Causes of Neurogenic Thoracic Outlet Syndrome

The anatomy and causes of neurogenic thoracic outlet syndrome are multiple and complex. Neurogenic thoracic outlet syndrome is by far the most common type of TOS. In general, patients experience a broad range of non-specific signs and symptoms. At the same time, doctors are often not aware of the diagnosis, or inexperienced in diagnosing patients with neurogenic TOS. Additionally, doctors cannot directly demonstrate the causes of neurogenic TOS in any one patient. Nonetheless, early clinical diagnosis of neurogenic TOS makes a critical difference in treatment outcomes. It is important to emphasize that modern imaging plays a most important role in each patient to prove the causes of neurogenic thoracic outlet syndrome.

All of the causes of neurogenic thoracic outlet syndrome have a common end-point: compression or tension of the brachial plexus. Put another way, neurogenic thoracic outlet syndrome occurs when anatomic structures in the thoracic outlet compress the brachial plexus. In detail, the brachial plexus begins as a large network of nerves arising from the spinal cord in the neck. After the brachial plexus branches and unites multiple times, it forms five terminal nerves. Finally, these nerves provide motor, sensory, and autonomic functions for each arm.

In general, compression or tension of the brachial plexus causes symptoms of neurogenic thoracic outlet syndrome. In detail, anatomic variants may narrow the normal tunnels of the thoracic outlet. These narrow tunnels may compress the brachial plexus. In addition, movement of the arms may cause compression or tension of the brachial plexus. Given these points, causes of neurogenic TOS may be static or dynamic. Firstly, static or fixed anatomic abnormalities narrow the spaces through which the brachial plexus must pass to reach the arm. Second, dynamic changes occur with arm motion, Which may result in brachial plexus compression or tension. As a result of either process, compression or tension on the brachial plexus may cause neurogenic thoracic outlet syndrome.

Neurogenic TOS is one form of a broader process physicians know as an entrapment neuropathy. Physicians are quite familiar with entrapment neuropathies occurring elsewhere in the body. Entrapment neuropathy occurs when muscles, bones, or soft tissues compress or stretch a nerve, resulting in neurogenic symptoms. Entrapment neuropathy occurs throughout the body. When a doctor diagnoses an entrapment neuropathy, she often orders an MRI to visualize the structures compressing the nerve.

It is important to realize that multiple mechanisms may cause compression or tension of the brachial plexus. In general, we find it helpful to divide these mechanisms into static and dynamic categories. To clarify our system, we have created a schematic, which you can review below. Specifically, static mechanisms consist of fixed, space-occupying lesions that narrow the tunnels through which the brachial plexus normally travels. In contrast, dynamic mechanisms are those where the tunnels are normal at baseline, but narrow on arm motion. Further, we can divide static mechanisms into developmental and acquired groups. In detail, developmental static elements include anatomic anomalies or variants that are present at birth. Conversely, acquired static elements appear during life. For example, a clavicle fracture represents an acquired static element. Another key point is that both static and dynamic mechanisms may contribute to nerve compression or tension in a patient.

Underlying Mechanisms of TOS

Since the early 1900s, TOS specialists have recognized a large number of anatomic variants in the thoracic outlet. We can divide these into soft tissue variants and bone variants. In general, soft tissue anomalies are common. These include muscle anomalies such as the scalene minimus. Likewise, patients often have fibrous bands. Specifically, fibrous bands arise from the lung apex, C7 transverse process, or cervical rib, and extend to the first rib. On the other hand, bony anomalies are less common.  Duplicated ribs, bifid ribs, and bony tubercles are quite uncommon. Conversely,  elongated C7 transverse processes and cervical ribs are fairly common. Overall, these two are present in about 1% of people. The relationship between cervical ribs and thoracic outlet syndrome is tenuous, at best. Many people have cervical ribs, most of whom do not have thoracic outlet syndrome.

Compression or tension on the brachial plexus can occur even with normal baseline thoracic outlet anatomy. So long as the thoracic outlet tunnels are normal, the brachial plexus should pass to the arm without difficulty. However, in some patients with normal tunnels at rest, arm motion causes compression or tension on the brachial plexus. In detail, the first rib serves as the ‘floor’ of one tunnel, the costoclavicular interval. This is the space between the collarbone (clavicle) and the first rib. The brachial plexus passes between these two bones. While the first rib moves little with arm motion, the clavicle moves quite a bit downwards towards the first rib. If the arm moves to certain positions, the clavicle and first rib can compress the brachial plexus. Moreover, this compression can occur even if the tunnels are normal at rest.

Bones can contribute to compression or tension on the brachial plexus in other parts of the thoracic outlet. For example, many people have cervical ribs, but few of these have thoracic outlet syndrome. While the cervical rib occupies space in the scalene triangle, brachial plexus compression occurs only in certain arm positions. This compression takes place between the cervical rib and the anterior scalene muscle. In this case, compression occurs between a soft tissue structure and a bony structure. In the same way, the retropectoralis space is located between two muscles and a bony structure, the coracoid process. Ordinarily, this space is wide enough for the brachial plexus to pass through unimpeded. However, in TOS patients, arm motion can move the brachial plexus against the coracoid process, creating tension on the plexus. As a result, neurogenic TOS can develop.

One point often overlooked is that doctors cannot directly see the causes of neurogenic TOS on physical examination. In general, a doctor can see the symptoms that result from brachial plexus compression or tension. However, the underlying causes of neurogenic TOS are not discernible on physical examination. As a result, modern imaging studies serve an essential purpose to clearly demonstrate these causes of neurogenic thoracic outlet syndrome.