A New Anatomical Variation in the Brachial Plexus Roots and its Implications for Neurogenic Thoracic Outlet Syndrome

Abstract

Neurogenic thoracic outlet syndrome (TOS), compression of proximal portions of the brachial plexus as they course through the superior thoracic outlet, is the most common type of TOS, and often results in pain, paresthesia, and paresis in the upper extremity. Typically impingement occurs as the neurovasculature passes between the anterior and middle scalene muscles, and this classic relationship is the foundation for clinical diagnosis. Positional testing, such as Adson's test, relies on vascular compromise of the subclavian artery in this space. Here we report an anatomical variant in which the superior trunk of the brachial plexus pierces the anterior scalene, while the subclavian artery travels without entrapment. Thirty cadavers at Midwestern University were assessed to determine the frequency of this variant. Twelve variations from classic anatomy were observed. In 10 (33.3%), the superior trunk pierced the anterior scalene muscle, and in one, the middle trunk also pierced the muscle belly. All cases occurred unilaterally, and almost exclusively on the left side. The high frequency of this variation and its potential to predispose patients to neurogenic TOS by compressing the superior and/or middle trunk suggests that current diagnostic methods may be insufficient to identify those who suffer from TOS symptoms. Due to a lack of vascular compromise, patients with this variant would likely not display a positive Adson's sign. Ultrasound may be used to determine whether this variation is present in patients who suffer from TOS symptoms but are without a diagnosis based on traditional positional testing.