Objectives We report two patients who underwent elective revision surgery were found to have reduced reservoir volumes and indented bottom shield of intrathecal drug delivery system (IDDS).

Case reports Case #1: A 59-year-old man stable on IDDS medications for chronic neck and upper back pain was found to have an irreversible reduction in reservoir volume from 40 to 28 cc on routine refill appointment. Despite having stable pain control, the reservoir volume decreased from 40 to 22 cc and subsequently to 17 cc when the patient had inadequate pain control. During replacement surgery, the posterior shield of the IDDS was found to be collapsed. The patient had exposure to hyperbaric oxygen treatments for chronic non-healing wounds of the lower extremities. Case #2: A 68-year-old woman had an IDDS implanted for chronic low back pain secondary to failed back surgery syndrome. The reservoir volume was found to be reduced to 15 cc of medication from 20 cc during the seventh year of therapy. There was a report of a fall. Routine reservoir exchange at the end of battery life revealed that the bottom shield of the reservoir was indented.

Conclusions Inability to refill the medication to full capacity should be an indication to investigate device deformation. Information about exposure to hyperbaric therapy or local trauma should be obtained.