Supplementary Material for: Idiopathic Acroosteolysis: A Novel Cutaneous Sign Can Help Identify the Condition Early

Acro-osteolysis (AO) is a rare condition characterized by resorption of the distal phalanges of the fingers and/or toes. It can be familial, idiopathic (IAO), occupational, or secondary. Other authors suggest a classification into primary (genetic disorders, lysosomal storage disorders) or secondary AO. Various skin and nail changes have been reported in this condition. However, the cutaneous change on the affected digit(s)/toe(s) during the natural course of AO has been poorly documented. A 5-year-old girl presented with a 3-month history of a distinct transverse boundary between normal skin proximally and affected crusted skin overlying osteolysis distally (‘split’ sign) on the plantar surface of the third toe. This boundary gradually elongated circumferentially to involve the dorsal surface. The mother gave a similar history of a delimitation line on the 2nd, 4th and 5th toes of the right foot with a duration of 3 months, one year, and two years, respectively, that disappeared before she noticed a shortening of those toes. X-rays revealed partial resorption of the terminal phalanx of the third toe and several lytic changes in the middle and terminal phalanx of the second, fourth, and fifth toes. The clinical features, radiology findings and a work-up that helped rule out conditions associated with AO (secondary AO) helped establish the diagnosis of IAO in our patient. This case study highlights that the natural course of IAO includes distinct skin findings, such as the ‘split’ sign that we describe. This sign can help identify the condition early.

肢端骨溶解症（Acro-osteolysis, AO）是一种罕见病症，以手指及/或足趾远节指（趾）骨吸收为核心临床特征。该疾病可分为家族性、特发性（Idiopathic Acro-osteolysis, IAO）、职业性及继发性四类；另有学者提出将其划分为原发性（涵盖遗传性疾病、溶酶体贮积症）与继发性肢端骨溶解症。目前已有文献报道该病可伴随多种皮肤及甲部异常，但关于其病程中受累指/趾的皮肤病变，相关记载却较为匮乏。

本次收治1名5岁女童，其第三趾足底可见一处清晰的横向分界：近端为正常皮肤，远端覆盖于骨溶解区域的受累皮肤呈结痂样改变，即所谓“分裂征”，该表现已有3个月病程。此分界逐渐沿环形扩展，最终累及足趾背侧。患儿母亲自述存在类似病史：其右脚第2、4、5趾曾出现分界线，病程分别为3个月、1年及2年，且在她察觉趾体缩短前，该分界线已自行消失。

X线检查结果显示：第三趾远节趾骨存在部分吸收，第2、4、5趾的中节及远节趾骨可见多处溶骨性改变。结合患儿的临床特征、影像学表现，以及排查所有与肢端骨溶解症相关的继发性病因后，最终确诊该患儿为特发性肢端骨溶解症。

本病例报告揭示，特发性肢端骨溶解症的自然病程中可出现特征性皮肤表现，如本次描述的“分裂征”，该体征可辅助临床早期识别该病。