Imaging of the sublingual and submandibular spaces

Aplasia of the salivary glands is a rare condition that usually affects the parotid and submandibular glands. Single or multiple glands may be absent or hypoplastic. Exact aetiology remains unknown; however, the condition has been attributed to first and second branchial arch abnormalities. Specifically, it has been associated with genetic syndromes involving craniofacial development such as Treacher-Collins syndrome, hemifacial microsomia and lacrimo-auriculo-dento-digital syndrome [3]. Some studies report increased incidence in trisomy 21 [4]. Although usually discovered incidentally on imaging, some cases are identified when patients present with dry mouth, dysphagia and dental disease. Imaging may show compensatory hypertrophy of other major salivary glands, and therefore it is important not to mistake normal gland tissue for other lesions such as tumour [5].

True dermoid cysts (also known as benign cystic teratomas), epidermoid cysts and teratoid cysts represent a spectrum of congenital and acquired cystic malformations sharing the common characteristic of a squamous epithelial lining. Dermoid cysts as a term refers to three histologically distinct processes classified based on whether they are lined with simple squamous epithelium (epidermoid), skin appendages (dermoid) or tissues of other major organs (teratoid). Dermoids and epidermoids arise from dermal elements of the first and second branchial arches [1, 6]. It is important to distinguish dermoid and epidermoid cysts apart from teratoid cysts, as the former harbour a lower risk of malignant degeneration [7]. Although rare within the head and neck, dermoid and epidermoid cysts have a predilection for the oral cavity, specifically the floor of the mouth [8]. Dermoid cysts typically present in the 2nd or 3rd decade of life as a slowly enlarging midline neck mass, which may progress to cause dysphagia, while epidermoid cysts manifest earlier in life, becoming evident in infancy.

On imaging, dermoid and epidermoid cysts can have a similar appearance. Some of the distinguishing features are derived from their unique histological composition. While both behave as cystic lesions on CT and MRI, dermoid cysts will demonstrate the hallmark finding of coalesced globules of fat resembling a “sac of marbles” in a medium (mixture of accessory dermal contents such as sebum and calcification) of low CT attenuation and variable signal on T1-weighted images (Figs. 3 and 4) [9]. Epidermoid cysts will distinctively show diffusion restriction on MRI [10]. The relationship of these cystic lesions to the mylohyoid muscle is important in guiding surgical intervention, as cysts superior to the mylohyoid muscle are removed preferably with an intraoral approach, while inferior lesions are excised through an external submandibular approach [11].

Among the differential diagnosis of cystic lesions in the sublingual space, a ranula is a commonly acquired lesion (Table 2). Traditionally considered as a post-inflammatory or post-traumatic sequela of glandular obstruction, a ranula is a mucous retention cyst arising from the sublingual or minor salivary glands [12].

On imaging, ranulas have the typical appearance of cystic lesions with fluid attenuation on CT, hyperintensity on T2-weighted MR sequences, and are anechoic on ultrasound (Fig. 5). MRI in particular can serve to assess the size, content, location and extent of the lesion, especially in preoperative planning [13]. Additionally, a ranula can be indistinguishable from an epidermoid cyst based on CT and sonography, but the absence of restricted diffusion on MRI establishes the diagnosis (Fig. 6) [14].

As the cystic lesion enlarges, it can “rupture” through its sublingual space boundaries and extend into the submandibular space through the posterior free edge of the mylohyoid muscle or mylohyoid boutonniere. Referred to as a plunging ranula, the herniated portion leaves behind a band-like portion of the cyst within the sublingual space, forming the “tail sign” (Fig. 7) [15]. On rare occasions, it can also extend into the parapharyngeal space or upper cervical soft tissues [16].

Cellulitis is a diffuse infectious process of the skin and subcutaneous tissues. In the oral cavity, infections typically arise from a glandular or dental aetiology and may be limited to a single compartment or be a trans-spatial process [17]. Most commonly diagnosed with CT, key findings include skin thickening, fat stranding and enhancement of fascial planes [18].

As infection progresses unimpeded, bands of fluid coalesce into well-defined rim-enhancing collections or abscesses (Fig. 8). Route of spread in dental infection is dictated by fascial planes and muscle attachments. In particular, the posterior attachment of the mylohyoid muscle is at the level of the third mandibular molar. Infections originating in the apex of posterior mandibular teeth (i.e. third molar) are inferior to the mylohyoid insertion and tend to involve the submandibular space [13].

A life-threatening condition, Ludwig angina presents as an intense, rapidly progressive cellulitis, typically originating in the sublingual and submandibular spaces, driven by odontogenic streptococcal infection; immunocompromised patients are more susceptible [13]. Imaging plays a role in identifying airway patency, drainable collections and evidence of gas-forming bacteria. Allowing for rapid evaluation, CT is preferred, which will show diffuse inflammatory changes and bands of fluid, representative of serosanguinous accumulations [19]. As a multispatial process, extension into the parapharyngeal fat/space increases the likelihood of pharyngeal involvement with significant increased risk of impending airway collapse and mediastinitis (Fig. 9). Clinical management involves prompt airway protection, intravenous antibiotic administration and surgical drainage [20].

Salivary stones (sialoliths) represent calcium concretions, most commonly within the submandibular ductal system, owing to the relatively higher calcium and phosphate salt concentrations of the gland’s secretions [21]. The resultant inflammation clinically manifests as glandular tenderness and swelling, particularly after meals. Sialadenitis is best evaluated with CT, which will show increased size and density of the gland, with ductal calcifications; post-contrast imaging will show diffuse, intense enhancement (Fig. 10) [2]. Advanced stages of sialadenitis can present with suppurative infections and abscess formation [22, 23]. The differential diagnosis of abscess within the salivary glands includes superinfected cysts in HIV patients, suppurative and necrotic lymph nodes, and cystic degeneration of malignancy; therefore, clinical context must be reviewed carefully, and follow-up to resolution may be warranted [22].

Ductal obstruction and calculi as small as 3 mm can be detected on ultrasound, although it is operator dependent [24]. Sialography is the reference standard imaging modality, as it can demonstrate ductal obstruction from multiple aetiologies, including recurrent infection, autoimmune processes and trauma (Fig. 11). MRI and conventional digital subtraction sialography have been found to be comparable in diagnostic performance [25].

Characterised by slow vascular flow, this category predominantly consists of venous-based lesions or venous malformations, which most frequently involve the floor of the mouth or buccal space when they occur in the head and neck [13]. Histologically, they are composed of a mass-like collection of venous sinusoids. On initial evaluation, typically by ultrasound, they appear as compressible mixed echogenicity structures with ill-defined margins and monophasic low velocity flow on Doppler interrogation. CT and MRI are used to evaluate spatial extent, visceral involvement and osseous destruction [27]. On MRI, signal patterns depend on vessel size and range from hyperintense venous lakes to more solid appearing lesions that are isointense to muscle on T2-weighted imaging. CT attenuation follows a similar pattern [13]. Characteristic features of any venous-based anomaly, such as phleboliths or diffuse venous phase enhancement, are also present [1].

Infantile haemangiomas are vascular tumours, rather than malformations, as they exhibit a distinct life cycle: manifest in infancy with initial rapid growth and proliferation, followed by involution in early childhood. Clinical presentation is usually sufficient in diagnosis without imaging assistance [1].

Contributing to the slow flow spectrum, lymphatic malformations, also known as lymphangiomas, are cystic structures with partial enhancement and more commonly occur in the submandibular space. Characteristic appearance is that of a non-enhancing multiloculated cystic lesion with fluid-fluid levels without phleboliths [18]. Unilocular lesions are less common and may be mistaken for a thyroglossal duct cyst or duplication cyst [10].

Mixed veno-lymphatic lesions share imaging features of both venous and lymphatic malformations, namely high signal on T2-weighted imaging attributed to the lymphatic component and homogenous enhancement attributed to the venous component (Fig. 12) [10].

Arteriovenous malformations (AVMs) may present at any age and largely involve the lingual artery and vein when they occur in the sublingual space. The lesion is characterised on imaging by enlarged, tortuous vessels consisting of a prominent feeding artery and a draining vein. Occasionally, these lesions may exhibit intraosseous extension [13].

Lipomas are the most common type of mesenchymal tumour; however, they rarely arise within the oral cavity. They are encapsulated mature adipose tissue and can be distinguished from surrounding fat based on the internal architecture, forming a well-circumscribed predominantly fat-containing lesion with fibrous septa on CT and MRI [18]. Lack of calcifications and cystic components distinguishes these lesions from dermoid cysts [13].

Pleomorphic adenomas are the most common benign primary lesions of the salivary glands, and occur in the parotid, sublingual and submandibular glands, in order of decreasing prevalence [15]. These tumours arise from salivary gland rests and are composed of epithelial, myoepithelial and stromal components. On CT, they usually appear as well-circumscribed homogenously enhancing masses, which demonstrate high signal on T2-weighted imaging (Fig. 13). As the lesion enlarges, areas of haemorrhage and cystic necrotic changes may develop, thus altering the classic imaging appearance [13]. Rapid growth should raise suspicion for malignant degeneration, which occurs in about 15% of cases [1]. Early changes are better delineated on MRI, with loss of homogenous high signal on T2-weighted images or rupture of the fibrous capsule [22].

Schwannomas and neurofibromas are rare within the sublingual and submandibular spaces. Arising from peripheral and sympathetic nerves, schwannomas involving the lingual and hypoglossal nerves can occur in the floor of the mouth. Neurofibromas are associated with known neurofibromatosis. On CT, these lesions are described as enhancing homogenous isoattenuating masses with distinct margins. On MRI, they demonstrate isointense signal on T1-weighted imaging and hyperintense signal on T2-weighted imaging, relative to the adjacent musculature. Larger lesions can have cystic components [13].

The mylohyoid muscle is traditionally depicted as a continuous muscular sling formed from two halves that join at the fibrous median raphe. However, with modern day CT and MRI, focal defects are becoming increasingly evident and have been reported in up to 77% of CT studies. During embryological development, the anterior and posterior muscular sheets fail to overlap with a resultant slit, allowing for protrusion of the sublingual space contents into the submandibular space (Fig. 17). Also known as the mylohyoid boutonniere, the protruding salivary gland tissue, fat, blood vessels or a combination of the three can present as a palpable “mass” on physical exam (Fig. 18) [32].

Symmetrical, diffuse and painless enlargement of the sublingual and submandibular glands may reflect response to a systemic process, such as diabetes mellitus, hypothyroidism, chronic alcoholism, malnutrition and certain drugs such as antibiotics, diuretics and antipsychotics. Bilateral and symmetric involvement should clue one into the possibility of a systemic diagnosis [22].