Q: What anti-seizure medicine may cause hyperammonemic encephalopathy, despite normal liver function, normal doses and normal serum levels?

Answer: Valproic acid (VPA)

The use of valproic acid (VPA) (Depakote) frequently causes hyperammonemia. Usually it is clinically not significant but in some people, hyperammonemia may be clinically evident as encephalopathy, which may become life threatening. Valproic acid-induced hyperammonemic encephalopathy may occur in people with normal liver function, with normal doses and normal serum levels.

Reference:

Wadzinski J, Franks R, Roane D, Bayard M. - Valproate-associated hyperammonemic encephalopathy. - .J Am Board Fam Med. 2007 Sep-Oct;20(5):499-502.