Anyone familiar with height-adjustable transport chairs?

After an extremely unpleasant trip to the ophthalmologist, I am wondering if anyone has suggestions for boosting a short person in a wheelchair? We do have a cushion, but it "squishes down" and really doesn't help. My mother is unable to use most of the diagnostic equipment for a routine eye exam because she sits too low in her transport chair, even with the devices maximally lowered. A well-meaning assistant tried to transfer her to the exam room chair, an iffy proposition owing to her severe and painful osteoarthritis. Well, she managed to get partially onto the chair, then began bellowing in pain. We won't be trying that again. I know there are chairs that can be raised and lowered, but I am guessing these are quite heavy, and beyond the ability of my 61-year-old bad back to lift in and out of a tiny car. I plan on researching this via Google, but this forum is so helpful that it's always my first stop. Thanks in advance for any suggestions!