Effects of intrathecal local anesthetics on spinal excitability and on the development of autotomy.

The effect of 500 micrograms intrathecal (i.t.) lidocaine and tocainide injected prior to unilateral sciatic nerve section on the development of autotomy behavior was examined in rats. Neither drug inhibited autotomy compared with saline controls. The effect of lidocaine and tocainide on the flexor reflex in decerebrate, spinalized rats was also examined. Both drugs at 500 micrograms i.t. totally blocked the flexor reflex and hyperexcitability following acute nerve section, with a longer duration of action with tocainide. Recovery from the drugs was followed by a prolonged period of reflex hyperexcitability, even without axotomy. Lower dose lidocaine (100 micrograms) briefly blocked the reflex and recovery was followed by reflex hyperexcitability. Very low dose i.t. lidocaine (1 and 10 micrograms) did not block the flexor reflex, but only induced a prolonged increase in reflex magnitude. The possible mechanisms underlying these observations are discussed. The results suggest that i.t. local anesthetic pretreatment alone may not be beneficial for the prevention of the development of postoperative or neuropathic pain.