Not having a medical background - I can tell you that I have a very healthy component of autonomic neruopathy which has pretty severely affected my ability to digest anything (gastroparesis, or essentially stomach paralysis due to nerve damage)... if I'm having a bad PN day - then my mouth and lips, and even tongue go numb (and the gastroparesis symtoms throughout my whole gut get worse - I think its just the disease rearing its ugly head in a broader way)..... (lips rather like a feeling of being at the dentist and getting a novacaine shot).... looking at the articles that Cyclops posted - this would tend to make sense - the "nerve" damage.....

Mine comes and goes as I said, as our PN tends to "web"... However, years ago I had a wisdom tooth out they told me before the surgery was going to damage the facial nerve and my face and lips (exactly half) would be numb but probably eventually come back.. took year to get it back, but it did come back... point? The feeling is much the same - and both are nerve damage....