Keywords
submandibular gland aplasia - sublingual swelling - mylohyoid
Introduction
The first case of bilateral submandibular gland aplasia was presented in 1885 by Gruber.[1] The submandibular salivary gland development is a finely regulated process, involving
well-timed sequences of cellular and tissue events. For this reason, different types
of structural congenital anomalies, ranging from asymptomatic anatomical variants
to alterations, are associated with syndromic conditions.[2] Off these major salivary gland aplasia is usually seen in association with congenital
disorders such as lacrimo-auriculo-dento-digital syndrome, mandibulofacial dysostosis,
Treacher Collins syndrome, hemifacial microsomia, and ectodermal dysplasia associated
with aplasia or dysplasia of lacrimal or thyroid glands.[2] However, isolated aplasia of major salivary glands is very rare entity; when present,
it most commonly affects the parotid gland. Clinically, patients may be asymptomatic
or may present with dryness of the mouth, have difficulty in chewing and swallowing,
and suffer from dental caries due to hypo function of salivary glands. Associated
pseudomasses can be seen as a result of compensatory hypertrophy of the other major
and minor salivary glands along with presence of accessory salivary glands.[3]
Case Report
A 55-year-old male presented with swelling in the floor of mouth for the last 6 months
and had mild difficulty in chewing food and swallowing. He had no history of prior
head and neck cancer or surgery. Bilateral nontender sublingual region masses were
noticed with bulge along both sides of midline in the floor of mouth and submandibular
gland salivary duct (Wharton duct) was not visualized on oral examination. No dental
caries were seen. No constitutional symptoms, local inflammatory signs, or regional
adenopathy were observed. Clinically and on ultrasonography from outside clinic, a
provisional diagnosis of ranula was considered with possibility of neoplastic conditions
as the differentials. He was referred for imaging prior to planned needle aspiration
cytology.
Computed tomography (CT) of the soft tissues of the neck with intravenous contrast
revealed absence of bilateral submandibular glands with normally enhancing bilateral
hypertrophied sublingual salivary glands that showed inferior lobulated projections
splitting the mylohyoid muscle protruding into the submandibular spaces (left >right)
([Fig. 1A] and [B]). No calculus, stranding, or ductal dilatation was noted. Similar findings were
confirmed on a noncontrast magnetic resonance (MR) study (T1-weighted, T2-weighted,
and fat-suppressed T2-weighted images in axial and coronal planes) ([Fig. 2A–C]). Additional finding of bilateral symmetrical enlargement of parotid glands and
accessory parotid glands was also observed on CT and MRI. The hypertrophied parotid
and accessory parotid glands were homogenous with distinct margins and no area of
differential echogenicity, attenuation, or intensity. No surrounding inflammatory
changes were seen. Patient was further advised for the sialography—gold standard investigation;
however, patient denied to undergo sialography. The final imaging diagnosis of bilateral
submandibular gland aplasia with associated hypertrophied bilateral parotid and sublingual
glands, accessory parotid glands, and splitting of mylohyoid sling by hypertrophied
sublingual salivary glands was made. This obviated the need for any intervention and
patient was sent home after appropriate counselling.
Fig. 1 Radiographic CT image of parotid, sublingual and submandibular salivary glands.
Fig. 2 (A and B) Schematic diagram of mylohyoid muscle and genioglossus muscle, sublingual and submandibular
spaces.
Discussion
Agenesis or aplasia of the major salivary glands is very rare entity and it most commonly
involves the parotid gland among the other major and minor salivary glands. When present,
it is usually associated with multiple other developmental head and neck anomalies.[2] Absence of submandibular gland may be associated with hypertrophy of the rest of
major or minor salivary glands. In previous case reports[4] of bilateral submandibular gland aplasia or agenesis, bilateral sublingual hypertrophy
accompanied this congenital absence, which was also seen in our patient.
Pathogenesis of bilateral salivary gland aplasia is unknown; however, it is most likely
owing to a disturbance during the fetal development. Major salivary glands development
is from the proliferation of oral ectoderm and outpouching at 4 to 8 weeks of in utero
development. Parotid gland develops first followed by submandibular gland and other
major and minor salivary glands.[2]
The mylohyoid muscle is classically thought of and depicted as a continuous muscular
barrier between the sublingual and submandibular spaces. The muscle, in fact, is often
discontinuous. Anatomic and surgical literature have demonstrated that the defects
may contain sublingual or submandibular salivary tissue, fat, blood vessels, or all
three components.[5]
[6]
[7] It inserts on the slightly obliquely oriented mylohyoid line on the medial surface
of the mandible, with the posterior aspect more cranial than the anterior aspect.
The muscle is thickest posteriorly, and thins as it approaches the mental tubercle.[7]
Submandibular gland aplasia may be associated with caries of mandibular permanent
incisor teeth, as well as other signs of salivary hypofunction, such as dry mouth,
disturbed oral sensation, and oral infections.[4] In a case reported by Mathison and Hudgins,[8] bilateral submandibular gland aplasia in a 34-year-old female patient with accompanying
nodular-sclerosing Hodgkin's lymphoma, the patient did not show any sign of salivary
hypofunction. Another study by Ahmed et al also did not show such clinical symptoms
in their 62-year-old male patient. Similarly, our patient did not suffer any major
problem related to salivary deficiency. As in the previous stated cases, we believe
this absence of oral symptoms is attributable to compensatory bilateral sublingual
gland hypertrophy.
White et al[3] reported the deficiencies in mylohyoid muscle in 77% of patients who presented with
submandibular gland aplasia. These defects may contain variable amounts of fat, blood
vessels, and salivary tissue. Accessory salivary tissue was identified in 37% of individuals
who underwent scanning. They found the hypertrophied sublingual glands located in
the mylohyoid defects are seen herniating into bilateral submandibular spaces to present
as palpable masses. However, this extremely rare entity has been reported only once
in literature by Ahmed et al.[9] They reported a case of nonsyndromic bilateral submandibular gland aplasia with
hypertrophied sublingual salivary tissue, the latter herniating through the mylohyoid
defect to present as a palpable mass on the left side with corresponding CT findings
in a pediatric patient who had bilateral palpable and visible masses with modified
Valsalva maneuver.
Incidental absence of the right submandibular gland in a 35-year-old woman, with a
“mass” in the right sublingual space, has been reported in previous literature.[10]
[11] There were no associated calculi or other lesions, and the location of mass was
above mylohyoid muscle sling. Their hypertrophy of the sublingual gland was seen in
this instance.
There are various imaging techniques that help in diagnosing salivary gland aplasia,
which include CT, MRI, ultrasonography, sialography, or nuclear medicine (99Tcm-pertechnetate)
studies.[10]
[12]
[13] Among various imaging techniques, sonography is cheap, noninvasive and widely available,
and useful imaging technique. As in our case, many radiologists may prefer MR and
CT imaging for better anatomical delineation and characterization of the abnormality.
However, an elaborate sonographic examination by an experienced radiologist may be
adequate. Inflammation—in particular, viral sialadenitis, tumors and dermoid cysts—should
be included in the differential diagnosis of sublingual gland enlargement. Of these,
bilateral enlargement is most likely to be seen in viral sialadenitis. Submandibular
gland lesions include various benign and malignant lesions, in which benign lesions
include pleomorphic adenoma, while malignant lesions include adenoid cystic carcinoma,
mucoepidermoid carcinoma, metastasis, and lymphoma.[14]
Sialosis[15] is a chronic, bilateral, diffuse, noninflammatory, nonneoplastic swelling of the
major salivary glands that primarily affects the parotid glands, but occasionally
involves the submandibular glands and rarely the minor salivary glands seen in patients
with diabetes, malnutrition, and alcoholism. In such cases, clinical and imaging findings
would be helpful in arriving at the correct diagnosis, where submandibular salivary
glands would be seen in its normal location, and are associated with gland heterogeneity.
Conclusion
Symmetrical submandibular gland aplasia associated with bilateral sublingual gland
hypertrophy should be kept in mind in patients with bilateral submandibular masses
to avoid unnecessary biopsy or surgical interventions. CT or MRI should be performed
before any intervention.[10]
[12] This prevented the need for any intervention and the patient was sent home after
appropriate counseling.
Authors' Contribution
Srividya B Thimmappa was involved in conceptualization, designing, definition of intellectual
content, literature search, data acquisition, data analysis, and manuscript preparation.
Abhishek Suman was involved in conceptualization, designing, definition of intellectual
content, literature search, data acquisition, data analysis, manuscript preparation,
manuscript editing, and review. Rashmi Dixit was involved in conceptualization, designing,
definition of intellectual content, literature search, data analysis, manuscript editing,
and review. Anju Garg edited and review the manuscript. Abhishek Suman, Rashmi Dixit,
and Anju Garg have provided guarantee for this manuscript.
