Nitrous exposure is not necessarily "toxic" in the same sense as other toxins because one normal exposure is enough to damage B12 actions in the body. Toxic usually applies to toxins/poisons, which need high doses say of Tylenol which can be toxic, or are not compatible with life such as pesticides/nerve gases/heavy metals. The B12 inactivation for nitrous was discovered long after it was in use for many decades. It is not commonly used now because of that.
So instead kids go to raves and huff it from balloons! But some hospitals still use it... they haven't kept up! Nitrous is NO, the nitrogen is very reactive and that is how it does its damage.

from http://en.wikipedia.org/wiki/Nitrous_oxide

Nitrates and nitrous have different chemistries. I have not seen any reports of nitrates used therapeutically affecting cobalamin metabolism.

Nitrates--nitrous--- and nitric oxide are all chemically different and different reactively.

http://en.wikipedia.org/wiki/Nitrate
organic nitrates have been used medically for decades with no connection to neuropathy directly. The conditions they treat may lead to poor circulation and hence PN but that is a very indirect effect, mostly in the elderly. Nitrates induce tolerance to vasodilation, and lose their effectiveness over time with daily doses. I really don't think your poppers are your culprit.