4.   Of the following, the local anesthetic with the LEAST neuro-toxic potential is?

1. Lidocaine
2. Tetracaine
3. Bupivacaine
4. Procaine

Answer:  Bupivacaine

Lidocaine, tetracaine, and procaine have all been reported to cause cauda equina syndrome after spinal anesthesia.  Bupivacaine is unique in not having done so.

Refs: 
1.  Nicholson M, Eversole U: Neurological complications of spinal anesthesia. JAMA 1946; 132:679-685 (tetracaine)

2.  Rigler ML, Drasner K, Krejcie TC, Yelich SJ, Scholnick FT, DeFontes J, Bohner D: Cauda equina syndrome after continuous spinal anesthesia. Anesth Analg 1991; 72:275-81 (lidocaine and tetracaine)

3.  Ferguson F, Watkins K: Paralysis of the bladder and associated neurological sequelae of spinal anesthesia (cauda equina syndrome). Br J Surg 1937; 25:735-752 (procaine

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