There is plenty of evidence in the medical literature to show that arterial compression does not equate to brachial plexus compression, although they may occur together, in varying degrees. Additionally, arterial compression by itself does not make the diagnosis of arterial TOS (the rarest form of TOS). Lesser degrees of arterial compression have been shown in normal individuals in various arm positions and are thought to be of little significance without the other criteria for arterial TOS.MRI scan can show the anatomy of the thoracic outlet, the soft tissues causing compression, and can show directly the brachial plexus compression. Classification
By structures affected and symptomatology
There are three main types of TOS, named according to the cause of the symptoms; however, these three classifications have been coming into disfavor because TOS can involve all three types of compression to various degrees. The compression can occur in three anatomical structures (arteries, veins and nerves), it can be isolated, or, more commonly, two or three of the structures are compressed to greater or lesser degrees. In addition, the compressive forces can be of different magnitude in each affected structure. Therefore, symptoms can be variable. Neurogenic TOS includes disorders produced by compression of components of the brachial plexus nerves. The neurogenic form of TOS accounts for 95% of all cases of TOS. Arterial TOS is due to compression of the subclavian artery. This is less than one percent of cases. Venous TOS is due to compression of the subclavian vein. This makes up about 4% of cases.
If left untreated, TOS can lead to neurological deficits as a result of the hypoperfusion and hypometabolism of certain areas of the brain and cerebellum.TOS has similar symptoms to pectoralis minor syndrome (PMS), which usually results from compression of the braxial plexus beneath the pectoralis minor muscle (while neurogenic TOS is caused by compression of the same nerves above the clavicle). Unlike TOS there is typically few headaches or neck pain in patients with PMS only, instead with pain in the chest area. Initially, it was believed that 95 percent of patients with TOS had nerve compression in the scalene area, but in the twenty-first century it is now recognized that the majority have nerve compression under the pectoralis minor, either by itself or in addition to the scalene area. One study of 100 patients diagnosed with neurogenic TOS found that 75 percent had neurogenic PMS and 30 percent in fact had PMS without TOS. Causes
TOS can be attributed to one or more of the following factors:
Congenital abnormalities are frequently found in persons with TOS. These include cervical rib, prolonged transverse process, and muscular abnormalities (e.g., in the scalenus anterior muscle, a sickle-shaped scalenus medius) or fibrous connective tissue anomalies. Trauma (e.g., whiplash injuries) or repetitive strain is frequently implicated. Rarer acquired causes include tumors (especially pancoast tumor), hyperostosis, and osteomyelitis

Diagnosis
Adsons sign and the costoclavicular maneuver lack specificity and sensitivity and should make up only a small part of the mandatory comprehensive history and physical examination undertaken with a patient suspected of having TOS.