Armpit sebaceous cyst: what the lump may mean

The everyday term sebaceous cyst commonly refers to an epidermoid cyst: a closed sac beneath the skin filled with keratin. It may feel round and mobile, grow gradually, and sometimes show a small central pore. If inflamed, it can become red, swollen and tender.

Not every armpit lump is a cyst. Possibilities include an inflamed hair follicle, abscess, hidradenitis suppurativa or an enlarged lymph node. Location, skin changes, texture and the history of the lump help a clinician distinguish among them; imaging may be considered when the examination does not settle the cause.

Do not squeeze, pierce or attempt to drain the lump at home. Pressure can rupture the cyst wall under the skin, intensify inflammation or introduce infection. Reduce friction from tight clothing, pause shaving over sore skin and use a warm compress for comfort without applying enough heat to burn.

A quiet, confirmed cyst may only need observation. A clinician may discuss an injection to calm selected inflamed cysts, drainage when an abscess or a tense infected collection is present, or removal of the cyst wall when a troublesome cyst repeatedly inflames or returns. Drainage can relieve pressure but does not necessarily remove the cyst lining.

Book an examination for a new lump that persists, enlarges, feels hard or fixed, or cannot clearly be identified as a skin cyst. Prompt review is warranted if redness is spreading, pain is escalating, pus is draining, fever develops, or arm movement becomes difficult.

Also seek assessment for repeated painful underarm nodules, tunnels beneath the skin or scarring, because that pattern can fit hidradenitis suppurativa and may need a broader treatment plan.

You could ask what features support a cyst rather than a lymph node or abscess, whether imaging is useful, and whether inflammation or infection is present. If a procedure is proposed, ask whether it is intended to settle inflammation, release a collection or remove the entire cyst wall, and what aftercare the wound will need.