A fluid-filled blister on the lower lip (mucocele): why does it happen, and why does it come back?

On the inner surface of the lips, cheeks and palate there are many small salivary glands that cannot be seen with the naked eye. Each has a fine duct that carries its secretion into the mouth. When you bite your lip, one of these ducts can tear. The gland carries on producing secretion, but the secretion now flows not into the mouth but into the tissue, forming a pocket. The fluid inside the blister is not water but a thick, slippery salivary secretion.

There are two types of mucocele. In the far more common type, the duct has torn and the secretion leaks into the surrounding tissue; this type is linked to injury. In the less common type, the duct has not torn but has become blocked, and the secretion collects inside the duct. In a study of 138 mucoceles in children, 96 per cent of cases were of the first type.

Its appearance is familiar: a dome-shaped, soft blister, usually clear or bluish. Its size can range from 1 millimetre to 2 centimetres. It appears most often on the inside of the lower lip; the inside of the cheek, the tongue, the gum and the floor of the mouth are the other places. It is seen most often between the ages of 10 and 20, and generally in people under 30. In the general population it is found in about 2.4 in every 1,000 people.

There is usually a history of biting, but it is not always remembered. In the same study of 138 cases in children, 87 per cent of patients described an injury, and the commonest site was the lower lip. Biting the lip while eating, a habit of biting or sucking the lip, a lip piercing, and a broken tooth edge that keeps rubbing against the lip are known triggers.

It is common for the blister to burst and go down on its own, and a large share of mucoceles clear up this way. But bursting does not get rid of the gland producing the secretion. If the damage to the duct has not healed, the gland carries on releasing secretion into the same pocket, and the blister swells up again within a few days or weeks. A blister that fills, goes down and then fills again is therefore the typical course of a mucocele. Bursting it at home with a needle does not work for the same reason; on top of that, it damages the tissue and opens the door to infection.

For a mucocele that keeps coming back or is large, treatment is removing the blister together with the small gland producing the secretion. A scalpel, a laser or freezing can be used; in children there is also a simpler method that passes a stitch through the blister to open a new drainage route. A review bringing together randomised studies found no significant difference in recurrence rate between removal with a scalpel, laser and this drainage method. So the choice of method is decided by the clinician's experience, where the blister is and the patient's age.

Even after removal it can come back, and this becomes more likely with age. At a children's hospital, recurrence was seen in 5 per cent of 492 patients whose mucocele had been removed. This rate was around 2 per cent in children under 7, rising to around 9 per cent at 13 and over. The size of the blister, how long it had been there, its location and the stitching technique were not found to be linked with recurrence. In teenagers and adults, a lip-biting habit that continues is a sensible explanation for recurrence.

A similar blister in the floor of the mouth, under the tongue, is called a ranula. A ranula comes not from the small glands but from the large salivary gland under the tongue, and it can be bigger. One type extends down from the floor of the mouth towards the neck and is noticed as swelling under the jaw. A large ranula can push the tongue out of place, make speaking and swallowing difficult, and affect breathing if it presses on the windpipe. The treatment options include drainage methods and removal of the gland it comes from; an analysis comparing ranula treatments found no single method clearly better at preventing recurrence.

The distinction below is not a firm diagnosis; it is a framework showing when waiting is reasonable and when an examination is needed.

The sequence below describes the steps followed for a fluid-filled blister on the lip or in the mouth. The first place to go is your dentist.

1. 1

   If it is affecting your breathing, emergency first

   If a swelling under the tongue that is growing fast is making it hard to breathe, do not wait for the next steps; call 112. A small mucocele on the lip does not cause a picture like this.

2. 2

   History: when it appeared, and what happened

   The clinician asks whether you bit your lip, how many times the blister has filled and gone down, and how long it has been there. If you took photos, bring them; a blister that fills and goes down may have gone down on the day of the examination.

3. 3

   Examination and telling it apart

   The blister's consistency, colour, whether it fades when pressed, and its location are assessed. A mucocele is usually diagnosed from its appearance and the history. For large swellings in the floor of the mouth or swellings extending into the neck, imaging may be requested.

4. 4

   Correcting the trigger

   If there is a broken tooth edge, a sharp filling or a denture edge rubbing against the lip, it is corrected. A lip-biting habit is discussed; as long as the trigger continues, the risk of recurrence remains whichever method is chosen.

5. 5

   Removal if needed

   For a mucocele that keeps coming back or is large, the blister is removed under local anaesthetic together with the small gland producing the secretion. Laser or freezing are among the options too. If the clinician considers it necessary, the removed tissue is sent for examination; the firm diagnosis is made by this examination.

6. 6

   Check-up

   A check-up appointment is given to look at healing and recurrence. If a new blister forms in the same place, report it; recurrence is seen more often with age.

These do not replace one another; which is suitable is decided by where the blister is, its size, whether it keeps coming back, and your age.

A mucocele is harmless in itself. Most problems come from trying to deal with it at home, or from taking something else that looks similar for a mucocele.

The course below is a general framework; it varies with where the blister is, its size and the method chosen.

1. The first few days

   The blister appears shortly after the bite. Not touching it and not biting it during this period keeps its chance of going down on its own.

2. The following weeks

   Some burst, go down and do not come back. A blister that fills, goes down and fills again should be shown to a clinician; as long as the gland stays in place, this cycle can continue.

3. After the procedure

   There is tenderness in the treated area for a few days. Follow the care advice your clinician gives you, and if there are stitches, find out when they will be taken out.

4. In the longer term

   Recurrence after removal is uncommon but possible, and a little more frequent with age. If you see a new blister in the same place, report it.

A small mucocele is often simply watched; the scope changes with whether a procedure is needed and where the blister is. These are the items that determine it:

Watching or a procedure

A mucocele that goes down on its own needs no further procedure; with one that keeps coming back or is large, removal comes up.

The method chosen

A scalpel, a laser, freezing or a drainage stitch need different equipment and time.

Examination of the tissue

Sending the removed tissue to pathology is a separate item.

Where the blister is

A mucocele on the lip and a ranula under the tongue or extending into the neck are very different scopes; a ranula needs surgical planning and sometimes imaging.

Correcting the trigger

Repairing a broken tooth or a sharp filling edge rubbing against the lip is a separate procedure.