A surgical instrument (1) for performing controlled myotomies, particularly for performing internal anal sphincterotomies, comprises a proximal handle portion (2), an elongate insertion shaft (3) connected with the handle portion (2) and extending distally therefrom, a distal incision tip (4) arranged at a distal end of the insertion shaft (3) and adapted to incise tissue distally to the insertion shaft (3), a lateral incision member (5) arranged at the insertion shaft (3) and adapted to incise tissue laterally to the insertion shaft (3), wherein the lateral incision member (4) is movable from a rest position, in which it is retracted towards or inside the insertion shaft (3) and an operational position in which it protrudes laterally from the insertion shaft (3).