THORACIC OUTLET SYNDROME (TOS) ⠀ ANTERIOR SCALENE SYNDROME ⠀ [ANATOMY AND PATHOLOGY] ⠀ The fibres of the brachial plexus and the subclavian artery must pass through a narrow aperture between anterior and middle scalene muscle. If these structures get entrapped it can lead to paresthesia or weakness in the upper limb. ⠀ The fibres of the subclavian vein, the suprascapular vein, the transverse cervical vein and the phrenic nerve must pass through a narrow aperture between the anterior scalene muscle and the sternocleidomastoid muscle. If these structures get entrapped it can lead to edema in the hands/fingers and also to problems with the thoracic diaphragm. ⠀ The fibres of the long thoracic nerve must pass through a narrow aperture between middle and posterior scalene muscle. If these structures get entrapped it can lead to a problem with the serratus anterior muscle. ⠀ Any increased tone in one of these muscles (usually scalene anterior), will provide enough compression to produce symptoms. Scarring and adhesions from injuries can be causes of compression, particularly if found at the attachments of the scalenes. Dysfunction or misalignent of the cervical or thoracic spine may also be a contributing factor to this syndrome. ⠀ Active trigger points in the scalene muscles can mimic a TOS and initiate similar symptoms, often characterized by numbness or tingling in the thumb or index finger .