Suprahyoid Spacesof the Head and Neck DavidM. Yousem ONE refers to the deep spacesof the component is fat. It is most mobile If,/HEN also the of the Y Y neck. one includes all of the tissue outside spacesof the head and neck becauseit is incom- the aerodigestivesystem mucosal and submucosal pletely ensheathed by the deep cervical fascia. structures.The major lesions of the aerodigestive Most head and neck radiologists use the displace- mucosal space(AMS) are surface epithelium neo- ment of the PPS to help decide in which space a plasms. Squamouscell carcinomas(SQCCA) ac- lesion resides.5,6Thus if the PPS fat is pushed count for most AMS neoplasms, and in conceft posteromedially by a mass, the assumption is that with minor salivary gland neoplasms and lym- the lesion arosein the masticator space.If the fat is phoma of Waldeyer's ring, they comprise over 957o pushedposterolaterally, an AMS massis suspected. of the neoplasms of the AMS. Thus, there is a If the mass is pushed anteriorly or anteromedially reasonablylimited differential diagnosisfor masses with predominantly an anterior direction, a carotid of the AMS. In contrast, the variety of lesions spacelesion is the likely culprit. (Distinguishing a found in the deep spacesof the head and neck is pre-styloid process mass from a post-styloid ca- astounding,and one is required to include spreadof rotid space mass requires visualization of the AMS lesions to the deep spaceswith one's differen- styloid processby computed tomography (CT) and tial diagnoses. the styloid musculature by magnetic resonance The classificationofregions ofthe deepspaces is imaging IMRII). If there is predominantly a medial not wholly arbitrary, yet engenderssome contro- displacement with some anterior component, a versy amongst head and neck radiologists. For the deep lobe parotid mass is the likely source. A radiologist who is not reading head and neck cases retropharyngeallesion will usually push the PPSfat on a daily basis, the model derived from the works anterolaterally.A perivertebral lesion may not af- of Harnsberger,Osborne, and Smokerl-4seryes as fect the fat at all, but if it does,there will usually be the most practical approach.This model divides the an anterior component of displacement. nonmucosalspaces ofthe neck into regions defined The second imporlant structure in the neck for lesions by layers of the deep cervical fascia. The deep localizing is the longus colli musculature complex. When these muscles are displacedposte- cervical fascia has superficial, middle, and deep riorly, the lesion is usually from the AMS layers, and these layers, in effect, form the bound- arising or the retropharyngealspace. If anteriorly, aries for different spacesof the head and neck. displaced a perivertebralsource is indicated.The muscles One separatesthe neck into the suprahyoid and them- selves are part of the perivertebral space so an infrahyoid compartments becauseof the termina- intrinsic longus colli mass is within the periverte' tion of some of the spacesat the hyoid level. In the bral space. suprahyoid region, the spacesof the neck include At this point, an analysis of each spacemay help the masticator space,the prestyloid parapharyngeal to gain confidence in diagnosing deep space le- space(PPS), the post-styloid parapharyngealspace sions. or carotid space,the parotid space,the retropharyn- geal space, and the perivertebral space. In the MASTICATORSPACE infrahyoid region, the visceral space (encompass- The masticator space encloses the muscles of ing the trachea,esophagus, thyroid and parathyroid mastication (medial and lateral pterygoids, masse- glands) comes into play, whereas the masticator ter, and temporalis), the neck and condyle of the space,the PPS, and the parotid spaceare no longer mandible, and for simplicity's sake, the buccal present. The carotid space,reffopharyngeal space, and the perivertebral spacespan the suprahyoidand infrahyoid compartments. From the Department of Radiology, Johns Hopkins Hospital, Before analyzing these spaces,it is helpful to Baltimore, MD. understandhow to localize a lesion to a particular Address reprint requests to David M. Yousem, MD, Depart- ment of Radiology, Johns Hopkins Hospital, 600 N. Wolfe St, space. Central to the suprahyoid spaces is the Houck B-112, Baltimore, MD 21287. prestyloid parapharyngeal space, which is rela- Copyright o 2000 by WB. Saunders Company tively unique among the spacesbecause its major 003 7- I 9 8X/00/3 s 0 t - 000 8$ I 0. 00/0 Seminarsin Roentgenology,Vol XXXV No 1 (January),2000:pp 63-71 64 DAVIDM. YOUSEM region. Many of these structuresare innervated by of the muscle after denervationinjuries, or bilateral branchesof the trigeminal nerve and are supplied or unilateral enlargementof the musclesin casesof by external carotid artery branches.The proximity bruxism or storagedisease infiltrations. of the pterygoid muscles to the AMS and the Of benign neoplasms,hemangiomas (eg, venous mandible to the retromolar trigone and floor of vascular malformations) in the pediatric age group mouth predisposesthis spaceto infiltration by AMS and schwannomasand mandibular bony neoplasms SQCCA. in the older population will predominate. The A lesion of the masticator space will displace hemangiomasmay spreadacross spaces of the neck PPS fat posteromedially and the longus muscula- (transspatial)disrespecting the deepcervical fascia. ture will be unaffected or displaced posteriorly. If The schwannomasare usually of mandibular nerve the lesion is primarily located in the masseteror origin and may be located anywhere from the temporalis muscle, the PPS fat may be unaffected. inferior alveolar canal, the soft tissue of the masti- The most common non-neoplasticlesions of the cator space,or the foramen ovale. Denervation of masticator spaceare odontogenicin origin.T-eInfil- the muscle or abnormal T2-weighted intensity and tration of the muscles by infectious spread from gadolinium enhancement of the masticator muscles dental caries or as a complication of tooth extrac- may be seen in conjunction with the neurogenic tions is not infrequently seen.An abscess,a mass tumor. Of benign mandibular tumors, ameloblasto- defined by peripheral contrast enhancement,may mas are most common. be presenton either side of the mandible or maxilla The most common malignancy to affect the or may straddle the bone (Fig 1). The presenceof masticator spaceis an AMS SQCCA. This is most edematouslinear stranding into the subcutaneous commonly manifest as infiltration of the pterygoid tissue or the PPS (if not the clinical symptoms) will muscles from tonsillar or retromolar trigone can- suggestthe inflammatory nature of the lesion. Of cers leading to clinical complaints of trismus.ro the noninflammatory non-neoplastic odontogenic Alternatively, mandibular invasion may occur from lesions,dentigerous cysts, aneurysmal bone cysts, cancers of these sites. Metastasesto the mandible and simple unicameral cysts may lead to evaluation and direct secondaryinvasion by AMS malignan- of masticator spacelesions.T The muscles are also cies are more common that primary malignancies not immune from pathology. One may see atrophy of the masticator space.The most common primary malignant masticator spacehistologies are rhabdo- myosarcomas(Fig 2), lymphomas, and bony sarco- mas. Tumors may spreadto or from the masticator space via perineural spread along the mandibular nerve." PRESTYLOIDPARAPHARYNGEAL SPACE The PPS contains fat, vessels, nerves, and on occasion, ectopic minor salivary gland tissue. The "styloid" of "prestyloid" refers to the styloid processand is a useful demarcationof the PPSfrom the carotid space.Although the styloid process is easily identified on Cl the signal void associated with it on MRI may simulate a vessel. Therefore one must identify the "styloid musculature," which includes the styloglossus, stylopharyngeus, and stylohyoid muscleson MRI to determineif a lesion is pre- or post-styloid. If the styloid musculatureis Fig 1. Masticator infection. This contrast-enhanced CT scan shows low density within the right masseter muscle displaced anteriorly, one should consider a carotid (arrowheadl as well as enlargement of the right pterygoid spacelesion. muscle. The parenchymal fat on the right side is displaced It is rare to see a primary PPS lesion. The posteriorly and medially (arrowl. The CT scan also shows infiltration of the subcutaneoustissue superficialto the right inflammatory lesions of this space ale usually masseter muscle, suggestive of an inflammatory process. arising from theAMS, the tonsillar crypts, the deep SUPRAHYOIDSPACES 65 To unequivocally identify a lesion of the PPS as arising in that space,one would like to seethe mass entirely surrounded by PPS fat. This is rarely demonstrated and often the PPS fat is either completely obliteratedor displacedanteriorly, simu- lating a parotid spaceor carotid spacelesion. The latter can be excluded if one can determine that the lesion is found anterior to the styloid musculature and/or styloid process. CAROTIDSPACE (POST-STYLOID PARAPHARYNGEALSPACE) The carotid spaceis a colloquialism used for the carotid sheath and adjacent structures.Some head and neck radiologists disdain the term "carotid Fig 2. Rhabdomyosarcomaof the masticator space.Unen- space" because the fascia around the sheath ap- hancedCT shows an infiltrative mass (asterisks)involving the pears to be incomplete above the angle of the pterygoid musculature,which demonstrates the characteris- mandible lie greater tic displacementof the parapharyngealfat