Nighttime Cough in Elderly Parents: Causes & Relief

Question
I am the care giver for both of my parents who are in their early nineties.  
They are both on blood pressure medicine and fluid pills.  My Mom is also on coumadin.  She is more frail than my Dad. At night, when they lay in bed, they both seem to have a cough that continues through the night and prevents them from getting proper rest.  I have taken them to their family ENT doctor many
times over the last few years for this problem.  He will give them a shot and prescribe antibiotics, allegra, and omeprazole.  After their doctor appt, they will be much better for about a week or two, and then they resume the cough.  I have mentioned this issue to their primary care doctor, their cardiologist, and
pulmonologist and they don't seem to be able to offer any help.  I have no idea how to help them with this problem.  Can you help?
Thanks

Answer
watch out for drug effects, especially ACE inhibitor blood pressure medicines.... they can cause a cough.  This also could be allergic, such as house dust mites.... getting pillow and mattress plastic covers may help this.  Keep bugging the family doctor to see about other causes like reflux, etc.