Quadrilateral space syndrome

Summary

Focal compressive neuropathy of the axillary nerve (± posterior circumflex humeral artery) within the quadrilateral space, classically in overhead/throwing athletes, causing posterior shoulder pain and teres minor-predominant denervation.

Key anatomy

Quadrilateral space boundaries

Superior: teres minor
Inferior: teres major
Medial: long head of triceps
Lateral: surgical neck of humerus

Axillary nerve
Posterior circumflex humeral artery (PCHA)

Pathogenesis

Chronic compression/entrapment of axillary nerve (± PCHA) at quadrilateral space, due to:
- Overhead/throwing sports (repetitive abduction + external rotation)
- Fibrous bands / scarring
- Paralabral cysts from posterosuperior labral tears
- Callus, exostosis, space-occupying lesion
Neurogenic ± vascular component (PCHA compression → positional ischaemia/aneurysm).

Clinical features

Demographic

Young or middle-aged overhead athletes (throwers, swimmers, volleyball, weightlifters)

Symptoms

Dull, aching posterior shoulder pain
Worse with abduction + external rotation
Often exercise-induced; may describe loss of throwing power/endurance

Signs

Weakness of shoulder abduction/external rotation (often subtle)
Atrophy of teres minor (± deltoid)
Sensory change over “regimental badge” area (axillary nerve territory) – variable
± Vascular findings if PCHA involved:
- Positional loss of radial pulse
- Arm fatigue, coolness, rarely distal emboli

Imaging

Radiograph

Often normal
Look for:
- Post-traumatic deformity, exostosis
- Degenerative change, enthesophytes, prior surgery

MRI

Core finding
- Isolated or dominant teres minor denervation:
  - Acute/subacute: T2/STIR hyperintense, swollen teres minor
  - Chronic: T1 hyperintense fatty replacement ± volume loss
Possible additional findings
- Deltoid involvement if more proximal axillary nerve involvement
- Underlying cause:
  - Paralabral cyst in posterosuperior quadrant
  - Scar tissue / mass along quadrilateral space
Must exclude other causes of teres minor atrophy:
- Prior surgery, trauma
- Massive rotator cuff tear (posterior cuff dysfunction)
- Brachial neuritis (Parsonage–Turner syndrome)
- Iatrogenic nerve injury

Vascular imaging

CTA / MRA / DSA (with arm neutral vs abducted/external rotated):
- Positional narrowing/occlusion of PCHA
- ± Aneurysm / irregularity of PCHA
Important in vascular-type QSS or suspected distal embolic phenomena.

Ultrasound

Can show:
- Teres minor atrophy (echogenic, thinned)
- Dilated or aneurysmal PCHA
- Dynamic compression with provocative positioning
Operator dependent; often adjunct to MRI/CTA.

Neurophysiology

NCS/EMG:

Axillary neuropathy localised to quadrilateral space
Abnormalities most marked in teres minor (and sometimes deltoid)
Rest of brachial plexus normal → helps distinguish from plexopathy (e.g. Parsonage–Turner).

Differentials (for isolated teres minor atrophy)

Quadrilateral space syndrome (axillary nerve entrapment)
Post-traumatic / post-surgical axillary nerve injury
Massive rotator cuff tear with posterior cuff dysfunction
Parsonage–Turner syndrome (brachial neuritis) – usually multifocal pattern, dramatic acute pain story
Idiopathic muscular atrophy

Quick contrast with Parsonage–Turner syndrome

QSS: athlete, chronic positional posterior shoulder pain, isolated teres minor ± deltoid denervation, discrete entrapment site.
PTS: sudden, severe shoulder/arm pain → later patchy weakness, multifocal muscle involvement across several nerves/plexus segments, no single compression point.

Management

Conservative first line

Activity modification (reduce overhead load)
Physiotherapy for scapulohumeral mechanics + rotator cuff strengthening
Analgesia
Interventions (case-dependent)
Target underlying lesion (e.g. paralabral cyst decompression)
Surgical decompression/release of quadrilateral space in refractory cases

Radiology reporting tips

Describe:

Pattern of muscle denervation (which muscles, acute vs chronic)
Evidence of focal axillary nerve compromise
Any mass / cyst / structural cause
Suggest:
Correlation with clinical exam + EMG
Consideration of vascular imaging if symptoms suggest ischaemia or emboli.