Difficult and rare forms of acne

Abstract Acne is the most common of skin diseases, being characterized as a chronic inflammatory disease of the pilosebaceous unit. Although acne is usually straightforward to diagnose and treat, there are patients with difficult or rare forms of acne. What seems to be ‘non-responding acne’ in a patient may be due to another acneiform disease that clinically mimics acne, thus misleading the clinician if not ruled out with scrutiny. Difficulties in the management of acne may be due to patient-related issues (low adherence to treatment or fear for side-effects), treatment-related issues (inappropriate treatment, dose or duration of treatment) or difficult-to-treat acne types (acne conglobata or acne fulminans). Rare forms of acne may be present in the context of complex syndromes, such as the synovitis, acne, pustulosis, hyperostosis, osteitis (SAPHO) syndrome, pyogenic arthritis, pyoderma gangrenosum, acne (PAPA) syndrome, pyoderma gangrenosum, acne and suppurative hidradenitis (PASH) syndrome, or congenital adrenal hyperplasia (CAH), in association with other systemic findings, often with a positive family history.