Medical device kit with endoscope accessory

A kit for use in an endoscopic procedure on a patient, the kit comprising:

• a positioning device configured for facilitating advancement of an endoscope through a body lumen in the patient, the positioning device comprising an over tube configured to slide over substantially an entire length of the endoscope and an annular balloon sized to substantially surround the endoscope and configured for positioning between the endoscope and the over tube to anchor the over tube to the body lumen;
• a coupler device configured for removable attachment to the endoscope, the coupler device comprising a main body comprising a substantially closed distal end, a proximal end configured for attachment to a distal end portion of the endoscope and a lateral surface between the proximal and distal ends, the coupler further including a passage for receiving an instrument and having an opening along a portion of the lateral surface, and a flexible membrane coupled to and at least partially surrounding the opening of the passage, the coupler device being configured to allow viewing of a surgical site;
• and
• a mechanism for articulating the instrument passing through the endoscope, wherein the mechanism causes movement of the flexible membrane relative to the opening of the passage.