I hope that you will be able to help us with our problem.
We are the Carters from New Zealand and we have a 4 year old cairn called Bonnie, she was the runt but seemed very healthy. When she was young and we would be out walking she would sometimes become unsteady on her hind legs and flop over on her side. This would only last for a few minutes and then she would continue on as if nothing had happened. She has always been a very sensitive dog and if we would lift her wrong she would yelp very loudly and approached from the rear she would cower down. We have over the years had her to several different Vets some who went as far as to give her x-rays but no firm diagnosis, most said it was psychological as she would readily jump down off a sofa so it "couldn't" be her back. She has, in the last 2 months worsened being able only to walk about 100 metres before flopping down with her back legs trembling. This only lasts about 2 minutes and she will get up and carry on as normal. We are at the moment under a Specialist Vet who has recognised that there is a problem and has done the "spinal dye exam" which was inconclusive. She is at the moment on a steroidal pain medication and another med for a "muscle junction" problem. She is not improving and is now barely able to eat without sinking slowly to the floor. We really need help as we fear our "little girl" may not get better.
As I am at work at the moment I do not have the names of the drugs or the tests carried out thus far but once we have established contact I can let you know the details. I am sure any ideas you may have would be beneficial so we look forward to your reply.

I had a similar problem with one of my dogs, an Afghan, who went from being able to race twice round a greyhound track to lying down when walking although her legs did not tremble. It got that it would take half an hour to do a ten minute walk. She also had the dye put down her spine which showed nothing. I ended up having an MRI scan done which showed one of her discs in her vertibrae was hooked over the top of the nerve and thus causing the problem. The problem disc was between her hip bones and beyond the point the dye can reach. She has had the disc removed. She could always jump on to a grooming table, top of a cage to sleep and in and out of a car without any problem. Apparently the spine is used differently to jump than it is to walk. Sue