Idiopathic Vestibular Syndrome

Idiopathic simply means no known cause.  How old is Rosie?

Typically this happens in older dogs (and sometimes you will hear it referred to as "Old Dog Vestibular";)

It can be triggered by an inner ear infection and simply by unknown, usually age-related infirmity. 

My Foxy had it a couple of times (when he was 18 and 19).  It's not uncommon.  Sometimes they use prednisone, sometimes nothing.

With Foxy it scared him to death -- almost literally.  It came on him during the night and I found him in the morning literally he had walked himself in circles into a tiny little ball of muscle spasms.  HOnestly he was SO scared to this day I don't know why he didn't have a heart attack or something (he didn't try to wake me up bless him ... he must have been like that for a couple of hours). 

Each dog reacts differently -- but try to keep him calm and let him know you think it will pass.  It should - truly.  It took Foxy a couple of weeks to get relaxed and back to normal. 

 My dog, Dancer, had it when she was about 16.  She had a residual head tilt afterward, but recovered enough to have good quality of life.  She was able to get around without stumbling and could do just about everything that she could do before, with the exception of jumping up on the couches.  So, we provided a comfy donut bed on the floor and she was just as happy.  She lived to be 17 1/2.