Suprahyoid spaces of the head and neck

W rHEN ONE refers to the deep spaces of the neck, one includes all of the tissue outside the aerodigestive system mucosal and submucosal structures. The major lesions of the aerodigestive mucosal space (AMS) are surface epithelium neoplasms. Squamous cell carcinomas (SQCCA) account for most AMS neoplasms, and in concert with minor salivary gland neoplasms and lymphoma of Waldeyer's ring, they comprise over 95% of the neoplasms of the AMS. Thus, there is a reasonably limited differential diagnosis for masses of the AMS. In contrast, the variety of lesions found in the deep spaces of the head and neck is astounding, and one is required to include spread of AMS lesions to the deep spaces with one's differential diagnoses. The classification of regions of the deep spaces is not wholly arbitrary, yet engenders some controversy amongst head and neck radiologists. For the radiologist who is not reading head and neck cases on a daily basis, the model derived from the works of Harnsberger, Osborne, and Smoker TM serves as the most practical approach. This model divides the nonmucosal spaces of the neck into regions defined by layers of the deep cervical fascia. The deep cervical fascia has superficial, middle, and deep layers, and these layers, in effect, form the boundaries for different spaces of the head and neck. One separates the neck into the suprahyoid and infrahyoid compartments because of the termination of some of the spaces at the hyoid level. In the suprahyoid region, the spaces of the neck include the masticator space, the prestyloid parapharyngeal space (PPS), the post-styloid parapharyngeal space or carotid space, the parotid space, the retropharyngeal space, and the perivertebral space. In the infrahyoid region, the visceral space (encompassing the trachea, esophagus, thyroid and parathyroid glands) comes into play, whereas the masticator space, the PPS, and the parotid space are no longer present. The carotid space, retropharyngeal space, and the perivertebral space span the suprahyoid and infrahyoid compartments. Before analyzing these spaces, it is helpful to understand how to localize a lesion to a particular space. Central to the suprahyoid spaces is the prestyloid parapharyngeal space, which is relatively unique among the spaces because its major component is fat. It is also the most mobile of the spaces of the head and neck because it is incompletely ensheathed by the deep cervical fascia. Most head and neck radiologists use the displacement of the PPS to help decide in which space a lesion resides. 5,6 Thus if the PPS fat is pushed posteromedially by a mass, the assumption is that the lesion arose in the masticator space. If the fat is pushed posterolaterally, an AMS mass is suspected. If the mass is pushed anteriorly or anteromedially with predominantly an anterior direction, a carotid space lesion is the likely culprit. (Distinguishing a pre-styloid process mass from a post-styloid carotid space mass requires visualization of the styloid process by computed tomography (CT) and the styloid musculature by magnetic resonance imaging [MRI]). If there is predominantly a medial displacement with some anterior component, a deep lobe parotid mass is the likely source. A retropharyngeal lesion will usually push the PPS fat anterolaterally. A perivertebral lesion may not affect the fat at all, but if it does, there will usually be an anterior component of displacement. The second important structure in the neck for localizing lesions is the longus colli musculature complex. When these muscles are displaced posteriorly, the lesion is usually arising from the AMS or the retropharyngeal space. If displaced anteriorly, a perivertebral source is indicated. The muscles themselves are part of the perivertebral space so an intrinsic longus colli mass is within the perivertebral space. At this point, an analysis of each space may help to gain confidence in diagnosing deep space lesions.