Case report: a case of subclavius posticus muscle mimicking a mass on mammogram

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.

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 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.

The subclavius posticus muscle: an unusual cause of thoracic outlet syndrome

We present the case of a 30-year-old female, complaining of thoracic outlet compression symptoms caused by a supernumerary muscle, the subclavius posticus, accompanied by a caudally inserted middle scalenus muscle on the second rib. This rare anatomic variant was clearly shown on CT angiography and MRI images and surgical treatment was successful. As first described by Rosenmuller in 1800, subclavius posticus is a supernumerary muscle originating from the cranial surface of the sternal end of the first rib, running laterodorsally beneath the clavicle, and inserting into the superior border of the scapula. Its role in thoracic outlet syndrome has been seldom demonstrated in living patients nor described in imaging, although it is theoretically easily recognizable on modern imaging modalities. It should be taken into account during workout of patients with thoracic outlet syndrome, since it can be potentially treated.