TOS is not an actual diagnosis according to the International Classification of Diseases (ICD), but a symptom complex. It is detected by the exclusion of other pathologies that can provoke neurological or vascular arm symptoms like cervical radicular syndrome and carpal tunnel syndrome.
Classification of TOS into further subcategories:
| TOS classification | Description |
| Neurological TOS | Entrapment of the brachial plexus |
| Vascular TOS | Entrapment of either an artery or vein |
| True TOS | TOS due to congenital abnormalities like an enlarged transverse process of C7, cervical ribs or enlarged scalene muscles |
| Symptomatic TOS | Intermittent and due to bad posture such as a lowered chest wall, drooping shoulders or a forward flexed neck |
According to Davidovic et al. (2003) the incidence of TOS is as high as 8% and it is said to be pretty rare in children with the ratio of males to females of 1:2 to 1:4.
Furthermore, 98% of the causes are neurogenic, while 2% are due to vascular compression.
Entrapment of neurological or vascular structures can happen in the following intervals:
| Site of entrapment | Description |
| Posterior scalene port / posterior scale triangle | Between the middle and posterior scalene muscle (no venous TOS possible as the subsclavian vein is not passing through this port) |
| Costoclavicular space | Interval between the clavicle and the first rib |
| Subcoracoid tunnel | Between the tendon of the m. pectoralis minor and the coracoid process |
| Study | |||||
| Watson et al. (2009) | |||||
| Watson et al. (2010) | |||||
| Davidovic et al. (2003) |