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.