An apparatus for sensing a property of a bladder wall, such as bladder-wall oxygen, comprises an elongate catheter (1) and an elongate sensor (2). The catheter (1) defines a path from a proximal end of the catheter to a distal end of the catheter. The catheter comprises a sensor channel for guiding the elongate sensor along at least a part of the path, the sensor channel opening at a sensor port (3) towards the proximal end of the catheter. The sensor channel comprises (i) an enclosed lumen portion (10), arranged to surround the sensor, and (ii) an open furrow portion (16). The furrow is located nearer to the distal end of the catheter than is the enclosed lumen. The furrow is arranged to allow the sensor (2) to exit the enclosed lumen (10) in a direction substantially parallel to, or tangential to, the path of the catheter (1) at a proximal end of the furrow.
