Necrobiosis lipoidica (NL) is a rare granulomatous noninfectious skin disease of unknown etiology and pathogenesis [1]. It was usually described in patients with diabetes mellitus. Rare cases of familial NL without impaired glucose tolerance have been published [2]. NL was also reported over surgical scars (appendectomy scar in a patient having morphea) [3].
Typical, NL lesions occur on the pretibial area as distinctive plaques with yellowish atrophic center and violaceous indurated periphery [4]. Microscopically, NL is typified by palisading necrobiotic non-caseating granulomas aligned parallel to the skin surface. Necrobiosis consists of eosinophilic, swollen, and degenerated collagen, and vascular changes are often seen [4, 5].
Therapeutic options are numerous, including topical or intralesional steroids, topical tacrolimus, photochemotherapy, antimalarial drugs, photodynamic therapy, fumaric acid, thalidomide, pentoxifylline, heparin injections, tumor necrosis factor (TNF) inhibitors, and excision followed by a graft. No treatment option is constantly effective [1].
Moreover, the skin is the most common site of extraintestinal involvement of Crohn’s disease (CD) (22–44% of patients), and skin lesions may be present during, after, and in rare cases, before the active stage of CD. Their exact etiology remains unknown; it is probably a multifactorial response due to immune mechanisms with a granulomatous type IV hypersensitivity reaction [6].
Cutaneous manifestations of CD have variable morphology [6]. The currently accepted classification is based on pathogenic mechanisms [7]: similarly to the pathogenic mechanism of CD, CD-specific manifestations are granulomatous (fissures, fistulae, oral involvement), and granulomatous lesions in a discontiguous site from the gastrointestinal tract are considered as metastatic CD. Reactive cutaneous manifestations arise from a different pathogenic mechanism (erythema nodosum, pyoderma gangrenosum, oral aphthae, erythema multiforme, cutaneous vasculitis, and so on); there are also cutaneous manifestations that are associated with CD, but without a well-described pathogenetic mechanism (palmoplantar pustulosis, vitiligo, palmar erythema, hidradenitis suppurativa) [7].