Difficult and rare forms of acne.

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, some patients have difficult or rare forms of acne. What seems to be "nonresponding acne" in a patient may be caused by 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 attributable 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 syndrome; pyogenic arthritis, pyoderma gangrenosum, acne syndrome; pyoderma gangrenosum, acne, and suppurative hidradenitis syndrome; or congenital adrenal hyperplasia, in association with other systemic findings, often with a positive family history.