To evaluate the influence of limb position changes on the incidence of incorrectly perceived or phantom sensations during regional anesthesia, Isaacson et al.  observed 40 women scheduled to undergo genitourinary procedures with subarachnoid anesthesia. The authors’ objective was to reevaluate the controversy of rigid versus  plastic influences on phantom sensations during regional anesthesia and to develop a better understanding of proprioceptive memory–imprinting processes.

Preoperative medication was limited to a maximum of 2 mg intravenous midazolam. After administration of subarachnoid block, patients were placed in the supine position and an opaque screen was placed in such a way as to prevent the patients from seeing their legs. Patients scheduled for procedures lasting 1 h or less were assigned to group 1 and were administered 1.5 ml lidocaine, 5%, in 7.5% dextrose; patients scheduled for procedures longer in duration than 1 h were assigned to group 2 and were administered 1.5 ml bupivacaine, 0.75%, in 7.5% dextrose. Then, patients in each group were randomized to one of four time groups: 1, 4, 7, and 10 min. The authors assigned each patient’s dominant leg as the leg to be flexed first (ipsilateral limb). The nondominant (contralateral) leg was completely extended. The patient’s legs were in this position for 1, 4, 7, or 10 min, depending on the time group to which the patient was randomized, and the patient was questioned regarding leg position. At the designated time, leg positions were switched simultaneously. Sensory testing to pin prick was performed 10 min after injection for each group and also was used during repositioning as a distraction to minimize movement cues.

The percentage of incorrect responses was analyzed using a logistic regression model with the independent variables of treatment and time. The authors found that inability to perceive a change in limb position was dependent on the point at which the position change was initiated, in relation to the onset characteristics of the local anesthetic. Patients incorrectly identified the position of the flexed-first limb more often than they did the position of the contralateral limb, leading the authors to conclude that proprioceptive memory involves a dynamic neuroplastic imprinting process (rather than a fixed body schema) that is influenced by limb position before onset of regional anesthesia.