A sheath for a medical instrument includes an elongate body extending from a proximal end to a distal face. The elongate body may be configured to axially extend from a first length to a second length greater than the first length. The sheath may include a first hollow lumen extending from a first end proximate the proximal end to a second end proximate the distal face, and a second hollow lumen extending from the proximal end to the distal face. The second lumen may be open at the distal face. The sheath may also include a transparent window positioned at the second end of the first lumen to seal the first lumen at the second end. The transparent window may be configured to transmit light therethrough.