83year old grandma's sugar dropped to 38. she started crying like a child and hitting my grandfather.was this typical or can it be a sign alzheimers?

Low blood glucose: Serum glucose levels this low commonly cause cognitive impairment. This would be especially pronounced in the elderly.

Due to low sugar: Having blood sugar that low can acutely alter brain function resulting in her unusual behavior. Alzheimers does not come on suddenly like this. The main sign of alzheimer's is chronic significant impairment of short-term (recent) memory.