Epidermoid and pilar cysts (sebaceous cysts)
What are epidermoid and pilar cysts?
A cyst is a sac that is filled by a liquid or a semi-solid.
Two of the most common types of cysts that form under the skin in various parts of the body are epidermoid and pilar cysts. They used to be called sebaceous cysts, but this term has been abandoned as they do not contain sebaceous fluid as was once thought.
The contents of both types of cyst are made up of soggy keratin, comprising the substance that hairs and the top layer of skin are made from. It resembles an oily toothpaste or soggy cheese.
The lining (sac) of the cysts is made from different substances.
The lining of an epidermoid cyst forms from cells that normally occur on the top layer of the skin (the epidermis). The lining of a pilar cyst is made up of cells similar to those found in the bottom of hair follicles.
What causes them?
Cells that are normally near to the surface of the skin are believed to get into deeper parts of the skin where they continue to multiply. These cells form a sac and make keratin - just as they would if they were on the top layer of skin.
What are the symptoms of these cysts?
Both types of cyst are common. They grow slowly and appear as round, domed bumps under the skin. Sometimes they are about the size of a pea, but they can reach several centimetres in diameter.
It is not possible to tell epidermoid and pilar cysts apart, except with the aid of a microscope.
Pilar cysts are most commonly found on the scalp. Epidermoid cysts are usually found on the face, neck and upper trunk.
Are they dangerous?
Neither type of cyst is usually harmful and, unless they are causing problems, can be left alone. They are not cancerous and cannot spread to other parts of the body or be passed on to others.
However, it is worth asking your doctor to look at them to confirm the diagnosis.
Sometimes the cysts can become infected, usually as a result of being bumped or "caught" on something like a comb.
Can they be treated?
Yes. If your cyst becomes infected, your GP might prescribe an antibiotic.
Both types of cyst can be removed under local anaesthetic, although you will be left with a scar. Occasionally the cyst can regrow, especially on the scalp or scrotum.