Acne keloidalis (AK) is one of the primary cicatricial alopecias and predominantly affects men of African descent. Reports in Asians are scant. This study aimed to retrospectively review the clinical and histopathological features of AK patients in southern Taiwan and identify the pathognomonic features of AK. There were 15 patients with histopathologically confirmed AK in National Cheng Kung University Hospital between 1988 and 2018. The median onset age was 24 years (range, 14–71). The male : female ratio was 14:1. In the acute stage of AK, the lymphocytic and neutrophilic peri-infundibular inflammatory infiltrates with microabscess formation and edema corresponded to the clinical finding of isolated papules or pustules. Subsequently, the inflammatory infiltrates involved the mid-dermis and the isthmus of hair follicles. The “spade sign”, a thin and dilated space resembling the shape of a balloon or spade symbol of playing cards at the level of lower isthmus, was identified in eight biopsies from five patients and may be a pathognomonic sign in the subacute stage of AK. At the chronic stage, the segments of hair shafts remained in the upper to mid-dermis and induced chronic inflammation and extensive fibrosis, resulting in the clinical keloid-like appearance. The restriction of inflammation and fibrosis in the upper to mid-dermis was another unique and pathognomonic feature of AK.
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