The interpretation of pain relief and sensory changes following sympathetic blockade

A comparative study of the effects of sympathetic blockade by stellate ganglion block (SGB) and intravenous phentolamine infusion (PhI) was carried out in 24 patients with presumed sympathetically maintained pain of an upper extremity. A total of 15 SGBs and 16 PhIs were performed, with seven patients undergoing both procedures. All patients developed a Horner's syndrome with SGB and nasal stuffiness and cardiovascular changes with PhI. Similar pain relief was obtained with SGB and PhI in six of the seven who underwent both procedures. Pre-procedure patient characteristics including age, sex, duration of pain, historical and physical examination features suggestive of the reflex sympathetic dystrophy syndrome, and sensory disturbances such as allodynia and hyperpathia did not predict pain relief from either procedure. Changes in skin temperature following the sympatholytic procedure did not correlate with pain relief. For PhI, pain relief correlated with the magnitude of decrease in systolic blood pressure. After SGB, changes in quantitative thermal sensory testing (QST) suggestive of a partial deficit in thermal sensation correlated with pain relief. In 20 normal controls, water bath immersion to cool the hand passively by 7 degrees C and warm the hand passively by 4 degrees C had small and selective effects on thermal QST thresholds, but did not produce a general impairment in thermal sensation. In conclusion, the diagnosis of sympathetically maintained pain based on the history and physical examination alone cannot be made with confidence and therefore a sympatholytic procedure is necessary. When SGB produces pain relief but PhI does not, systemic absorption of local anaesthetic and/or sensory blockade by spread to somatic nerves may be the reason. Thus, PhI appears to be a less sensitive but more specific test than SGB. These two procedures provide complementary information and both may be needed to establish the diagnosis of sympathetically maintained pain.