Thoracic outlet syndrome (TOS) is a controversial topic in the literature; many proponents support the existence of the condition, but some strongly vocal opponents doubt the validity of TOS as a medical entity. Even the name itself has been questioned because the actual site of pathology is technically the thoracic "inlet," not the "outlet." The primary controversy seems to center around the lack of objective criteria for diagnosis and the confusion with multiple types or clinical presentations.
Since TOS involves proximal neurovascular structures (see the image below), symptoms often are confused with various distal compression neuropathies or cervical radiculopathies.
In addition, surgical treatment has been known to have devastating complications, which further fuels debate with the opponents of recognition of this entity. Conservative treatment appears to be the most universally accepted approach, with even surgeons recommending a prolonged trial before any operative procedure.
A review of the pathomechanics and dysfunctional body habitus associated with this type of presentation suggests that the term "postural TOS" may be more appropriate, particularly when no neurologic or vascular abnormality is documented (or testing results are inconclusive). 
Thoracic outlet syndrome (TOS) involves compression, injury, or irritation to the neurovascular structures at the root of the neck or upper thoracic region, bounded by the anterior and middle scalenes; between the clavicle and first rib (with possible enlargement/hypertrophy of the subclavius); or beneath the pectoralis minor muscle. Some authors define the thoracic outlet as an opening bordered by the first rib laterally, the vertebral column medially, and the claviculomanubrial complex anteriorly. The syndrome of compression at this site could be primarily neurologic, involving the brachial plexus, most often the lower trunk or medial cord; alternatively, it could involve compression of the subclavian artery and/or vein. [2, 3] Thrombosis, embolus, or aneurysm of these vessels is a less likely possibility.
In addition, imaging with diagnostic ultrasound has been able to identify deformation of the pectoralis minor in TOS that may be related to the pathophysiology, due to excessive tightness or shortening of the muscle that impinges on the neurovascular bundle. 
One proposed classification system has broken TOS into the following 3 categories:
True neurogenic TOS: The brachial plexus is injured in these cases as documented by electromyography (EMG) and/or nerve conduction studies.
Nonspecific or disputed TOS: Patients have symptoms, but there are no abnormal tests to document the lesion clearly. This category is by far the most common type of TOS seen in the clinical setting.
A new category is recommended for cases in which no objective neurologic or vascular abnormality can be identified. These cases could be termed postural thoracic outlet syndrome, which may be more appropriate, owing to the dysfunctional body habitus observed almost universally in these cases. 
Many authors have discovered accessory cervical ribs associated with TOS  ; however, they have noted tough fibrous bands coming off the accessory ribs that are believed to be more responsible for the pathology. There have even been reports of bony fusion of variant cervical ribs, resulting in bifid ribs with attached fibrous bands.  The bands cause tethering of the brachial plexus, which results in traction and, therefore, symptoms. Other authors report compression or irritation of the neurovascular bundle more distally under the pectoralis minor muscle or from anterior displacement of the humeral head.
Additionally, clavicle fractures can result in plexopathy from expanding hematomas or pseudoaneurysms that compress the plexus, with variable latent periods following the fracture. Delayed onset of symptoms may suggest exuberant callus from the healing fracture site. Nonunion of the fracture site also can result in direct compression by the lateral fragment, which is pulled inferiorly.
Trapezius weakness due to spinal accessory nerve injury (following cervical lymph node biopsy) also has been implicated as a cause of TOS. This results in "droopy shoulder" with secondary compression of the neurovascular bundle, which is particularly aggravated with arm elevation (abduction). 
The inability to make a definitive and accurate diagnosis makes determination of the exact prevalence of this condition impossible. The prevalence of nonspecific thoracic outlet syndrome (TOS) has been reported as high as 23% of soft-tissue injuries of the cervical spine. TOS is overlooked or misdiagnosed commonly, especially upon presentation to the emergency department. The true or classical neurogenic or vascular TOS incidence is considered rare, with only 1 case per million population estimated for the neurogenic type. Some believe that TOS is the most common cause of acute arterial occlusion in the upper limbs of adults younger than 40 years. It is more common with occupations or activities that involve prolonged posturing of the neck, such as secretaries, cashiers, machine operators, surgeons, truck drivers, and overhead work or lifting. There is no increased incidence in athletes.
No known mortality is associated directly with thoracic outlet syndrome (TOS). Morbidity often encompasses debilitating functional loss of the involved upper extremity, loss of livelihood or occupation, especially if the work involves overhead activity (eg, hairdressing, carpentry, painting). True neurogenic TOS causes neurologic deficit. Depending on the amount of nerve injury, there usually is weakness of the hand and sensory deficits in the lower trunk distribution. Devastating complications, usually to the brachial plexus, have been reported from surgical treatment for TOS.
Females are diagnosed more commonly with thoracic outlet syndrome than males, with some reports of a 9:1 female-to-male ratio. The shape of the chest wall is believed to predispose women by encouraging closure of the thoracic outlet. Large, pendulous breasts have been particularly implicated, with these adding to the anterior forces on the chest, leading to drooped shoulder posturing and further closing the outlet. 
Age of onset of thoracic outlet syndrome is from the second to the eighth decade, with a peak occurring in the fourth decade. In the younger age group, there appears to be a greater likelihood of anatomic or structural abnormality, such as cervical rib variants and fibrous bands.
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