Background: Sympathetically maintained pain (SMP) can occur in patients with neuropathic pain. Sympathetically maintained pain is frequently diagnosed clinically by assessing the analgesic effect of an appropriate sympathetic block (SB). The diagnostic value of such blocks depends on both the degree of sympathetic activity disruption achieved and its duration without unintentional concomitant sensory block.

Methods: This pilot study evaluated the rate of diagnostically valuable SBs performed by experienced anesthesiologists in 19 patients (stellate/thoracic blocks: n = 11, lumbar blocks: n = 12). Monitoring included pain rating before SB; 10 and 30 minutes; and at 1, 3, and 6 hours after SB; bilateral skin temperature 30 minutes before SB through 120 minutes after SB; and detection of bilateral thresholds for cold, warmth, tactile, and vibration stimuli before and after.

Results: Ten (43%) of 23 SBs were not eligible for SMP diagnosis (4 had insufficient skin temperature increase; and 6 had cold or tactile detection threshold increases in the painful area). In 11 of the SBs, no significant sensory threshold change was detected; however, 2 individuals demonstrated marked reductions in the cold or tactile sensory thresholds. Sympathetically maintained pain was diagnosed in 3 (25%) of the 12 patients who had at least 1 SB with the required skin temperature increase without concomitant somatosensory block.

Conclusions: Sympathetic blocks are useful in the diagnosis of SMP. However, their value is limited by the potential for false positives (unintentional sensory block) or false negatives (insufficient SB). Adequate monitoring of the sympathetic and somatosensory function for a minimum of 90 minutes after the intervention is essential to ensure that a valid diagnosis of SMP is made.