8.3 Thoracic Outlet Syndrome (TOS) Evaluation

8.3 Thoracic Outlet Syndrome (TOS) Evaluation

Thoracic Outlet Syndrome (TOS) is a complex neurovascular entity caused by dynamic extrinsic compression of the brachial plexus, subclavian artery, or subclavian vein as they traverse the cervico-axillary canal from the superior mediastinum and neck into the upper extremity. Because the thoracic outlet contains both neural and vascular structures tightly packed between osseous, muscular, and ligamentous boundaries, postural changes of the shoulder and neck can mechanically narrow these spaces. Non-invasive vascular laboratory testing provides objective hemodynamic documentation of dynamic vascular compromise and guides medical, physical therapy, and surgical interventions.

Regional Anatomy: The Three Thoracic Outlet Spaces

The neurovascular structures pass through three sequential anatomic compartments before entering the upper arm:

Mediastinum / Neck
        │
        ▼
[1. Interscalene Triangle] ──► Subclavian Artery & Brachial Plexus
        │                      (NOTE: Subclavian Vein is EXCLUDED)
        ▼
[2. Costoclavicular Space] ──► Subclavian Artery, Subclavian Vein & Brachial Plexus
        │
        ▼
[3. Subpectoral / Pectoralis Minor Space] ──► Axillary Artery, Axillary Vein & Brachial Plexus
        │
        ▼
Upper Extremity

1. Interscalene Triangle

• Anatomic Boundaries:
  • Anterior Border: Anterior scalene muscle (originates from C3-C6 transverse processes, inserts onto scalene tubercle of first rib).
  • Posterior Border: Middle scalene muscle (originates from C2-C7 transverse processes, inserts onto superior surface of first rib).
  • Inferior Border: Superior surface of the first thoracic rib.
• Traversing Structures: The five roots and three trunks of the brachial plexus (C5-T1) and the subclavian artery.
• CRITICAL ANATOMIC FACT: The subclavian vein DOES NOT pass through the interscalene triangle. The subclavian vein courses anterior to the anterior scalene muscle, lying between the anterior scalene and the clavicle / subclavius muscle. Consequently, isolated muscular hypertrophy of the scalene muscles can compress the subclavian artery and brachial plexus, but cannot directly compress the subclavian vein within this triangle.

2. Costoclavicular Space

• Anatomic Boundaries:
  • Superior / Anterior Border: Middle third of the clavicle, subclavius muscle, and costoclavicular ligament.
  • Inferior / Posterior Border: Superior surface of the first rib and insertion of the anterior scalene muscle.
• Traversing Structures: The subclavian artery, subclavian vein, and cords of the brachial plexus.
• Clinical Significance: This is the primary compression site for Venous TOS (Paget-Schroetter syndrome). Dynamic downward and backward movement of the clavicle (as in the military brace posture) directly pinches the subclavian vein against the first rib.

3. Subpectoral / Pectoralis Minor (Retropectoral) Space

• Anatomic Boundaries:
  • Anterior Border: Tendon of the pectoralis minor muscle (inserting onto the coracoid process of the scapula).
  • Superior Border: Coracoid process.
  • Posterior Border: Second through fourth ribs and chest wall.
• Traversing Structures: The axillary artery, axillary vein, and peripheral divisions of the brachial plexus.
• Clinical Significance: Hyperabduction of the arm ($180^\circ$) stretches and tensions the neurovascular bundle beneath the pectoralis minor tendon against the coracoid process.

Clinical Classification of Thoracic Outlet Syndrome

Thoracic outlet syndrome is categorized into three distinct pathophysiological subtypes, each presenting with unique clinical demographics and diagnostic findings:

1. Neurogenic TOS (nTOS)

• Prevalence: Accounts for approximately $90\% - 95\%$ of all clinical TOS cases.
• Pathophysiology: Mechanical compression of the lower trunk of the brachial plexus (formed by nerve roots C8 and T1) within the interscalene triangle or costoclavicular space. Frequently triggered by whiplash neck trauma, repetitive overhead work, or congenital fibrous bands extending from a cervical rib.
• Clinical Signs & Symptoms: Paresthesias, numbness, burning, and pain radiating down the medial arm, forearm, and into the fourth and fifth fingers (ulnar nerve distribution). In advanced cases, motor involvement produces intrinsic hand muscle weakness and wasting of the thenar and hypothenar eminences (Gilliatt-Daniels hand).
• Diagnostic Role: Resting duplex ultrasound is normal. Dynamic vascular tests may show positional arterial compression, but this does not confirm nTOS. Diagnosis is clinical and multidisciplinary; electrodiagnostic studies, blocks, and other tests may support evaluation or exclude alternatives, but no single test is universally required or definitive in every presentation.

2. Venous TOS (vTOS / Paget-Schroetter Syndrome)

• Prevalence: Accounts for approximately $3\% - 4\%$ of TOS cases.
• Pathophysiology: Extrinsic compression of the subclavian vein within the costoclavicular space between the clavicle, subclavius muscle, and first rib. Repetitive arm motion in overhead athletes (baseball pitchers, competitive swimmers, weightlifters, rowers) or manual laborers causes perivenous fibrosis, intimal trauma, and acute extensive deep vein thrombosis—a syndrome termed effort thrombosis or Paget-Schroetter syndrome.
• Clinical Signs & Symptoms: Sudden onset of prominent upper extremity swelling, cyanosis, heaviness, dull aching, and visible dilated subcutaneous collateral veins across the anterior chest wall and shoulder girdle (Urschel's sign).
• Diagnostic Role: Duplex ultrasound confirms acute, non-compressible, intraluminal thrombus within the axillary-subclavian vein with loss of normal respiratory variation and cardiac pulsatility.

3. Arterial TOS (aTOS)

• Prevalence: The rarest subtype, accounting for approximately $1\% - 2\%$ of all TOS cases, but by far the most limb-threatening.
• Pathophysiology: Repetitive extrinsic mechanical compression of the subclavian artery within the interscalene triangle or costoclavicular space. In almost all cases ($> 95\%$), aTOS is caused by an underlying congenital bony abnormality—most commonly a complete or incomplete cervical rib (an anomalous extra rib arising from the seventh cervical vertebra, C7), an abnormally elongated C7 transverse process, or an ununited/malunited first rib or clavicle fracture.
• Pathologic Progression: Pulsatile contact against the cervical rib traumatizes the subclavian arterial wall, leading to focal stenosis followed by turbulent post-stenotic jetting. This promotes post-stenotic subclavian artery dilatation and true aneurysm formation. Mural thrombus lines the aneurysm wall, shedding micro- and macro-emboli into the distal brachial, forearm, and digital arteries.
• Clinical Presentation: Exertional arm claudication, weakness, coldness, pale or cyanotic digits, ischemic digital ulcers, absent or diminished radial pulses, and acute upper extremity ischemia from digital embolization.

Provocative Positional Examination Protocols

Non-invasive vascular testing for TOS utilizes dynamic positional maneuvers designed to systematically stress each of the three anatomic compartments while recording physiological arterial signals:

1. Instrumentation & Signal Detection

• Photoplethysmography (PPG): A PPG sensor is secured to the fleshy pulp of the index finger (Digit 2) or thumb (Digit 1) of the symptomatic limb. PPG provides continuous, motion-tolerant visual tracking of digital microvascular blood volume pulses during postural shifts.
• Continuous-Wave (CW) Doppler or Duplex Ultrasound: Alternatively, a $5 - 8\text{ MHz}$ CW Doppler pencil probe or a high-frequency linear array duplex transducer is positioned over the radial artery at the wrist. Duplex imaging can also directly interrogate the subclavian and axillary arteries to visualize focal compression, aliasing, and velocity acceleration.

2. Standardized Maneuver Sequence

The examination begins with a resting baseline recording, followed by systematic testing through four standardized provocative positions. In each position, the limb is held for at least $30\text{ to }60\text{ seconds}$ while continuously recording waveforms:

Diagnostic Criteria & Waveform Analysis

Interpretation of dynamic TOS testing relies on comparing the amplitude and morphology of the Doppler or PPG signal during provocative positioning to the baseline resting waveform:

• Normal Response (Negative Maneuver):
  • The pulsatile contour of the digital PPG or the triphasic radial Doppler waveform remains stable and robust throughout all provocative maneuvers, showing minimal or no attenuation in peak systolic amplitude.
• Hemodynamically Positive Maneuver:
  • Waveform Attenuation: A marked reduction in systolic waveform amplitude (greater than $50\%$ drop relative to baseline) or the appearance of a severely damped, low-resistance, blunted parvus-tardus contour during the maneuver.
  • Complete Flow Cessation (Flatline): Complete obliteration of the pulsatile PPG signal into a flatline trace, or total absence of audible/spectral Doppler flow at the radial artery during the maneuver, with prompt recovery to a normal robust pulsatile contour immediately upon returning the arm to the neutral resting position.
  • Duplex Ultrasound Signs: Direct imaging of the subclavian or axillary artery demonstrates focal extrinsic caliper compression, color aliasing, and a peak systolic velocity (PSV) increase greater than $100\%$ ($2:1$ velocity ratio) across the compression site during the maneuver.

Radial Artery PPG Tracings During Provocative TOS Testing:

Neutral Baseline:          Adson Maneuver (Partial Compression):     Wright Maneuver (Complete Flatline):

    /\      /\      /\             /\_     /\_     /\_  
   /  \    /  \    /  \           /   \   /   \   /   \  
  /    \  /    \  /    \         /     \ /     \ /     \         ─────────────────────────────────────
─/──────\/──────\/──────\─     ─/───────\───────\───────\─       (Total cessation of arterial flow;
(Robust pulsatile amplitude)    (Marked amplitude dampening >50%)  flatline digital perfusion)

The Crucial Clinical Caveat: Asymptomatic Compression vs. True TOS

A critical concept tested on the CCI RVS examination is the distinction between anatomical vascular compression and clinical Thoracic Outlet Syndrome:

Integrating symptoms, hemodynamics, and anatomy

Document which position changes the waveform and whether it reproduces the patient's usual symptoms. Positional signal loss can occur in healthy people, and symptom reproduction plus signal loss still does not independently establish the full syndrome or determine surgery. Identify direct arterial or venous pathology, consider competing neurologic/musculoskeletal causes, and correlate with the clinical assessment and appropriate anatomic imaging. Do not diagnose neurogenic TOS solely from vascular waveform changes.

Clinical Management & Surgical Correlation

• Arterial TOS: Requires prompt surgical decompression because of the imminent risk of aneurysm formation, thrombosis, and distal embolization. Surgical intervention entails resection of the cervical rib (if present) or first thoracic rib, combined with anterior scalenectomy and arterial reconstruction (aneurysm resection with interposition vein grafting or bypass).
• Venous TOS: Acute effort thrombosis is managed with catheter-directed thrombolysis followed by urgent or semi-elective first rib resection and costoclavicular decompression to eliminate the extrinsic bone pinch and prevent recurrent thrombosis.
• Neurogenic TOS: Initial management is conservative, including physical therapy for postural correction and scalene stretching. Patients with debilitating symptoms refractory to conservative measures undergo transaxillary or supraclavicular first rib resection and anterior/middle scalenectomy to decompress the brachial plexus.