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Journal of the Anatomical Society of India

Anatomy of Inter-scalene Triangle and Its Role in Thoracic Outlet Compression Syndrome

Author(s): Savgaonkar M G, Chimmalgi M, Kulkarni U K

Vol. 55, No. 2 (2006-07 - 2006-12)

Savgaonkar M G, Chimmalgi M, Kulkarni U K(1)
B J Medical College, Pune and Govt Medical College Miraj(1)

Abstract:

Interscalene triangle, costoclavicular space and retro-pectoralis minor space are three areas that could be responsible for upper extremity neurovascular compression symptoms. Interscalene triangle also forms the chosen site for brachial plexus blocks in shoulder surgeries. In the present study an attempt has been made to note the anatomy and variations in the inter-scalene triangles and summarize the possible role of the variations in causing thoracic outlet compression syndrome.

The study was conducted in 77 adult cadavers of western India origin using dissection method. The scalene muscles exhibited variations in their insertions being fleshy, tendinous or aponeurotic. Width of the base of the triangle varied from 0 to 2.5cm. Scalenus minimus muscle was found in 5% cases and cervical rib was found bilaterally in 2 cadavers. In addition there were neurovascular variations like subclavian artery or trunks of brachial plexus piercing through the scalene muscles. Fibrous band was found in a single case connecting the inner margins of the first rib. Presence of any or combination of these variations added with dynamic demands of the upper extremity predispose to thoracic outlet compression syndrome.

Key words: Inter-scalene triangle, scalenus anterior, scalenus medius, subclavian artery, Thoracic Outlet Compression Syndrome

Introduction:

Thoracic outlet compression syndrome (TOCS) is a complex of signs and symptoms caused by compression of neurovascular structures in the cervicobrachial junction. Although anatomical nomenclature of the upper end of thorax is ‘thoracic inlet’, and perhaps syndrome ought to be called as ‘Thoracic Inlet compression syndrome’, clinically accepted terminology is the erroneous ‘thoracic outlet compression syndrome’ and will be adhered to in the present study. Three spaces that could be responsible for upper extremity neurovascular compression symptoms are:

  • Inter-scalene triangle
  • Costo-clavicular space and
  • Sub or retro-pectoralis minor space (Atasoy 1996).

Congenital variations in bony and fibro-muscular structures in these areas compounded with trauma, inflammation, wrong posture or dynamic demands of upper extremity predispose to the onset of TOCS (Roos 1976). The inter-scalene triangle is selected in the present study owing to its importance in the onset of TOCS and for being a chosen site of brachial plexus block. An attempt has been made to (a) note the anatomy and variations in the inter-scalene triangles of cadavers using dissection method and (b) summarize the possible role of variations in causing TOCS. Anatomy of interscalene triangle (IST): IST is bounded by scalenus anterior muscle anteriorly, scalenus medius posteriorly and first rib between their insertions inferiorly (fig. 1).

Fig.1 Inter-scalene triangle. a. scalenus medius b. scalenus anterior c. phrenic nerve. d. trunks of brachial plexus e. subclavian artery f. subclavian vein g. first rib.

Scalenus anterior muscle arises from anterior tubercles of transverse processes of C3-6 vertebrae. It gets inserted into scalene tubercle on the first rib.

Scalenus medius arises from posterior tubercles of transverse processes of C2-7 vertebrae. It gets inserted on the superior surface of the first rib behind the groove for subclavian artery.

Enclosed within this triangle are:
(a) In the upper part of the triangle, ventral rami of C3, 4, and 5 taking part in the formation of phrenic nerve.
(b) In the lower part are the upper, middle and lower trunks of brachial plexus along with the subclavian artery.

Materials and method:

This study was conducted in the department of Anatomy at B J Medical College, Pune and Government Medical College, Miraj. Dissection was carried out on 154 inter-scalene triangles in seventy-seven adult cadavers of western India origin. Following details of inter-scalene triangle were studied: (A) insertions of scalene muscles (B) width of the base of the triangle(C) neurovascular variations (D) presence of scalenus minimus (E) presence of cervical rib and (F) bilateral symmetry.

Observations:

(A) Insertions of scalene muscles: In 53 (34.42%) cases insertions of both scalenus anterior and scalenus medius were fleshy. In 50 (32.47%) cases insertions of both the muscles were tendinous. In 29 (18.83%) cases insertion of one of the scalenes was tendinous and that of the other fleshy. In 22 (14.28%) cases insertions of scalenus medius alone or both the scalene muscles were expanded to become aponeurotic.

In 44 (28.57%) cases (28 bilateral and 16 unilateral) the insertions of scalenus anterior and medius merged with each other reducing the interscalene ‘triangle’ to narrow interscalene ‘space’.

Among these in 10 cases (6.49%) the insertion of scalenus medius overlapped the insertion of scalenus anterior forming a ‘V’ and narrowing the space further. In another 10 (6.49%) cases the two insertions were membranous and joined to form a u-shaped sling. Subclavian artery arched over the sling in these cases.

(B) Base of the triangle: Width of bases of triangle ranged from 0 to 2.5 cm with an average of 0.9 cm. Forty-six (59.74%) of these cadavers showed equal widths on both sides and in the remaining 31widths were unequal.

(C) Neurovascular variations: In 12 (7.79%) cases (bilateral in 6) subclavian artery pierced through scalenus anterior muscle. Upper trunk of brachial plexus pierced through scalenus anterior in 10 cases (8 bilateral) and medius in 4 cases (2 bilateral). In one case middle trunk was found to be piercing scalenus anterior. In two cases, both present unilaterally on the right side, the lower trunk was traversing through the fibers of scalenus anterior. In both these cases subclavian artery was highly arched and was accompanying the middle trunk of brachial plexus. In one case all the three trunks were piercing scalenus medius muscle. In 6 cases (3.9%) fibers of the two scalene muscles were intermingling at higher level, upper trunk being above these fibers in 4 cases and traversing through these fibers in 2 cases.

(D) Scalenus minimus muscle: Scalenus minimus is reported to be extending from transverse process of C7 or occasionally C6 vertebra to get inserted into inner border of first rib. It separates the subclavian artery from T1 root or lower trunk of brachial plexus. In our study, scalenus minimus muscle was found in 8 (5.19%) cases, presenting bilaterally in 2 cadavers. In all the cases its lower end was tendinous and unyielding. It bisected the lower part of the triangle narrowing both the compartments.

(E) Cervical rib: In 2 cadavers (2.59%) cervical rib was present bilaterally. Length of the cervical rib was 7 cm in one cadaver and 2.5 cm in the other. In the former, the rib was cartilaginous as confirmed by roentgenogram. It was interesting to note that scalenus medius muscle was primarily inserted into the cervical rib. From here, few flattened scalenus medius fibers extended downwards and forwards towards the first rib, resembling in appearance with the external intercostal muscle of a typical intercostal space. Similar to external intercostal membrane, the scalenus medius fibers also became aponeurotic towards the anterior end (fig. 2). Remark: Change in the direction of fibers of scalenus medius between cervical rib and the first rib to resemble those of external intercostals muscle makes the authors believe that rib influences the development of the muscles in thoracic region.

(F) Fibrous band: In one case on the right side, a fibrous band was found connecting across the inner margins of the first rib extending to the adjacent superior surface. Posteriorly it was attached close to the posterior tubercle and anteriorly to scalene tubercle. Subclavian artery was found to be arching over this band.

(G) Bilateral symmetry: Interscalene triangles were bilaterally symmetrical in their boundaries, contents and disposition in 34 of 77 cadavers (44.16%).

Fig. 2: Figure showing cervical rib on the right side of neck. Fibers of scalenus medius are seen sweeping from cervical rib towards the first rib where they resume the appearance of the external intercostal muscle.

Discussion:

Almost all the variations noted above can be potential cause of TOCS. Although each of these variations has been mentioned in the literature, exact incidence has not been forthcoming.

(A) Scalene muscles: Scalene muscles cause TOCS in one of the following ways: (i) Tough and unyielding tendinous insertions, against which the soft neurovascular structures may be forced during certain movements as seen in 51.3% of our cases. (ii) Aponeurotic insertion of single scalene with sharp sickle shaped margin (14% of our cases) or both the scalenes joining to form u-shaped sling (6.49 % of our cases) cause subclavian artery to be highly arched, which might result in kinking of the artery or thrombus formation with distal embolization. (iii) Triangle can be narrowed to form slit like inter-scalene space by approximation of insertions of the two scalenes (28.5% of our cases) or the insertions may overlap (6% of our cases) resulting in high arching and scissoring effect (Thomas 1983). (iv)Either of the scalene muscles may be larger than normal thus narrowing the space. (v) TOCS may also result from spasm of scalenus anterior resulting in ‘scalenus anticus syndrome’.

(B) Neurovascular structures: The contents are subjected to compression by altering their course. Subclavian artery may pierce through scalenus anterior (7% cases); upper trunk may pass through scalenus anterior (10 cases) or scalenus medius (4 cases) or in relation to intermingling fibers of the two scalenes (6 cases); middle (1 case) or lower trunk (2 cases) may pierce through one of the scalenes. All the brachial plexus trunks were found to be piercing through scalenus medius in one case. Similar variations have been noted by Atasoy(1996) although incidences have not been mentioned. In all these cases the muscle bundles and thick fibrous tissue coverage may cause adhesion of nerve to muscle resulting in symptoms (Atasoy 1996).

(C) Scalenus minimus: Incidence of its presence is variably reported as 30-50% by Atasoy (1996). In the present study it was found in only 5% of our cases which is considerably lesser incidence as compared to the figures quoted by others. When present, scalenus minimus predisposes to TOCS by narrowing the triangle (Atasoy 1996).

(D) Cervical rib: Incidence of cervical rib is reported to be 0.5-0.6% (Atasoy 1996) with 50-80% of these being bilateral. In our study, it was present bilaterally in 2 cadavers (2.59%). Only 10% of the cases with cervical rib will ever become symptomatic.

(E) Fibrous bands: In our study we found one case with fibrous band stretching across first rib. Such a fibrous band is reported as the commonest variety encountered during surgery. Various fibrous bands have been reported by other authors in this region arising from first rib, C7 vertebra, scalene muscles, cervical rib, Sibson’s fascia, etc. (Roos 1976, Poitevin 1988).

Clinical symptoms of TOCS can be neural or vascular. In neural symptoms, sensory precede the motor, both of which precede sympathetic symptoms. Typically, thenar wasting with ulnar sensory loss is seen (compare with carpal tunnel syndrome where thenar wasting is with median sensory loss). Vascular symptoms are ischemic pain and rarely necrotic changes with arterial involvement. With venous involvement, dull aching pain along with venous congestion is seen. Diagnosis is mainly based on clinical and electrophysiological criteria. Imaging techniques are now found useful in locating the cause for neural or vascular compressions. MRI can also demonstrate the effect of dynamic demands of the limb on the compressed structures (Demondion 2000). Management of these cases is mainly conservative treatment like avoiding exacerbating postures. However, surgical treatment is indicated if there is threatened ischemia or progressive neurological deficit. Currently popular techniques are transaxillary first rib resection or cervical scalenectomy or combination of both. With the current anatomical knowledge it would be apt to add that presence of a fibrous band, cervical rib and/or scalenus minimus are kept in mind during surgery.

Limitations of this study are:

  1. Measurements of costoclavicular space or retro-pectoralis minor space was not included in the study.
  2. Measurement of thickness of scalenus anterior was not included in the study due to variable state of preservation of the cadavers.

Thus the study of interscalene triangle shows that variations in the region are a rule rather than exceptions. Added with the various dynamic demands of upper extremity probability of TOCS is far greater than its actual incidence.

References:

  1. Atasoy E. Thoracic outlet compression suyndrome. Orthopedic Clinics of North Am. 1996; 27 (2): 265- 303
  2. Demondion X, Boutry N, Drizenko A, Paul C, Francke JP and Cotton A. Thoracic outlet: anatomic correlation with MR imaging. Am J of Roentgenology 2000; 175: 417-422
  3. Poitevin L. Proximal compression of the upper limb neurovascular bundle: an anatomic research study. Hand Clinics 1988; 4: 575
  4. Roos D B. Congenital anomalies associated with thoracic outlet syndrome. Am J Surg 1976; 132: 771- 778
  5. Thomas GI, Jones TW, Starney LS et al. The middle scalene muscle and its contribution to the TOC. Am J Surg 1983; 145: 589-592
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