A dermoid cyst is a cyst which lies deep to the skin and is lined by skin. So a dermoid cyst can be called an epidermal cyst.
These cysts are lined by squamous epithelium and these contain pultaceous or tooth paste like material which contain desquamated epithelial cells.
There are four types of dermoid cyst-
- Sequestration dermoid
- Implantation dermoid-Acquired variety
- Teratomatous dermoid.
This is a congenital variety of dermoid cyst, which is formed by inclusion of epithelium buried at the line of embryonic fusions. So these are found along the lines of fusion of the two embryonic segments.
This cyst is lined by stratified squamous epithelium with hair, hair follicles, sebaceous glands and sweat glands. It contains white pultaceous tooth paste like desquamated material with or without hairs. It is the mixture of sebum, sweat and desquamated epithelial cells.
Common sites are:
- At the midline of the body particularly in the neck.
- External angular-just above the outer canthus of the eye-at the line of fusion of the frontonasal and maxillary processes.
- Post auricular-behind the ear at the site of fusion of the mesodermal hillocks.
- On the skull at the site of fusion of the skull bones.
- At the midline of the face particularly at the root of the nose.
At the line of embryonic fusion, a few ectodermal cells are sequestrated into the deeper layer. Ultimately these cells proliferate and liquefy to form a sequestration dermoid cyst. Such cyst lies almost near the mesoderm from where the bones develop, that is why indentation is often found in the underlying bone. Sometimes the cyst starts in the mesoderm so that there may be prolongation of the cyst through the bone and a portion of the cyst may remain intracranial.
The cyst may be noticed at birth, but it is usually seen a few years later-the time taken to form the cyst.
A painless swelling, which is slowly growing is the main symptom. Cosmetic disfigurement is the main complaint. Such cyst hardly becomes big enough to cause any serious mechanical disability and rarely may become infected.
- Site: The site is very peculiar as this must be at the embryonic fusion.
- Size and shape: Such cyst hardly attains a size bigger than 2 cm in diameter. It is usually ovoid or spherical in shape.
- Surface: is smooth
- Punctum: A dermoid cyst does not have puctum which is often found in sebaceous cyst.
- Consistency: Such cyst feels soft. It may indent with pressure as the content is thick pultaceous material, mixture of sebum, sweat and desquamated epithelial cells.
- Compressibility or reducibility: This cyst can neither be compressed nor be reduced (cf. meningocele).
- Relations: Dermoid cyst is not attached to the skin. So the skin can be lifted off the cyst easily. This cyst is also free from underlying structures. There may be bony indentation when the bone lies exactly deep to the cyst. This can be palpated by moving the base of the cyst with a finger.
- Transillumination test-is always negative.
- While lying above cranial bones such cyst may have intracranial connection.
This may show a depression in the bone underlying the cyst or a gap. Such gap may be present when there is an intracranial extension or a fibrous band may pass through this bony gap and connect the cyst with the underlying durameter.
- Pressure symptoms to the surrounding structures
Complete excision of the cyst is the treatment of choice. This should be done under general anaesthesia as the cyst has to be dissected from the sensitive pericranium moreover there may be intracranial extension.
If preliminary X-ray shows a gap in the underlying bone, the operation has to be delayed to give an opportunity for spontaneous closure.
If there be intracranial extension, osteoplastic flap should be removed for excision of the intracranial part.