The answer is not "arm numbness." It is location plus trigger plus vessel or nerve behavior. Arm overhead narrows the outlet; the pattern tells you whether the lower brachial plexus, subclavian vein, or subclavian artery is being squeezed.
From the Attending
Three structures, three stories. If the hand tingles and weakens, that is nerve. If the arm swells and turns blue, that is vein. If the hand turns cold and pale, that is artery. The trigger is overhead position. The answer is which structure. Every time.
Mechanism theater · make the squeeze visible
The outlet is a moving doorway
The brachial plexus trunks and subclavian artery pass between the anterior and middle scalenes, above the first rib. The subclavian vein travels more anteriorly. Raising the arm, depressing the clavicle, hypertrophying scalenes, or adding a cervical rib makes one of those corridors smaller.
Mechanism theater · the moving doorwayOutlet open
1Interscalene96%OPEN
2Costoclavicular94%OPEN
3Subcoracoid95%OPEN
Neutral arm. The lower trunk, artery, and vein all glide through three corridors with room to spare.
Three corridors, one question
SpacesInterscalene triangle · costoclavicular space · subcoracoid (pectoralis minor) space.
NeurogenicAbout 95 percent of TOS. Lower trunk C8 to T1: numbness along the ulnar border and medial forearm, grip fatigue, thenar wasting (Gilliatt-Sumner hand).
VascularVein → swollen, blue, heavy arm from effort thrombosis (Paget-Schroetter). Artery → cold, pale, pulseless hand with digital ischemia.
PearlArm overhead reproduces it. Numb hand points to nerve, blue arm to vein, white hand to artery.
Neurogenic dominates; the vascular two are the urgent traps.
Tap the button to walk one corridor at a time. Each act squeezes a single space while the others stay open, and the readout shows how tight that corridor gets.
Memory hooks
From the Attending
The scalene triangle sits between two muscles and one bone. Anterior scalene in front, middle scalene behind, first rib below. The plexus and artery thread through that triangle. The vein runs in front of the anterior scalene. When overhead position narrows the gap, whatever runs through it gets squeezed. Know your anatomy, know which structure fails. That distinction drives everything.
Symptom explorer · tap where the patient hurts
The region tells you the type
Different TOS types produce symptoms in different zones. Tap a body region below to see which TOS pattern localizes there and what clinical findings you would expect.
Tap a body region to see which TOS type produces symptoms there and what clinical findings differentiate it from mimics.
Pattern triage · sort the type before choosing the test
Choose the bottleneck before choosing the test
Board stems usually give you the trigger and one decisive physical or imaging finding. Use the battle tabs to compare types, then test yourself with the decision questions below.
Neurogenic TOS
FrequencyMost common type (over 90% of TOS cases)
StructureLower brachial plexus: C8 to T1 fibers compressed between anterior and middle scalenes
Stem clueUlnar-side paresthesia, medial forearm pain, grip fatigue worse with overhead activity, normal distal pulses
TestsPositive Adson test (scalene narrowing), positive Roos / EAST test (3 min overhead fatigue), positive Wright test (hyperabduction)
ManagementPosture correction and physical therapy first, unless progressive neurologic deficit or structural lesion
Venous TOS
Also calledPaget-Schroetter syndrome (effort thrombosis of axillosubclavian vein)
StructureSubclavian vein compressed at the costoclavicular space
Stem clueAcute arm swelling, cyanosis, heaviness after repetitive overhead effort; duplex shows axillosubclavian thrombosis
Classic patientYoung overhead athlete (pitcher, swimmer, rower) with sudden arm swelling after competition
ManagementUrgent anticoagulation, vascular evaluation for thrombolysis and decompression. Not routine stretching.
Arterial TOS
FrequencyRarest TOS type (under 5%), but most dangerous
StructureSubclavian artery compressed at scalene triangle, often by cervical rib
Stem clueCold pale hand, pulse loss with provocative position, digital ischemia, poststenotic dilation on imaging
DangerPoststenotic dilation, aneurysm formation, distal embolization to digits
ManagementVascular imaging and surgical evaluation. Anticoagulation alone does not fix the structural compression.
Cubital tunnelUlnar at elbow: ring and little finger symptoms worse with elbow flexion, medial epicondyle tenderness
A patient gets arm paresthesia only when the arm is abducted. Which extra clue moves the case from common neurogenic compression to urgent vascular evaluation?
A young overhead athlete has acute arm swelling and cyanosis after practice. Duplex ultrasound shows axillosubclavian thrombosis. What is the pattern?
Memory hook: overhead trigger
"Arm up, outlet down." Overhead abduction lifts the clavicle, tightens the scalenes, and narrows every corridor. The trigger is the same for all three types. The pattern depends on which structure sits in the narrowest spot.
Memory hook: Paget-Schroetter
"Blue arm after the game." The young athlete who comes in with a swollen blue arm after overhead sports has effort thrombosis of the subclavian vein. It is venous TOS until ultrasound says otherwise. Do not send this patient home with stretches.
Memory hook: Pancoast trap
"Weight loss + eye signs + lung shadow = not TOS." If the stem gives you lower plexus symptoms PLUS Horner syndrome PLUS apical opacity, the answer is Pancoast tumor. TOS does not cause weight loss or miosis. The eye catches the mimic.
Memory hook: subclavian steal
"Dizzy with arm use = steal, not squeeze." Subclavian steal is a vertebrobasilar flow problem from proximal subclavian stenosis. The blood runs backward through the vertebral artery. The patient gets dizzy, not numb. Different vessel, different direction, different answer.
Mimic lab · flip before you commit
The wrong diagnosis has a favorite location
If the stem localizes to elbow, wrist, cervical root, lung apex, or vertebral flow, the outlet is no longer the best answer. Flip each card and say why before revealing.
Anatomy strip
Thoracic outlet region: the problem is corridor size, not a distal wrist lesion.Lower trunk C8 to T1 explains medial forearm and ulnar-side hand symptoms.Cervical ribs can crowd the outlet and make vascular or neurogenic compression more likely.The artery and plexus share the scalene triangle; the vein sits anterior and behaves differently.
Board walkthrough · one case at a time
Commit, then reveal the chain
Use the answer tools before committing: right-click or long-press to cross out, double-click or double-tap to highlight. Each case shuffles answer choices and each restart remixes the order without repeating until the set is exhausted.
Question 1 of 6
Cross out: right-click or long-press. Highlight: double-click or double-tap. These gestures will not submit the answer.
Reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO. Content references: Gray's Anatomy, Greenman's Principles of Manual Medicine, clinical board preparation resources. All vignettes are original compositions for educational use.
Medically reviewed by Fatima Ali, DO and Kaitlyn Cocuzzo, MD · Last updated July 2, 2026 at 4:04 PM ET
