What Causes Disorientation, Poor Memory And Loss Of Balance In An Elderly Person?

My father, 84 yrs old, has had signs of disorientation, memory difficulty, and physical balancing difficulty while standing or walking, over the last several months. While apparent before an operation about 10 months ago, since the operation it appears somewhat worse. The operation to remove arthritic build-up in upper spinal column. He also has lower back pain sometimes. He has had e-rays and scans - nothing appears. Do you have any ideas what the cause could be?