We have tried everything to get 18 month old 2 sleep- nothing works. only sleeps for 90 minutes at a time. what could cause this? could it be medical?

Many possibilities: If a toddler is having trouble sleeping, the first thing to do is to take the child to the doctor for evaluation. It is important to rule out an ear infection and sleep apnea due to large adenoids or tonsils. Other symptoms could be allergies, pain, low iron stores causing restless leg syndrome, night terrors. Often sleep problems are behavioral, but you need to rule out physical problems first.