Nerve Conduction Studies in Neurogenic Thoracic Outlet Syndrome
Nerve conduction studies (NCS) are frequently abnormal in neurogenic thoracic outlet syndrome (nTOS), but importantly, normal electrodiagnostic studies do NOT exclude the diagnosis.
Key Electrodiagnostic Findings
The most consistent and sensitive NCS abnormality in nTOS is reduced amplitude of the medial antebrachial cutaneous (MABC) sensory nerve action potential, which was abnormal in all tested patients in multiple studies 1, 2, 3. This finding is so reliable that it should be included as a screening method in suspected nTOS 2.
Additional common NCS abnormalities include:
- Decreased ulnar sensory nerve action potential amplitude 1, 3
- Reduced median motor compound action potential (CMAP) amplitude 1
- Decreased ulnar motor CMAP amplitude 1, 3
On needle EMG, the most frequently affected muscles are:
- Abductor pollicis brevis (APB) - showing reduced recruitment or denervation 1, 2, 3
- First dorsal interosseous 1
- Adductor digiti minimi 1
Critical Clinical Caveat
However, nTOS can absolutely present with completely normal electrodiagnostic studies 4. Two well-documented cases demonstrated patients with classic nTOS symptoms, clear anatomical compression on CT angiography and MRI showing neurovascular bundle compression, who had entirely normal electrophysiology including normal MABC studies 4. These patients were successfully diagnosed based on clinical presentation and imaging findings alone.
Diagnostic Algorithm
When nTOS is suspected:
Perform comprehensive electrodiagnostic testing including:
- MABC sensory nerve conduction (most sensitive)
- Ulnar and median sensory studies
- Median and ulnar motor studies
- Needle EMG of intrinsic hand muscles (especially APB)
If NCS/EMG shows abnormalities (decreased amplitudes as above): This provides supportive evidence for nTOS, particularly when showing T1-predominant lower trunk involvement 2
If NCS/EMG is completely normal: Do NOT exclude nTOS. Proceed with imaging (CT angiography and brachial plexus MRI) to identify anatomical compression 4
Unlike most entrapment neuropathies, electrodiagnostic studies in nTOS provide only "clues" rather than definitive diagnosis 1
Why NCS May Be Normal
The compression in nTOS is often positional or intermittent, occurring primarily in the costoclavicular space with provocative maneuvers 5. Standard electrodiagnostic testing performed with the arm in neutral position may miss this dynamic compression. Additionally, early or mild cases may not yet show measurable nerve conduction abnormalities despite causing significant symptoms.
Bottom Line
Abnormal NCS findings (particularly reduced MABC amplitude) strongly support nTOS diagnosis, but normal studies do not rule it out. The diagnosis ultimately rests on the combination of clinical presentation, provocative maneuvers, diagnostic injections, and imaging findings 4, 6. Electrodiagnostic studies serve as one piece of evidence rather than the gold standard they represent in other entrapment syndromes 1.