What is Left Scalene Anticus Syndrome (Left Anterior Scalene Syndrome)?
Left scalene anticus syndrome is an older term for neurogenic thoracic outlet syndrome (nTOS) affecting the left side, where the anterior scalene muscle compresses the brachial plexus and/or subclavian artery as they pass through the interscalene triangle. This condition represents compression of neurovascular structures between the anterior scalene muscle, middle scalene muscle, and first rib 1.
Anatomical Basis
The interscalene triangle (also called the scalene triangle) is formed by:
- Anterior border: Anterior scalene muscle
- Posterior border: Middle scalene muscle
- Inferior border: First rib
The brachial plexus trunks and subclavian artery pass through this confined space 1. When the anterior scalene muscle becomes hypertrophied, develops abnormal tone, or has anatomical variations in its insertion, it can narrow this space and compress the neurovascular structures 2, 3.
Clinical Presentation
Patients typically present with chronic arm and hand paresthesia, numbness, or weakness 1. The symptoms reflect compression primarily of the lower trunk of the brachial plexus, though arterial compression can occur simultaneously, creating mixed presentations 2.
Key Pathophysiological Features
The anterior scalene muscle shows distinctive changes in this syndrome:
- Type 1 (tonic) fiber predominance reaching 85.1% (versus normal distribution)
- Type 1 fiber hypertrophy reflecting chronic increased tone or sustained motor neuron stimulation
- This unique fiber composition allows the muscle to sustain prolonged contraction, contributing to persistent compression 3
Anatomical Variations and Risk Factors
Several anatomical factors predispose to this syndrome 1, 2:
- Cervical ribs
- Anomalous first rib
- Fibromuscular bands
- Variations in anterior scalene muscle insertion
- Presence of scalenus minimus muscle
- Post-traumatic changes from clavicular or rib fractures
Repetitive upper-extremity activities (swimming, throwing, overhead work) can trigger symptoms even without obvious anatomical abnormalities, as repetitive stress leads to muscle hypertrophy and fibrosis 1.
Diagnostic Approach
Clinical Diagnosis
The diagnosis remains primarily clinical, as there is no definitive gold standard test 2. Look for:
- Reproducible paresthesias in the affected upper extremity
- Weakness in hand intrinsic muscles
- Symptoms provoked by arm elevation or specific positions
- Relief with arm positioning changes
Diagnostic Testing
Anterior scalene muscle block (ASMB) serves as a functional diagnostic tool 4, 5, 6:
- Performed under ultrasound guidance with local anesthetic injection into the anterior scalene muscle
- Positive test: Temporary relief of symptoms after injection, confirming the muscle's role in compression
- Technical success rate approaches 100% with ultrasound guidance 5
- Common side effect: Temporary partial brachial plexus block occurs in approximately 31% of cases 5
Imaging Studies
According to ACR guidelines 1:
First-line imaging:
- Chest radiography: Identifies osseous abnormalities (cervical ribs, first rib anomalies, bone lesions)
- MRI chest without and with IV contrast: Delineates anatomy in neutral and abducted positions, shows fat effacement adjacent to brachial plexus, identifies causative lesions
Additional studies when indicated:
- CT chest with contrast for anatomical detail
- MRA for arterial involvement
- Ultrasound duplex Doppler for real-time assessment
Treatment Considerations
Conservative Management
Initial approach focuses on physical therapy, posture correction, and activity modification.
Surgical Intervention
Scalenotomy (anterior scalene muscle transection) is the primary surgical treatment 7:
- Effectiveness in uncomplicated cases: 93.7% success rate
- Effectiveness with vascular complications: Only 51.1% success rate
- Critical timing: Surgery performed in initial, uncomplicated stages prevents severe vascular complications 7
Alternative surgical approaches 2:
- Transaxillary approach for first rib excision (greatest field of view for vascular decompression)
- Supraclavicular approach for scalenotomy (preferred when anterior scalene muscle is primary compressor)
- Combined approaches when extensive decompression needed
Important Clinical Pitfalls
Delayed diagnosis leads to worse outcomes: Once vascular complications develop (thrombosis, aneurysm formation, distal embolization), surgical success rates drop significantly 7
Proximity to parietal pleura: The anterior scalene muscle inserts on the superior, posterior, and inferior surfaces of the first rib, immediately adjacent to the parietal pleura, creating pneumothorax risk during surgery 8
Body size considerations: Smaller individuals (particularly Japanese populations) have narrower scalene triangles (8.2 mm vs 10.7 mm in Western populations), potentially increasing compression risk 8
Diagnostic confusion: Symptoms may overlap with cervical radiculopathy, brachial plexitis, or deeper pathology like Pancoast tumors, requiring comprehensive imaging to exclude alternative diagnoses 1, 2