Sebaceous Cyst

The Concept

A 'sebaceous cyst' is a common skin cyst arising from a blocked pilosebaceous (hair follicle) unit, filled with keratin. The name is something of a misnomer — the contents are mostly keratin (epidermoid or pilar cyst), not sebum — but the term is entrenched. It occurs on hair-bearing skin, commonly the scalp, face, neck and scrotum.

Clinical Features — the Punctum

It presents as a firm, round, smooth swelling within the skin that moves with the skin (confirming its cutaneous plane). Its pathognomonic feature is a central punctum — a small dark dot on the surface marking the blocked follicular opening. The contents are a cheesy, keratinous, often foul-smelling material.

Complications & Management

Complications include infection (a red, painful, discharging cyst), ulceration, a sebaceous horn, and (rarely) an ulcerating proliferative form called Cock's peculiar tumour. Treatment is complete surgical excision, removing the entire cyst wall — because leaving any of the lining leads to recurrence. An infected cyst is usually treated (incision/drainage and antibiotics) before definitive excision when the inflammation has settled.

Sebaceous CYST VS Lipoma

A common exam contrast is the sebaceous cyst versus the lipoma. The sebaceous cyst is in the skin (moves with the skin, has a central punctum, contains cheesy keratin), whereas the lipoma is subcutaneous (skin moves freely over it, no punctum, soft and lobulated with a slip sign). This distinction of plane and the presence or absence of a punctum is usually enough to tell them apart clinically, and it is a favourite bedside question.

The Bottom Line

A sebaceous cyst is a keratin-filled cutaneous cyst with a central punctum that moves with the skin, treated by complete excision of the cyst wall to prevent recurrence.

A Note on Managing the Infected CYST

The management of an infected sebaceous cyst follows a logical sequence: while acutely inflamed and infected, the cyst is treated with antibiotics and, if it has formed an abscess, incision and drainage — but definitive excision is deferred until the inflammation settles, because attempting to remove the whole wall in the middle of acute infection is difficult and tends to leave fragments behind. Once quiet, the cyst is excised completely with its wall to prevent recurrence — a sequence that mirrors the general surgical principle of not doing definitive surgery in infected, inflamed tissue.

The punctum and skin attachment distinguish it from lipoma.

• 'Sebaceous cyst' = keratin-filled cyst from a blocked pilosebaceous unit (mostly epidermoid/keratin, not sebum); scalp, face, neck, scrotum.
• Firm, round swelling in the skin, moving with the skin, with a pathognomonic central punctum; cheesy keratinous contents.
• Complications: infection, discharge, sebaceous horn, Cock's peculiar tumour.
• Treat by complete excision of the whole cyst wall (to prevent recurrence); settle infection first if present.