That Lump Under Your Skin: How Epidermal Cysts and Lipomas Form, and How to Tell Them Apart by Touch

You are washing your neck, or scrubbing your back, and your fingers catch on a small round lump you never noticed before. Most people jump straight to searching the internet, but lumps under the skin are common, and the vast majority turn out to be harmless.

The two you will hear about most often are epidermal cysts and lipomas. From the outside they can look like the same kind of round bump, but they form in different tissue and feel noticeably different once you press on them. Let's go through how each one develops.

What Should You Suspect First When You Feel a Lump?

The first thing to check is size, whether it hurts when pressed, and how long it has been there. If there is no pain and the size hasn't changed much over several months, it is most likely benign.

There are many types of lumps that can form under the skin, but the two that come up by far the most in clinical practice are epidermal cysts and lipomas. Both grow slowly and cause almost no pain, so they can be easy to confuse just by looking at them. Understanding how each one actually forms makes telling them apart much easier.

Why Epidermal Cysts Form

An epidermal cyst forms when skin cells from the surface get trapped and curl inward instead of shedding normally. Skin cells are supposed to form on the surface and eventually flake off, but if the opening of a hair follicle gets blocked or compressed, those cells cannot escape and instead pile up in a pocket right underneath.

Think of a balloon that keeps filling up with dead skin cells. The inner wall is made of the same type of cells as the skin's outer layer, and inside, old skin cells and oil clump together into a paste-like material. That's why squeezing an epidermal cyst often releases a white, foul-smelling mass.

These used to be commonly called sebaceous cysts, but since the main ingredient is dead skin rather than sebum, epidermal cyst is now the more accurate term. If you look closely at the surface, you can often spot a tiny pinpoint opening at the center, which is the original follicle opening the skin cells used to pass through.

Why Lipomas Form

A lipoma is exactly what the name suggests: a lump made of clustered fat cells. There is normally a thin, even layer of fat spread under the skin, but for reasons that aren't fully clear, a group of fat cells starts multiplying abnormally and forms a mass wrapped in a thin membrane.

Picture a comforter that should have evenly spread stuffing, but instead has a clump bunched up on one side, poking outward. This mass of fat is surrounded by a thin fibrous capsule, which gives it a reasonably defined border from the surrounding tissue.

The exact cause of lipomas is still not fully understood, but they often run in families, which points to a genetic component. Unlike epidermal cysts, lipomas have nothing to do with hair follicles or skin cells. The biggest difference is that a lipoma is purely a problem within the fat tissue itself.

The Difference You Can Feel

The two lumps feel quite different under your fingers. An epidermal cyst is packed with compacted skin cells, so it feels firm and doesn't give much when pressed. A lipoma, on the other hand, is made of soft fatty tissue, so it feels squishy and yields like dough when you press on it.

• Firmness: An epidermal cyst feels about as firm as a rubber ball, while a lipoma presses like soft dough. That difference comes straight from what's inside, dead skin cells versus fat.
• Boundary: An epidermal cyst is fused to the skin above it, so it moves together with the skin when you pinch it. A lipoma sits in a deeper layer, so you can often shift the lump side to side while the skin stays put. That's because an epidermal cyst originates from a follicle and stays connected to the epidermis, while a lipoma forms in a deeper layer that has nothing to do with the epidermis.
• Surface opening: An epidermal cyst often shows a tiny pinhole on the surface, while a lipoma is smooth with no opening at all. This comes down to whether the old passage the skin cells once used to exit through is still there.
• Inflammation: An epidermal cyst can suddenly swell up and become painful if the trapped skin cells get infected by bacteria, while a lipoma rarely becomes inflamed and tends to stay in more or less the same state. That's because a cyst has an opening connecting it to the outside world, which makes it easier for bacteria to get in, while a lipoma is a sealed structure where that's far less likely.

How They Look From the Outside

Epidermal cysts tend to show up on the face, neck, and upper back, areas with a high density of hair follicles. Sizes range from a few millimeters to a few centimeters, and they often grow slowly larger over time.

Lipomas tend to appear on the back, shoulders, arms, and back of the neck, areas with a thicker fat layer. They generally grow larger than epidermal cysts and usually keep the skin's normal color while pushing upward from underneath as a smooth, rounded bulge.

For both types, the skin color usually stays more or less normal. The exception is that an epidermal cyst can turn red and swollen locally if it becomes inflamed, while a lipoma typically just changes in size over time without any color change.

Where Each One Tends to Show Up

Epidermal cysts can form anywhere there are hair follicles, which is why they show up even in exposed areas like the face. Since follicles are the root cause, areas with a higher density of follicles tend to be more prone to them.

Lipomas are more common on the trunk and limbs, where the fat layer is thicker, and while most people only develop one, some people develop several at once. Because the cause is fat tissue proliferating, lipomas are relatively more common on the trunk than on the face, where the fat layer is thin.

Are There Situations That Actually Call for Treatment?

Most epidermal cysts and lipomas are small and symptom-free, so there is usually no urgency to remove them. That said, a few situations are worth getting checked out.

• The lump suddenly grows fast, or becomes hard and fixed in place so it no longer moves under your finger. Benign lumps normally grow slowly, so a rapid change in a short period of time warrants a check to rule out other tissue changes.
• There is pain when pressed, or the area suddenly turns red and swollen. This can signal inflammation or infection developing inside an epidermal cyst, and if left alone it can fester and rupture.
• It's in a visible spot like the face or neck and is bothering you cosmetically. In this case, the visit isn't about danger, it's simply because you want it removed.
• A lipoma is larger than 5 centimeters or keeps growing. It's uncommon, but a large, fast-growing fatty mass may need to be distinguished from other types of tissue, so a checkup is recommended just to be safe.

How Doctors Confirm It With Testing

A visual and physical exam alone can usually tell the two apart fairly well, but when a definitive answer is needed, ultrasound is the go-to tool. Ultrasound lets a doctor look at the tissue under the skin in real time, which helps distinguish whether the contents are more like a semi-liquid mass of dead skin cells or actual fat tissue.

The brightness and border shape that show up on the screen differ between epidermal cysts and lipomas, because ultrasound waves reflect differently depending on the tissue's density and composition. If the findings are unclear or unusual, a biopsy may be used to confirm the diagnosis.

At the end of the day, these two lumps form from different tissue and behave differently, but they share the fact that both are common and almost always benign. If you notice a lump you're curious about, it's worth getting it checked in person rather than guessing on your own. It's the more accurate route, and it will put your mind at ease.

References

Korean Dermatological Association, American Academy of Dermatology (AAD), and Ministry of Food and Drug Safety (MFDS).