A cyst is a lump just underneath the skin, usually filled with fluid like a blister, but sometimes with air or semi-solid material.
They are relatively common and not usually something to be concerned about, but it’s important to check that they are not something more serious – see your GP if you have any sort of lump so it can be properly diagnosed.
Cysts are sometimes confused with boils or skin abscesses. A cyst may go on to become a boil or abscess, which are painful collections of pus that indicate an infection.
What do they look like?
Cysts are normally round or dome-shaped. They tend to be white or yellow, sometimes with a small dark plug through which you might be able to squeeze out pus. Cysts are usually relatively small, ranging from pea-sized to a few centimetres across.
A cyst shouldn’t normally be painful, but they can become tender or sore if they become infected. Foul-smelling pus coming out of the cyst is another sign of infection.
What types of cysts are there?
There are many different types of cyst, usually differentiated by where they appear.
Epidermoid cysts are one of the most common kind. These are normally found on the face, neck, chest, shoulders, or on the skin around the genitals. They’re most likely to affect young and middle-aged adults, and are more likely to appear on people who suffer with acne.
Cysts that form around hair follicles are known as pilar cysts, and are mainly found on the scalp. These typically affect middle-aged adults, particularly women. Unlike epidermoid cysts, they can be hereditary.
A cyst that forms on the eyelid is called a chalazion, or meibomian cyst.
There are also internal cysts, which can be more serious; these aren’t easily diagnosed, and are usually only detected by an imaging scan like an MRI or ultrasound.
How does a cyst form?
Cysts can be caused by infection, clogging of sebaceous glands, inflammatory conditions, or an injury that breaks a vessel.
Anyone can develop a skin cyst, but you're more likely to have one if you've been through puberty, you have a history of acne, or you've injured the skin – for example, if you've damaged a hair follicle.
How are cysts treated?
Cysts are usually harmless; small cysts that aren't painful or causing significant problems don’t need any medical treatment. Holding a warm flannel or towel against the skin will encourage the cyst to heal and reduce any inflammation.
Don't be tempted to burst the cyst. If it's infected, you risk spreading the infection, and if the sac is left underneath the skin, it can grow back. See your GP if you think the cyst is infected – you may need a course of antibiotics.
A large cyst causing problems or other symptoms can be surgically removed, or they can be drained. In the latter case your GP will use a local anaesthetic to numb your skin, before making a tiny cut and squeezing the cyst out.