A bifurcated anterior scalene muscle: A case report

Introduction 

The literature indicates that, whilst anatomical anomalies can represent normal variations ranging from minor to major, they still carry some risk of predisposing to illness, symptomatology and clinical investigation that may require case management including surgery.1 The interscalene triangle between the anterior and middle scalene muscles is part of the thoracic outlet, which allows the passage of the subclavian artery and brachial plexus from the root of the neck to the axillary region (Fig. 1). Anomalies at the thoracic outlet, which may cause compression of the subclavian artery and/or brachial plexus, have been cited in the literature.2 Among the causes responsible for narrowing the thoracic outlet are anterior scalene muscle, cervical ribs, clavicular deformity, anomalous blood vessels and congenital fibrous bands across the thoracic outlet.3

View Large Image Download to PowerPoint

Figure 1. Normal anatomy of thoracic outlet.

Case report 

During regular dissection of the scalene muscles of one male cadaver by first-term students, the right anterior scalene was found to be split into two bellies, an anterior and a posterior, both attached to the first rib (Figure 2, Figure 3). The right subclavian artery was noted passing between the two bellies of the anomalous muscle (Figure 2, Figure 3). The trunks of the brachial plexus were in their normal anatomical position in the scalene triangle between the anterior and middle scalene muscles. In this case, the trunks of the brachial plexus were seen passing between the posterior belly of anterior scalene and the middle scalene muscles (Figure 2, Figure 3). The subclavian vein was found anterior to the anterior belly, which separates it from the subclavian artery (Figure 2, Figure 3). The phrenic nerve was seen crossing over the anterior surface of the common stem of the anterior scalene and then running over the anterior surface of the anterior belly (Figure 2, Figure 3).

View Large Image Download to PowerPoint

Figure 2. Dissection of the scalene triangle. AS: anterior scalene, ab: anterior belly, pb: posterior belly, MS: middle scalene, SA: subclavian artery, SV: subclavian vein, and BrP: brachial plexus.

View Large Image Download to PowerPoint

Figure 3. The relationship of the brachial plexus to the scalene muscles. AS: anterior scalene, ab: anterior belly, pb: posterior belly, MS: middle scalene, SA: subclavian artery, SV: subclavian vein, and BrP: brachial plexus.

Discussion 

A study of developmental anomalies at the thoracic outlet involving 200 cases reported that, in 66% of the cases, the single or multiple abnormalities that were recognized represented developmental variations.2 There are few references to bifurcation of the anterior scalene in the literature; however, one reference by Macalister in 1875 cited that the anterior scalene is sometimes located behind the subclavian artery or may be split into two parts for the artery to pass between them.4

In another study, dissection of both sides of the necks of 51 cadavers revealed one instance in which the subclavian artery pierced the substance of the anterior scalene rather than passing posterior to it with an accompanying poststenotic dilatation.5 The same study also reported the presence of a scalenus minimus in 46% of sides (28% bilaterally). In 1992, Makhoul and Machlender described the scalenus minimus muscle (an offshoot of anterior scalene, in an evolutionary context) as a segment of the scalenic muscle mass. The scalenus minimus must be distinguished carefully from the split anterior scalene.

In a third study, an abnormally anterior insertion of the anterior scalene was demonstrated with compression of the subclavian vein.6 In 1989, Fuss et al. found an anterior scalene with two insertions, one at the scalene tubercle and one at the costochondral border of the first rib. This caused an opening for the T1 spinal nerve root of the brachial plexus. An aponeurotic membrane, attached to the inner margin of the first rib and narrowing the superior thoracic aperture, prevented the T1 nerve root from passing behind the scalene tubercle insertion of the anterior scalene.7 Bortolani et al. reported a case of hypertrophic anterior scalene muscle causing compression of subclavian artery, and causing upper limb intermittent claudication at any sustained upper extremity activity such as lifting a weight or opening and closing the hand with the arm abducted. Simple anterior scalenotomy was followed by prompt recovery of symptoms.8

The clinical significance of the anterior scalene anomaly in this report cannot be underestimated. Although the clinical records for this case were not available, it could be argued that the extra belly of the anterior scalene narrows the scalene triangle between the anterior and middle scalene muscles. Consequently, it could compress the subclavian artery between its two bellies, thus causing vascular TOS. Similarly, the brachial plexus could be compressed between the posterior belly of the anterior scalene and the middle scalene producing neurogenic TOS. Since the anterior and middle scalene muscles act to elevate the first rib during respiration, it could also be argued that persistent contraction of the scalene muscles could aggravate the symptoms. Both vascular and neurogenic TOS conditions may require surgical intervention to relieve the pressure and alleviate the symptoms.

It must be stated however that the mere presence of an abnormality or a variation does not by itself imply that symptoms will be apparent. Where variations do occur, a secondary precipitating factor, e.g., drooping of the shoulder girdle may cause compression of structures to a degree that elicits the neuromuscular deficits.5 Therefore, in order to reliably correlate the structure and function of the superior thoracic aperture (thoracic outlet), a thorough medical history and a careful physical examination with state-of-the-art diagnostic imaging techniques must be considered by the astute clinician.

Summary 

This is a case report about a right side anterior scalene muscle found in one male cadaver that splits into two bellies, an anterior and posterior, enclosing the right subclavian artery. The trunks of the brachial plexus run in the space between the posterior belly of the anterior scalene and the middle scalene muscles. The clinical implication of this anomaly involves the possible risk of developing vascular or neurogenic TOS with compression of the subclavian artery between the two bellies of the anterior scalene and/or compression of the trunks of the brachial plexus between the posterior belly of anterior scalene and middle scalene muscles.

Conclusion 

Chiropractors who may encounter patients with TOS in their clinics should be familiar with the anatomic variations and anomalies that may lead to this clinical condition.

Conflict of interest statement 

None declared.