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Ranula is categorized into three types: sublingual, plunging, or mixed. The majority is of the sublingual type and plunging ranula is far less frequently encountered. Ranula mostly occurs unilaterally, and to our knowledge, bilateral presentation was reported only in three literatures [1, 2, 3].

Since the etiology of the ranula is unknown, its surgical treatment is still controversial. However, ranula formation has recently been speculated to be associated with trauma and/or congenital anomalies of sublingual gland, and to be resulted from mucous extravasation into the ambient tissues from a traumatized sublingual gland or duct [4, 5].

We here report a case of bilateral plunging ranula that occurred asynchronously following the excision of a median cervical cyst, and successfully treated via cervical and intraoral surgical approaches, respectively.

A 5-year-old girl, who had undergone the removal of a median cervical cyst by Sistrunk method at the age of 2, had swelling in the left submandibular region 9 months before, and was observed at a nearby clinic as lymphadenitis. Because of progressively increased swelling at mealtime over a year without intraoral presentation, she consulted Kochi Medical School Hospital. Magnetic resonance imaging (MRI) revealed a plunging ranula and sublingual fluid retention was found extending to the left submandibular region beneath the mylohyoid muscle (Fig. 1). Excision of ranula and removal of the sublingual gland were performed via a cervical approach. Skin incision was made along the previous incisional line. The cyst could be relatively easily dissected from the submandibular gland and therefore this gland and Wharton's duct remained. The cyst lining was partially ruptured due to a tight adhesion to the hypoglossal nerve. Histological examination revealed pseudocyst that has no lining with epithelial component.

Although recurrent legion had not been encountered, contralateral ranula was found by follow-up MRI 6 months after the surgery. After a month of observation, sublingual phlegmone occurred and was treated with antibiotics infusion, and consequently, ranula has tentatively decreased in size. After an additional 4 months of observation, we selected surgical treatment for this right lesion because it was confirmed to expand by MRI (Fig. 2) and demonstrated cervical swelling. Removal of the sublingual gland with an evacuation of cyst was undergone via an intraoral approach. Cyst lining histology in this side was pseudocyst as well. Follow-up at 9 months showed no evidence of recurrence.

Treatment of plunging ranula is still obscure due to association with a confusion of its pathogenesis. Recent reports described ranula formation was mostly related to trauma and/or congenital anomaly of the sublingual gland and that mucous extravasation from severed duct or sublingual gland presented as ranula [4, 5]. In addition, a number of literatures indicated that excision of the sublingual gland appeared to be essential for cure, otherwise, recurrent lesion was frequently encountered [4, 5, 6, 7, 8].

In the present case, magnetic resonance exhibited fluid retention, which arose from sublingual point extending to the submandibular space and this may support the speculation of literature. The primary left lesion, however, was treated via cervical approach because the previous incisional line was located fairly left-deviated. This approach has an advantage that the submandibular gland can be seen if this gland has any responsibility to ranula formation [9]. Intraoperatively, the submandibular gland was observed to have no relation to ranula and thereby remained. Cyst was mostly removed with total resection of the sublingual gland and no epithelial component was confirmed to form cyst lining by a frozen section. Following this result, the contralateral plunging ranula was treated by simple evacuation with the sublingual gland removal. Another reason was involved in selection of this approach that further extension of cervical wound should be avoided. Even though different surgical approaches were selected on both sides, excision of the sublingual gland was accomplished and this may lead to a satisfactory result that no recurrent lesion has been encountered to date.

The reason for an asynchronous presentation of bilateral plunging ranula is unknown. In the past, only three cases of bilateral presentation were reported [1, 2, 3]. Two of them [1, 2] presented asynchronous occurrence as in the case presented. The first surgery seems rather negative for the pathogenesis of the second ranula [2]. In this case, the previous injury to the mylohyoid muscle by Sistrunk procedure might cause sublingual gland herniation through the mylohyoid muscle and produce a latent susceptibility to the development of plunging ranula [2, 10], though time lag between both sides remains unclear.

As management of plunging ranula, excision of the sublingual gland appears to be essential for cure, regardless of whether via intraoral or cervical approach. Intraoral excision was considered more advisable because of its less invasiveness.