Birklein et al.4 separated CRPS patients (type I and II) into 2 duration subtypes based on a 3-month CRPS duration cutoff (acute CRPS, <3 months, and chronic CRPS, >3 months) when looking at keratinocyte and mast cell proliferation in CRPS skin biopsies and identifying the cellular source of upregulated TNF-alpha, IL-6, and tryptase. Here, IL6 is linked to complex regional pain syndrome.