Branchial/Thyroglossal/Dermoid Cysts
- These are three congenital cysts that show up because something didn't fully close or migrate during development — a leftover from how the neck is built.
- Location is the whole game. Each one has a favorite address, and where it sits usually tells you which one it is before you even look inside.
- Thyroglossal duct cyst: midline, near the hyoid bone, and it moves when you swallow or stick out your tongue.
- Branchial cleft cyst (usually second): off to the side, classically just below the angle of the jaw, deep to the front edge of the sternocleidomastoid muscle.
- Dermoid: typically midline (often floor-of-mouth or near the eyebrow), and it may carry fat — sometimes even tiny fat globules that float, the famous "sack of marbles."
When you were a tiny embryo, your neck was assembled out of a surprising number of little arches, pouches, and ducts — imagine flat-pack furniture, but biology. Most of those parts dissolve or migrate exactly on schedule and you never think about them again. Congenital neck cysts are the screws that got left in the bag. A bit of tissue that should have closed up, or finished its road trip, instead stuck around and quietly filled with fluid. Years later it shows up as a painless lump, and a radiologist gets to play detective.
The good news: these three have such reliable home addresses that location does most of the diagnostic work. Learn the map and you're 80% of the way there.
Thyroglossal duct cyst: the one in the middle that swallows
The thyroid gland doesn't start where it ends up. It begins life high at the back of the tongue and then migrates down the front of the neck, trailing a little tube behind it called the thyroglossal duct. Normally that tube melts away. When a piece of it refuses to leave, it can fill with fluid — and because the path ran right past the hyoid bone, that's where the cyst usually parks.
Two tells make this one easy. First, it's midline (or just barely off it). Second, because it's tethered to that old tongue-to-thyroid track, it famously moves upward when the patient swallows or sticks out their tongue. It's the only neck mass that does a little dance on command.
Before anyone operates on a midline neck cyst, confirm there's a normal thyroid gland in the usual spot. Rarely, the "cyst" is the patient's only functioning thyroid tissue that never finished migrating — and removing it is a problem you do not want to cause.
Branchial cleft cyst: the one off to the side
The embryonic neck has a stack of structures called branchial (or pharyngeal) arches, separated by clefts. The second cleft is the troublemaker — the vast majority of branchial cleft cysts come from it. When that cleft doesn't close, the leftover space can balloon into a cyst.
Its address is so consistent it's almost a cheat code: lateral, classically near the angle of the mandible, sitting deep to the sternocleidomastoid, lateral to the carotid space, and posterior to the submandibular gland. Picture it nudging in among those structures like a beach ball wedged between three pieces of furniture. Understanding which compartment it lives in is easier once you know your neck spaces.
In an adult — especially someone older than about 40 — a "new branchial cleft cyst" deserves real suspicion. A cystic neck mass at that age can be a cystic lymph node metastasis, classically from an HPV-related oropharyngeal squamous cell carcinoma, masquerading as something benign. Don't reflexively call it congenital; that distinction is covered under suppurative adenitis vs nodal mets.
Dermoid: the one with stuff inside
A dermoid is an inclusion cyst — skin elements that got trapped where they shouldn't be along an embryonic fusion line. Because it's lined by skin-type tissue, it can contain the things skin makes: keratin, sebaceous gunk, and sometimes fat. That fat is the giveaway. On imaging, fat is the loud, recognizable houseguest — low density on CT, bright on certain MRI sequences — that the other two cysts simply don't bring.
In the head and neck, dermoids favor the midline (the floor of the mouth and the area around the eyebrow/orbit are classic). The signature finding, when present, is small rounded globules of fat floating within the cyst — described as a "sack of marbles." It's one of those phrases that sounds made up until you see it, and then you never forget it.
If a midline neck or floor-of-mouth cyst contains fat, you're almost certainly looking at a dermoid (or its cousin, the epidermoid — same idea, minus the skin appendages and usually minus the fat). Fat content is the feature that pulls dermoid out of the lineup.
A cheat sheet for the three
| Cyst | Where it lives | The tell |
|---|---|---|
| Thyroglossal duct | Midline, at/near the hyoid | Moves with swallowing/tongue protrusion; confirm normal thyroid first |
| Branchial cleft (2nd) | Lateral, deep to SCM near jaw angle | Beware the adult cystic node mimic |
| Dermoid | Midline (floor of mouth, brow) | May contain fat; "sack of marbles" |
When the picture gets muddy
All three are happiest when they're calm, simple, and fluid-filled — thin-walled and uniform inside. The moment one gets infected, the rules blur: the walls thicken, the contents get murky, the surrounding fat gets dirty-looking, and suddenly your tidy congenital cyst looks like an abscess. That's normal and expected — infection is the most common reason these finally announce themselves. Just remember that an inflamed cyst can imitate scarier things, and a scarier thing (that cystic node) can imitate a calm cyst.
So the takeaway is the one we started with: read the address first. Midline and dancing when you swallow? Thyroglossal. Off to the side under the SCM? Branchial. Midline with fat floating inside? Dermoid. Get the location, and these three mostly diagnose themselves.