Dimethadione is the primary metabolite of trimethadione. Dimethadione causes depression of neuromuscular transmission. Dimethadione primarily decreases transmitter release from the nerve terminal[1].
[1]. M T Alderdice, et al. Comparison of the effects of trimethadione and its primary metabolite dimethadione on neuromuscular function and the effects of altered pH on the actions of dimethadione.