Cross-sectional imaging plays an important role in the evaluation of the retropharyngeal space (RPS) and the prevertebral space (PVS). Because of their deep location within the neck, lesions arising within these spaces are difficult, if not impossible, to evaluate on clinical examination. This article details the cross-sectional anatomy and imaging appearances of primary and secondary diseases involving the RPS and PVS, including metastasis and spread from adjacent spaces. The role of image-guided biopsy is also discussed.
Key Points
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Because of the deep location of the retropharyngeal space and prevertebral space within the neck, lesions arising within these spaces are difficult, if not impossible, to evaluate on clinical examination.
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Cross-sectional imaging plays an important role in the evaluation of the retropharyngeal space and prevertebral space and consists of various modalities: plain radiography, fluoroscopy, multidetector computed tomography, magnetic resonance imaging, ultrasonography, and positron emission tomography/computed tomography.
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Knowledge of the normal anatomy of these spaces, the common lesions affecting them, and the imaging and biopsy techniques used to evaluate such lesions will aid the head and neck surgeon who encounters them in clinical practice.
Introduction
The retropharyngeal space (RPS) extends from the skull base to the upper mediastinum, and the prevertebral space (PVS) extends from the skull base to the coccyx. Diseases of these spaces are uncommon but can result in significant morbidity. As these lesions are inaccessible to clinical inspection, cross-sectional imaging plays an important role in the evaluation of the RPS and PVS and consists of various modalities. Diseases in the RPS and PVS include primary tumors, direct spread of tumors from adjacent spaces, metastasis, congenital/developmental lesions, inflammation, and infection ( Table 1 ).
| Primary lesions |
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| Direct spread |
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| Nodal metastasis |
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| Other |
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Imaging techniques
The RPS and PVS can be evaluated by plain radiography, fluoroscopy, multidetector computed tomography (MDCT), magnetic resonance imaging (MRI), ultrasonography, and positron emission tomography/computed tomography (PET/CT). Plain radiographs have been replaced, for the most part, by the more advanced modalities of MDCT and MRI because these provide a more comprehensive evaluation of the neck.
Plain radiographs obtained in anteroposterior and lateral projections may be used to detect radio-opaque foreign bodies and, in the lateral plane, to assess for thickening of the PVS soft tissues ( Fig. 1 ). On plain radiography, a PVS thickness of less than 6 mm at the level of C3 is considered normal in children ; in adults, a PVS of less than 6 mm at C2 and less than 22 mm at C6 is within normal limits. The differential diagnosis of a widened PVS includes edema, hematoma, abscess, tumors, and surgical reconstruction.
Fluoroscopic evaluation of the upper aerodigestive tract after ingestion of contrast material can reveal defects in the posterior pharyngeal wall or fistulous tracts ( Fig. 2 ) and also be used to assess the swallowing function.
MDCT provides images in the axial plane at a 1.25-mm thickness. Computer manipulation allows reconstruction of the images in any plane deemed necessary. Different window settings can highlight either the soft tissues or the bony structures of the head and neck, including the skull base and vertebral column. MRI acquires images in axial, coronal, or sagittal planes and, because of better contrast resolution between various tissues, can aid in identifying subtle changes, such as perineural tumor spread. Commonly used MR sequences include T1 weighted without and with contrast, which delineate the anatomy and pathologic conditions; the T2-weighted sequences show the water content of a region and is helpful in the differentiation between various lesions. The administration of intravenous contrast is important for both MDCT and MRI to determine the extent of disease, which is important for staging and treatment planning.
The RPS may be evaluated with ultrasonography by placing a transducer in the oral cavity and oropharynx; this technique is used for fine-needle aspiration (FNA) biopsy to evaluate RPS nodes. PET/CT allows the detection of disease in the RPS and PVS as well as distant disease spread throughout the body ( Fig. 3 ). Lesions with increased glucose use, such as tumors, take up fluorodeoxyglucose (FDG) when imaged with PET. Because PET images do not provide adequate anatomic detail, they are fused with a CT scan for lesion localization (ie, the PET/CT study [see Fig. 3 A]).
Anatomy
The RPS extends from the clivus to the upper mediastinum, lies posterior to the pharynx and esophagus, and is anterior to the prevertebral musculature. It is bounded by the buccopharyngeal fascia anteriorly, the prevertebral fascia posteriorly, and the carotid space laterally ( Fig. 4 A). The very thin alar fascia, a part of the deep layer of the deep cervical fascia, extends from the medial border of the carotid space on either side and divides the RPS into 2 components: the anteriorly positioned true or proper RPS and the posteriorly situated danger space. The true RPS extends from the clivus inferiorly to a variable level between the T1 and T6 vertebrae where the alar fascia fuses with the visceral fascia to obliterate the true RPS. The danger space extends further inferiorly into the posterior mediastinum to the level of the diaphragm and is named as such because it provides a conduit for spread of infection from the pharynx to the mediastinum (see Fig. 4 B).
The RPS can be further divided into the suprahyoid and infrahyoid RPS, each with different contents. The suprahyoid RPS contains fat and lymph nodes, whereas the infrahyoid RPS contains only fat and, thus, can be involved only by non-nodal disease. The suprahyoid retropharyngeal nodes lie medial to the medial aspect of the internal carotid artery at the level of the transverse process of the atlas and are divided into medial and lateral groups. The medial group of nodes is not consistently present and is located anterior to the medial parts of the longus colli muscles. The lateral group, also known as the nodes of Rouvière, lies ventral to the longus colli muscles. Retropharyngeal nodes are normally present in children and then atrophy in puberty. Small nodes may be present in two-thirds of asymptomatic adults and, when present, can be visible on imaging. Normal retropharyngeal nodes should be less than 1 cm in diameter.
The PVS is situated between the prevertebral fascia anteriorly and the vertebral bodies posteriorly (see Fig. 4 ); the contents include the prevertebral muscles and fat. Controversy exists in the literature regarding the lateral aspect of the PVS and whether or not the contents of a paravertebral space (ie, the scalene muscles, levator scapulae muscle, splenius capitis and splenius cervicis muscles, brachial plexus nerve roots, and vertebral artery and vein) should be included with the PVS to form a combined perivertebral space. Because the deep layer of the deep cervical fascia attaches to the transverse process for the cervical vertebrae, separating the PVS and the paravertebral space, the term PVS ( pre vertebral space) is used in this article for identifying the region between the carotid sheaths laterally, the prevertebral fascia anteriorly, and the vertebral bodies posteriorly.
Primary tumors of the RPS and PVS
Primary tumors of the RPS are extremely uncommon. Lipoma ( Fig. 5 ) is the most common primary neoplasm. Lipomas of the RPS appear elliptical on axial sections, conforming to the shape of the RPS. A lipoma may be differentiated from air ( Fig. 6 ) on CT by use of the appropriate window width and window level settings because air is less dense (more black) than fat (as measured on CT with Hounsfield units). On MRI, fat has a hyperintense appearance on T1-weighted images and is suppressed with fat saturation techniques, often used in postcontrast T-weighted sequences.
