21. Lump in the floor of mouth

A 9-year-old boy attended complaining of a persistent single swelling in the floor of his mouth causing some difficulties with eating, swallowing and speaking. There was no previous trauma to this area. None of his close relatives had similar lesions. Examination of the skin below the chin revealed a small swelling first noticed by his mother 3 weeks previously (Figure 1). There was no redness, tenderness or sinus tract or cervical lymph node enlargement.

Oral examination revealed a large dome-shaped, non-tender, fluctuant, transparent swelling in the floor of his mouth (Figure 2). This swelling extended bilaterally, beneath the salivary ducts, and pushed the tongue upwards.

Radiography and ultrasonography were undertaken and the lesion, when removed under GA, was found to be composed of granulation tissue lining a cavity containing mucus, muciphages and inflammatory cells. There was no recurrence by two-year follow-up.

(a) Dermoid cyst is a true hamartoma caused when skin and skin structures become trapped during foetal development. The dermoid cyst presenting in the floor of the mouth is rare but represents the second most common after the lateral eyebrow dermoid cyst. There are 3 types: i) dermoid or compound; ii) teratoid and iii) epidermoid cysts, according to the presence or not of skin and skin appendages within the fibrous cystic wall. The patient's lesion has some features in common with the dermoid cyst, such as the location, age and sex predilection, but was different in consistency and histological characteristics. The lesion was more fluctuant and composed of granulation tissue without an epithelial lining, while the dermoid cyst (especially the epidermoid type) has a more rubbery consistency and its cyst wall is fibrous with an epithelial lining. Both lesions show a cystic structure on ultrasonography. The CT scan can differentiate the two lesions using the pathognomonic criterion of a ‘sack of marbles’ appearance seen in dermoid cysts.