Structure
The neck has a superficial fascia and a deep cervical fascia. The superficial fascia is subcutaneous fatty tissue containing platysma, the external and anterior jugular veins, cutaneous nerves and superficial lymph nodes. Surgical flaps in the neck are raised in the plane deep to platysma.
The deep cervical fascia has three layers and a condensation around the great vessels.
| Layer | Encloses | Upper attachment | Lower extent |
|---|---|---|---|
| Investing | Sternocleidomastoid, trapezius, parotid and submandibular glands | Superior nuchal line, mastoid, zygomatic arch, lower border of mandible, hyoid | Manubrium, clavicle, acromion, spine of scapula |
| Pretracheal | Infrahyoid muscles (muscular part); thyroid, trachea, oesophagus (visceral part) | Hyoid and thyroid cartilage; buccopharyngeal part to skull base | Fibrous pericardium in the superior mediastinum |
| Prevertebral | Vertebral column, prevertebral and scalene muscles, deep muscles of the back of the neck | Base of skull | Upper thoracic vertebrae, blending with the anterior longitudinal ligament |
| Carotid sheath | Carotid arteries, internal jugular vein, vagus nerve | Base of skull around the carotid canal and jugular foramen | Root of the neck |
The layers in detail
Investing layer
The investing layer encircles the whole neck like a collar deep to platysma. It splits to enclose sternocleidomastoid and trapezius and forms the roof of both the anterior and posterior triangles. Above the manubrium it splits into two sheets attached to the front and back of the jugular notch, enclosing the suprasternal space, which contains the jugular venous arch. Between the parotid and the mandible a thickening forms the stylomandibular ligament.
Pretracheal layer
The pretracheal layer lies in the front of the neck only. Its muscular part surrounds the infrahyoid muscles. Its visceral part surrounds the thyroid gland, trachea and oesophagus, and continues behind the pharynx as the buccopharyngeal fascia. The thyroid sheath is attached to the larynx, which is why the thyroid moves upwards on swallowing.
Prevertebral layer
The prevertebral layer wraps the vertebral column and the muscles attached to it: longus colli, longus capitis, the scalenes, levator scapulae and the deep back muscles. It forms the floor of the posterior triangle. The cervical and brachial plexus roots and the phrenic nerve lie deep to it. As the brachial plexus and subclavian artery pass into the axilla they carry a sleeve of this fascia with them, the axillary sheath.
Carotid sheath
The carotid sheath is a tube of fascia formed by contributions from all three layers. It contains the common and internal carotid arteries medially, the internal jugular vein laterally and the vagus nerve between and behind them. Deep cervical lymph nodes lie along it, the ansa cervicalis lies in its anterior wall, and the cervical sympathetic trunk lies behind it on the prevertebral fascia.
Spaces of the neck
The potential spaces between the fascial layers decide how infection spreads. A thin alar fascia, often described as part of the prevertebral layer, lies between the buccopharyngeal and prevertebral fascia and divides the space behind the pharynx into two.
- Retropharyngeal space: between the buccopharyngeal fascia in front and the alar fascia behind. It runs from the skull base to the upper thorax, where the alar and visceral fascia fuse, and contains the retropharyngeal lymph nodes.
- Danger space: between the alar and prevertebral fascia. It runs from the skull base to the diaphragm, giving infection a direct route into the posterior mediastinum.
- Prevertebral space: deep to the prevertebral fascia, extending along the length of the vertebral column.
- Pretracheal space: within the visceral compartment in front of the trachea, opening into the anterior mediastinum.
- Parapharyngeal space: an inverted cone lateral to the pharynx, from the skull base to the hyoid, communicating with the retropharyngeal and submandibular spaces.
Clinical relevance
Neck infections follow the fascial planes. A retropharyngeal abscess is most common in young children, whose retropharyngeal nodes suppurate after upper respiratory infection; it presents with fever, neck stiffness, drooling and a bulge of the posterior pharyngeal wall. Infection in the danger space or pretracheal space can cause descending necrotising mediastinitis, which carries a high mortality.
Infection deep to the prevertebral fascia, classically tuberculosis of the cervical spine, is confined in the midline and bulges forwards behind the pharynx, or tracks laterally to point in the posterior triangle or along the axillary sheath into the axilla.
Other consequences of the fascial anatomy:
- Thyroid and laryngeal swellings move on swallowing because the pretracheal fascia binds the thyroid to the larynx.
- The investing layer limits swelling, so bleeding or oedema deep to it can compress the airway after thyroid surgery.
- A brachial plexus block in the interscalene or axillary approach relies on local anaesthetic spreading within the prevertebral fascia and axillary sheath.