Diagnostics. 2017 Jul 4;7(3). pii: E40. doi: 10.3390/diagnostics7030040.

Ultrasonographic Diagnosis of Thoracic Outlet Syndrome Secondary to Brachial Plexus Piercing Variation.

Leonhard V, Caldwell G, Goh M, Reeder S, Smith HF.

PubMed

 

Abstract

Structural variations of the thoracic outlet create a unique risk for neurogenic thoracic outlet syndrome (nTOS) that is difficult to diagnose clinically. Common anatomical variations in brachial plexus (BP) branching were recently discovered in which portions of the proximal plexus pierce the anterior scalene. This results in possible impingement of BP nerves within the muscle belly and, therefore, predisposition for nTOS. We hypothesized that some cases of disputed nTOS result from these BP branching variants. We tested the association between BP piercing and nTOS symptoms, and evaluated the capability of ultrasonographic identification of patients with clinically relevant variations. Eighty-two cadaveric necks were first dissected to assess BP variation frequency. In 62.1%, C5, superior trunk, or superior + middle trunks pierced the anterior scalene. Subsequently, 22 student subjects underwent screening with detailed questionnaires, provocative tests, and BP ultrasonography. Twenty-one percent demonstrated atypical BP branching anatomy on ultrasound; of these, 50% reported symptoms consistent with nTOS, significantly higher than subjects with classic BP anatomy (14%). This group, categorized as a typical TOS, would be missed by provocative testing alone. The addition of ultrasonography to nTOS diagnosis, especially for patients with BP branching variation, would allow clinicians to visualize and identify atypical patient anatomy.

KEYWORDS: anatomical variation; anterior scalene muscle; brachial plexus; middle trunk; neurogenic thoracic outlet syndrome; provocative testing; superior trunk; ultrasound

PMID: 28677632

 

Supplement:

Neurogenic thoracic outlet syndrome (nTOS) is a common neurologic syndrome resulting in pain, numbness, and/or weakness in the arm, forearm, and hand. This condition affects 0.3-8.0% of the U.S. population, and is generally caused by impingement of nerves traveling from the brachial plexus in the neck, through a region referred to as the thoracic outlet. Despite its prevalence, this condition is notoriously difficult to diagnose in the clinical setting. However, we have recently discovered a previously unknown cause of nTOS and an effective method of identifying it in patients.

We are currently studying how anatomical variations in neck musculature and nerve pathways may contribute to nTOS, and investigating the utility of ultrasound to diagnose such variations. We have found anatomical variations that are linked with nTOS. The classic understanding of the anatomy of this region is that the nerves of the brachial plexus travel between the scalene (neck) muscles without impingement on their way out to the arm, and traditional tests for diagnosing nTOS presume this pattern. However, we have discovered that parts of the brachial plexus often pierce the anterior scalene muscle belly and travel through it, resulting in neurologic impingement of the nerves by the muscle fibers and potential predisposition for nTOS symptoms.

Using a large sample of cadavers, we discovered that as many as 50% of people may have a clinically relevant variation in the neck, in which some of the brachial plexus pierces the anterior scalene muscle (Fig. 1). This surprisingly common pattern has profound clinical implications, because it may result in impingement of the nerves and consequently predispose patients to nTOS. To determine whether these variations were significantly correlated with nTOS symptoms, we recruited a sample of volunteer subjects who filled out a detailed questionnaire describing any neurogenic neck or arm symptoms they had, including those typically found in nTOS, and subjected them to traditional nTOS diagnostic positional testing. The brachial plexus and scalene muscles of each subject were then evaluated using ultrasound (Fig. 2). Ultrasound was found to reliably reveal brachial plexus piercing variations, and that these variations are found at significantly higher rates in subjects with nTOS symptoms. In fact, the ultrasound results were more reliable at diagnosing nTOS than the traditional tests.

These findings have important implications for the diagnosis and treatment of nTOS, and reveal a previously unknown explanation as to why nTOS has historically been difficult to diagnose. This study also demonstrates that ultrasound can help clinicians diagnose cases of nTOS caused by these anatomical variations, in conjunction with patient history and symptoms. These findings were reported in the journal Diagnostics, in a special issue dedicated to the diagnosis and treatment of thoracic outlet syndrome.

 

 

Figure 1 caption: Anatomical relationships between the proximal brachial plexus and scalene musculature identified in the cadaveric component of this study: (A) Classic anatomical relationship between the brachial plexus and anterior scalene muscle: Superior, middle, and inferior trunks of the brachial plexus travel with the subclavian artery through the interscalene gap, between the anterior and middle scalene muscles; and (B) Superior piercing variant: The superior trunk of the brachial plexus pierces the anterior scalene muscle; (C) Multiple piercing variant: The superior and middle trunks of the brachial plexus pierce the anterior scalene muscle. AS = anterior scalene; C5 = anterior ramus of C5; C6 = anterior ramus of C6; IT = inferior trunk; MS = middle scalene; MT = middle trunk; SA = subclavian artery; ST = superior trunk.

 

Figure 2 caption: The multiple piercing variant, identified using ultrasonography in the present study: (A) unlabeled; and (B) labeled. Note that the superior and middle trunks are not separated from the anterior scalene in this condition, visible as a lack of hyperechoic fascia. AS = anterior scalene; C5 = anterior ramus of C5; C6 = anterior ramus of C6; IT = inferior trunk; MS = middle scalene; MT=middle trunk; SCM = sternocleidomastoid; ST= superior trunk. The green outlines demarcate the trunks and roots of the brachial plexus.

 

 

Citation: Leonhard V, Caldwell G, Goh M, Reeder S, Smith HF. 2017. Ultrasonographic diagnosis of thoracic outlet syndrome secondary to brachial plexus piercing variation. Diagnostics 7:40. doi:10.3390/diagnostics7030040