ABSTRACT & COMMENTARY
Electrodiagnosis of Thoracic Outlet Syndrome
By Michael Rubin, MD
Professor of Clinical Neurology, Weill Cornell Medical College
Dr. Rubin reports no financial relationships relevant to this field of study.
Thoracic outlet syndrome is difficult to accurately diagnosis and requires adherence to rigorous clinical and electrophysiological criteria to avoid errors and unnecessary surgery.
Tsao BE, et al. Electrodiagnostic features of true neurogenic thoracic outlet syndrome. Muscle Nerve 2014;49:724-727.
Thoracic outlet syndrome (TOS) refers to disorders attributed to compromise of the neurovascular structures in that area, including arterial TOS, venous TOS, true neurogenic TOS (TN-TOS), or combinations of the above. TN-TOS, also known as Gilliat-Sumner hand, involves motor and sensory loss in the arm and hand due to stretching of the lower trunk of the brachial plexus over a fibrous band, arising from the end of a rudimentary cervical rib and attaching to the upper surface of the normal first thoracic rib. Anterior scalene muscle compression of the lower trunk may also be causative. Electrodiagnostic studies are required in all patients in whom this diagnosis is considered, and a specific set of findings should be sought. What are they?
To determine the electrodiagnostic features of TN-TOS, retrospective review of the Cleveland Clinic Electrodiagnostic Laboratory database was undertaken for the years 1975-2008, and all patients with surgical verification of TN-TOS and preoperative electrodiagnostic studies were included for study. Nerve conduction studies were performed using standard techniques and distances, limbs were warmed and kept at a minimum surface temperature of 34° C, needle electromyography (EMG) was performed using concentric needle electrodes, and all examiners had completed fellowship training at the Cleveland Clinic.
Among 32 patients who met inclusion criteria, all were referred for nonspecific hand weakness and atrophy, with symptoms on the dominant side in 81% (n = 26). Age ranged from 17-77 years, and symptom duration from 2 months to 20 years (mean = 61 months). Handgrip or fine distal motor weakness was described in 97% (n = 31), and intrinsic hand muscle atrophy, particularly of the thenar muscles, was present in all. Pain, paresthesiae, or numbness of the medial forearm or hand was noted in 97% (n = 31). Cervical spine radiographs, performed in 29 patients, identified a cervical rib or elongated C7 transverse process in 83% (24 of 29), which was bilateral in 66% (n = 19) and on the asymptomatic side in a single patient. Surgical exploration revealed a tendonous band in all patients, which was resected, with concomitant first thoracic rib resection in 10 patients.
Nerve conduction studies revealed that the median compound motor action potential (CMAP) amplitude recording from abductor pollicis brevis was decreased for age in 91% and by side-to-side comparison in 97%. Ulnar CMAP values, recording from abductor digiti minimi, were decreased in 3.1% and by side-to-side comparison in 38%. Among sensory nerves, the medial antebrachial cutaneous sensory nerve action potential (SNAP) amplitude was the most sensitive, being low for age in 84% and by side-to-side comparison in 95%, with ulnar SNAP amplitude decreased in 6.3% and 78%, respectively. Chronic motor axon loss was evident on needle EMG, particularly in T1/median nerve innervated intrinsic hand muscles, and less so in C8 median or ulnar innervated hand and forearm muscles, with spontaneous activity in the form of fibrillation potentials noted in 50% in the former and in 33% in C8/ulnar intrinsic hand muscles. TN-TOS affects T1 > C8 fibers, and comparison to the unaffected side is critical for accurate diagnosis.
One of the most controversial diagnoses in medicine, and frequently over diagnosed, TOS is often diagnosed and treated surgically with poor results. Careful adherence to objective clinical criteria is necessary to avoid this pitfall, as patients with nonspecific arm numbness, tingling, and pain are often given this label. Electrodiagnostic studies are crucial to accurate diagnosis, and the authors are to be commended for outlining their findings in this retrospective study. Physicians would be well advised to rigorously adhere to these criteria before referring such patients for surgery.