Carotid space

3.14: Carotid space

Madhavi Kandagaddala, Arunima Patra, Matthew Monachen


The carotid space (CS) is a paired tubular space in the supra and infra-hyoid neck, encased by the carotid sheath. In surgical parlance, it is commonly referred to as the ‘post-styloid or retro-styloid parapharyngeal space’ or simply as the ‘carotid sheath’. Although abnormalities of the CS are less frequently encountered, they represent a formidable diagnostic and treatment challenge. The differentiation of a post-styloid lesion from a pre-styloid lesion is the foremost step in assessing and managing lesions in this region, as the contents of each space differ.The most common lesions in the CS are nerve sheath tumours and paragangliomas, followed by metastatic nodes and pseudo-lesions. This chapter will discuss the imaging approach to these lesions, with an emphasis on clinical relevance.


The nomenclature of the suprahyoid CS has always been controversial. Historically, surgical and anatomic literature has been of the opinion that the suprahyoid CS is the same as the post-styloid parapharyngeal space (PPS). This is based on three observations:

  • The consensus in the anatomic literature that the posterior layers of the carotid sheath and posterior boundary of the PPS are one and the same.
  • The consensus in the surgical literature that on approach to the PPS, the tensor-vascular-styloid fascia serves as a demarcation between the superficial pre-styloid compartment and deeper post-styloid compartment of the PPS.
  • Experimental evidence indicated that above the carotid bifurcation, the carotid sheath did not act like a space that allowed the spread of infection.

Some radiologists suggest that the carotid sheath and its contents (in the suprahyoid neck) can be considered a distinct ‘carotid space’. Although this may not necessarily reflect the underlying anatomy, it is of benefit to the radiologist, and hence the clinician in their ability to narrow the list of differential diagnoses of lesions that are localized to this space on imaging. The only disadvantage is that surgeons are more familiar with a pre-styloid/post or retro-styloid terminology. One acknowledges that what radiologists call the suprahyoid CS and surgeons call the post-styloid PPS are the same space. This chapter uniformly uses the term ‘post or retro-styloid carotid space’ or simply ‘carotid space’ for the suprahyoid and infra-hyoid extension of the carotid sheath and its contents.


Extent and division

The CS extends from the jugular foramen-carotid canal above to the arch of the aorta below, i.e., from the skull base to the superior mediastinum. It is broadly divided into suprahyoid and infra-hyoid segments. The suprahyoid segment is further divided into the nasopharyngeal and oropharyngeal segments. The infra-hyoid segment likewise is split into the cervical and mediastinal segments (Fig. 3.14.1). These four segments are named after the region of the neck/mediastinum in which they lie.

Fig. 3.14.1 Sagittal section along the course of the carotid artery demonstrating the broad divisions of the carotid space – supra hyoid (nasopharyngeal and oropharyngeal segments) and infra-hyoid (cervical and upper mediastinal segments).


The natural boundary of the CS is the carotid sheath which receives a contribution from all three layers of the deep cervical fascia (Fig. 3.14.2). In the infra-hyoid neck, the sheath is more substantial and closely adherent to the vessels it contains. In the suprahyoid neck, however, it is less constant, as the middle layer of the deep cervical fascia of the neck is sometimes deficient, especially on the medial side. The contributions to the carotid sheath are as follows:

  • Anterior margin: stylopharyngeal aponeurosis or tensor-vascular-styloid fascia1 (part of the middle layer of the deep cervical fascia that separates the pre-styloid and post-styloid PPS).
  • Medial margin: cloison sagittale2 (a sagitally oriented fascia that separates the midline deep neck spaces from the lateral ones; it arises from either the middle or deep layer of the deep cervical fascia)
  • Posterior margin: prevertebral fascia and/or alar fascia (from the deep layer of the deep cervical fascia)
  • Lateral margin: fascia of the sternocleidomastoid muscle (from the superficial layer of the deep cervical fascia)

Fig. 3.14.2 Axial schematic image at the level of infra-hyoid neck showing composition of the carotid sheath from all three layers of the deep cervical fascia. Source: (Image courtesy by Dr. Praveen Kumar C.)


As the CS spans the entire length of the neck, it is expected that its relations with the surrounding viscera will be different. The hyoid bone is at the approximate level of the bifurcation of the common carotid artery (Fig. 3.14.3). For convenience sake, the suprahyoid and infra-hyoid segments are considered separately. The relations are tabulated (Table 3.14.1)

Fig. 3.14.3 Axial sections T2-weighted MRI (A) and CT (B) images of the carotid space at the level of the hyoid bone. The common carotid artery has just divided into the external and internal carotid arteries. (1 Hyoid bone, 2 Submandibular gland, 3 External Carotid Artery, 4 Internal Carotid Artery, 5 Internal Jugular Vein, 6 Posterior cervical space, 7 Sternocleidomastoid muscle).

TABLE 3.14.1

Relations of the Carotid Space

Suprahyoid Carotid Space (Figs 3.14.4 and 3.14.5) Infra-Hyoid Carotid Space (Figs 3.14.6 and 3.14.7)
Anterior Masticator space, parapharyngeal space Anterior cervical space
Posterior Peri vertebral space Peri vertebral space
Medial Retropharyngeal space Retropharyngeal and visceral space
Lateral Parotid space Posterior cervical space

Fig. 3.14.4 Axial sections supra-hyoid carotid space at the level of the nasopharynx. T2-weighted MRI (A) shows the relationship of the left carotid space to the surrounding spaces. Brown ghosted image is superimposed on masticator space, yellow ghosted image is superimposed on parotid space, green ghosted image is superimposed on parapharyngeal space, red ghosted image is superimposed on carotid space, orange ghosted image is superimposed on retropharyngeal space, blue ghosted image is superimposed on peri-vertebral space. CT images (B) shows the relationship of the right carotid space (outlined by red line) to the styloid process ( black asterix) and stylohyoid muscle.

Fig. 3.14.5 Axial sections of the supra-hyoid carotid space at the level of the oropharynx. T2-weighted MRI (A) image shows the relationship of the carotid space to surrounding muscles, especially the posterior belly of digastric on its anterolateral aspect. The CT (B) image shows the relationship of the carotid space to the stylohyoid and styloglossus muscles just below the level of the styloid process (1 Masseter, 2 Medial pterygoid, 3 Pharyngeal constrictor muscle, 4 Longus colli muscle, 5 Posterior belly of digastric, 6 Sternocleidomastoid muscle, 7 Obliquus capitis inferior muscle, 8 Stylohyoid, 9 Styloglossus muscle).

Fig. 3.14.6 Axial section T2-weighted MRI (A) through the upper cervical infra-hyoid neck showing the relationship of the left carotid space to the adjacent neck spaces. Red ghosted image is superimposed on left carotid space. Brown ghosted image is superimposed on visceral space, green ghosted image is superimposed on anterior neck space, yellow ghosted image is superimposed on the retropharyngeal space, blue ghosted image is superimposed on peri-vertebral space and pink ghosted image is superimposed on posterior cervical space. The axial CT (B) image at the upper cervical infra-hyoid level shows relationship of carotid space with 1 Sternohyoid muscle, 2 Thyroid cartilage, 4 sternocleidomastoid muscle.

Fig. 3.14.7 Axial section T2-weighted MRI (A) and CT (B) images through the lower cervical infra-hyoid neck showing the relations of the left carotid space (outline in red) Common carotid artery, Internal Jugular Vein, Vertebral artery, Trachea, Lung apex.


The contents of the CS can be easily recalled by remembering the extent of this space. As it extends from the jugular foramen–carotid canal above to the arch of the aorta below, by default it contains the structure exiting the jugular foramen and carotid foramen respectively. Exiting the jugular foramen is the internal jugular vein, the glossopharyngeal, vagal, and spinal accessory nerves. Exiting the carotid foramen is the internal carotid artery (ICA) with its accompanying sympathetic plexus. In addition to this, the hypoglossal nerve exits the adjoining hypoglossal canal and enters the CS just below the skull base. Embedded in the anterior wall of the carotid sheath is the Ansa cervicalis, a bundle of nerve fibres from the first three cervical spinal nerves. It is present only in the suprahyoid neck. The deep cervical lymph nodes accompany the neurovascular bundle.

Given that this space is called the ‘carotid space’, the carotid artery is at the centre of the space with the Jugular vein postero-lateral to it, reflecting the approximate position of their respective foramina on the skull base. The 9, 12 and 11th cranial nerves leave the CS by piercing the anterior wall of the carotid sheath at the level of the soft palate to innervate the tongue and sternocleidomastoid muscle respectively; hence they are absent in the infra-hyoid CS. The vagus nerve that has to reach the mediastinum and beyond, traverses the entire length of the CS in the posterior notch formed between the carotid artery and internal jugular vein. The sympathetic plexus is not covered by the carotid sheath; hence it is not a true content of the CS; however, it courses close to the CS along its posterior and medial aspect (Fig. 3.14.8). To summarize, the contents of the CS are tabulated in Table 3.14.2.

Fig. 3.14.8 Axial schematic representation of the contents of the left carotid space at suprahyoid (A) and infra-hyoid (B) levels. The suprahyoid space contains 9–12 cranial nerves, whereas only 10th nerve continues in the infra-hyoid space. The sympathetic trunk runs just medial to the carotid sheath at both levels.

TABLE 3.14.2

Contents of the Carotid Space

Suprahyoid Neck Infra-Hyoid Neck
Arteries Internal carotid artery Internal carotid arteryExternal carotid artery
Veins Internal jugular vein Internal jugular vein
Nerves Cranial nerves IX, X, XI, XII Cranial nerve X
Nodes Upper deep cervical lymph nodes (level II) Middle and lower deep cervical lymph nodes (levels III and IV)
Sympathetic plexus Not encased by the carotid sheath. It runs posterior and medial to the carotid space None

Lymph nodes – The middle and lower deep cervical lymph nodes (levels III and IV) are associated with, but not a content of the infra-hyoid neck CS. The nodes of the suprahyoid and infra-hyoid CS, as enumerated above, are known as the deep cervical nodes as they lie along the course of the Internal Jugular Vein on either side. Nodal masses can arise at any level in the CS and are part of the host of differential diagnoses of lesions of the CS. Nodal anatomy and pathology are dealt with in a separate chapter elsewhere in this textbook.

Chemoreceptors – A small cluster of chemoreceptors and supporting sustentacular cells, known collectively as the carotid body, is located in the adventitia at the bifurcation of the common carotid artery. It is commonly accepted that it is not seen on routine imaging; however, carotid body tumours can arise from it and splay the internal and external carotid arteries. Glomus vagale arises from similar but distinct chemoreceptor cells located along the inferior ganglion of the vagus nerve. As the vagus nerve is located posteriorly, this tumor displaces the carotid arteries anteriorly without splaying the carotid bifurcation. The skull base chemodectomas arise from similar but distinct chemoreceptor cells situated along the Jacobsons nerve (Glomus tympanicum), Arnold’s nerve (Glomus jugulotympanicum) and Jugular bulb (Glomus Jugulare).


  • Knowledge of the relationship of the cranial nerves to the vessels within the carotid space is extremely important to assess the accurate origin of the pathologies in carotid space.
  • The suprahyoid space contains 9–12 cranial nerves, whereas only the 10th nerve continues in the infra-hyoid space.
  • The sympathetic trunk runs posterior and medial to the carotid sheath.

Imaging approach and imaging modalities

CS lesions arise from the structures within or closely related to the space, namely the carotid artery, internal jugular vein, lower cranial nerves, sympathetic plexus, paraganglion cells and lymph nodes. The most common masses in the CS are nerve sheath tumours and paragangliomas. The other primary CS pathologies include vascular abnormalities such as dissection, aneurysms & thrombosis and lymphadenopathy. In addition, normal vascular variants may mimic space-occupying lesions, and there may be a secondary invasion of the CS by abnormalities in the adjacent spaces. Abnormalities of the CS are enlisted in Table 3.14.3.

TABLE 3.14.3

Abnormalities of Carotid Space

Origin Pathologies

  1. A. Vascular origin

    1. 1. Carotid artery
    2. 2. Internal jugular vein
    3. 3. Paraganglioma

  • Aneurysm/pseudoaneurysm
  • Dissection
  • Thrombosis
  • Ectasia
  • Arteriosclerosis
  • Arteritis
  • Encasement by direct squamous cell carcinoma.
  • Thrombosis/thrombophlebitis
  • Tumour invasion
  • Meningioma (from jugular foramen)
  • Glomus jugulare
  • Glomus vagale
  • Carotid body tumour

  1. B. Neurogenic origin

  • Lower cranial nerve (IX, X, XI, XII) schwannoma
  • Cervical sympathetic trunk schwannoma
  • Lower cranial nerve neurofibroma
  • Perineural tumour spread: from nasopharyngeal carcinoma and non-Hodgkin lymphoma
  • Meningioma

  1. C. Inflammatory/infective

  • Abscess
  • Skull base osteomyelitis

  1. D. Nodes

  • Metastatic nodes from pharyngeal squamous cell carcinoma, minor salivary gland carcinoma, mucoepidermoid carcinoma of the parotid.
  • Lymphoma
  • Extension of retropharyngeal nodes
  • Inflammatory

Imaging approach

The main approach to the lesions arising within the CS is to systematically narrow down the differentials and arrive at an appropriate and definitive diagnosis. It is therefore essential to identify the space of origin of the lesion (prestyloid vs. retrostyloid), followed by its morphology and the pattern of displacement of the vessels.

Step 1: Space of origin of the lesion: Retro-styloid vs. pre-styloid space

Mass lesions originating in the retro-styloid space are closely related to the carotid artery and jugular vein and displace the adjacent structures in different directions depending on the level of the lesion. They displace the PPS fat and ICA anteriorly. In contrast, the pre-styloid tumors displace the PPS fat and ICA posteriorly (Fig. 3.14.9). Also, mass lesions in the retro-styloid space displace the styloid process anterior and lateral, the posterior belly of the digastric muscle and the parotid gland laterally. The retropharyngeal space and anterior compartment of the prevertebral space may also be encroached from a lateral to medial direction.

Fig. 3.14.9 Pre-styloid versus Post-styloid space lesions. The T2W (A) and STIR (B) axial images of the neck demonstrates the anterior displacement of the parapharyngeal space fat (PPS) ( white arrow) and the internal carotid artery (ICA) ( yellow dotted arrows) by the mass lesions originating in the post-styloid space. The pre-styloid tumours displaces the PPS fat and ICA posteriorly ( red dotted arrows).

Step 2: Morphology of the lesion

The next step is to assess the size, solitary or multiple, location within the CS, internal morphology, that is solid or solid-cystic, vascularity, enhancement pattern, intracranial extension and associated bone changes. These features help in an accurate prediction of their histological diagnosis.

Step 3: Displacement of the vascular structures

The final crucial step is to assess the displacement pattern of the vascular structures within the CS. Tumours in the retro-styloid space are closely related to the carotid artery and jugular vein and displace them in different directions depending on the level and the structure of origin, providing clues to the origin of the lesion. Carotid body tumours cause widening and splaying of the internal and external carotid arteries. Tumours arising from the vagus nerve splay out the carotid arteries and jugular vein resulting in anterior and medial displacement of the CCA or ICA and posterior and lateral displacement of the IJV. Tumours arising from the cervical sympathetic chain displace both CCA or ICA, and IJV together anteriorly and laterally (Fig. 3.14.10). Imaging approach of the CS lesions is summarized in the flow chart (Fig. 3.14.11).

Fig. 3.14.10 Patterns of vessel displacement by the tumours in the left carotid space. Vagus nerve is located posterior to and in-between carotid artery and IJV. Vagal tumours such as paragangliomas and schwannomas displace the artery anteromedially and IJV posterolaterally (A). Superior sympathetic chain is located along the medial border of the carotid sheath. Tumour arising from it displaces both the ICA and IJV anterolaterally without splaying (B). Carotid body tumours cause widening and splaying of the internal and external carotid arteries (C).

Fig. 3.14.11 Flow chart.

Goals of imaging of the CS lesions.

Clinical evaluation of CS is quite limited, especially if lesions are small in size or located higher in the neck (close to skull base); hence imaging plays a key role in diagnosis and preoperative assessment and in surveillance. The main goals of imaging are summarized in Table 3.14.4.

TABLE 3.14.4

Goals of Imaging of the Carotid Space Lesions


  1. 1. Localization

  • Pre-styloid or retro-styloid space: based on the direction of displacement of parapharyngeal fat and carotid space contents.
  • Suprahyoid or infra-hyoid compartment

  1. 2. Structure of origin

  • Based on relationships with carotid vessels.
  • The angle of circumferential contact with ICA: for carotid body tumors.

  1. 3. Morphology

  • Shape: round, oval or irregular. Suspect intracranial extension if dumb-bell shaped.
  • Size (three dimensions)
  • Solid vs. cystic/necrotic
  • Margins (smooth contour vs. invasive margins)
  • Enhancement (early or delayed on dynamic imaging)
  • Intra-tumoral tortuous shunts and vessels/flow voids
  • Internal calcification.

  1. 4. Relationship with skull base

  • Distance from skull base
  • Intracranial extension
  • Relationship with jugular foramen/hypoglossal canal
  • Pattern of skull base involvement: permeative destruction favours paraganglioma, hyperostosis favours meningioma
  • Smooth widening of skull base foramina: neurogenic tumours

  1. 5. Adjacent neck spaces

  • Spread into/from adjacent neck spaces: masticator space, parotid space and pharyngeal mucosal space.
  • Involvement of the mandible.

  1. 6. Synchronous lesions

  • Coexisting paragangliomas in bilateral carotid spaces.
  • Visceral metastasis in malignant paragangliomas.

  1. 7. Cervical lymph nodes

  • Number and level
  • Solid vs. necrotic
  • Extracapsular spread
  • Unilateral or bilateral

Role of imaging modalities

Several radiologic imaging and functional imaging techniques are available to evaluate CS lesions. The cross-sectional modalities are the primary anatomic imaging to identify their nature and stage these lesions. Isotope imaging is used to assess multiplicity and metastasis. Dynamic angiography is seldom performed, but it is valuable preoperatively for surgical planning and often for preoperative tumour embolization.

Ultrasound (USG)

Ultrasound neck is the first-line of examination for the investigation of neck swellings, particularly more useful in nonpulsatile lesions. A high frequency (above 7.5 MHz) linear transducer is ideal for assessing the CS lesions and their relation to the neck vessels. Greyscale ultrasound is used to differentiate between solid versus cystic lesion, delineation of tumour margins, its size and location. Doppler examination is useful for assessing lesion vascularity and helps differentiate paragangliomas from nodal metastasis, vessel thrombosis, vascular anomalies, and aneurysm. The role of ultrasound is restricted to lesions in infra-hyoid neck space. It has limited value in poorly accessible lesions located higher in the neck, close to the skull base. It is also of limited value for accurate assessment of the extent and origin in larger lesions and the evaluation of synchronous lesions.

Ultrasound-guided transcervical fine needle aspiration (FNA) cytology or biopsy.

Based on the clinical and imaging information, a correct preoperative diagnosis can be made in 90%–95% of patients with a CS mass. Guided biopsy/FNAC is safe if meticulously done, and may be performed when imaging findings are atypical for paragangliomas or nerve sheath tumours and to confirm nodal metastasis.

Computed tomography (CT)

MRI with its high soft tissue resolution and non-ionizing nature is considered the imaging modality of choice for head & neck masses. However, long procedure time, costs, and limited availability in developing countries often make CT the imaging modality of choice, despite the use of ionizing radiation. Thin sections (2.5–3 mm) are acquired after contrast administration, from skull base to thoracic inlet in the axial plane and reformatted into coronal and sagittal sections. CT accurately and quickly assesses the location, extent, morphology, intra-lesional calcifications and bony destruction.

Dynamic CT.

The rate of contrast enhancement on dynamic CT (early filling and late equilibration phases) helps to differentiate paragangliomas from nerve sheath tumours. After a bolusi.v.injection of 100 mL of iodinated contrast at the rate of 1 mL/s, initial scan is performed within the first 10–20 seconds for the vascular filling phase, followed by repeat scanning for the equilibration phase at 60 s. Examination is best performed with 2.5 mm thick contiguous slices and a small field-of-view covering the region of interest. Attenuation versus time graphs depicting the sequential enhancement patterns of the tumors can also be plotted.

Nerve sheath tumours have a paucity of veins and show delayed enhancement due to gradual accumulation of contrast in their extracellular space. On the other hand, paragangliomas show early enhancement due to rapid intravascular accumulation of contrast. Acquisition of routine CT only in the late phase will show a higher degree of enhancement in the otherwise hypo vascular neurogenic tumours, giving a false impression of paragangliomas. Therefore, the distinction between these two lesions based only on the degree of enhancement is difficult on the routine contrast-enhanced CT.

CT angiography contributes to preoperative vascular assessment by visualizing the anatomical relations with the ICA, ipsilateral and contralateral venous return as well as tumours at other possible sites.

Magnetic resonance imaging (MRI)

MRI is the imaging modality of choice for characterization of CS lesions due to its superior contrast and soft-tissue resolution. It provides a more reliable assessment of the relation of the CS tumours with the vessels, relationship with the skull base, intracranial extension and detection of small tumours at other possible sites.

MR protocol in the assessment of CS lesions is the multi-planar acquisition of the images in thin sections (3 mm) from skull base to thoracic inlet. The sequences acquired are T1-weighted (T1W), T2-weighted (T2W), Short Tau Inversion Recovery (STIR) and T1W fat-suppressed sequences, post-gadolinium fat-suppressed T1W sequence, and diffusion-weighted imaging.

Conventional T1W and T2W sequences allow better delineation of the contour of a mass compared to CT. Smooth contours can be useful to differentiate benign lesions from the invasive margins of malignant paragangliomas or a vascular metastasis with extra-capsular spread. Infiltrative margins, tumour spread into adjacent spaces or low T2W signal can predict malignancy, and diffusion-weighted imaging further helps in differentiating benign from malignant lesions.

Moderate-to large-calibre, rapid flow vessels associated with paragangliomas are easily identified by their flow voids, using flow-sensitive sequences or by MR angiography (phase contrast, 3D TOF or gadolinium-enhanced).

Conventional angiography

Non-invasive CT and MR angiogram have replaced conventional arteriography as the primary diagnostic tool for tumour vascularity. Hence, conventional arteriography is no longer indicated for the diagnosis of paraganglioma, but is reserved for preoperative endovascular embolization to minimize intraoperative blood loss or for preoperative balloon occlusion test. It may show dilated feeder arteries and rapid venous return. Although it may be less often used for carotid body tumour because of concern of reflux of embolization material into the ICA, but is performed much more frequently for very large paragangliomas of the vagus nerve or lesions close to the skull base. Conventional angiography for carotid body tumours demonstrates the ‘Lyre sign’ due to splaying of the internal and external carotids arteries.

Radionuclide imaging

It is complementary to radiological imaging for paragangliomas and provides specific information about local staging, multifocal lesions, metastasis and assessment of post-treatment residual or recurrent lesions. This is due to expression of specific somatostatin receptors by the paragangliomas, which other neck lesions such as nerve sheath tumours lack.

Paragangliomas are somatostatin receptor (SSR-2) expressing neuroendocrine tumours. 68Ga-Labeled Somatostatin Analogue PET-CT is the radiotracer of choice, with the highest sensitivity (close to 100%) for the detection of small lesions and multifocal forms not visible on morphological imaging. It is superior to other radiotracers, especially for multiple small-sized lesions.

In absence of available 68Ga-DOTA PET, a combination of 18F-FDOPA and 18F-FDG PET/CT is recommended, especially for patients with multifocal tumours. There is a promising role for using 177Lu-DOTATATE for both diagnosis and highly targeted radionuclide therapy of these tumours.


Mar 25, 2024 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Carotid space

Full access? Get Clinical Tree

Get Clinical Tree app for offline access