three trunks:

C5 and C6 form the upper trunk.

C7 forms the middle trunk.

C8 and T1 form the lower trunk.

Each trunk splits into an anterior division and a posterior division.

The divisions join into three cords:

Lateral cord

Medial cord

Posterior cord

The three cords form five terminal nerves:

Musculocutaneous nerve

Median nerve

Axillary nerve

Radial nerve

Ulnar nerve

Brachial plexus segmental anatomy

The five terminal nerves extend down the arm. There are additional smaller nerves arising from the brachial plexus, which have been excluded for simplicity. It is worth remembering that each of the five major terminal nerves contains a mix of somatic and autonomic nerves, both motor and sensory. Given the complex branching pattern of the brachial plexus, as well as the complex mixture of nerve fibers within each branch, it should be easy to understand why compression or tension on the brachial plexus would produce a complex clinical picture.

Anatomy of the thoracic outlet

Doctors have classically defined the thoracic outlet as the space at the top of the rib cage, where the neck joins the chest. Doctors use the term outlet because blood vessels pass from the chest into the neck and shoulders through this space. At the same time, anatomists have classically defined the same space as the thoracic inlet. Anatomists use the term inlet because the esophagus and trachea pass from the neck into the chest. This point remains somewhat pedantic, and is unimportant in our discussion of anatomy. Our first media shows how the classically-defined thoracic outlet crosses the midline in front of the spine.

Thoracic outlet rib cageThoracic outlet rib cage

The great vessels (one subclavian artery and vein on each side) exit the chest through this space. The nerve roots exit the cervical spine and form one brachial plexus on each side. The vessels and plexus align on each side, pass over the top of the chest, and enter each arm. 

Thoracic outlet borders skeletalThoracic outlet borders and contents

The image below demonstrates the relationship between the bony thoracic outlet and the soft tissues that pass through it.

TOS Anatomy Plexus, Vein and Artery

Tunnels of the Thoracic Outlet

Three vital structures (brachial plexus, subclavian artery and vein) follow a pathway through three anatomic tunnels to each arm. Narrowing of any of these tunnels may compress or stretch the nerves or vessels, causing TOS. We can better understand TOS by understanding these pathways.

As we move from the neck to the arm, the three tunnels in this pathway have been classically defined as:

  • The scalene triangle

  • The costoclavicular interval

  • The retropectoralis space

Scalene triangle

After the nerve roots exit the spine, they form the brachial plexus. The subclavian artery exits the chest, after which it travels with the brachial plexus. These two structures enter the scalene triangle. The scalene triangle is defined by the following structures:

  • Anterior: Anterior scalene muscle
  • Posterior: Middle scalene muscle
  • Inferior: First rib

The subclavian vein takes a different path than the subclavian artery. The subclavian vein exits the chest and passes anterior to the anterior scalene muscle. Thus, only two of the three vital structures pass through the scalene triangle.

TOS Anatomy-Scalene TriangleTOS Anatomy-Scalene Triangle

Costoclavicular Interval

After the subclavian artery and brachial plexus exit the scalene triangle, they align with the subclavian vein. The three structures then enter the costoclavicular interval. The costoclavicular interval is defined by the following structures:

  • Superior: Clavicle
  • Inferior: First rib
TOS Anatomy-Costoclavicular IntervalTOS Anatomy-Costoclavicular Interval

Retropectoralis Space

The subclavian artery and vein exit the costoclavicular interval and enter the retropectoralis space. The retropectoralis space is defined by the following structures:

  • Anterior: Pectoralis minor muscle
  • Posterior: Subscapularis muscle
  • Superior: Coracoid process of the scapula

The retropectoralis space is an inverted triangle, with the narrowest area at the bottom and the widest area at the top, just beneath the coracoid process.

TOS Anatomy-Retropectoralis SpaceTOS Anatomy-Retropectoralis Space

These anatomic tunnels normally conduct the three vital structures from the neck and chest to each arm. TOS occurs when narrowing of one or more of these tunnels becomes severe enough to cause symptoms. Symptoms develop when compression or stretching damages one of the three vital structures. If we hold a clear understanding of anatomy, we can grasp the changes in anatomy that cause TOS.