Orion had no medical problems until shortly after he turned one year old.  He began limping on one of his hind legs, then started acting like his lower back hurt.  We took him to the vet, but tests and x-rays didn't show anything abnormal.  Steroids helped for a little while, but didn't eliminate the problem.  We took more x-rays, but still nothing showed up.  I was especially concerned because Orion might be a Manx, and Manxes sometimes have spinal deformities and problems with the nerves in the hindquarters. 
This dragged on for a month and a half and it was killing us to see him in pain - the poor guy would growl and hiss when he had to walk, despite being on medication.  Finally, we took him to see a veterinary neurologist in Phoenix, Arizona.  There, he had more x-rays and bloodwork, a spinal tap and analysis of spinal fluid, and a myelogram.  All were normal.  The location of the pain was definitely lumbar vertebra 4 and 5, but there was no visible cause.

Not knowing what else to do, the neurologist suggested doing an MRI, but first put him on antibiotics, since that was the one thing no one had tried.  There really was no indication of an infection, but it couldn't hurt.  Unbelievably, after three doses of the antibiotic, he was back to normal.  It was like a miracle!   If we'd tried antibiotics first, we would have saved him some pain and ourselves around $1,300 and a lot of stress, but we live and learn...

He must have had some odd type of internal abscess or other infection that just didn't show up on any of the tests we did.  Thinking back, we remembered suspecting he may have been stung by a wasp while playing on the roof one morning before all this began, and I suppose that could have caused an abscess or something similar.