To the Editor:—
I read with interest the article by Isaacson et al. 1that examined phantom limb sensations during subarachnoid block. Abnormal phantom sensation has been described previously, not only during spinal 2and epidural anesthesia 3,4but also during brachial plexus blocks 3,4and intravenous regional anesthesia. 4I would like to comment on a few issues.
First, the period of clinical observation after subarachnoid injection seems to be too short to permit a full form of phantom sensations. With both epidural anesthesia and peripheral nerve blocks, phantom sensations are reported 20–30 min after the onset of anesthesia. Although the onset time of subarachnoid block is more rapid than that of these other forms of anesthesia, more information might have been gained with a longer period of observation. Second, the authors conclude that proprioceptive memory involves a dynamic neuroplastic imprinting process that is influenced by limb position before the onset of regional anesthesia, rather than the classic “fixed body” schema. 2,5This is consistent with a previous study with spinal anesthesia in which Moriyama et al. 5noted that the incidence of false answers was related to the perceived position of the lower limb before the block and was not influenced by subsequent general anesthesia. These authors argued that when input from the limb was blocked, the “short-term” memory became a more persistent “long-term” memory.
There is perhaps no absolute contradiction between the classic “body schema”2and the “additive neuroplastic process.”1The reappearance of phantom limb pain after administration of a regional anesthetic supports some role for transient deafferentation produced by the block. The “flexed” position in the “body schema” concept does not represent a position of rest but is the memory of an archaic “tetrapod” schema disinhibited by anesthesia. Regardless, understanding this process remains an exciting challenge.