Video: Compartments of the neck

Unless you have an unusually long neck and got teased about it at school, you've probably not thought about this part of your body for more than two seconds especially its compartments. But clinically, compartments of the neck are very important because they provide potential routes of infection between the head and the rest of the body. In this tutorial, we will be discussing the cervical fascia and the compartments of the neck.

For this tutorial on compartments of the neck, we'll be looking at this transverse section through the neck. We'll begin by identifying different layers of fascia in the neck specifically the superficial layer and the deep layer which is further subdivided into the superficial, middle, and deep layers with the carotid sheath sometimes identified as a separate division. For easier reference, the superficial layer of cervical fascia is also simply called the superficial cervical fascia while the deep layer is commonly referred to as the deep cervical fascia. We will take a look at the subdivisions and compartments each of these layers form.

We'll start with the superficial layer of cervical fascia which is actually the subcutaneous tissue of the neck. Simple as it is, it'll allow us to jump right into the subdivisions of the deep layer of the cervical fascia which are the superficial, middle, and deep layers. First, there is the superficial layer of deep cervical fascia which is associated with the muscles immediately deep to the skin. We'll then move on to the middle layer of the deep cervical fascia which is subdivided into the strap muscle fascia, the visceral fascia, the buccopharyngeal fascia forming the visceral compartment, and the retropharyngeal space. We'll then tackle the deep layer of the deep cervical fascia with special emphasis on one of its parts called the alar fascia which contributes to the bounding of two cervical spaces – the danger space and the vertebral compartment. We'll finally tackle the carotid sheath and the vascular compartment it forms. Finally, we will, of course, finish up with some clinical notes.

Before we get into the juicy stuff, let's take a general overview of the cross-section that we're looking at today.

We're used to looking at anatomical structures from the anterior, posterior, lateral, or medial aspect so working with the cross-section can feel a bit unfamiliar. Let's spend a minute familiarizing ourselves with the image we'll be working with today. First up, bang in the middle, we have a cervical vertebra. Even if we didn't already know we were looking at the cervical region, we can tell that this is a cervical vertebra because of its typical bifid spine and the grooves for spinal nerves on the transverse processes.

The cervical vertebra is also a good indicator of the orientation of this transverse section. We know that the vertebral spine projects posteriorly and the vertebral body faces anteriorly which means the anterior aspect of the neck is at the top of the image and the posterior aspect is at the bottom. The last thing we need to bear in mind is that we're looking at this section from the bottom up so the left side of the image actually represents the right side of the body and vice versa. Finally, we need to figure out which vertebral level we're looking at.

In this section, we have the trachea. The trachea is immediately inferior to the larynx which terminates at around the C6 vertebral level. We can also see the thyroid gland. The isthmus connecting the two lobes of the thyroid gland is visible. The isthmus is normally located just below the thyroid cartilage of the larynx which means we're looking at the C5 or C6 vertebral level. Mystery solved!

Before we discuss individual compartments, we need to define what a compartment actually is. A compartment is a section of the body bounded by fascia. It may contain any combination of muscles, nerves, vessels, viscera, and adipose tissue. Fascia refers to a band or sheet of connective tissue normally consisting of collagen which separates organs or muscles. In the neck, the fascia is referred to as cervical fascia.

Alright, now that we've covered a few definitions and orientated ourselves, let's go to the organization of the cervical fascial layers. We need to understand the general organization of fascia in the neck before we dive into any specifics. There are two main divisions – the superficial and deep cervical fascia.

The superficial cervical fascia or, perhaps more accurately, the subcutaneous cervical tissue is exactly what it says on the tin. It is the most superficial layer of fascia immediately deep to the dermis of the skin. This layer of fascia is not a sheet but rather a zone of loose connective tissue which often contains a lot of adipose tissue especially in females. The superficial cervical fascia encircles the platysma muscle.

The rest of the fascia in the neck is classed as deep cervical fascia. The deep cervical fascia is further subdivided into superficial or investing layer of deep cervical fascia surrounding the trapezius and sternocleidomastoid muscles; the middle layers of the deep cervical fascia surrounding the structures anterior to the cervical vertebrae; and the deep layers of deep cervical fascia which encircle the structures lateral and posterior to the cervical vertebrae as well as the vertebrae themselves. The carotid sheath is also part of the deep cervical fascia and we'll see why you might want to talk about it separately towards the end of this tutorial. So let's jump in and start with the superficial layer of the deep cervical fascia.

The superficial layer of the deep cervical fascia surrounds all structures of the neck deep to the skin and subcutaneous cervical tissue. It creates a sheath around the trapezius and sternocleidomastoid muscles. Unlike the other two layers of the deep cervical fascia, this layer does not contribute to any of the compartments of the neck. At a few vertebral levels superior to our cross-section, the superficial layer encloses the submandibular gland and forms the fibrous capsule of the parotid gland.

Superiorly, the superficial layer of the deep cervical fascia attaches to the superior nuchal lines of the occipital bone, the mastoid processes of the temporal bone, the zygomatic arches, the inferior border of the mandible, and the hyoid bone. Inferiorly, its attachments are the manubrium, the clavicles, and the acromions and the spines of the scapulae. Posteriorly, the superficial layer attaches to the spinous processes of the cervical vertebrae.

Now that we've covered the superficial layers of fascia which do not contribute to any neck compartments, let's move on to the middle layer of the deep cervical fascia and the compartments of the neck that it forms. The middle layer of the deep cervical fascia, in some sources known as pretracheal fascia, surrounds the structures in the anterior portion of the neck. It is subdivided into strap muscle fascia and visceral fascia. On occasion, you may encounter the term pretracheal fascia only for the small part of the visceral fascia overlying the anterior aspect of the trachea.

The strap muscle fascia, also known simply as the muscular layer, encloses four pairs of muscles. These include the sternohyoid, the sternothyroid, the thyrohyoid, and the omohyoid muscles, which are collectively known as infrahyoid or strap muscles. We can't see the thyrohyoid muscle in our cross-section because it is located higher up in the neck at around the C3 to C4 vertebral level.

The visceral fascia is located deep to the strap muscle fascia. Superiorly, it attaches to the base of the skull posteriorly and the hyoid bone anteriorly and laterally. Inferiorly, the visceral fascia fuses with the fibrous pericardium in the mediastinum. The posterior aspect of the visceral fascia overlying the pharynx and esophagus which is the part of the upper digestive tract that we see in our image is continuous with the buccopharyngeal fascia. Superiorly, it extends over the buccinator muscle. In some sources, you may encounter the term buccopharyngeal fascia being used as the name for the whole of the visceral fascia.

The visceral fascia forms the visceral compartment of the neck and creates a sheath of varying thickness around the various organs within it. Quite helpfully, the name of this compartment tells you exactly what's in it – the viscera or organs of the neck. Luckily, there aren't many organs in the neck which means less memorizing for you.

In this cross-section, we can see that the visceral compartment contains the trachea, the esophagus, the thyroid gland, and the recurrent laryngeal nerves. Structures enclosed in the visceral compartment that we can't see at this vertebral level are the larynx and the pharynx. Bordered by the buccopharyngeal fascia which we've already seen anteriorly; the vascular compartments laterally, and the alar fascia, both of which we'll learn about in a bit; posteriorly is the retropharyngeal space. Superiorly, it is limited by the base of the skull and inferiorly extends to the superior mediastinum. Here it is limited by the fusion of the alar fascia to the buccopharyngeal fascia between the C6 and T4 vertebral levels. The retropharyngeal space is filled with loose connective or areolar tissue. It contains the retropharyngeal lymph nodes and the sympathetic trunks.

That concludes the section on the middle layer of deep cervical fascia. So let's move on to the last division – the deep layer of the deep cervical fascia and its associated compartments.

The deep layer of the deep cervical fascia, also known as the prevertebral fascia or sometimes the perivertebral fascia, is a tubular sheet of connective tissue which forms the largest compartment of the neck – the vertebral compartment. Within the vertebral compartment, another subdivision can be identified. The segment of the deep layer of the deep cervical fascia anterior to the vertebral body is known as the alar fascia. To complicate things further, we have some terminology variation here too.

The term prevertebral fascia which we encountered earlier describing the whole vertebral compartment is also commonly used to describe the deep layer minus the alar fascia. The prevertebral fascia attaches to the skull base superiorly. Inferiorly, it blends with the endothoracic fascia which is the thin layer of fascia on the inner surface of the thoracic wall separating it from the pleura. Inferiorly, it also attaches to the anterior longitudinal ligament at around T3.

The alar fascia is a sheet of connective tissue between the visceral and vertebral compartments. It is separated from the pharynx or esophagus and visceral fascia by the retropharyngeal space anteriorly and from the vertebral bodies by the danger space posteriorly. Superiorly, it is attached to the base of the skull and normally extends inferiorly to the level of C7 but it can range between C6 and T4 vertebral levels. There it blends with the visceral fascia of the middle layer. Laterally, it is attached to the transverse processes of cervical vertebrae.

Although danger space sounds like the name of a Tom Cruise film, in reality, it's a lot less exciting. Also sometimes known as the alar space, the danger space is just a potential space in the neck. Its name indicates the danger of potential infection spreading through the space. It is bound by the alar fascia anteriorly and the prevertebral fascia which covers the vertebral body and longus colli muscle posteriorly. It contains – well, nothing really. However, it is not empty. Remember, it is a potential space. It's filled with loose connective tissue but unlike most of the other spaces of the body, the danger space does not contain any muscles, viscera, nerves, or vasculature. It extends from the skull base down into the posterior mediastinum to the level of the diaphragm.

Now there's a slight complication regarding the danger space. The danger space cannot be distinguished from the retropharyngeal space using medical imaging and so they are sometimes considered a single entity.

We're now moving on to another structure – this one that you can see now highlighted in green known as the vertebral compartment. The vertebral compartment is the main compartment formed by the deep layer of the deep cervical fascia or the prevertebral fascia. The name of this compartment is hardly creative but it gives you a good idea what you should expect to find in it. The vertebral compartment contains the cervical vertebrae, the muscles of the vertebral column which surround it, the phrenic nerves, the cervical spinal cord, and the cervical spinal nerve roots.

Sometimes there are two divisions identified within the vertebral compartment – prevertebral and perivertebral. However, the perivertebral division is sometimes also known as the paraspinal division. In addition, the contents of each space vary from source to source so here we will consider the vertebral compartment as a whole. To make things a little easier, we'll look at the muscles anterior, lateral, and posterior to the cervical vertebrae separately.

Anterior to the vertebrae, we find the prevertebral muscles. These muscles include the longus colli muscle which we can see in our transverse section. Other anterior cervical muscles are the longus capitis, the rectus capitis anterior muscles, and the rectus capitis lateralis muscles – all of which are found at the higher vertebral levels, and are therefore, not visible in our section.

The muscles lateral to the vertebrae are the scalenes. We have the anterior scalene, the middle scalene, and the posterior scalene muscles. The latter could be considered as part of the posterior cervical muscles but may be easier to remember if you think of all the scalenes together. The phrenic nerves also contained in the vertebral compartment sit anterior to the anterior scalene muscles.

The posterior muscles in the vertebral compartment are essentially all of the deep cervical muscles with the addition of the levator scapulae muscles. The deep cervical muscles include the splenius cervicis, the splenius capitis, the semispinalis cervicis, the semispinalis capitis, and the multifidus.

Let's now take a look at the last component of the deep cervical fascia – the carotid sheath.

The carotid sheath is a paired tube formed of fibrous connective tissue. It is considered part of the deep cervical fascia; however, it is unclear whether it contains contributions of all three divisions of the deep layer or if it should be grouped with just the deep or just with the superficial layer. The carotid sheath surrounds the vascular compartment which is a bilateral compartment located posterolateral to the visceral compartment. In our cross-section, we can see that each vascular compartment only contains a vagus nerve, the common carotid artery, and internal jugular vein. Structures in the vascular compartment that you can't see are the internal carotid artery which is more superior than the level of our cross-section, some deep cervical lymph nodes, the carotid sinus nerve, and the carotid periarterial plexuses which contain sympathetic nerve fibers. Superiorly, the carotid sheath attaches to the skull base and inferiorly extends down to the root of the neck.

And that wraps up all the anatomy we had to cover in this tutorial. Now let's see an example of how the compartments of the neck are important medically by looking at retropharyngeal abscesses.

An abscess is a buildup of pus, in this case, in the retropharyngeal space. It is relatively uncommon and therefore may not receive an early diagnosis. It is caused by infections in the upper respiratory tract, the molars, and the middle ear, or sometimes an injury. The symptoms include a stiff neck, pain, fever, difficulty swallowing, and enlarged cervical lymph nodes.

As this is deep in the neck, a physical examination may not be enough for a diagnosis so a CT scan or an x-ray will often be used to confirm. The treatment normally includes surgical intervention to remove the abscess and administration of antibiotics. Now the reason why this kind of abscess is so dangerous is because if left untreated, the infection can spread and eventually lead to death.

We're now looking at a representation of the danger space. An abscess in the retropharyngeal space could easily penetrate the walls of the danger space and make its way down into the mediastinum. Here it could cause mediastinitis, inflammation of the mediastinum. Mediastinitis, if left untreated, can lead to sepsis and multiple organ failure. If physicians were unaware of the compartments of the neck and therefore potential infection pathways, they would never be able to make the connection.

Alright, now that we've covered all the aspects of this topic, let's summarize what we learned today.

We started by looking at the most superficial layer of the cervical fascia, the subcutaneous cervical tissue, which was just a layer of fat below the skin and we saw that it envelops the platysma muscle. Next, we talked about the deep cervical fascia which forms all the compartments in the neck. The superficial layer of the deep cervical tissue is deep to the subcutaneous cervical tissue and forms sheaths for the trapezius and the sternocleidomastoid muscles.

The middle layer of the deep cervical fascia was further subdivided into strap muscle fascia which unsurprisingly surrounds the strap muscles and the visceral fascia which forms the visceral compartment of the neck. The visceral compartment contains the trachea, the esophagus, the thyroid gland, the recurrent laryngeal nerves, the larynx, and the pharynx. The fascia posterior to the pharynx is known as the buccopharyngeal fascia.

We then moved on to the retropharyngeal space which is bordered by the buccopharyngeal fascia anteriorly and the alar fascia posteriorly. It contains areolar tissue, retropharyngeal lymph nodes, and the sympathetic trunks. We then looked at the deep layer of the deep cervical fascia which forms the vertebral compartment and the danger space.

Anteriorly bound by the alar fascia, the danger space contains no more than areolar tissue. The vertebral compartment, in contrast, is the largest compartment of the neck containing numerous muscles. We identified the prevertebral muscles, the longus colli, the longus capitis, the rectus capitis anterior, and the rectus capitis lateralis anterior to the vertebra. Laterally, we saw the scalenes – the anterior, the middle, and the posterior scalenes – with the phrenic nerve running anterior to the anterior scalene.

Posterior to the vertebra, we found the levator scapulae and the deep cervical muscles. These include the splenius cervicis, the splenius capitis, the semispinalis cervicis, the semispinalis capitis, and the multifidus. Finally, we saw the paired vascular compartments located posterolateral to the visceral compartment. Enclosed by the carotid sheath, this compartment contains the vagus nerve, the common and internal carotid arteries, the internal jugular vein, the deep cervical lymph node, the carotid sinus nerve, and the carotid periarterial plexuses. And we finished up by looking at the retropharyngeal abscess and a possible route for the infection to spread into the mediastinum.

Alright, that's the end of the tutorial. We hope you learned a lot today and we look forward to seeing you in our other tutorials. Hope you enjoyed it and happy studying.