What Are Dermoid, Epidermoid, 'Sebaceous' & Pilonidal Cysts?

No, these terms do not mean the same thing, though patients and even some healthcare providers often use them interchangeably. The easiest way to understand the difference is by looking at how they form, where they are located, and what is inside them.

Note: This page focuses on skin (cutaneous) cysts and lumps near the tailbone. Dermoid cysts can also form internally (such as in the ovaries or brain), but those involve different evaluation and treatment.

A dermoid cyst is a developmental cyst (formed before birth) that contains true skin layers and skin appendages like hair and oil glands ^[1]. An epidermoid cyst contains only surface skin cells and flaky keratin ^[2]. A “sebaceous cyst” is usually an inaccurate term for an epidermoid or pilar cyst ^[3]. Finally, pilonidal disease is an acquired, hair-associated inflammatory condition near the tailbone, rather than a true developmental cyst ^[4]. While most of these lumps are benign (non-cancerous), understanding exactly which type you have is important because it changes how the lesion should be evaluated and treated ^[5].

Summary Comparison of Common Skin and Tailbone Cysts

Dermoid Cysts vs. Epidermoid Cysts

Both dermoid and epidermoid cysts are closed sacs or lumps under the skin, but they differ in their origins and contents:

• Dermoid Cysts: These are developmental anomalies that form before birth when layers of skin get trapped along the lines where the embryo’s face or body fuses together ^[6]. Because they contain true skin layers, they can hold skin appendages like hair follicles, sweat glands, and sebaceous (oil) glands ^{[1] [2]}. While they form before birth, they may not be noticed until years later.
• Epidermoid Cysts: Sometimes called epidermal inclusion cysts, these sacs are lined with normal surface skin cells (squamous epithelium) but lack deeper skin structures like hair or oil glands ^{[1] [2]}. They are filled with flaky material called keratin ^[5]. Many are acquired later in life when a hair follicle gets blocked or a minor injury pushes surface skin cells deeper into the tissue ^[7].

The “Sebaceous Cyst” Misnomer

The term sebaceous cyst is a widely used but imprecise clinical label ^[5]. True cysts originating from the sebaceous (oil) glands do exist, but they are relatively rare.

When people refer to a “sebaceous cyst,” they are almost always looking at an epidermoid cyst or a pilar cyst (a cyst on the scalp) ^{[3] [8]}. Because the term is non-specific and unreliable, dermatologists and pathologists classify skin cysts based on pathology—what is seen under a microscope when the tissue is removed ^{[5] [8]}.

Tailbone Lesions: Pilonidal Disease vs. Congenital Tracts

Confusion often arises when a lump, dimple, or sinus (a small skin tract or tunnel) is found near the tailbone (the sacrococcygeal region). Not every lump near the tailbone is a cyst; proper diagnosis requires an in-person medical examination. A tailbone lesion could be a superficial acquired condition or a deeper congenital one:

• Pilonidal Disease: This is an acquired inflammatory condition, not a developmental cyst ^[4]. It happens when loose hair and debris get trapped in the natal cleft (the buttock crease) and irritate the skin, causing a chronic sinus tract or an acutely painful abscess ^{[14] [13]}. It most frequently affects adolescents and young adults ^[15].
• Congenital Dermoid Cysts or Dermal Sinuses: A small subset of midline lower-back lesions are developmental abnormalities. A dermal sinus is a tract or opening in the skin, while a dermoid is a closed cyst. Unlike superficial pilonidal disease, some congenital dermal sinuses or dermoids can connect to deeper structures, including the presacral space (the area in front of the sacrum) or even the spinal canal ^{[16] [17]}. Most dermoids do not involve the spine, but clinical evaluation is important to identify those that do.

Why Examination and Imaging Matter

Because a superficial pilonidal lesion and a deeper congenital dermoid can look similar from the outside, a careful clinical examination is critical ^[14]. Doctors look for red flags: a dimple, skin opening, or tuft of hair that has been present since birth raises more concern for a congenital tract than a routine pilonidal cyst ^[16].

Uncomplicated superficial pilonidal disease is usually diagnosed by a physical exam and does not routinely require advanced imaging. However, if a doctor assumes an atypical tailbone dimple is just a routine pilonidal abscess, a standard superficial drainage procedure may not address a deeper tract ^[18]. In rare cases, unrecognized connections to the spinal canal can lead to serious complications like meningitis (inflammation around the brain and spinal cord) or nerve damage ^[16].

For deep masses, atypical lesions, or lumps accompanied by neurological symptoms, imaging studies are used to check for deeper connections ^[16]. An ultrasound may be used to look at superficial tracts, while a dedicated MRI is generally the most informative test to evaluate for extension into the spinal canal ^{[19] [20]}.

General Treatment and Safety Guidance

Treatment depends entirely on the type of cyst, its location, and whether it is inflamed. Small, asymptomatic cysts may simply be monitored. Symptomatic or growing dermoid and epidermoid cysts are often completely surgically excised. Pilonidal disease is managed differently; an acute abscess requires medical drainage, while chronic disease may require hygiene measures or surgical removal ^{[18] [21]}.

Never attempt to pop, squeeze, or self-drain a cyst. Squeezing a cyst can rupture the sac wall beneath the skin, causing severe inflammation, pain, and a higher risk of infection.

When to Seek Urgent Care
While most cysts are benign and slow-growing, you should seek prompt medical assessment if you develop a fever along with rapidly increasing redness, warmth, severe pain, swelling, or pus.

Seek urgent or emergency care if a lesion near your spine or tailbone is accompanied by new leg weakness or numbness, difficulty walking, numbness in the “saddle” area (groin and buttocks), or a new loss of bowel or bladder control. While these symptoms do not definitively prove a spinal connection, they require immediate medical evaluation to rule out serious neurological involvement ^[16].