An anterior fistula opening below the dentate line is a low (simple) fistula. Low fistulas are laid open by fistulotomy, which is the standard approach. A seton is mainly used first for high or complex fistulas that involve a lot of sphincter. A defunctioning colostomy is only for extrasphincteric or complex disease. Lateral sphincterotomy treats fissures, not fistulas.