A guide catheter (200) has a pre-set shape for insertion into a nasal cavity. Said guide catheter (200) comprises a proximal end, a distal end, and a lumen extending from the proximal end to the distal end. Said guide catheter (200) incorporates one or more preformed curves or bends, wherein the curved or bent regions form a deflection angle selected from: 30 degrees, 45 degrees, 60 degrees, 70 degrees, 90 degrees, 120 degrees and 135 degrees. A system for diagnosing and/or treating sinusitis or another disorder affecting the nose, paranasal sinuses or other anatomical structures of the ear, nose or throat comprises: a guide catheter (200) as described above a balloon catheter (204) that can be advanced along the guide catheter (200), wherein the balloon catheter (204) comprises an inflatable balloon and optionally a guidewire (202) that can be inserted through the lumen of the guide catheter (200).
