Intravenous phentolamine test--an aid in the evaluation of patients with persistent pain after low-back surgery?

BACKGROUND: Persistent pain following surgery in the treatment of chronic low-back pain patients is still relatively frequent. Most of these patients with persistent pain have clinical signs of neuropathic pain. The neuropathic pain might be sympathetically maintained pain (SMP) or sympathetically independent pain (SIP). Systemic administration of phentolamine, a competitive alpha-adrenergic antagonist, has been used as a diagnostic tool to identify patients with SMP. METHODS: Thirty-seven patients with persistent pain after low-back surgery (lumbar laminectomy, with or without discectomi, or a posterior fusion, with or without decompression) received intravenous phentolamine (0.5 mg/kg over 30 min) in a single-blind, placebo-controlled manner. Prior to this infusion the patients were classified clinically into different pain groups based on physical examination and imaging findings. An opioid epidural test blockade was used as a control. RESULTS: Clinical classification divided the patients into nociceptive pain (n = 7), neuropathic pain (n = 22) and mixed pain (n = 8). In the phentolamine test there were only one responder, 34 non-responders and 2 patients were placebo-responders. In the control epidural blockade there were 11 non-responders, 23 fentanyl/local anaesthetic-responders and 3 placebo-responders. CONCLUSIONS: SMP is either an uncommon cause of persistent pain in this type of failed back surgery patients or the phentolamine test, as we performed it, was unable to identify SMP.