An extendable medical lead comprises a lead body and a sheath defining a cavity that encloses a length of the lead body. The length of the lead body enclosed within the sheath may be coiled or otherwise gathered such that when extended, the length of the enclosed section of the lead body is greater than the length of the sheath. The sheath may include a seal to help prevent contaminant entry into the cavity in order to help reduce tissue in growth around the length of the lead body disposed within the sheath. A portion of the length of the lead body enclosed within the sheath exits the cavity through an aperture defined by the seal when a tensile force is applied to the lead body.