Teaching Case

A 24-year-old man presented with a painless swelling in the right submandibular region.

A 24-year-old man presented with a painless swelling in the right submandibular region. The swelling was first noticed about 9 months before and had increased at a very slow pace to the present size. The skin over the swelling was healthy with no redness or local rise in temperature. The swelling was fluctuant and non-tender on local examination. No cervical lymph nodes were palpable. Intra-oral examination was unremarkable with good oral hygiene. His systemic examination and haematological examinations were within normal limits. The patient was evaluated with a high-frequency ultrasonogram of neck which revealed a unilocular smooth walled cystic lesion in the right submandibular region. This was located antero-inferior to the right submandibular gland which appeared normal in size and echotexture. An MRI of neck was suggested for further evaluation. Since the patient was claustrophobic we could not do a complete study. However, a limited study was performed using coronal fat-supressed T2-weighted images which revealed a unilocular, homogeneous, well circumscribed, fluid-filled lesion along the floor of mouth in the right sublingual space. The lesion was seen extending through a mylohyoid defect into the right submandibular space anterior to the right submandibular gland. Based upon the imaging findings, diagnostic possibility of a diving or plunging ranula was considered. The patient was subsequently operated. Intra-operatively the lesion was seen to extend into the submandibular space through a dehiscence in the mylohyoid muscle. The mucocele was succesfully evacuated intra-orally and the sublingual salivary gland was excised. Histopathological diagnosis was pseudocyst without an epithelial lining.

Ranula is a mucous retention cyst of the floor of the mouth. The term ranula is derived from the Latin word rana, meaning frog. It is named so because of its characteristic bluish translucent colour that somewhat resembles the underbelly of a frog. These retention cysts originate within the sublingual space from obstruction of the sublingual or minor-salivary-glands. While simple ranulas are confined to the sublingual space, plunging-or-diving ranulas tend to grow and spill into one or more adjacent spaces such as submandibular or parapharyngeal space. Simple ranulas are confined above the mylohyoid muscle, whereas plunging ranulas extend into the neck via one of the two routes: (1) either through a dehiscence in the mylohyoid muscle; or (2) posterior to the mylohyoid muscle and anterior to submandibular gland. Extravasation of saliva from the sublingual gland due to obstruction of its ducts appears to be the causative factor of a plunging ranula. Partial or complete obstruction of the sublingual gland duct results in proximal duct dilatation which eventually ruptures allowing secretions to escape in surrounding soft tissues. The escaped salivary secretions incite a chronic inflammatory response resulting in condensation of connective tissue at the periphery of the collection. The histological appearance of the plunging ranula is characteristic, devoid of epithelium or endothelium; lined by a fibro-connective tissue containing chronic inflammatory cells, hence these are often referred as pseudocysts.

Ranulas are little more common in females and are mostly encountered in second-third decades of life. Clinically simple ranulas present as a painless swelling along the floor of mouth. If large, these may interfere with speech, mastication, and swallowing because of the supero-medial displacement of the tongue. In contrast, plunging ranulas produce a swelling in the neck with or without swelling in the mouth. Ranulas are mostly unilateral demonstrating no side preference. On ultrasonography, these appear as a thin-walled unilocular cystic mass. On CT-scan they are seen as a well-circumscribed non-enhancing fluid-attenuation mass. If the cyst has been infected the contents may demonstrate increased attenuation, and occasionally may show thin rim-enhancement. At MR-examination it demonstrates fluid signal-intensity and smooth thin walls. MR-imaging plays an important role in providing precise information regarding the boundaries and extent of the lesion thereby assisting the operating surgeons in better planning the surgical approach. The differentials include dermoid, epidermoid-cyst, lipoma, cystic hygroma, cervical abscess, thyroglossal duct cyst and 2nd brachial cleft cyst. These entities differ in appearance on CT-MR imaging from a plunging ranula. Dermoid cyst and lipoma classically demonstrate intra-lesional fat. Cervical abscess is associated with extensive fat stranding and is usually associated with cavitory-tooth-disease. Thyroglossal duct cyst is usually midline, while the 2nd-brachial cleft cyst is classically located along anterior border of sternocleidomastoid muscle. Cystic hygroma characteristically presents early in life, 90% lesions manifesting before 2-years of age. It is infiltrative in nature and does not respect fascial planes presenting as a lobulated, poorly circumscribed, septated mass.

Intraoral excision of the ipsilateral sublingual gland and pseudocyst is considered an effective and safe method for the treatment of plunging ranula.