Desperately seeking answers for husband's dizziness

I apologize that this is long; I want to make sure to include all the details.

My husband has been having a problem for a few years now. It used to happen just once in a while, but lately it's been almost a weekly thing.

Here's what happens:
In the late morning or early afternoon, he'll start feeling generally unwell. Within a few hours it turns into this feeling of dizziness and/or lightheadedness accompanied by nausea. He gets slight relief if he sits in air conditioning or lays in bed. Then he wakes up in the morning feeling almost hung over, which he describes as a feeling very similar to the dizziness, but less so.

We can't figure out what triggers it. It happens on weekend days and weekdays. It happens regarless of what he eats or how much water he drinks. He has a fairly healthy lifestyle--we eat very well and he exercises at least 5 days a week. He doesn't use drugs or tobacco, only occasionally has a little caffeine, and even more rarely has one or two alcoholic drinks.

He went to an ear, nose and throat doctor and had a hearing test and balance test (VNG). He got his symptoms after the VNG. He also gets them after flying if he doesn't take dramamine. But of course that's not the only time it happens. The ENT doc didn't think it had anything to do with his ears, but said there was an abnormality in the VNG and ordered an MRI to rule out a brain tumor. We also know his cortisol levels are low (which he has begun treating), which apparently can cause these symptoms. But then I wonder why it isn't daily, or why it doesn't happen when he's feeling particularly stressed.

It turns out this ENT doc is a jerk and I'd like him to go to a different one. He hasn't had the MRI yet because we don't trust this doctor. I'd also like him to see a neurologist and a cardiologist.

We just can't figure out what this is. The symptoms started about the time my own health began deteriorating, which is of course stressful for him. But that could be a coincidence. Maybe it's sleep? He usually gets 7-7.5 hours during the week, 8-9 on the weekends.

I'd really appreciate any feedback any of you can offer. This is becoming really disruptive. My health prevents me from working, and we depend on his income. I also really depend on him when I'm not feeling well. I'm worried we'll soon both be in bad shape. And of course I hate seeing him feeling like that.

Thank you in advance for your help. Tweet