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. We all used to know them as 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 soggy cheese.
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.
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, we might prescribe an antibiotic.
Both types of cyst can be removed under local anaesthetic although you will be left with a small scar. Occasionally the cyst can regrow, especially on the scalp or scrotum.