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Subclavian triangle

The subclavian triangle, also known as the supraclavicular or omoclavicular triangle, is a distinct anatomical region within the posterior triangle of the neck, serving as a key landmark for vascular, neural, and lymphatic structures in the supraclavicular fossa.^[1] It is bounded superiorly by the inferior belly of the omohyoid muscle, inferiorly by the middle third of the clavicle, anteriorly by the posterior border of the sternocleidomastoid muscle, and posteriorly by the anterior border of the trapezius muscle, with the floor formed by the first rib and the first digitation of the serratus anterior muscle.^[2]^[3] The primary contents include the third part of the subclavian artery and vein, trunks of the brachial plexus, suprascapular and transverse cervical vessels, the phrenic nerve, and supraclavicular lymph nodes.^[1] This triangle plays a critical role in clinical practice due to its proximity to vital neurovascular elements, making it susceptible to injury during neck surgeries such as scalenectomy or central venous catheterization.^[4]^[5] Enlarged supraclavicular lymph nodes within the triangle, particularly Virchow's node on the left side, serve as an important indicator of distant metastases from abdominal malignancies like gastric or ovarian cancer, known as Troisier's sign.^[1] Additionally, the region's vessels are often accessed for reconstructive procedures^[2] or vascular grafting,^[6] while its nerves, including the brachial plexus, are relevant in diagnosing thoracic outlet syndrome.^[7] Understanding the subclavian triangle's anatomy is essential for surgeons navigating the root of the neck to avoid complications like phrenic nerve damage or vascular compromise.^[1]

Anatomy

Definition and location

The subclavian triangle is defined as the smaller, inferior subdivision of the posterior triangle of the neck, a key anatomical region in the lateral aspect of the cervical area.^[8] It is also referred to as the omoclavicular triangle or supraclavicular triangle, reflecting its positional relationship to the clavicle and surrounding structures.^[9] This triangle serves as an important landmark in neck anatomy due to its superficial accessibility and proximity to major neurovascular elements.^[10] Positioned within the supraclavicular fossa, the subclavian triangle lies immediately superior to the clavicle and lateral to the sternocleidomastoid muscle, extending from the lower neck into the upper shoulder region.^[11] This location places it at the junction between the cervical and thoracic regions, making it a transitional zone visible in the supraclavicular depression when the arm is in a neutral position.^[9] The triangle's orientation is such that it forms the most inferior aspect of the posterior triangle, facilitating its role in surgical and diagnostic approaches to the neck.^[8] The posterior triangle of the neck, bounded by the sternocleidomastoid anteriorly, trapezius posteriorly, and the clavicle inferiorly, is divided into superior and inferior parts by the inferior belly of the omohyoid muscle.^[10] The superior portion is known as the occipital triangle, while the inferior portion constitutes the subclavian triangle, highlighting the omohyoid's role as a natural divider in cervical topography.^[8] This subdivision aids in understanding the regional organization of neck structures, with the subclavian triangle being notably smaller and more delimited.^[9] The size and shape of the subclavian triangle exhibit variability influenced by the extent of muscular attachments, such as those of the sternocleidomastoid and trapezius to the clavicle, as well as arm positioning during examination.^[1] Such variations underscore the importance of considering patient posture in anatomical assessments.^[11]

Boundaries

The subclavian triangle, also known as the omoclavicular or supraclavicular triangle, is defined by distinct anatomical boundaries that delineate its region within the posterior triangle of the neck. The medial boundary is formed by the posterior border of the sternocleidomastoid muscle, which separates it from the anterior triangle.^[12] The lateral boundary is the anterior border of the trapezius muscle, providing a posterior limit to the triangular space.^[12] The superior boundary consists of the inferior belly of the omohyoid muscle, which divides the posterior triangle into the subclavian and occipital subdivisions.^[10] The inferior boundary is the middle third of the clavicle, marking the base of the triangle at the root of the neck.^[12] The floor of the subclavian triangle is composed of the first rib and the first digitation of the serratus anterior muscle, covered by the prevertebral layer of the deep cervical fascia.^[9] This muscular base supports the neurovascular structures passing through the region. The roof is formed by the skin, superficial fascia, platysma muscle, and investing layer of the deep cervical fascia, creating a superficial covering that is traversed by the supraclavicular nerves originating from the cervical plexus.^[12] These nerves pierce the roof to innervate the skin over the clavicle and upper chest.^[12] The apex of the subclavian triangle is located at the point where the inferior belly of the omohyoid muscle crosses the middle third of the clavicle, converging the superior and inferior boundaries.^[10] This configuration gives the triangle its characteristic shape, with variations in size depending on the extent of muscular attachments.^[12]

The subclavian triangle, also known as the supraclavicular triangle, contains critical neurovascular and lymphatic structures that facilitate blood supply, innervation, and drainage to the upper limb and adjacent regions. These contents are enclosed by the triangle's boundaries and lie superficial to the first rib and scalene muscles.^[13]

Vascular contents

The primary vascular structure is the third part of the subclavian artery, which extends laterally from the anterior scalene muscle to the outer border of the first rib, where it continues as the axillary artery. This segment curves upward and may ascend as high as 4 cm above the clavicle, positioning it vulnerable during neck procedures.^[14]^[15] The subclavian vein runs parallel and inferior to the artery, typically positioned posterior to the clavicle but with occasional variability where it ascends into the triangle proper. The external jugular vein terminates by draining into the subclavian vein at its angle of union with the internal jugular vein, often within or near the inferior aspect of the triangle.^[13]^[10] Additional vessels include the transverse cervical artery and vein, which cross the triangle to supply the trapezius and surrounding muscles, and the suprascapular (also called transverse scapular) artery and vein, which pass inferiorly to reach the scapula via the suprascapular notch. These branches arise from the thyrocervical trunk of the subclavian artery and accompany their venous counterparts.^[10]^[13]

Nervous contents

The trunks of the brachial plexus (formed by the C5–T1 roots) traverse the triangle, passing superior to the third part of the subclavian artery en route to the axilla, providing motor and sensory innervation to the upper limb. The phrenic nerve, arising from C3–C5, descends on the anterior scalene muscle within the region.^[1] The nerve to the subclavius muscle, a branch from the brachial plexus (typically C5-C6), descends within the triangle to innervate the subclavius muscle, which stabilizes the clavicle during arm movements.^[16]^[17]

Lymphatic contents

Supraclavicular lymph nodes are situated within the triangle, primarily along the superior surface of the middle third of the clavicle, and serve as key drainage points for the upper limb, thorax, and portions of the head and neck. These nodes filter lymph from superficial and deep cervical chains before it enters the venous system. On the left side, the triangle is in close proximity to the thoracic duct, which empties into the junction of the left subclavian and internal jugular veins nearby.^[8]^[16]^[13]

Clinical significance

Surgical procedures

The subclavian triangle serves as a critical anatomical landmark in central venous catheterization via the supraclavicular approach, where the subclavian vein is accessed just lateral to the clavicular head of the sternocleidomastoid muscle, facilitating catheter insertion into the central venous system while minimizing risks to adjacent structures like the brachial plexus and subclavian artery.^[18] This technique involves needle puncture at the apex of the triangle, guided by the intersection of the sternocleidomastoid and omohyoid muscles, allowing for straightforward advancement of a guidewire into the superior vena cava using the Seldinger method.^[19] Ultrasound guidance enhances precision by visualizing the vein's position relative to the pleura and nerves within the triangle, reducing complication rates compared to landmark-based methods.^[20] Supraclavicular lymph node biopsy utilizes the subclavian triangle for sampling enlarged nodes, particularly in oncologic staging, through a small incision along the clavicle that exposes the superficial lymphatics without deep dissection into the underlying vessels or nerves.^[21] The procedure targets nodes in levels IV and Vb, which drain thoracic and abdominal malignancies, and can be performed under local anesthesia with ultrasound to confirm node location and avoid vascular injury.^[22] Core needle biopsy is preferred over excisional methods for its lower morbidity, providing histological diagnosis while preserving the triangle's structural integrity.^[23] In vascular surgery, exposure of the third part of the subclavian artery—located lateral to the anterior scalene muscle within the subclavian triangle—is achieved through a supraclavicular incision that divides the platysma and retracts the sternocleidomastoid, allowing direct access for aneurysm repair or bypass grafting in cases of occlusion.^[24] This approach avoids the need for clavicular resection, which is required for more proximal segments, and facilitates proximal and distal control of the artery while protecting the adjacent phrenic nerve on the scalene surface.^[25] Intraoperative heparinization and vascular clamps are applied to manage blood flow during reconstruction, with graft materials like Dacron commonly used for durability.^[26] Modified radical neck dissection in oncology incorporates the subclavian triangle to remove supraclavicular lymph nodes (level Vb) while preserving non-lymphatic structures such as the sternocleidomastoid muscle, internal jugular vein, and spinal accessory nerve, reducing postoperative morbidity compared to classical radical techniques.^[27] The dissection begins at the mastoid tip and extends inferiorly into the triangle, ligating lymphatic trunks along the omohyoid and clavicle to clear metastatic spread from head and neck cancers, with sentinel node mapping sometimes employed to limit resection extent.^[28] This preserves shoulder function and cosmetic outcomes, achieving oncologic control through comprehensive nodal clearance.^[29] Surgical interventions in the subclavian triangle carry risks of brachial plexus injury from direct needle or instrument trauma during catheterization or biopsy, potentially causing transient or permanent neuropathy in the upper extremity.^[30] Phrenic nerve damage, which courses along the anterior scalene, can lead to ipsilateral diaphragmatic paralysis and respiratory compromise, particularly in patients with underlying pulmonary disease.^[31] Ultrasound guidance mitigates these hazards by enabling real-time visualization of neural and vascular elements, significantly lowering incidence rates of pneumothorax and vascular puncture.^[32]

Pathological conditions

The subclavian triangle, particularly its supraclavicular fossa, is a site for lymphadenopathy where enlarged lymph nodes can signal serious underlying conditions. On the left side, Virchow's node—an enlarged supraclavicular lymph node—classically indicates metastatic abdominal malignancies, such as gastric cancer, due to tumor spread via the thoracic duct to the jugulosubclavian junction.^[33] This finding, also known as Troisier's sign, may present as a palpable, firm mass and is associated with advanced disease, potentially causing compressive symptoms like thoracic outlet syndrome from mass effect on adjacent structures.^[34] In head and neck cancers, such as nasopharyngeal carcinoma, supraclavicular lymphadenopathy is classified as N3 in the TNM staging system, denoting regional nodal involvement beyond level V and indicating a poorer prognosis.^[35] Vascular pathologies in the subclavian triangle often involve the subclavian artery, where aneurysms or thrombosis can manifest as a pulsatile supraclavicular mass or symptoms of upper extremity ischemia, including pain, pallor, and coolness due to embolization.^[36] These conditions frequently arise in the context of thoracic outlet syndrome (TOS), a rare disorder (1-2% arterial form) where bony anomalies like a cervical rib compress the subclavian artery against the first rib or scalene muscles, leading to post-stenotic dilatation, mural thrombus, or acute limb-threatening ischemia.^[37] Infections affecting the region include suppurative lymphadenitis, typically caused by Staphylococcus aureus or Streptococcus pyogenes, which can progress to abscess formation in the supraclavicular fossa following upper respiratory tract infections, presenting with localized swelling, erythema, tenderness, and fever.^[38]^[39] On the left side, thoracic duct obstruction or injury near its termination at the subclavian vein junction can result in chylothorax, characterized by chyle accumulation in the pleural space, often due to malignancy, thrombosis, or iatrogenic trauma, leading to dyspnea and nutritional deficits from lymphatic leakage.^[40] Diagnostic evaluation of the subclavian triangle includes palpation of the subclavian artery pulse to assess for coarctation of the aorta, where a diminished or delayed left radial/femoral pulse compared to the right (brachiofemoral delay) signals narrowing distal to the left subclavian origin, often with upper limb hypertension.^[41] Additionally, supraclavicular fullness may indicate underlying mediastinal masses, such as lymphomas or thymic tumors, warranting imaging to evaluate for compressive or infiltrative pathology.^[42]

References

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