A cyst is a closed pocket of tissue that can form almost anywhere in your body. It can be filled with fluid, air, pus, or semi-solid material, and most cysts are benign. They range from tiny sacs you’d never notice to large growths that cause pain or interfere with organ function. Understanding what a cyst is, what causes one, and when it needs attention can save you a lot of unnecessary worry.
How a Cyst Forms
The cells that make up the outer wall of a cyst are distinct from the tissue around them. They create a sealed pocket, almost like a balloon, that gradually fills with whatever substance the surrounding tissue produces. On the skin, that’s often a thick, waxy material made by oil glands. In the lungs, cysts tend to fill with air. In organs like the kidneys or liver, they’re typically fluid-filled.
Several things can trigger cyst formation. A blocked duct or gland is one of the most common causes: when oil, mucus, or other fluid has no way to drain, it backs up and forms a pocket. Chronic inflammation can also prompt the body to wall off tissue. Foreign material lodged under the skin, like a splinter, can trigger a cyst to form around it. And in some cases, genetics play a direct role. Autosomal dominant polycystic kidney disease, for instance, involves inherited gene mutations that cause hundreds of cysts to develop in the kidneys over time. Researchers have identified this as a two-step process: a person inherits one faulty copy of the gene, and then a second mutation in individual cells triggers each cyst to form independently.
Common Types of Cysts
Cysts show up in a wide variety of locations, and each type has its own characteristics.
- Epidermoid cyst: The most familiar skin cyst, sometimes called a sebaceous cyst. It develops in the top layer of skin when cells that normally shed get pushed deeper instead, forming a sac filled with soft, cheese-like material. These are slow-growing and painless unless they become infected.
- Pilar cyst: Forms in a hair follicle, most commonly on the scalp. Similar to an epidermoid cyst in appearance but develops from different cells. Also sometimes called a sebaceous cyst.
- Ganglion cyst: A firm, round lump that appears along a joint or tendon, most often on the wrists, hands, or fingers. It’s filled with a thick, jelly-like fluid and can fluctuate in size.
- Baker’s cyst: A fluid-filled swelling behind the knee, usually linked to arthritis or a cartilage tear. It can cause tightness and discomfort, especially when bending or straightening the leg.
- Ovarian cyst: Extremely common. Studies estimate that between 5% and 17% of postmenopausal women have ovarian cysts, and they’re even more frequent in women of reproductive age, where small cysts form and resolve naturally with each menstrual cycle.
What a Cyst Feels Like
Most cysts grow slowly and cause no pain at all, at least at first. A skin cyst typically feels like a smooth, firm marble just beneath the surface that you can move slightly with your fingers. It’s usually the same color as your skin, though it may develop a small dark opening at its center.
Pain tends to show up only when a cyst grows large enough to press on surrounding tissue, or when it becomes inflamed or infected. An infected cyst turns red, swollen, and tender, and it may feel warm to the touch. Internal cysts, like ovarian cysts, can go undetected for months or years. When they do cause symptoms, it’s often a dull ache or a feeling of fullness or pressure in the area.
How Cysts Differ From Abscesses and Tumors
People often confuse cysts with abscesses or tumors, but they behave differently. An abscess is a pocket of pus caused by an active infection, usually bacterial. It’s red, swollen, painful, and warm from the start. A cyst, by contrast, grows slowly and isn’t usually painful unless it becomes enlarged or infected. When a cyst does get infected, it essentially becomes an abscess.
A tumor is a solid mass of tissue, not a fluid-filled sac. Both cysts and tumors can be benign or, rarely, malignant. On imaging like ultrasound or CT scans, cysts that appear uniform and fluid-filled are almost always benign. If a cyst contains solid components, that’s when further evaluation is needed to rule out something more serious. A biopsy, where a small tissue sample is removed and examined under a microscope, is the most reliable way to determine whether any growth is benign or malignant.
How Cysts Are Diagnosed
Your doctor can often identify a skin cyst through a simple physical exam. For cysts deeper in the body, imaging is the standard first step. Ultrasound is particularly useful because it can clearly show whether a lump is fluid-filled (likely a cyst) or solid (possibly a tumor). MRI and CT scans provide more detailed views when needed, especially for cysts in the brain, abdomen, or pelvis.
If there’s any uncertainty about what’s inside a cyst, a fine needle aspiration can be performed. A thin needle is guided into the cyst, often with ultrasound imaging, to withdraw fluid or cells for analysis. This serves double duty: it can relieve pressure from a large cyst while also providing a sample for testing.
Treatment and Recurrence
Many cysts need no treatment at all. Small, simple ovarian cysts under 3 centimeters in postmenopausal women, for example, carry a low risk of malignancy and don’t require follow-up. Slightly larger ones, up to 5 centimeters, can be monitored with a repeat evaluation in four to six months. If the cyst stays the same size or shrinks, it’s generally safe to stop monitoring after a year.
When treatment is needed, the approach depends on the cyst’s type, size, and location. Draining a cyst with a needle provides fast relief but doesn’t remove the sac wall, which means the cyst often refills. Surgical excision, where the entire sac is removed, is more definitive. Even so, recurrence is possible. Studies on surgically removed cysts in certain locations show recurrence rates ranging from 1% to 30%, depending on how completely the cyst wall and any connected tissue are removed.
Skin cysts that aren’t bothering you can safely be left alone. If one becomes infected, antibiotics or drainage may be needed before the cyst can be surgically removed. Cysts that are painful, growing rapidly, or in a cosmetically sensitive spot are reasonable candidates for removal.
Signs a Cyst Needs Attention
Most cysts are harmless, but certain changes warrant a closer look. Rapid growth over days or weeks is one red flag, especially if the cyst is also becoming painful. Redness, warmth, and swelling suggest infection. Any cyst that appears to have solid components on imaging, rather than being purely fluid-filled, needs further evaluation to rule out malignancy.
For internal cysts, symptoms like persistent pelvic pain, bloating that doesn’t go away, or difficulty with urination or bowel movements can indicate that a cyst is large enough to press on nearby organs. Sudden, severe abdominal pain could signal a ruptured ovarian cyst or ovarian torsion, both of which require immediate medical care.