Hairpin (looped) vessels are characterized by vessels that resemble hairpins, consisting of a U-shape with a sharp bend at one end. These looped hairpin vessels can appear distorted when the hairpin configuration twists around its own axis.
Hairpin vessels are found in melanocytic and nonmelanocytic tumors. Relatively thin, uniform hairpin vessels are often seen in keratinizing tumors, such as seborrheic keratoses (SKs). In these benign keratinizing tumors, a whitish halo often surrounds the vessels. This whitish halo probably represents viable tumor keratinocytes surrounding the dermal papilla, which contains the hairpin vessel. In a study of pigmented SKs, hairpin vessels were seen in 63% of lesions; more specifically, these vessels were seen in 50% of plaque-type SKs and 43% of papular/nodular SKs but were only seen in 6% of patch type SKs. In another study looking at lesions with vascular dermoscopic structures, 51.2% of SKs contained hairpin vessels, conferring a PPV of 70%. In keratoacanthomas, hairpin vessels are located peripherally, with a yellow keratotic center. In contrast, hairpin vessels in melanomas are often surrounded by a pink halo. Hairpin vessels may also be observed in warts, SCCs, BCCs, dermal nevi, and Spitz nevi.