Panes J, Reinisch W, Rupniewska E, Khan S, Forns J, Khalid JM, Bojic D, Patel H. Burden and outcomes for complex perianal fistulas in Crohn's disease: systematic review. World J Gastroenterol. 2018 Nov 14;24(42):4821-34. doi: 10.3748/wjg.v24.i42.4821