Thoracic Outlet Syndrome—Postural Type: Ultrasound Imaging of Pectoralis Minor and Brachial Plexus Abnormalities

Abstract

Thoracic outlet syndrome (TOS) is a controversial diagnosis that sometimes is referred to as disputed or nonspecific neurogenic TOS because diagnostic test results typically are negative or inconclusive [1-10]. A review of the pathomechanics and associated dysfuncional body habitus suggests that use of the term “postural TOS” is more appropriate for this ommon type of presentation [10]. Postural TOS is thus recommended in cases without bjectifiable neurologic or vascular abnormality. In contrast, the less-common type of TOS nvolves compromise of the neurovascular bundle (NVB), which includes the brachial lexus (BP), the axillary or subclavian artery, and the axillary or subclavian vein [1-8,10]. Compression in all types of TOS may occur superiorly at the interscalene space, the costoclavicular space, or beneath the pectoralis minor muscle (PMM). The PMM location is recognized as a common and important site of compression in both the postural and nonpostural types of TOS [1-3,9-13]. The findings on an electrodiagnostic examination (EDX) typically are normal in patients with postural TOS because there is no axonal damage and no significant conduction block or demyelination [1,8,10]. Results of blood-flow studies usually are normal as well, and although they may reveal mild vascular compromise during stress maneuvers [1,2], this finding may not be clinically significant because it also can be seen in asymptomatic persons [1,7-9]. In addition, it is important to consider differential diagnoses and rule out the following conditions: cervical radiculopathy, peripheral nerve entrapment (median or ulnar), peripheral polyneuropathy, cervical cord lesion, or superior sulcus (Pancoast) tumor. The EDX may reveal the first 3 categories, but cervical magnetic resonance imaging (MRI) and a chest radiograph will be needed if the other conditions are considered. Neuromuscular ultrasound (NMUS) is a new imaging modality for examining nerve structures within the thoracic outlet area [14,15]. NMUS has unique applications for the postural type of TOS; as some researchers have noted, “evaluation of proximal nerves is often problematic when using electrodiagnostic techniques and may be more easily studied with ultrasound” [16]. My experience with this type of postural TOS [1,2,9,12,17,18] suggests that the PMM is contributory, and NMUS now appears to support these prior clinical impressions. Palpation of increased tension and trigger points in the PMM, combined with shoulder protraction, suggests that the PMM has shortened. The hypothesis herein is that the effects of a tight PMM can be observed to affect the NVB and explain the symptoms. The goal of this article is to present and analyze imaging findings in patients with TOS who were examined with NMUS while in neutral and stress positions, while monitoring for symptoms.