What is a pilar cyst?
A pilar cyst, also known as a trichilemmal cyst, is a keratin-filled cyst that originates from the outer hair root sheath. Keratin is the protein that makes up hair and nails. Pilar cysts are most commonly found on the scalp and are more frequent in middle-aged females. They often run in the family, as they have an autosomal dominant pattern of inheritance (ie the tendency to the cysts can be is passed on by a parent to their child of either sex, and the child has a 1 in 2 likelihood of inheriting it).
What are the clinical features of pilar cyst?
Pilar cysts may look similar to epidermoid cysts and are often incorrectly termed sebaceous cysts. Pilar cysts present as one or more firm, mobile, subcutaneous nodules measuring 0.5 to 5 cm in diameter. There is no central punctum, unlike an epidermoid cyst. Pilar cyst can be painful if inflamed.
What does pilar cyst compare to epidermoid cyst?
|Pilar cyst||Epidermoid cyst|
|Most common site||Scalp (90% of pilar cysts), scrotum||Face, neck, trunk|
|Origin||Outer root sheath||Epithelium or hair follicle infundibulum|
|Cyst wall||Thick and less prone to rupture||Delicate and prone to rupture|
|Histology||Granular layer absent||Granular layer present|
What are the histological findings in pilar cysts?
The pathology of a pilar cyst is characteristic. The wall of the cyst is stratified squamous epithelium (skin) that has a palisaded outer layer, which resembles the that of outer root sheath of a hair follicle. The inner layer does not have a granular layer. The cyst shows very dense pink keratin on haematoxylin and eosin staining.
What is the treatment for pilar cysts?
It is not necessary to remove pilar cysts if they are not causing symptoms. However, incision and drainage under local anaesthesia provides comfort, and elective excision before rupture prevents scarring.
Surgical treatment involves either of the following methods:
- Enucleation of the cyst, ie removal without cutting into it and leaving surrounding skin intact
- Incision followed by expression of contents and removal of cyst wall – this is often best achieved with a surgical punch with the cyst expressed through the hole