Differential:ThoracicOutletSyndrome

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Thoracic outlet syndrome (TOS) is a collection of symptoms arising from compression of the neurovascular bundle — the brachial plexus, subclavian artery, subclavian vein, and/or the lymph duct from the arm — as these structures exit the thorax by crossing over the first rib, passing through the interscalene triangle, and traveling beneath the clavicle. It is not a well-defined specific disease but a syndromic label applied to a heterogeneous group of presentations. Its management is frustrated by controversy, variable surgical outcomes, and, most importantly, by the nearly universal failure to identify myofascial trigger points (TrPs) as a primary or major contributing cause of both the pain and the entrapment itself.

A key clinical principle from Travell & Simons: scalene TrP tension is the most common identifiable cause of the neuromuscular compression that produces TOS symptoms, and this cause is amenable to conservative treatment without surgery. Patients with documented cervical ribs have obtained relief from thoracic outlet entrapment symptoms through inactivation of scalene TrPs alone. No surgical report reviewed by Travell & Simons described examining patients preoperatively for TrPs as a potential major contributor to symptoms.

Contents

Anatomy of the Thoracic Outlet

The thoracic outlet is, anatomically, the superior border of the thorax. Three specific compression sites are relevant:

  1. The interscalene triangle — bounded by the scalenus anterior anteriorly, the scalenus medius posteriorly, and the first rib inferiorly. Both the brachial plexus and the subclavian artery pass through this triangle. TrP-related tension in either scalene muscle narrows this space and elevates the first rib, increasing compression.
  1. The costoclavicular space — between the first rib and the clavicle. The subclavian vein and lymph duct pass over the first rib anterior to the scalenus anterior attachment; the neurovascular bundle traverses the space behind the clavicle. Elevated first rib or depressed clavicle narrows this space.
  1. The subcoracoid space — beneath the pectoralis minor at the coracoid process. The axillary artery and the three cords of the brachial plexus pass beneath the pectoralis minor tendon. Pectoralis minor TrP shortening or round-shouldered forward coracoid depression narrows this space.

The lower trunk of the brachial plexus (formed from C8 and T1) is most vulnerable. The T1 nerve courses cephalad to hook over the first rib, where its fibres and those of C8 are wedged between the subclavian artery and the rib attachment of the scalenus medius. Any elevation of the first rib from scalene TrP tension forces the lower trunk to angulate more sharply, and also wedges the subclavian artery more tightly against the lower trunk.

Classification of TOS Presentations

Type Structures compressed Predominant symptoms Primary myofascial cause
Neurogenic TOS Lower trunk brachial plexus (C8–T1) Numbness, tingling, dysesthesia — predominantly ulnar (4th–5th digits); dropping objects; grip weakness Scalenus anterior and/or medius TrPs (true TOS); or pseudo-TOS quadrad
Arterial TOS Subclavian or axillary artery Pallor, coolness, Raynaud phenomena, arm fatigue, reduced radial pulse Scalene TrPs (subclavian artery entrapment between first rib and scalenus anterior tendon); pectoralis minor TrPs (axillary artery behind pectoralis minor)
Venous/lymphatic TOS Subclavian vein and/or lymph duct Morning hand puffiness (dorsum of hand and bases of fingers), rings tight on waking, resolves through the day; subclavian vein thrombosis in severe cases Scalenus anterior TrPs; subclavius muscle TrPs (costoclavicular syndrome)
Pseudo-TOS No true outlet entrapment; TrP referral pattern mimics TOS Arm and shoulder pain in TOS distribution, without objective neurovascular deficit Pectoralis major, latissimus dorsi, teres major, subscapularis (the pseudo-TOS quadrad) — see below

Neural involvement is far more common than arterial involvement in clinical TOS series. Venous and lymphatic compromise are seen primarily with costoclavicular syndrome involvement of the subclavius muscle.

Myofascial Causes of TOS

True TOS: Scalene TrPs

Scalene muscle TrP tautness directly elevates the first rib and narrows the interscalene triangle, producing genuine neurovascular entrapment. This is the most important and most commonly overlooked cause of TOS:

  • Scalenus anterior — the most commonly involved muscle; its TrP taut bands elevate the first rib, narrowing the interscalene triangle and compressing the subclavian artery against the first rib
  • Scalenus medius — the larger and arguably more important muscle; its anterior insertion anomalies may place the lower trunk of the brachial plexus and subclavian artery in direct contact with the muscle's margin; in cadaveric studies, the lower trunk rested on the inferior margin of the scalenus medius in practically all specimens
  • Scalenus minimus — a variable accessory muscle present in 50–75% of individuals, attaching from C6–C7 to the pleural dome fascia and first rib; when hypertrophied (as in habitual paradoxical breathers), its TrPs can make a significant contribution to TOS; not inactivated by spray and stretch — requires injection

The mechanism: TrP taut bands in the scalene muscles increase resting muscle tension → first rib is elevated → interscalene triangle narrows → lower trunk of brachial plexus and subclavian artery are compressed → neurological and vascular symptoms develop.

Articular dysfunction of the first rib (fixation and elevation at the costotransverse joint) commonly accompanies scalene TrP activity and may compound the entrapment. Treatment of the elevated first rib (postisometric relaxation technique with downward pressure on the rib head) also releases scalene TrP tension. See Muscle:Scalene for full treatment protocol.

Myofascial Pseudo-TOS: The Quadrad

Four muscles — when at least three of them simultaneously harbour active TrPs — produce a clinical presentation that closely mimics TOS and also mimics multilevel cervical radiculopathy, bursitis, and tendinitis around the shoulder. Because these muscles are rarely examined by surgeons as likely sources of TOS symptoms, their involvement accounts for a large proportion of patients who fail TOS surgery or conservative treatment not directed at TrP identification.

The four muscles:

Muscle TrP pain pattern contribution Role in pseudo-TOS
Muscle:Pectoralis_Major Anterior chest, shoulder, inner arm Strong medial rotator; its sternal head refers pain into the axilla and inner arm in TOS distribution
Muscle:Latissimus_Dorsi Posterior axilla, subscapular, inner arm to ring and little fingers Medial rotator and adductor; refers pain in lower trunk (C8–T1) distribution; its humeral depression mimics TOS arm symptoms
Muscle:Teres_Major Posterior shoulder, posterior arm Medial rotator synergist; satellite of latissimus TrPs; contributes to restricted abduction and lateral rotation
Muscle:Subscapularis Posterior shoulder, wrist band, anterior axilla The most critical of the four; severe medial rotation restriction mimics frozen shoulder; subscapularis TrPs can cause a posterior shoulder and wrist band of pain closely resembling a neurological distribution

All four of these muscles are strong medial rotators and adductors. The pseudo-TOS is especially prevalent in patients after cerebrovascular accidents (CVA/stroke), who have selective spasticity of medial rotators; these patients often develop severe restriction of shoulder mobility, subluxation, and reflex sympathetic dystrophy as a result.

A pseudo-TOS clinical clue: Electrodiagnostic testing should be negative in pseudo-TOS. True neurogenic TOS from scalene entrapment produces abnormal needle EMG in more severe chronic cases; pseudo-TOS does not produce objective neurological deficit.

For additional causes of myofascial pseudo-TOS, the following muscles also produce referral patterns that may be diagnosed as TOS:

  • Muscle:Pectoralis_Minor — its TrPs can cause true axillary artery entrapment beneath the coracoid, or refer pain in a pattern mimicking medial cord compression
  • Muscle:Trapezius — TrPs may contribute to symptom complex diagnosed as TOS
  • Muscle:Levator_Scapulae — TrPs may contribute to symptom complex diagnosed as TOS

Subclavius Muscle and Costoclavicular TOS

The costoclavicular syndrome is caused by compression of the neurovascular bundle between the clavicle and the first rib when the shoulders are drawn backward and down (military brace posture, heavy backpack). The subclavius muscle — attaching from the middle third of the clavicle to the first rib and its cartilage — when shortened by TrPs, elevates the first rib in the costoclavicular space, compressing the subclavian vein. Among 200 surgical TOS patients, an abnormality of the subclavius muscle system was found in 19.5% — the only finding that correlated with the clinical picture (subclavian vein stricture and thrombosis). An exostosis at the subclavius tubercle was observed in 15.5% of patients, suggesting chronic abnormal muscle tension. See Muscle:Subclavius.

Structural and Developmental Contributors

Structural anomalies can intensify TOS symptoms by further narrowing an already TrP-compromised outlet — but are rarely sufficient by themselves to explain the symptoms:

  • Cervical rib — found in 0.17% of 40,000 consecutive chest X-rays of army recruits. When present, all structures crossing over it are more sharply angulated than over the first rib; cervical rib intensifies symptoms caused by scalene TrP elevation. Notably, patients with documented cervical ribs have obtained relief from thoracic outlet entrapment symptoms through inactivation of scalene TrPs alone, without rib removal.
  • Deformed first rib — found in 0.25% of the same series.
  • Supernumerary scalene muscles — found in 10% of 200 surgical TOS cases.
  • Developmental scalene muscle variations — found in 43%.
  • Fibrous bands — the most commonly found soft-tissue anomaly at operation in severe TOS; associated with brachial plexus nerve roots; may have sharp edges that increase nerve vulnerability.
  • Abnormally narrow interscalene space — congenital narrowing makes neurovascular structures more vulnerable to TrP-related compression.

TOS characteristically first appears in early adulthood — precisely when individuals begin to develop active TrPs — suggesting that structural anomalies alone are insufficient without the trigger of TrP tension.

Subjective Presentation and History

Neural vs Vascular vs Lymphatic Symptoms

Neurological symptoms (most common):

  • Numbness, tingling, and dysesthesia — predominantly in the ulnar distribution (4th and 5th digits, ulnar side of the hand, occasionally the forearm)
  • Unexpectedly dropping objects from the hand — grip failure from lower trunk (C8–T1) dysfunction
  • Mild hypesthesia to light touch, pinprick, and temperature in the little finger

Arterial symptoms:

  • Pallor, coolness, or Raynaud-like phenomena of the hand
  • Arm fatigue or pain with use
  • Reduced or absent radial pulse on provocative manoeuvres

Venous/lymphatic symptoms:

  • Diffuse hand puffiness distal to the wrist, especially over the bases of the four fingers and the dorsum of the hand — appearing on waking and resolving through the day
  • Rings feeling tight in the morning
  • Finger stiffness on waking — not solely from oedema, but also from myofascial tautness of finger extensors with an autonomic reflex component
  • In severe cases (subclavian vein thrombosis): persistent unilateral arm oedema

Sleep disruption: When night pain is severe, the patient is likely to sleep sitting upright propped on pillows. This prevents the sustained shortening of the scalene muscles that occurs when lying flat, as the chest and shoulders ride up around the neck.

Radial vs Ulnar Hand Distribution: A Critical Differentiator

The distribution of hand symptoms is one of the most clinically useful history elements for distinguishing true TOS from myofascial scalene pain without entrapment:

Feature Scalene TrP referral (myofascial, no entrapment) True neurogenic TOS (lower trunk entrapment)
Hand symptoms Ulnar side — 4th and 5th digits, ulnar palm, occasionally forearm
Quality Aching pain; may include referred "numbness" sensation without objective hypoesthesia True sensory deficit; numbness and dysesthesia; mild hypesthesia to light touch and pinprick
Motor deficit None Grip weakness; dropping objects; may progress to intrinsic hand muscle wasting in severe chronic cases
Oedema Possible diffuse hand oedema if venous component
Electrodiagnostics Normal — no EMG denervation; no prolonged nerve conduction latency Needle EMG positive in severe chronic cases; F-wave testing next most sensitive; nerve conduction studies often unreliable

Practical clinical rule: Pain on the radial side of the hand indicates a myofascial referred pain source from scalene TrPs. Pain on the ulnar side with puffiness of the hand suggests true brachial plexus and subclavian vein entrapment.

Clinical Examination

Scalene-cramp Test

This test elicits or increases pain from active scalene TrPs by forcefully contracting the muscle in its shortened position.

Procedure:

  1. Patient rotates the head fully to the side of the pain
  2. Then actively pulls the chin down into the hollow above the clavicle by flexing the head and neck

During the final part of this movement, the anterior and middle scalene muscles contract forcefully in the shortened position, evoking a local cramp-like pain at the TrP and potentially referring pain to a distance.

Interpretation: Positive — local ache at TrP with referred pain in the scalene pattern → active scalene TrPs confirmed. The pain can be immediately relieved by spray and stretch applied to the activated TrP. If the patient is already in severe pain before the test, the additional pain of the test may not be perceived; try the Scalene-relief Test first.

Scalene-relief Test

This test identifies whether pain is caused or aggravated by clavicular pressure on structures passing over an elevated first rib.

Procedure:

  1. Patient places the painful forearm across the forehead
  2. Raises and pulls the shoulder forward to lift the clavicle off the underlying scalene muscles and brachial plexus (maximising clearance beneath the clavicle and protracting the scapula)

Interpretation: Pain relief ensuing immediately or within a few minutes = positive for scalene/TOS source. None of the positions in this test should affect pain caused by cervical radiculopathy — the Scalene-relief Test can therefore help distinguish scalene TOS from cervical disc disease. This is also the position patients spontaneously adopt for relief (arm raised, shoulder forward).

Finger-flexion Test

This test detects the reflexly inhibited finger flexion that occurs with active scalene TrPs.

Critical technique requirement: The metacarpophalangeal (MCP) joints must be actively held straight in full extension throughout — this requires forceful extensor digitorum contraction. The test is invalid if the MCP joints are allowed to flex freely (a tight fist can be made normally even with scalene TrPs).

Procedure:

  1. Patient actively holds MCP joints in full extension
  2. Attempts to flex all fingertips to firmly touch the volar pads of the MCP joints

Interpretation:

  • Normal: All fingertips press firmly against the MCP volar pads
  • Positive extensor digitorum TrPs: One or more specific finger(s) fail to flex fully — corresponding to which extensor digitorum compartment is involved
  • Positive scalene TrPs: All four fingertips fail to touch the volar pads — general inhibition of finger flexors by scalene TrP activity on the same side

This positive test is not due to oedema — it is restored to normal immediately after spray and stretch of the involved scalene muscles. Scalene TrPs are frequently the key to forearm extensor digitorum TrPs.

Provocative Vascular Manoeuvres

Standard TOS provocative manoeuvres (Adson, costoclavicular, hyperabduction) have proved disappointingly unreliable for vascular responses in normal populations:

  • Adson manoeuvre: 13.5% false positive for vascular responses in normal extremities
  • Costoclavicular manoeuvre: 47% false positive in normal extremities
  • Hyperabduction manoeuvre: 57% false positive in normal extremities

These tests are considerably more reliable for neurological than vascular responses. Their limitation is that identifying a structure suffering compression does not identify the cause — the cause may be either anatomical or TrP origin.

The Roos test (arms abducted to 90°, elbows bent 90°, held up as in "stick 'em up") was the only provocative manoeuvre found helpful by Roos as a neurological test for operative selection; it reproduces neurological symptoms of lower trunk compression. However, this test was also proposed as a selection criterion for surgery, and Roos' series showed only 22% good relief from surgical decompression in his total group — suggesting that unidentified scalene TrPs were responsible for many of the symptoms in patients selected by this criterion.

First Rib Palpation

Because the first rib lies under the clavicle, it is most easily palpated posteriorly at the head of the rib, at its articulation with the first thoracic vertebral segment. Elevation of the first rib at the costotransverse joint can be detected on palpation and is commonly observed with scalene muscle involvement. Treatment combines postisometric relaxation of the scalene muscles with gentle downward pressure over the head of the first rib.

Differential Diagnosis

The TOS differential diagnosis must be worked through in two directions: structural causes of genuine entrapment, and myofascial sources mimicking TOS without entrapment.

Structural TOS Mimics

  • C5–C6 radiculopathy — produces a pain complaint similar to scalene TrPs; both may be present simultaneously because the neuropathy encourages development of forearm TrPs, and middle scalene TrPs may cause entrapment of those same nerve fibres. Forearm TrPs induced by nerve compression persist after successful surgery and must be inactivated separately.
  • Carpal tunnel syndrome — may occur as a concurrent entrapment with TOS, or scalene TrPs may produce symptoms mimicking carpal tunnel syndrome. Loss of normal mobility of the structures forming the carpal tunnel (from oedema reflexly originating from scalene TrPs) can make a major contribution to carpal tunnel entrapment. Both should be assessed.
  • Cervical articular dysfunctions — anterior and/or middle scalene TrPs are commonly associated with C4, C5, and C6 articular dysfunctions; treat with muscle energy technique.

Myofascial Mimics of TOS

Condition Key feature distinguishing it from true TOS
Muscle:Scalene TrPs — pure pain referral without entrapment Radial hand pain (thumb and index finger); no objective neurological deficit; positive Scalene-cramp Test; positive Finger-flexion Test; no ulnar distribution
Pseudo-TOS quadrad (Muscle:Pectoralis_Major, Muscle:Latissimus_Dorsi, Muscle:Teres_Major, Muscle:Subscapularis) At least three of the quadrad have active TrPs; severe medial rotation restriction and shoulder mobility loss; electrodiagnostics negative; no hand oedema; reproduces patient's pain on TrP palpation
Muscle:Pectoralis_Minor TrPs Axillary artery entrapment or medial cord compression pattern; coracoid process tenderness; forward shoulder posture
Muscle:Subclavius TrPs Costoclavicular pattern; morning hand oedema; venous rather than neurological predominance
Omohyoid TrPs Tense omohyoid can act as a constricting band across the brachial plexus; stands out prominently when head tilted to the opposite side; may be mistaken for upper trapezius or scalene muscle

Treatment

Conservative Approach

Conservative treatment is generally more successful than surgical treatment, with considerably less danger. Success rates ranging from 9% to 83% have been reported; success was strongly and inversely related to the severity of symptoms initially.

The cornerstone of effective conservative treatment is specific identification and inactivation of TrPs in the scalene muscles and all associated muscles — not general stretching or physiotherapy without TrP-directed examination. Most conservative treatment reports describe procedures (scalene stretching, myofascial release) that are known to inactivate scalene TrPs, but rarely identify whether TrPs were specifically examined or treated.

Required components of comprehensive conservative treatment:

  1. Specific scalene TrP examination and inactivation — spray and stretch, postisometric relaxation, trigger point pressure release, and/or injection (see Muscle:Scalene)
  2. Treatment of first rib elevation — postisometric relaxation technique with downward pressure over the head of the first rib (see Muscle:Scalene)
  3. Pseudo-TOS quadrad assessment — if the quadrad muscles are involved, treat each one; subscapularis is particularly critical
  4. Correction of paradoxical breathing — scalene overload from accessory muscle breathing perpetuates TrPs; diaphragmatic breathing retraining is essential in patients with this pattern
  5. Correction of body asymmetry — lower limb-length discrepancy as small as 1 cm (3/8 in) can perpetuate scalene TrPs despite all other treatment; ischial lift for small hemipelvis
  6. Elevation of the head of the bed — 8–9 cm (3–3½ in) at the head end; frequently, scalene TrPs cannot be permanently relieved without this. Two pillows are not equivalent — they flex the neck, shortening the anterior scalene
  7. Postural correction — forward round-shouldered posture perpetuates pectoralis minor and scalene TrPs; correct forward head position
  8. Body warmth maintenance — chilling increases skeletal muscle irritability and aggravates TrP activity

Surgical Approach

Surgery for TOS has a controversial reputation. Surgical success rates range from 24% to 90% depending on criteria, patient selection, and surgeon skill — but overall, approximately half of operative interventions are successful. Occasionally results are tragic.

Key observations from the surgical literature:

  • Not one surgical report reviewed by Travell & Simons described examining patients preoperatively for TrPs
  • Among Roos' series, 22% of his total surgical group obtained good relief; one of his selection tests was supraclavicular pressure on the scalene muscle — which likely identified patients with scalene TrPs that could have been treated conservatively
  • Among 50 patients comprehensively evaluated for possible TOS, only 12 met TOS diagnostic criteria; of 7 operated on, only 4 obtained complete relief; of non-TOS patients, 57% reported good results from conservative management
  • Many surgical failures and partial successes likely had unidentified TrPs as a major pain contributor

Before any surgical referral:

  • Positive electrodiagnostic findings are required before considering surgical release of the suprascapular ligament
  • Cervical rib resection should not be undertaken on radiographic findings alone — inactivating scalene TrPs should be the first step, even in the presence of a confirmed cervical rib
  • MRI or CT identifies structural anomalies; nerve conduction and needle EMG assess neurological compromise severity

References

  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1: The Upper Half of Body. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 20 (Scalene Muscles), Section 11 (Differential Diagnosis — Thoracic Outlet Syndrome).
  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1. 2nd ed. Chapter 18 (Overview of the Upper Back, Shoulder, and Arm Region — Myofascial Pseudothoracic Outlet Syndrome).
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