The authors present imaging anatomy of the parapharyngeal space. Imaging approach is discussed in terms of the effect it has on differential diagnosis and diagnosis by the otolaryngologist. Neoplastic and congenital lesions are discussed along with other neck diseases occurring in this space.
Key Points
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The division of the parapharyngeal space into the prestyloid and retrostyloid spaces, as divided by the tensor–vascular–styloid fascia, is significant to the surgical approach of head and neck surgeons, because this fascial plane acts as a landmark to the great vessels and cranial nerves located just deep to this plane in the carotid sheath.
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Given the significantly varied imaging differential diagnoses of the parapharyngeal and carotid spaces, it is advantageous in imaging to approach these spaces as separate imaging entities to increase the precision of diagnoses.
Normal imaging anatomy of the parapharyngeal space
The parapharyngeal space is a largely fat-filled spaced in the suprahyoid neck that has been variably described by anatomists, head and neck surgeons, and radiologists. The bilateral parapharyngeal spaces are regions in neck that have historically been subdivided by surgeons into prestyloid and retrostyloid compartments. The more anterior prestyloid compartment lies deep to the masticator space and lateral to the pharyngeal mucosa, with the deep lobe of the parotid extending into its lateral aspect. The posterior retrostyloid compartment corresponds with the carotid sheath and its components, and has more recently been termed the carotid space. The craniocaudal extent of the parapharyngeal space runs from the skull base to the angle of the mandible.
The division of the parapharyngeal space into the prestyloid and retrostyloid spaces, as divided by the tensor–vascular–styloid fascia, is significant to the surgical approach of head and neck surgeons, because this fascial plane acts as a landmark to the great vessels and cranial nerves located just deep to this plane in the carotid sheath. However, from an imaging perspective, we along with other authors favor separating the parapharyngeal space from the carotid space given the significantly varied imaging differential diagnoses of these 2 regions. We approach these spaces as separate imaging entities. This point is worth emphasizing because, to a large degree, the existing literature on the “parapharyngeal space” includes more general and broad definition of this space, which includes portions of the deep lobe of the parotid gland (an obvious component of the parotid space) and carotid space. In the era of cross-sectional imaging, we are able to reliably separate these spaces and will do for the purpose of this review to increase the precision of image-specific differential diagnoses.
Defined as such, we consider the parapharyngeal space as bilateral, crescent-shaped, fat-filled regions extending from the skull base to the hyoid bone ( Fig. 1 ). The parapharyngeal space contacts the skull base in a triangular region along the inferior surface of the petrous temporal bone; there are no significant bony foramina. Inferiorly, the parapharyngeal space fat is contiguous with the fat of the posterior margin of the submandibular space. The incomplete fascial planes surrounding the parapharyngeal space are complex and include the middle layer of the deep cervical fascia separating the parapharyngeal space from the medially located pharyngeal mucosal space, the medial slip of the superficial layer of the deep cervical fascia separating the parapharyngeal space from the masticator and parotid spaces, and the combined fascia of the carotid sheath and the deep layer of the deep cervical fascia separating the parapharyngeal space from the carotid and retropharyngeal spaces posteriorly.
The parapharyngeal space is of particular interest to head and neck radiologists given its high degree of conspicuity. Fat is easily delineated from other soft tissues on both magnetic resonance imaging (MRI) and computed tomography (CT), and the parapharyngeal space is accordingly relatively easily identified, even with significant displacement and distortion by mass effect arising from lesions in the bordering spaces of the neck. Although the frequency of lesions primary to the parapharyngeal space is relatively rare (and discussed in depth below), the importance of the parapharyngeal space as an imaging sign revealing the site of origin of adjacent neck pathology cannot be overstated. The parapharyngeal space is surrounded by multiple adjacent spaces in the suprahyoid neck, including the pharyngeal mucosa space, masticator space, parotid space, carotid space, and the lateral margin of the retropharyngeal space. Lesions arising from these spaces tend to displace the parapharyngeal fat in predictable patterns—a point that can be of critical importance in localizing the space of origin of a neck lesion. For instance, a carotid space mass pushes the parapharyngeal space fat anteriorly ( Fig. 2 ). A lesion in the masticator space pushes the parapharyngeal space fat posteriorly ( Fig. 3 ). Similarly, lesions in the pharyngeal mucosal space and lateral retropharyngeal deviate the parapharyngeal fat laterally and anterolaterally, respectively.
Furthermore, clinical evaluation of the parapharyngeal space is limited, further improving the utility of cross-sectional imaging. Typically, only large masses within the parapharyngeal space are palpable by bimanual examination given its location lateral to the pharyngeal mucosa and deep to the masticator space and mandibular ramus. These lesions can grow to a relatively large size before clinical detection. For this reason, imaging can add significantly to the diagnosis and treatment plan of a patient with a suspected parapharyngeal space lesion.
Imaging strategies for parapharyngeal space lesions
The choice of imaging modality to evaluate this region is limited to CT or MRI given absence of a reliable sonographic window to this relatively inaccessible space. Contrast-enhanced CT is often more easily accessible, requires less patient cooperation, and can be done in patients with contraindications to MRI such as implanted pacing devices or cochlear implants. However, beside the radiation exposure and potential toxicities of iodinated contrast, contrast-enhanced CT generally provides less information than gadolinium-enhanced MRI of this region, which in most cases is the preferred technique for evaluating primary lesions of the parapharyngeal space. Although gadolinium-enhanced MRI is not without its own potential toxicities, lesions of this region are generally better characterized with the information provided by MRI in 3 planes. At our institution, suspected or known parapharyngeal masses are evaluated with the following sequences from the skull base to just below the hyoid bone: Axial T2- and T1-weighted, fat-saturated post-contrast images; coronal T1-, T2-fat-saturated, and T1-weighted fat-saturated post-contrast images; and sagittal T1-weighted fat-saturated post-contrast images. The use of fat-saturated pre-contrast T1-weighted imaging should be abandoned; the fat is a critical element of evaluating lesions in this region.
Before evaluating the imaging characteristics of a lesion primary to the parapharyngeal space, it is critical to confirm that the space of origin is in fact the parapharyngeal space and not secondary extension of an adjacent lesion from a bordering space. Demonstrating a margin of fat separating the lesion from the adjacent spaces of the neck is necessary to confidently place a lesion in the parapharyngeal space. Failure to visualize an intervening fat plane between a lesion and an adjacent space on any section implies that the lesion is not primary to the parapharyngeal space, or has invaded a contiguous space. A classic example of this imaging dilemma is a benign mixed tumor arising from the deep lobe of the parotid gland. Although on initial review of MRI ( Fig. 4 ) this lesion may seem to be located primary in the parapharyngeal space, close examination of the posterolateral margin of the parapharyngeal mass shows no clear fat plane and suggests a site of origin within the deep lobe of the parotid gland. Although the imaging features are consistent with a benign mixed tumor arising from salivary gland tissue (pleomorphic adenoma), the space of origin is the parotid gland with secondary deformation and mass effect on the parapharyngeal space.
