Hi Grom,
The part that hits me is the displacement in 45 % cadavers and in 21 % TOS patients, where the vascular comes through the wrong scalense and it is vised in between..
makes sense to why some of us are vascular or start as neuro and develope vascular problems.
Di

Anterior scalene
The anterior scalene muscle has a constant site of origin from the third through the sixth vertebrea, but its point of insertion, generally on the scalene tubercle of its instertion on the first rib, can vary.
The insertion of the tubercle is between the subclavian artery and vein, with the expansion of the plural dome. Variants include insertion behind the artery and brachial plexus, or an extended area of insertion behind the artery, between the artery and brachial plexus, or extended area of insertion that includes the entire base of the scalene triangle.

The latter variant may result in the anterior and middle scalene muscles forming a vise around the neurovascular bundle. The insertion of the anterior scalene muscle merges with the middle scalene muscle in 20% of individules. In half of all individules, the insertions are overlapping the first rib, while the other half of all individules they are joined in a common insrtion.

Deep into the anterior scalene muscle lie the subclavian artery and nerve trunks of the brachial plexus. Usually the nerves pass through a slit formed by the anterior and middle scalene muscles. IN some cases, however, the fifth and sixth cervical roots actually pass between bundles of the anterior scalene muscle rather than the hiatus between the anterior and middle scalene muscles. In one study this variant was seen in 45% of cadavers and 21% of TOS patients.