Pilar Cysts (Trichilemmal Cysts)

What a pilar cyst is

Each hair grows from a follicle, which is wrapped in an outer root sheath. A pilar cyst forms when cells from this sheath grow inward to form a sac. The lining produces a dense, compact keratin, which builds up slowly and stretches the sac into a firm ball. Because the lining makes keratin differently from the skin's surface, the contents are firmer and less smelly than those of an epidermoid cyst.

About 70% of people with pilar cysts have more than one, and a strong family tendency is common, inherited in an autosomal dominant pattern (one parent with the trait can pass it to half of their children).

What pilar cysts look and feel like

• Round, dome-shaped lump, usually 0.5 to 5 cm (about 0.2 to 2 inches) across
• Firm or rubbery, with a smooth surface
• Moves slightly when pushed
• Skin over the top is normal or shiny, and hair may be thinner over large cysts
• No central opening or pore, unlike many epidermoid cysts
• Usually painless unless knocked, combed hard or inflamed

Pilar cyst vs other scalp lumps

The term "sebaceous cyst" is often used loosely for both pilar and epidermoid cysts, even though neither comes from oil glands.

Complications

• Inflammation or rupture: if the cyst is knocked or squeezed, keratin leaks into surrounding skin, causing a red, painful, swollen lump that looks infected but is often sterile inflammation.
• Infection: less common, with pus, warmth and spreading redness.
• Proliferating trichilemmal tumor: rarely, a pilar cyst grows quickly into a larger lobulated mass, usually in older women. It is usually benign but can recur, and very rarely becomes malignant, so fast-growing cysts should be removed and examined.

Diagnosis

A doctor usually recognizes a pilar cyst by its look, feel and location. An ultrasound can help when the diagnosis is uncertain. After removal, the cyst may be sent to a lab, particularly if it was large, unusual or growing fast.

Treatment and removal

Leaving it alone

Small, painless cysts can safely be left. They will not go away by themselves, and they may slowly enlarge.

Surgical removal

Pilar cysts are among the easiest cysts to remove, because their tough wall separates cleanly from surrounding tissue:

1. The area is numbed with local anesthetic. Hair is usually parted rather than shaved.
2. A small incision is made over the cyst.
3. The cyst is squeezed or dissected out whole, wall and all.
4. The wound is closed with stitches, staples or sometimes left to heal if very small.

The procedure takes about 15 to 30 minutes in a clinic. Stitches come out in 7 to 14 days. Removing the entire wall makes recurrence unlikely; if part of it remains, the cyst can return. Insurance may not cover removal for cosmetic reasons alone.

Inflamed cysts

A red, swollen cyst may be treated with a steroid injection, warm compresses, or drainage, with antibiotics only if there are signs of infection. Full removal is usually done a few weeks later once the inflammation settles.

What not to do

Do not squeeze or try to cut out a cyst at home. This can cause inflammation, infection, scarring and recurrence.

Outlook

Pilar cysts are harmless. People who are prone to them often develop new cysts over the years, but each one can be removed simply. Malignant change is extremely rare.

Frequently asked questions

What causes pilar cysts?

Pilar cysts form from cells of the hair follicle's outer root sheath that grow into a sac filled with keratin. The tendency is often inherited, so several family members may have them. They are not caused by poor hygiene, hair products or diet.

Can a pilar cyst go away on its own?

No. Because the cyst has a wall that keeps producing keratin, it generally stays or slowly grows. Occasionally it shrinks after draining, but it usually refills. Complete surgical removal of the cyst wall is the only way to get rid of it permanently.

Is a pilar cyst dangerous?

No. Pilar cysts are benign and almost never become cancerous. Have a lump checked if it grows quickly, bleeds, ulcerates or feels hard and fixed, since a rare variant called a proliferating trichilemmal tumor or another type of lump may need removal and testing.

What is the difference between a pilar cyst and a sebaceous cyst?

"Sebaceous cyst" is an older catch-all name. Pilar cysts come from hair follicle roots, usually on the scalp, and have firm contents without a central pore. Epidermoid cysts, the other type often called sebaceous cysts, come from the skin's surface layer, often have a pore, and contain softer, smelly material.

Does pilar cyst removal leave a scar?

There is a small linear scar, usually hidden by hair on the scalp. Careful closure and following wound-care instructions keep it small. Hair usually grows normally around the scar.