What are epidermoid and pilar cysts?
By definition, cysts are closed sacs that have two main features:
- A lining
- Contents that are liquid or semi-solid
The lining. Differences between the lining of epidermoid and pilar cysts can be seen under the microscope:
- The lining of an epidermoid cyst looks like the epidermis (the outermost layer of cells in the skin)
- The lining of a pilar cyst is made up of cells like those found in the roots of hairs
The contents. Both types of cyst contain a cheesy material, looking rather like toothpaste. This is made of soggy keratin - the material that makes up hair and the outer layer of the skin.
In the past, pilar and epidermoid cysts were wrongly known as ‘sebaceous’ cysts but this term should be used only for a quite different and much less common type of cyst that is filled with a clear oily liquid made by sebaceous (grease) glands.
Epidermoid and pilar cysts are common, benign, and not contagious.
What causes these cysts?
Epidermoid cysts affect young and middle aged adults. They can come up after a hair follicle has been inflamed, so they are common in acne.
Pilar cysts affect women more often than men, and tend to come up in middle age. They run strongly in families (see below).
Are they hereditary?
Epidermoid cysts by themselves are usually not hereditary, but they may be part of several rare conditions that are. Pilar cysts run strongly in some families, being inherited as an autosomal dominant trait - which means that there is a 1 in 2 chance that each child of an affected parent will inherit the condition.
What are their symptoms?
Both types grow slowly. Some become infected (red and sore) from time to time. They may then discharge cheesy foul-smelling pus. Those on the scalp can catch on the comb: others may look embarrassing.
What do these cysts look like?
They are round, sometimes dome-shaped bumps, lying just under the skin surface. Some are yellow or whitish. A small dark plug is often present, through which it may be possible to squeeze out some of the cyst’s contents. The cysts range in size from those that are smaller than a pea to those that are several centimetres across.
They can occur anywhere on the skin, but:
- Pilar cysts are most common on the scalp, where several can often be found.
- Epidermoid cysts are most common on the face, neck and upper trunk.
How will they be diagnosed?
Your doctor will make the diagnosis on the appearance of the bump. If there is any doubt, the cyst can be removed and checked under the microscope.
Can they be cured?
Yes. There are several simple and effective ways of removing them under local anaesthetic. However it is fairly common for new cysts to grow at a later date, especially on the scalp or scrotum.
How can they be treated?
- Epidermoid and pilar cysts are harmless, and small ones that give no trouble can safely be left alone.
- Your doctor may give you an antibiotic if your cyst becomes infected.
- Both types of cyst are easy to remove under a local anaesthetic but this does leave a scar.
Reasons for removal include the following:
- If the cyst is embarrassing and easily seen by others.
- If it interferes with everyday life, for example by catching on your comb.
- If the cyst becomes infected.
It is important that the doctor removes the whole of the lining during the operation as doing so cuts down the chance of the cyst growing back.
What can I do?
If you find any sort of lump in your skin, you should consult your doctor. Epidermoid and pilar cysts are not dangerous, but your doctor should be asked to look at them to make sure that the diagnosis is right.