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Excellent anatomical discussion, but not completely on point as relates to Thoracic Outlet Syndrome (see my references below).
Etiology of TOS lies within functional relationships of the major muscles of the shoulder hemi-girdles. Let me explain.
Major range of motion for each shoulder hemi-girdle is elevation & depression; (abduction & adduction).
These motions are imbued by complimentary contractions/relaxations of major muscle systems of each shoulder hemi-girdle.
Superior trapezium (plural) muscles draw shoulder hemi-girdles upward; into shrug postures.
Pectoralis minor muscles draw shoulder hemi-girdles downward into sloped shoulder morphologies. These two muscle systems work in concert.
When one of these two muscle systems is hyperdynamic compared to the other, then the shoulder hemi-girdle becomes dysfunctional & Thoracic Outlet Syndrome (TOS) can result.
When the Pectoralis minor muscle is relatively (compared to Superior trapezius) hyperdynamic, the shoulder hemi-girdle is pulled/shifted downward during arm abduction. This has TOS consequences.
Remember that the acromion-clavicular (AC) joint is small & is a universal joint with potential subluxation in all directions.
Downward shift of distal clavicle & anteriorly draws proximal clavicle closer to first rib; evoking Costco-Clavicle Dimension (true Thoracic Outlet) closure & symptoms of TOS.
During closure of the Thoracic Outlet, a sub-clavicle bruit can often be ascultated; as the subclavian artery is squeezed shut by scissor-like pinch of clavicle & 1st rib.
During progressive arm abduction simultaneous palpation if the radial pulse will sense pulse extinguishment at same degree of abduction whereat the bruit occurs. Search #TOS for dramatic cine radigraphic (w. dye) views.
An adept ultrasound technician can often demonstrate subclavian artery impingement during arm abduction above 90 degrees; a skill not widely possessed by ultrasound technicians.
When arm abducts, superior tubercule of proximal clavicle can be palpated & observed to rotate anteriorly & as distal clavicle dips lower than acromion; an event able to be captured (AP view) radiologically.
Anatomical review shows that this clavicle shift first impinges that part of brachial neuroplexus tract which gives rise to ulnar nerve (ulnar paresthesias characterize TOS).
On the other hand, if the Superior trapezius (singular) is hyperdynamic (compared to Pectoralis minor), then during arm abduction distal clavicle rotates/shifts posterior-inferior relative to acromion.
These disjunctions in acromion & clavicle symmetry can be appreciated by placing a finger on top of each bony prominence of AC bones & to observe their relative shifts in real time during arm abduction.
Posterior-inferior shift of the distal clavicle causes brachial plexus impingement within plexus part giving rise to radial nerve. Radial paresthesias are less frequently observed than ulnar paresthesias.
When distal clavicle resection was popular (~15 years ago; for Shoulder Impingement Syndrome) discomforting radial paresthesias were frequently reported post-operatively.
One way to observe relative dominance of S. trapezius vs. ipsilateral P. minor is to have patient stand with arms outstretched (90 degrees).
At this posture, the clinician evokes passive resistance to forced abduction & adduction; noting which muscle system is relatively weaker.
TOS represents effects of a “crossed syndrome” (after V. Janda) between the S. trapezius & the P. minor muscles.
Resection of the P. minor off attachment site on the coracoid process became a popular TOS corrective surgery about 15 years ago, & a new procedure ....
that replaced a failed butchering/flaying operation intended to “free up” Scalenes impinging cervical nerves; until Scalenus Anticus Syndrome lost favor as the theoretical cause of TOS.
The idea that 2 soft tissues (Scalene muscles) pinched another soft tissue (neural tracts) was a non-starter theory to begin with. The true Thoracic Outlet is a bony scissors acting upon the neural brachial plexus.
There are various causes of shoulder crossed muscle syndromes as described above. Tilted shoulders due to scoliosis (commonly seen in pwHSD) is one such factor; ...
whereby the head chronically tilts toward lower shoulder side; leading to disuse weakness of the S. trapezius muscle on the lower shoulder side.
On the higher shoulder side, the S. trapezius becomes toned & more massive from chronically supporting a 10# head tilted contra-laterally when upright.
Tilted shoulder hemi-girdles lead to a crossed muscle syndrome of the Superior trapezium from side to side.
Inequality of S. trapezium tone/mass is easily noted by grasping the leading edges of each S. trapezius & comparing them. Oft there is a 2-3x girth differential.
An hyperdynamic S. trapezius begets an hyperdynamic ipsilateral P. minor because the brain desires opposing muscles to be equally toned & balanced.
Once again, an hyperdynamic P. minor muscle can induce excessive downward shift of distal clavicle during arm abduction & thereby incite symptoms of TOS.
Please note that there are various causes of shoulder hemi-girdle dysfunction & TOS development. Another etiology is pitching overhead:
Severe (Subclavian vein blood clots) TOS occurs in major league pitchers. My recent TOS “threadreader unrolled” (11/22/19) discusses details of this interesting phenomenon.
Observations & theories discussed herein are mostly my own & based on countless patients I have studied over the years.
About 15 years ago, wanting to know more about shoulder function, I collected & studied 100 patients with unilateral chronic aching shoulder pain; many with symptoms of TOS & CRPS.
I was surprised to learn how common this pain pattern was. It seemed these patients had formerly complained &, being told X-rays were normal, they stopped complaining.
My method of study was to suspend 3-5# weights from their forearms & hang the arms at stations before, beside, & behind their trunks. At same time, I noted SaO2 & temperature readings in fingers.
These vital function measurement transducers were sensitive enough to inform me which postures impinged blood flow within the Subclavian artery.
These measurements + sensitive palpation of shoulder joints & auscultation of the Subclavian artery enabled deduction of shoulder hemi-girdle musculoskeletal bio-mechanics.
A shoulder function mostly overlooked is that of the acromio-clavicular (AC) joint. This universal joint allows distal clavicle to shift/rotate in 4 directions: up, down, anterior, & posterior.
The clavicle is about 18 cm long. A few mm of distal inferior shift translates (pure geometry) to ~30% closure of proximal costo-clavicular dimension (true Thoracic Outlet, ~1.0 cm in life).
Another interesting concept is that the Levator scapulae muscle is the driver/director of scapula rotation within the coronal plane.
This long narrow muscle is not fashioned for lifting, but for driving/directing scapula rotational shifts via insertion upon Superior Scapula Spine (SSS, shaped like guitar pick).
As the scapula rotates during cranking of shoulder hemi-girdle, the Levator scapulae, if spastic, twangs as its tendon stumbles over the SSS.
This sudden tendon catch & release emits a grossly audible snapping sound I named the “Levator snap”; confirmed by palpation & auscultation.
Entheses where Levator scapulae inserts upon SSS is a premiere Fibromyalgia pain point; depicted in almost every Fibromyalgia (FM) drug ad (I saved them).
The Levator scapulae muscle chronically & painfully spasms as it struggles to keep bendy body towers of pwHSD & FM upright & vertical; a job outside its repertoire.
When examining body part functions in real time, the clinician must regard, at all times, the unseen gorilla in the exam room: Gravity.
Repetitive joint dysfunctions within ambient gravity field generate painful spastic soft tissues of FM; resulting in neural (autonomic) impingements at floppy joints; inducing dysautonomias.
Shoulder mechanism weighs ~15#, & the only central osseous attachment of this marvelous bio-mechanical structure is at the sterno-claviclar joint. Muscles, tendons, & ligaments hold everything upright.
Another clue to shoulder girdles potentiated for TOS is ancestory. Persons of Viking-Irish heritage oft have sloped shoulder morphology that portends a tighter Thoracic Outlet.
Another clue: awareness that a major aircraft builder is proximate a clinic that specializes in TOS; epidemic amongst electricians who work daily with arms overhead installing wires in airplanes.
Overhead work requires that shoulder girdles be stabilized via S. trapezium chronic contraction, which leads to spasm & correlate hyperdynamic development of opposing P. Minor muscles.
Once again, hyperdynamic function of downward draw of distal clavicle as imposed by an hyperdynamic P. minor muscle is a dominant theme in evolution of TOS.
Within these tweets, I have discussed the cure for TOS. Cures follow rationally upon knowledge of true causes.
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