Deep Dive: Anatomical Contributors to TOS

Each piece of anatomy has its own way of contributing to the compression either by way of genetics or by acquired changes from activity or injury. Please note that the presence of any of the below anatomy or anatomical changes by itself is not diagnostic of TOS.

Cervical Rib There are complete and incomplete cervical ribs. The presence of cervical ribs alone is not diagnostic of NTOS. In fact, the majority of patients with NTOS do not have cervical ribs. Only around 10% or less of NTOS patients have them, and they are not required for diagnosis. Cervical ribs alone do not cause NTOS, but rather they predispose a person to the development of NTOS. In contrast, the majority of ATOS patients do have cervical ribs, but even still, most people with cervical ribs do not develop ATOS.

(complete) a complete cervical rib connects to the first rib by a joint or by fusion. Only about 30% of cervical ribs are complete. It originates at the C7 level of the spine and usually occupies space between the anterior and middle scalene muscles within the scalene triangle. This can reduce the space within the scalene triangle and contribute to compression of the brachial plexus nerves and axillary-subclavian artery.

(incomplete) An incomplete cervical rib (aka short cervical rib) has a joint but is connected to the first rib only by a fibrous band. It originates at the C7 level of the spine, but unlike a complete cervical rib, it is mainly the fibrous band that occupies the space between the anterior and middle scalene muscles within the scalene triangle. It is this band that narrows the scalene triangle and can contribute to compression of the brachial plexus nerves and axillary-subclavian artery.

Elongated C7 Transverse Process an elongated C7 transverse process also originates at the C7 level of the spine. It is considered elongated if it extends past the T1 transverse process which is the spinal level directly below it. Much like with an incomplete or short cervical rib, it is connected to the first rib by a fibrous band. Again, it is this band that narrows the scalene triangle and can contribute to compression of the brachial plexus nerves and axillary-subclavian artery.

Anterior & Middle Scalene Muscles These muscles can contribute to compression within the scalene triangle both by congenital anomalies and by acquired changes in them due to activity or injury.

Congenital Anomalies Anomalies which people can be born with include these two muscles being fused together or having fibrous bands connecting them, being larger in size than normal, originating from different cervical levels, or attaching to other ribs such as the 2nd or 3rd ribs. These scenarios can decrease the amount of space within the scalene triangle thus leaving less space for the brachial plexus nerves and axillary-subclavian artery to travel through.

Activity/Injury-Related Changes A hyperextension neck injury, such as whiplash, injures muscle fibers. As part of the healing process, scarred muscle fibers replace what were once healthy muscle fibers. This can cause the muscle to become very inelastic and tight. Tight muscles in spasm narrow the scalene triangle and can compress any nerves running directly through the muscles. Since these muscles also attach to the first rib, when tight, they can pull up on the first rib causing it to be higher than normal which can also narrow the scalene triangle. Activities such as working out or bodybuilding can also change the scalene muscles and cause them to be hypertrophic or enlarged which can lead to a narrowed scalene triangle as well. The narrowed scalene triangle can contribute to compression of the brachial plexus nerves and the axillary-subclavian artery. However, an enlarged anterior scalene muscle can also contribute to compression of the axillary-subclavian vein by encroaching on the space where it travels between the collarbone and first rib.

Extra Scalene Muscle (scalene minimus) this smaller extra scalene muscle usually originates from the C7 transverse process and attaches to the first rib. It can narrow the scalene triangle and contribute to the compression of the brachial plexus nerves, axillary-subclavian artery, or axillary-subclavian vein. Some sources state that this can be found in 25-50% of people, but other quotes are all over the map. It seems to be something that a fair number of people have for whom it does not cause any problems.

First Rib An anomalous first rib can contribute to or predispose someone to any of the types of TOS. There are several different attributes that can make a first rib anomalous such as being wider or thicker than normal, being fused to the second rib, being free floating and not attaching to the sternum, or being narrower and higher than normal. There could also be a fracture of the first rib which can result in bone spurs or fragments. Any of these anomalies can cause the scalene triangle and/or the costoclavicular space to become narrowed which can contribute to the compression of the brachial plexus nerves, axillary-subclavian artery, or axillary-subclavian vein.

Fibrous Bands there are 9 different classified fibrous bands that are known to possibly be present within the thoracic outlet. These bands are classified based on location, point of origin, and point of attachment. These bands can narrow the scalene triangle and/or costoclavicular space which can contribute to the compression of the brachial plexus nerves, axillary-subclavian artery, or axillary-subclavian vein.

Costoclavicular Ligament the positioning of this ligament can vary in people. If the ligament is situated further away from the sternum, it can narrow the space that the axillary-subclavian vein has to travel through between the anterior scalene and this ligament which can contribute to compression.

Subclavius Muscle this muscle can be impacted by both congenital anomalies and acquired changes. It can be larger than normal or become thickened or enlarged from injury or activity such as bodybuilding. If this muscle is enlarged or thickened, it narrows the costoclavicular space between the collarbone and first rib and can contribute to compression of the axillary-subclavian vein.

Pec Minor Muscle this muscle can be impacted by both congenital anomalies and acquired changes. It can be larger than normal or become thickened or enlarged from injury or activity such as bodybuilding. If this muscle is enlarged or thickened, it narrows the subcoracoid space underneath it where the brachial plexus nerves, axillary-subclavian artery, and axillary-subclavian vein travel through which can contribute to compression of these structures. Some sources also believe that the forward hunched posture which the body subconsciously goes into in an attempt to open the thoracic outlet area in NTOS patients can eventually shorten this muscle causing it to become tight which can contribute to compression of the structures.

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Deep Dive: Anatomical Contributors to TOS