The term ‘thoracic outlet syndrome’ (TOS) was originally coined in 1956 by RM Peet to indicate compression of the neurovascular structures in the interscalene triangle possibly corresponding to the etiology of symptoms. Since Peet provided this definition, the condition has emerged as one of the most controversial topics in musculoskeletal medicine and rehabilitation. This controversy extends to almost every aspect of the pathology including the definition, incidence, pathoanatomical contributions, diagnosis, and treatment.

Controversy with this diagnosis begins with the definition because the term TOS only outlines the location of the problem without actually defining what causes the problem. TOS encompasses a wide range of clinical manifestations due to compression of nerves and vessels during their passage through the cervicothoracobrachial region.

 

Investigators identify two main categories of TOS: the vascular form (arterial or venous), which raises few diagnostic problems, and the neurological form, which occurs in more than 95-99% of all cases of TOS. Neurological forms are classified either as ‘‘true’’ neurological forms associated with neurological deficits (mostly muscular atrophy), or "disputed" neurological forms (with no objective neurological deficit). The disputed neurological forms add to the controversy of the TOS topic due to the absence of objective criteria to confirm the diagnosis.

 

Relevant Anatomy

 

The neural container described as the thoracic outlet is comprised of several structures, and is divided into two main sections by the first rib. The proximal portion consists of the interscalene triangle and the costoclavicular space, while the axilla comprises the distal aspect of the canal. The proximal portion has more clinical relevance since there is a higher potential for neurovascular compression at that site.

More specifically, the thoracic outlet includes three compact compartments: the interscalene triangle, the costoclavicular space, and the throaco-coraco-pectoral space. The interscalene triangle is bordered by the anterior scalene, middle scalene, and the medial surface of the first rib. The trunks of the brachial plexus and the subclavian artery travel through this triangle. The costoclavicular space is bordered anteriorly by the middle third of the clavicle, posteromedially by the first rib, and posterolaterally by the upper border of the scapula. The borders of the thoraco-coraco-pectoral space include the coracoid process superiorly, the pec minor anteriorly, and ribs 2-4 posteriorly.

Certain anatomical abnormalities can be potentially compromising to the thoracic outlet as well. These include the presence of a cervical rib, congenital soft tissue abnormalities, clavicular hypomobility, and functionally acquired anatomical changes.

 

Cervical ribs form off of the 7th cervical vertebra and are found in approximately 1% of the population, with only 10% of these people ever experiencing adverse symptoms. Soft tissue abnormalities may create compression or tension loading of the neurovascular structures found within the thoracic outlet. Researchers have found different congenital scalene morphologies in individuals with TOS such as hypertrophy or a broader middle scalene attachment on the 1st rib. Another complicating soft tissue anomaly found are fibrous bands that increase the stiffness and decrease compliance of the thoracic container, resulting in an increased potential for neurovascular load. These soft tissue abnormalities are usually detected with magnetic resonance imaging.  Lastly, Laulan and her colleagues introduce a mechanism of functional acquired anatomical changes that occur from compensation and repetitive activities (usually overhead). In this population, upper limb dysfunction or muscle imbalances of the neck and shoulder region are considered responsible for TOS.