Epidermal inclusion cysts are the most common cutaneous cysts. Milia are small epidermal inclusion cysts. Pilar cysts are usually on the scalp and may be familial.
Benign cutaneous cysts are classified according to histologic features of the cyst wall or lining and anatomic location. On palpation, a cyst is firm, globular, movable, and nontender; cysts usually vary in size up to 5 cm in diameter. There are several types of benign cutaneous cysts:
Epidermal inclusion cysts
Milia
Pilar cysts (trichilemmal cysts)
Epidermal inclusion cysts (epidermoid cysts) seldom cause discomfort unless they have ruptured internally, causing a rapidly enlarging, painful foreign body reaction and abscess. Epidermal inclusion cysts are often surmounted by a visible punctum or pore; their contents are white, cheesy, and malodorous.
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Milia are minute superficial epidermal inclusion cysts that are most often on the face and scalp.
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Pilar cysts (trichilemmal cysts) may appear identical to epidermal inclusion cysts, but 90% are on the scalp. There is often a family history of pilar cysts; inheritance is autosomal dominant (1).
General reference
1. Hörer S, Marrakchi S, Radner FPW, et al: A monoallelic two-hit mechanism in PLCD1 explains the genetic pathogenesis of hereditary trichilemmal cyst formation. J Invest Dermatol139(10):2154-2163.e5, 2019. doi: 10.1016/j.jid.2019.04.015
Treatment of Cutaneous Cysts
Cyst excision or dissection, if desired
Milia evacuation
Troublesome cysts can be removed. To prevent recurrence, the entire cyst and its wall should be removed. Ruptured cysts can be incised and drained but may recur if the wall is not eventually removed. Antibiotics are not needed unless cellulitis is present.
Milia may be evacuated with a #11 blade and comedone extractor.
