Dermoid cyst: a painless midline neck lump
A neck dermoid cyst is a benign pocket of skin, formed when skin-forming tissue is trapped as the embryo fuses, that typically appears in the midline.
Remember
A painless, doughy (soft rather than hard) lump in the midline beneath the chin that does not rise when the tongue sticks out suggests a dermoid cyst. A thyroglossal duct cyst, a sac left along the path the thyroid took as it descended from the tongue base, usually follows tongue movement.
Memory aid
a dermoid is a small pocket of skin sealed in a bag
Think of the lump as a small pocket of skin sealed in a bag, so it feels doughy and grows slowly, but is not tied to the tongue or the thyroid.
A dermoid cyst is a benign cyst. It is congenital, meaning it begins before birth, even though it is often not noticed until later. It forms when skin-forming tissue becomes trapped as the structures of the embryo fuse, and the trapped tissue stays as a closed pocket. Dermoid means skin-like, so a dermoid is a pocket of skin in the wrong place.
Because the trapped tissue behaves like skin, the wall of the cyst contains skin structures such as hair follicles and sweat glands. The cyst itself collects keratin (the tough protein of skin and hair), sebum (the oily secretion of skin) and sometimes hair. These contents help explain the soft, doughy feel.
The lump typically appears as a painless swelling in the midline, particularly beneath the chin (the submental region). It grows slowly, can reach a considerable size before it causes symptoms, and is most commonly seen in young adults.
Movement is the examination clue. A thyroglossal duct cyst moves upwards when the tongue sticks out (and also on swallowing), and a thyroid swelling moves upwards on swallowing. A lump that follows neither movement is more likely to be a dermoid.
A true dermoid also differs from a teratoid cyst (a cyst containing derivatives of all three germ layers, the three primitive tissue sheets that form every organ). Teeth are not typical contents of a dermoid in the neck (a cervical dermoid).
Key facts
- Origin
- skin-forming tissue trapped as the structures of the embryo fuse, giving a benign cyst
- Site
- typically a painless lump in the midline, particularly beneath the chin
- Feel and growth
- soft and doughy rather than hard, and slow-growing, so it may only be noticed in young adulthood
- Contents
- keratin, sebum and sometimes hair, with hair follicles and sweat glands in the wall
- Movement
- does not rise when the tongue sticks out or on swallowing, unlike a thyroglossal duct cyst or a thyroid swelling
- Teratoid cyst
- contains derivatives of all three germ layers, whereas a true dermoid contains skin structures
Exam traps
- A thyroglossal duct cyst is also painless and midline, so it is the classic confusion, but it moves upwards when the tongue sticks out. A lump that does not move points away from it.
- A midline neck cyst containing tissue from all three germ layers, such as teeth, is a teratoid cyst, not a dermoid, because a true dermoid of the neck contains only skin-type tissue.
- A branchial cleft cyst (a congenital cyst left when part of the neck apparatus of the embryo fails to close completely) lies on the side of the neck along the front edge of sternocleidomastoid (the prominent muscle on either side of the neck), not in the midline.
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