What is the definition or description of: cystic acne?

Severe, scarring: Acne cysts are large nodules developing deep within the dermis, or second layer of skin. They are painful, disfiguring, and may cause deep scars. There is often a genetic predisposition to cystic acne. Treatment usually requires systemic medication including oral isotretinoin, and should be started early, before the scarring is established.