Key Points

Introduction and overview

The mylohyoid muscle divides the lower part of the oral cavity into 2 spaces: the sublingual space, which is located superior to the muscle, and the submandibular space, inferior to the muscle but superior to the hyoid bone. A wide range of pathologic processes may involve these spaces. They include lesions that arise uniquely in this location (eg, ranula, submandibular duct obstruction) as well as inflammatory processes, vascular abnormalities, and various malignancies that may also occur elsewhere in the head and neck. Some lesions that arise in superficial tissues may be easily diagnosed at physical examination. However, computed tomography (CT), magnetic resonance imaging (MRI), or ultrasonography (US) may be necessary for a reliable assessment of lesion extension to deeper structures. This article outlines the radiologic anatomy of the region, describes the various pathologic processes that may affect it, and discusses the use of imaging in their evaluation.

Anatomy

The mylohyoid muscle is an anterior suprahyoid muscle located deep or superior to the anterior belly of the digastric muscle. The mylohyoid muscle separates the sublingual space from the submandibular space and is a key landmark in imaging of the oral cavity and upper neck ( Fig. 1 A). Surgical approaches are chosen based on the relationship of a lesion to the mylohyoid muscle.

( A ) The sublingual ( shaded in green ) and submandibular ( shaded in blue ) spaces with their contents. The mylohyoid muscle ( arrows ) separates the sublingual space from the submandibular space and is a key landmark in imaging of the oral cavity. Coronal T1-weighted magnetic resonance (MR) image ( B ) showing the sublingual and submandibular spaces. The platysma forms the superficial margin of the submandibular space ( arrowhead ).

The sublingual space is a potential space without a fascial lining. The sublingual space is superomedial to the mylohyoid muscle and lateral to the genioglossus and geniohyoid muscles in the oral cavity. The major contents of the sublingual space are the sublingual salivary gland; the submandibular duct (Wharton duct); the deep portion of the submandibular salivary gland; and the lingual nerve, artery, and vein. The sublingual space communicates with the submandibular space at the posterior margin of the mylohyoid muscle. However, there can be a defect in the midportion of the muscle (boutonnière). The normal sublingual gland or sublingual lesions can pass through this defect to reach the submandibular space as well. The deep portion of the submandibular gland wraps around the posterior edge of the mylohyoid muscle, so a small part of the gland is cranial to the muscle in the sublingual space. The rest of the gland is located in the submandibular space. Contents of the submandibular space are the anterior belly of the digastric muscle, submandibular nodes, submandibular gland, and facial vein. The platysma forms the superficial margin of the submandibular space. Further, sublingual lesions are classically considered to extend into the submandibular space at the posterior edge of the muscle.

In the United States, CT is the modality of choice for imaging the floor of the mouth because of the widespread availability of CT scanners and the short examination time. CT is particularly useful for evaluating acute inflammatory processes because it is capable of depicting mandibular cortical bone erosion and destruction, cutaneous changes, and submandibular duct calculi. MRI provides better soft tissue resolution than CT and is particularly useful for staging oral cavity malignancies that involve the floor of the mouth and complex disease processes that extend through multiple anatomic spaces.

The mylohyoid muscle and the spaces can be seen on axial, sagittal, and coronal images on CT and MRI. However, the shape of the muscle sling is best seen on CT and MRI in the coronal plane (see Fig. 1 B).

Lesions

Although the submandibular and sublingual spaces are a small region, many different pathologic processes may occur there: cystic lesions, inflammatory conditions with various causes (eg, infection, obstruction of the main submandibular duct), rare vascular lesions, and benign and malignant neoplasms ( Box 1 ).

Cystic lesions

Cystic lesions in the sublingual/submandibular region are usually slow growing and often cause signs and symptoms only after they are large. Most often cysts in the floor of the mouth are benign and arise from salivary glands. The more common cystic lesions include ranulas and dermoid or epidermoid cysts and rarer lesions include false sialoceles, branchial cleft cysts, and thyroglossal duct cysts. Vascular anomalies and malignancies may manifest as cystic lesions, and, in these circumstances, specific imaging criteria and contrast-enhanced MRI may provide clues to the diagnosis.

Ranulas

A ranula is a mucous retention cyst or mucocele that arises from a sublingual gland or minor salivary gland and thus has a peripheral epithelial layer. Ranulas are characterized as either simple ( Fig. 2 A) or plunging (diving). They typically result from trauma or inflammation of the salivary glands. A plunging or diving ranula (see Fig. 2 B) develops after a simple ranula ruptures. The ruptured ranula usually extends posteriorly from the sublingual space into the submandibular space. Less commonly, it may extend anteriorly through a mylohyoid defect into the anterior submandibular space. Because the extension lacks an epithelial lining, a plunging ranula is classified as a pseudocyst. The lesion usually measures less than 6 cm at its maximum diameter in the submandibular space, with a narrower tail extending into the floor of the mouth. All ranulas are homogeneous, well-defined masses with fluid density on CT or extremely high T2-weighted signal intensity on MRI. It is essential to differentiate ranulas from other cystic masses before surgery because the surgical approach to ranulas differs from approaches to other types of masses.

Ranulas are homogeneous, well-defined masses with fluid, extremely high T2-weighted signal intensity on MRI. Simple ranulas ( A ) are confined to the sublingual space. Plunging ranula ( B ), also known as diving ranula, dissects along facial planes beyond the confines of the sublingual space, around the posterior edge of the mylohyoid muscle ( arrows ).

Congenital Cysts

Dermoid/epidermoid

Dermoid/epidermoid cysts derive from ectodermal tissue trapped during embryonic development; those with skin appendages on their capsules are termed dermoid, and those without are termed epidermoid. Although these cysts are mostly found in the midline of the floor of the mouth (that is, between the genioglossus muscles), they can occur also in the sublingual or submandibular space. On CT or MRI, dermoid cysts typically look like inhomogeneous masses, reflecting mixtures of keratin or fat with their contents. In contrast, epidermoid cysts are likely to be more homogeneous and, when occurring in the sublingual space, may be indistinguishable from simple ranulas.

Thyroglossal duct and branchial cleft cysts

Thyroglossal duct cysts ( Fig. 3 A) are the most common congenital neck cysts, and occur anywhere along the course of the thyroglossal duct, presenting as midline neck masses. Although they are mostly located at or just below the hyoid bone, they occasionally occur above the hyoid, in the submandibular space.

Thyroglossal duct cysts ( A ; arrows ) are the most common congenital neck cysts, present as midline neck masses. Although they are mostly located at or just below the hyoid bone, they occasionally occur above the hyoid, in the submandibular space. Branchial cleft cysts ( B ; star ) are lateral neck masses that characteristically displace the sternocleidomastoid muscle posterolaterally ( arrows ), the carotid and jugular vessels posteromedially ( arrowheads ).

Branchial cleft cysts (see Fig. 3 B), properly called second branchial cleft cysts, arise from the remnant of the second branchial apparatus. On images, they characteristically displace the sternocleidomastoid muscle posterolaterally, the carotid and jugular vessels posteromedially, and the submandibular gland anteriorly. The contents of the cysts are known to vary from watery fluid to a gelatinous, mucoid material.