Abstract

Background: This study aims to determinate the rate of thoracic outlet anomalies by means of analysis of cadavers.

Methods: Supraclavicular incisions were applied by two anatomists and two thoracic surgeons in the thoracocervicoaxillary region of both extremities (n=40) in twenty cadavers (7 females, 13 males; mean age 46). The formation and type of fibrous bands, cervical ribs, C7 long transverse processes and anomalies of the clavicles, scalenus anterior and scalenus medius muscles, brachial plexus, subclavian arteries and veins were evaluated. The type and formation of fibrous bands were classified using Roos' classification.

Results: Anomalies were found in 34 (85%) of extremities. The type 3-band was most frequently (15%) observed and all of them were on the right extremity. The type 4-band was rarely seen (2.5%). Two bands (type 9 and type 11) in the same extremity were notified in one cadaver. (2.5%). The occurrence rate of cervical rib and C7 long transverse process was 10%. Some fibers of m. scalenus medius emerged from a cervical rib in one extremity (2.5%). The arteria subclavia anterior passed through the scalene muscle in three extremities (7.5%). In 10% of extremities the C5 truncus passing through the anterior scalene muscle and upper truncus of brachial plexus passing anterior scalene muscle via perforation was found in 7.5% of patients.

Conclusion: In our population, brachial plexus and subclavian artery variations are frequently observed. Therefore these types of anomalies should be taken into consideration to prevent morbidity and complications when muscle division or blockage applications are performed.

Thoracic outlet syndrome (TOS) is characterized by compression on the subclavian vessels and brachial plexus at the superior aperture of the chest. The diagnosis or treatment of the thoracic outlet syndrome is complicated.[1] Factors which cause pressure on the thoracic outlet include bony tissue and soft tissue related anomalies. Bone anomalies include cervical rib, abnormal first rib and long C7 cervical transverse processes while soft tissue factors are ligaments, fibrous bands and scalene muscle anomalies resulting in symptoms due to compression.[2-4]

Functional symptoms become apparent when these anomalies are demonstrated in operation with various combinations. If myofascial anomalies impinge on soft and sensitive nerves of the plexus, nerve compression causes pain and gradual progression to compression neuropathy. Abnormal anatomic structures create the basic problem of mechanical compression or irritation of the sensitive cervical nerves. Since appropriate dissection of the anomalies are required for sustained relief[5] it is helpful to know compressing structures. The thoracic outlet region is rich in anatomical variations making proper characterization of this region necessary. Special attention was given to Roos' classification of abnormal anatomy in the upper thorax.[6]

There is more need for cadaver studies for characterization of these anomalies. The aim of our study is to determine the rate of anomalies in the thoracocervicoaxillary region in cadavers.

Methods

Subjects
Cadavers were provided by Ankara University School of Medicine, Department of Anatomy. Observations were noted without any medical history. The thoracic outlets of 20 cadavers (7 females, 13 males; mean age 46) were dissected by two anatomists and two thoracic surgeons. Supraclavicular incisions were performed on both extremities (n=40).

Dissection technique
The Platysma, sternocleidomastoideus (SCM), omohyoid and scalene muscles were dissected with subcutaneous fat tissue after supraclavicular incisions. Clavicula was evaluated with respect to any structural anomaly. The presence of cervical ribs was noted. Sternocleidomastoid muscles were cut. The scalenus anterior and scalenus medius muscles were palpated and described with macroscopic observation. The brachial plexus (BP) was reached between the scalene muscles using blunt dissection. Relationships of the subclavian artery and subclavian vein between muscles were determined. Anomalies of the first rib were evaluated. Dissection was extended to the neck through the posterior triangle thoracic outlet and adjacent structures. Absence of C7 long transverse process formation was documented. Presence or absence of compression on the brachial plexus was recorded, including origin from an anterior or posterior site, level of plexus involved (T1, C8-T1, C7-C8-T1 or more distal) and the cause (muscle anomaly or band anomaly).

Demographic data; age and laterality of anomalies were also noted. The formation and type of fibrous bands were evaluated via Roos' classification (Table 1).[6]

Table 1: Congenital bands and ligaments described by Roos

Results

Fibromuscular bands
The type 3 band was most frequently observed and all of them were on the right extremity (15%, n=6/40). The type 4 band was the rarest anomaly (2.5%, n=1/40). The other bands (types 12, 9, 2, 11 and 1) were found in rates of 10%, 10%, 7.5%, 5% and 5% respectively. The formation of two band-anomalies (type 9 and type 11) was found in only one extremity (2.5%; Table 2).

Table 2: Fibromuscular band types in 40 cervical dissection

The anomalies had no statistically significant difference with respect to extremities and sex (p=0.48, p=0.33).

Brachial plexus
It was observed that the C5 trunk passed through the anterior scalen muscle (ASM) in 10% of extremities by perforation and the upper truncus of the BP passed through the ASM by perforation in 7.5% of dissections.

Scalene muscle anomaly
Some fibers of m. scalenus medius emerged from a cervical rib in one extremity (2.5%).

Subclavian artery and vein
In three extremities (7.5%) the arteria subclavia anterior passed through scalene muscle (Fig. 1).

Fig 1: A lateral view of the right side of the neck region depicting an anomalous right subclavian artery piercing through the scalenus anterior muscle. BP: Brachial plexus; A; Subclavian arteria; AS: Scalenus anterior; *: Fibromuscular band type 3.

Bone anomalies
Cervical ribs and cervical long transverse processes each of these occurred in 10%. Clavicula and first rib anomalies were not observed. Anomalies are summarized in table 3 with the exception of fibromuscular bands.

Table 3: Anatomical variations in cadaveric dissections

Discussion

In our anatomical study the existence of a cervical rib was demonstrated in 10%. A cervical rib was present at a rate of 63.8% in our surgical series, which consisted of 206 cases.[1] Cervical ribs are encountered at a rate of 7.5 to 9% in surgical cases. Cervical ribs may occur in 0.5% of the general population and may be symptomatic in 10 percent of people.[5,7-9] Our surgical and cadaver series may suggest higher rates of anomalies and variations than those of other series in our population. This situation may reflect ethnic differences. However, no discussions on thoracic outlet susceptibility to genetic variation between populations could be found in the English literature. If cervical ribs are present, the pleural dome ascends higher in the neck so it becomes more superficial. Such displacement could explain the compression of the subclavian artery and brachial plexus when a load is carried on the shoulders.[10] The brachial plexus is usually formed between the 4^th and 8^th cervical nerves when a complete cervical rib is found. The first thoracic ventral ramus may also continue to join the plexus but it must ascend a considerable distance to do so. The inferior trunk of the brachial plexus must take the most acute course of the plexus relative to the cervical rib, predisposing it to compression and traumatic neuritis.[9]

It was reported that the frequency of fibrous bands was 98% in surgical series, but it was 33% in cadaver series. In our study fibrous bands were found in 55% of the subjects. The frequency of type 3 bands was 15% but it was 17% in Roos series.[6] In our study two band-anomalies (type 9 and type 12 band anomalies together) were found in 10% of the subjects. Type 9 and type 10 anomalies were found in 1%, although a type 12 band was not observed in 98 cadavers.[10] High rates of type 9 and type 12 bands compared with other studies may be due to the small cadaver sample. The type 3, 1, 6 and 7 fibrous bands were observed most frequently in studies. Most of these series were surgical series.[5,11] Thoracic outlet syndrome development is less than 1% in people who have these bands. In our series types 3, 2, 9 and 12 band anomalies were observed with decreasing frequency, unlike Roos series where types 3, 5, 6 and 1 were observed in that order.[6] It is thought that the existence of bands and a combination of repetitive predisposing factors could induce TOS development more effectively than the type of fibrous bands do. The addition of factors such as micro traumas, muscle hypertrophies, inflammatory reactions onto these morphological variations can result in TOS formation.[12]

In our study, it was observed that the C5 trunk passed through the ASM in 10% of extremities and the upper trunk of the BP passed through the ASM in 7.5%. In a study by Harry et al.[13] the root of C5 passed through the ASM in 13% of a 51 cadavers. It was observed that the roots of C5 and C6 passed in front of the ASM in one case in Roos series.[5] In an analysis of 93 cadavers by Natsis et al.[14] a variation in which the C5 trunk did not perforate the ASM although it passed anterior to the trunk in 3.2% of cadavers, and a variation in which the upper truncus of the BP passed the ASM was recorded. In many studies anomalies related to the root of C5 are generally reported as root fibers of C5 passing in front of the ASM.

Anomalies observed in people such as friction around scalene muscles or neck movements which can induce symptoms may be related to UT involvement of the BP. It was shown that if a nerve is compressed over a long period, vascular support of the BP roots in the endoneurium and mesoneurium could be damaged.[15] Roos et al.[5] reported that the upper plexus type of TOS occurs due to anatomical variations in the relation between the roots of the BP and the scalene muscles. Anomalies of the brachial plexus mentioned in our study also predispose to development of upper brachial plexus type neurogenic disorders in housewives, hairdressers, teachers and people who work with computers.

Arterial thoracic outlet anomalies were generally characterized on the right extremity in our study. These anomalies may involve the subclavian artery passing posterior to the esophagus, between esophagus and trachea.[16-18] In our study the arteria subclavia passed through scalene muscle in three extremities (7.5%). One of the factors which causes thoracic outlet syndrome is the compression of subclavian artery caused by hypertrophic muscle with fibrotic bands. In these types of anomalies, there are no symptoms observed in neutral position of the anterior scalene muscle, although pain can occur with pressure on the artery and complaints of arm weakness can be observed.

In our cadaver population, the frequent occurrence of brachial plexus and arterial anomalies suggests that we should be careful when exploration of penetrating trauma of the neck is performed for diagnosis and treatment of TOS. The route of the subclavian artery through the ASM by perforation is important particularly with respect to exploration of vascular structures during TOS surgery. The types of anomalies should be kept in mind in order to prevent morbidity and complications when muscles are divided.

Declaration of conflicting interests
The authors declared no conflicts of interest with respect to the authorship and/or publication of this article.

Funding
The authors received no financial support for the research and/or authorship of this article.

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