3.20: Posterior cervical space

Anita Nagadi

Introduction

The posterior cervical space lies along the posterolateral aspect of the neck and is also commonly referred to as the posterior triangle of the neck by the surgeons.

Extent and division

The posterior cervical space extends from the level of the skull base down to the clavicles. This space lies deep to the sternocleidomastoid and trapezius muscles, but superficial to the perivertebral space. The space is small cranially and gradually expands in a caudal direction where it is finally related to the middle third of the clavicle.

Boundaries

Superficial to the PCS lie the sternocleidomastoid and trapezius muscles which are lined by the superficial layer of deep cervical fascia (SL–DCF). Along the deeper aspect, the PCS is separated from the perivertebral space by the deep layer of deep cervical fascia (DL–DCF).

Anteromedially, the PCS is related to the carotid space which is lined by contributions of all the three layers of the deep cervical fascia.

Relations

Anterolateral – Sternocleidomastoid
Posterolateral – Trapezius
Anteromedial – Carotid space
Medial and deeper aspect – Perivertebral space

The contents of the PCS are fat, lymph nodes and nerves.

Several groups of lymph nodes lie in the posterior cervical region. Those that lie posterior to the posterior border of the sternocleidomastoid are classified as level V lymph nodes. The lymph nodes that lie deep to the sternocleidomastoid are classified as level II to level IV lymph nodes. The craniocaudal landmarks that help separate these levels are the inferior margin of the hyoid and cricoid cartilages. The lymph nodes that lie above the level of the inferior margin of the hyoid cartilage are labelled as level II, those that lie beneath the inferior margin of the cricoid cartilage are labelled as level IV and the ones in between are level III lymph nodes.

The main nerves to traverse the posterior cervical space are the spinal accessory nerve (XI) and the dorsal scapular nerve. The distal portions of the brachial plexus also traverse the lower PCS.

The spinal accessory nerve runs along the deep surface of the sternocleidomastoid and crosses the posterior cervical space in an oblique direction to run along the deep surface of the trapezius.

The trunks of the brachial plexus emerge between the anterior and middle scalene muscles and divide into divisions and cords within the PCS before continuing into the axilla. The dorsal scapular nerve and the cutaneous branches of the cervical plexus also traverse the PCS.

The third part of the subclavian artery briefly traverses the inferior aspect of the PCS as it emerges behind the anterior scalene muscle and continues distally to pass over the first rib.

Key points from anatomy

The clinicians refer to the PCS as the posterior triangle of the neck bordered by the sternocleidomastoid anteriorly and the trapezius muscle posteriorly. The triangle is further divided by the inferior belly of the omohyoid into a superior occipital triangle and inferior subclavian triangle (Figs 3.20.1–3.20.5).

Fig. 3.20.1 Axial CT section just beneath the skull base. The posterior cervical space is narrow at this level and related anterolaterally to the sternocleidomastoid which is enveloped by the SL-DCF ( *purple line*) and anteromedially to the carotid sheath. Along the deeper aspect of the PCS lies the perivertebral space enveloped by the DL-DCF ( *green line*). Note the contributions of both the SL-DCF and the DL-DCF to the carotid sheath.

Fig. 3.20.2 Axial CT section at the level of the body of mandible. The posterior cervical space is related anterolaterally to the sternocleidomastoid and posterolaterally to the trapezius muscle which are enveloped by the SL-DCF ( *purple line*) and anteromedially to the carotid sheath. The contents of this space are predominantly fat and lymph nodes.

Fig. 3.20.3 Axial CT sections at the level of the hyoid and cricoid cartilages. The posterior cervical space (PCS) gradually increases in size from the cranial to caudal aspect. It is related anterolaterally to the sternocleidomastoid and posterolaterally to the trapezius muscle which are enveloped by the SL-DCF ( *purple line*). Along the deeper aspect lies the perivertebral space ( *green line*).

Fig. 3.20.4 The PCS is inferiorly limited by the clavicles. The trunks of the brachial plexus emerge at the lateral border of the anterior scalene muscle and divide into the divisions and cords in the PCS before entering the axilla. Note the inferior belly of the omohyoid muscle which traverses the space dividing it into the superior suboccipital and inferior subclavian triangle (surgical parlance).

Fig. 3.20.5 Coronal CT reformat of the neck demonstrating the various cervical lymph node levels. The lymph nodes that lie anterior to the posterior border of the sternocleidomastoid are termed as level II when above the inferior border of the hyoid and level IV when lying beneath the inferior border of the cricoid cartilage. The lymph nodes in between these are termed as level III. Axial CT in the lower neck demonstrates level V which lies posterior to the posterior border of sternocleidomastoid. A line drawn from the posterior border of the anterior scalene muscle to the posterior border of sternocleidomastoid also helps demarcate level V lymph nodes which lie in the PCS.

KEY POINTS FROM ANATOMY

• The predominant masses to affect the PCS are lymph node enlargement, either metastatic or infective.
• The level of lymph node stations involved may harbour clues to the possible site of primary in the case of head and neck cancers.
• Masses that arise within the PCS elevate the sternocleidomastoid and flatten the perivertebral space and displace the carotid space anteromedially.
• The spinal accessory nerve may be damaged or resected during head and neck surgery, with resultant atrophy of the sternocleidomastoid and trapezius.
• The cutaneous branches of the cervical plexus emerge from the posterior aspect of the sternocleidomastoid at the junction of its upper and middle third, a site known as the nerve point of the neck. These can be anaesthetized using a superficial cervical plexus block under ultrasound guidance.
• A greater auricular nerve (cutaneous branch of the cervical plexus) neuroma may be uncommonly encountered on ultrasound following neck dissection.