A comedo, or blackhead, may be defined as a plug of sebaceous material found in a duct of a sebaceous gland. It is most commonly seen as the elementary lesion of acne vulgaris, but it may also be present in linear nevi, postradiation atrophy, hidradenitis suppurativa, and tar acne.1-3
A single comedo may have more than one opening on the skin. The term "double comedo'' (Ormsby, as quoted by Ormsby and Montgomery4) was used to describe a comedo having two openings which were connected beneath the skin (Fig. 1b). No name was given to the comedo which had more than two openings on the skin.
The term "grouped comedones'' (Fig. 1e) is used to describe a patch of numerous blackheads, each of which has a single opening on the skin (Thin, as quoted by Ormsby and Montgomery).
A giant comedo (Fig. 1g) describes any huge blackhead
WATSON JB. Monoporous and Polyporous Acne. AMA Arch Derm. 1959;80(2):167–170. doi:10.1001/archderm.1959.01560200035003
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