Neurogenic thoracic outlet syndrome due to subclavius posticus muscle with dynamic brachial plexus compression: a case report

Prevalence and anatomy of the anomalous subclavius posticus muscle and its clinical implications with emphasis in neurogenic thoracic outlet syndrome: Scoping review and meta-analysis

BACKGROUND

The subclavius muscle runs underneath the clavicle. However, there have been a few reports of a duplicated subclavius muscle which is commonly referred to as subclavius posticus muscle due to its orientation being posterior to the proper subclavius muscle. Its occurrence seems to potentially create a narrowing interval at the superior thoracic aperture as it crosses over the brachial plexus. It might also have functional influence on the shoulder girdle.

PURPOSE

To provide comprehensive gross anatomy description of the variant "subclavius posticus muscle" and to investigate its reported clinical implications with emphasis on its involvement in causing brachial plexus compression.

BASIC PROCEDURES

A scoping review with meta-analysis of the gross anatomy of the subclavius posticus muscle was conducted along with investigating its correlation to the thoracic outlet syndrome. Forty-seven articles were pooled through two rounds of the selection process. The relevant information was extracted and meta-analyzed.

MAIN FINDINGS

The scoping review and meta-analysis of the 47 articles revealed a total prevalence of 11/2069 (4.9%); 10/1369 (5.1%) in cadaveric studies, and 1/700 (5.0%) in MRI studies. The subclavius posticus muscle is a short triangular muscle with an average length of 12 cm and an average width of 1 cm. It originates from the sternal end of the first rib in most cases with reported variants of one case originating from the costoclavicular ligament and one case where it was fused with the proper subclavius muscle. Its insertion is more variable: on the superior border of the scapula with variable length in 71.35%, on the coracoid process of the scapula in 25.42%, and on the clavicle in 0.90%. The subclavian nerve seems to be the dominant nerve supply with a 57.6% prevalence, while 25.8% are innervated by the suprascapular nerve. Other reported nerves were the nerve to the myolohyoid (4.5%), accessory phrenic nerve (4.5%), and a direct branch from the brachial plexus (2.0%). The blood supply was reported (only once) to be from the suprascapular artery. However, the venous drainage was not established at all.

PRINCIPAL CONCLUSIONS

The subclavius posticus muscle is a variant muscle occurring with a reported overall prevalence of 4.9%. It can compress the brachial plexus as it runs across at the space of the superior thoracic aperture with exquisite contact, and this can lead to a neurogenic thoracic outlet syndrome. It also can be involved in the vascular thoracic outlet syndrome. Lastly, it is important not to omit its potential influence in shoulder joint instability to some extent.

Bilateral absence of subclavius muscles with thickened costocoracoid ligaments: a case report with the clinical-anatomical correlation

Anatomical variation is defined as normal flexibility in the topography and morphology of body structures. Such variations are not uncommon in muscles, particularly in the upper limb. Subclavius muscle (SM) has a proximal attachment to the first costochondral junction and the muscle fibers are then directed upwards and laterally to get attached distally to the subclavian groove of the clavicle. Having similar attachments as the subclavius, the costocoracoid ligament (CCL) is the thickening of the proximal part of clavipectoral fascia extending up to the coracoid process. Both SM and CCL help in the maintenance of smooth movements of the pectoral girdle and both may not always be present. Absent SM may be due to anomalous development from the muscle matrix that also forms the inferior belly of the omohyoid apart from the SMs. This anatomical variation may be associated with thickened CCLs and can be correlated to the smooth functioning of the pectoral girdle.

Paralysis of the Upper Extremity Due to Acute Thoracic Outlet Syndrome Caused by the Subclavius Posticus Muscle: A Case Report With Literature Review

The subclavius posticus muscle is a rare aberrant muscle that traverses from the costal cartilage of the first rib posterolaterally to the superior border of the scapula. We report a patient having persistent paralysis of shoulder abduction with wrist and finger extension after a humeral neck fracture. Electromyography (EMG) examination revealed injuries to several upper extremity peripheral nerves, including the radial, axillary, and musculocutaneous nerves. Magnetic resonance imaging (MRI) performed at 10 months post-injury showed severe entrapment of the left brachial plexus by the subclavius posticus muscle at the thoracic outlet. The diagnosis of brachial plexus injury due to a rare abnormal subclavius posticus muscle was typically delayed until the MRI was performed for unexplained multiple peripheral nerve palsy. Resection of the aberrant muscle and brachial plexus decompression did not yield significant improvement in the patient’s radial nerve palsy until 6 months after surgery. Entrapment of the brachial plexus caused by the subclavius posticus muscle can cause symptoms of acute thoracic outlet syndrome following trauma to the upper extremity. In a case of inexplicable multiple peripheral nerve injuries in the upper extremity that are not proportional to the degree of trauma, MRI imaging along with EMG is required.

The anatomy and variation of the coracoid attachment of the subclavius muscle in humans

The functions of the subclavius muscle (SM) are described as stabilization of the sternoclavicular joint (SCJ) and resisting elevation of the lateral end of the clavicle. During systematic cadaveric dissections, we observed additional fibrous structures, previously described as variants of the anatomy, extending from the SM and inserting into the coracoid process (CP). Due to the high incidence of these structures in our dissections, we hypothesized that the attachment at the CP is more common than appreciated and that, as a corollary, the function of the SM was (or has been) more complex than simply depressing the clavicle and generating stability at the SCJ. For our investigation, fifty‐two upper extremities of 26 human cadavers were dissected. The SM was demonstrated from costal to clavicular attachment. We documented additional fibrous structures apparently derived from the SM inserting into the CP. Measurements of the length of the SM, the length of its attachment, and the length of the clavicle were taken in situ, with the specimens supine and the upper extremity in the anatomical position. Variations in the anatomy of the SM and its coracoidal attachment were recorded, and potential correlations were investigated. For documentation purposes photographs and video sequences of passive motion of the shoulder girdle of the specimens were taken. In 49 of the 52 specimens we found additional fibrous structures passing from the SM to the CP. We differentiated three types: (1) a strong cord‐like structure; (2) a small or thin cord‐like structure or structures; and (3) a planar twisted sheet‐like structure. The SM and its extension to the CP appears to contribute to a ‘functional scapular suspension system’ together with the other muscles enveloped by the clavipectoral fascia (pectoralis minor, coracobrachialis and the short head of the biceps brachii). This system assists in the control of the position of the scapula in relation to the thorax, particularly in elevated positions of the upper extremity. We speculate that the differentiation of the fibrous structure depends on the functional demands of the individual.

Level of Evidence: Basic science study.

Rare Case of Dynamic Thoracic Outlet Syndrome Resolved by an Isolated Subclavius Posticus Muscle Resection: A Case Report

CASE

We reported a dynamic neurogenic left thoracic outlet syndrome (TOS) with a permanent abduction of the fifth left finger. Preoperative magnetic resonance imaging (MRI) assessed the presence of subclavius posticus muscle (SPM). Because of a nonoperative treatment failure, we performed a brachial plexus neurolysis and SPM resection. Immediate postoperative assessment showed an immediate disappearance of the Wartenberg sign.

CONCLUSION

SPM constitutes an underestimated cause of TOS. A careful MRI reading is necessary to make correct diagnosis. Nonoperative treatment includes physiotherapy and can be proposed in first instance. When nonoperative treatment fails, brachial plexus exploration with release of the SPM may result in resolution of symptoms.

Improved functional outcome in NTOS patients following resection of the subclavius muscle with radiological signs of nerve impingement: indication of participation of the subclavius in brachial plexus compression

OBJECTIVE

Both clinical and radiological reports have suggested that the subclavius, a muscle in the costoclavicular space of the thoracic outlet, participates in neurogenic thoracic outlet syndrome (NTOS) in some instances, especially during movements narrowing the costoclavicular space. Magnetic resonance imaging can identify subclavius muscles with signs of nerve impingement, yet the impact of the subclavius in such situations remains unclear. Therefore, the authors investigated whether dividing or sparing the subclavius characterized by nerve impingement on MRI would affect surgical outcomes.

METHODS

In this retrospective nonrandomized study, authors analyzed all NTOS patients with a subclavius muscle characterized by nerve impingement on MRI (loss of normal fat planes surrounding the brachial plexus) in the period between March 2010 and November 2016. Patients were divided into two groups: the sparing group, in which patients had undergone conventional supraclavicular scalenectomy and first rib resection (FRR), and the dividing group, in which patients had undergone scalenectomy, FRR, and subclavius dividing using a modified supraclavicular incision. The Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire, a shoulder range of motion subscale (DASH items 6, 12-15, and 19) concerning overhead activities that can significantly narrow the costoclavicular space, postoperative MRI studies, and patient self-assessments were used to assess surgical outcomes. Univariate and multivariate analyses were conducted to identify independent factors associated with subscale scores.

RESULTS

From a total of 261 patients screened, 71 were eligible for study inclusion. Compared with the sparing group (33 patients), the dividing group (38 patients) had similar postoperative DASH scores and self-assessments but better subscale scores (9.50 ± 2.76 vs 11.94 ± 2.87, p = 0.0005). Postoperative MRI on hyperabduction showed that the brachial plexus became surrounded by normal fat tissue in the costoclavicular space in the diving group but still had signs of impingement from the untreated subclavius muscle in the sparing group. This observation agreed with a better functional recovery in terms of overhead activities in the dividing group, which was reflected by better subscale scores. Multivariate analyses indicated that the type of treatment and symptom duration prior to surgery influenced the subscale scores independently.

CONCLUSIONS

This study revealed that an untreated radiological nerve-compressing subclavius muscle could lead to a relatively lower degree of recovery in the ability to perform overhead activities for NTOS patients postoperatively, suggesting that such subclavius muscles may participate in positional brachial plexus compression during movements narrowing the costoclavicular space. Dividing the muscles could decompress the costoclavicular space more effectively and may lead to better functional recovery.

The prevalence and characteristics of the subclavius posticus muscle in the adult population on MRI

PURPOSE

The subclavius posticus muscle (SPM) is an aberrant muscle, and the presence of the SPM has a clinical significance, since the SPM might lead to thoracic outlet syndrome (TOS). To date, no radiological study has been evaluated the prevalence of the SPM in the adult population. In this study, we aimed to assess the prevalence of the SPM using magnetic resonance imaging (MRI).

METHODS

We retrospectively reviewed brachial plexus, neck, and thoracic MRI of the patients who underwent MRI for various pathologies. Only the patients whose MRI examinations did not have sufficient image quality and the patients who had any pathology that directly involved the thoracic outlet region were excluded from the study. The observers assessed images in the coronal plane to detect the presence of the SPM. The thickness of the muscle and the distance between the SPM and brachial plexus were also noted.

RESULTS

MRI examinations of 350 patients, 174 males and 176 females, were evaluated. SPM was detected in 29 patients (8.3%). In 6 patients (1.7%), we identified bilateral SPM. The mean thickness of the muscle was 6.10 ± 1.60 mm (range 2.5-10 mm). In 21 sides (60%), the brachial plexus and SPM were touching each other. In the remaining 14 sides (40%), the mean distances between the SPM and brachial plexus were 2.32 ± 0.62 mm (range from 1.7 to 3.6 mm).

CONCLUSIONS

We conducted the first radiological study that evaluated the prevalence of the SPM, and we also assessed the thickness of the SPM and the distance between the SPM and brachial plexus. We highlight that our study might be accounted as a first step for future studies to assess the role of the SPM in TOS.

An Aberrant Subclavius Posticus Muscle: A Case Report

OBJECTIVE

An aberrant subclavius posticus muscle was found during routine dissection of the left infraclavicular fossa of a 60-year-old male cadaver.

PRESENTATION

This aberrant muscle arises antero-medially, from the costoclavicular ligament, runs postero-laterally, over the trunks of the brachial plexus, and postero-lateral has a common insertion with the omohyoid muscle.

CONCLUSION

The presence of such an aberrant muscle may cause a dynamic compression of the subclavian artery and brachial plexus.

Choosing Surgery for Neurogenic TOS: The Roles of Physical Exam, Physical Therapy, and Imaging

Neurogenic thoracic outlet syndrome (nTOS) is characterized by arm and hand pain, paresthesias, and sometimes weakness resulting from compression of the brachial plexus within the thoracic outlet. While it is the most common subtype of TOS, nTOS can be difficult to diagnose. Furthermore, patient selection for surgical treatment can be challenging as symptoms may be vague and ambiguous, and diagnostic studies may be equivocal. Herein, we describe some approaches to aid in identifying patients who would be expected to benefit from surgical intervention for nTOS. We describe the role of physical examination, physical therapy, and imaging in the evaluation and diagnosis of nTOS.