From time to time I have had many issues descending stairs.  Rheumatoid Arthritis (RA) has affected my feet, ankles and knees. Up is not as much of a problem for me as down.  My biggest fear is falling down a flight of stairs and then ending up in worse condition than I started.  The problem is that I cannot trust the foot that is bearing all the weight not to give out.  The solution that has worked for me is to descend the stairs backward.  It changes my center of gravity and if I feel like I am losing my balance I lean forward. It has worked wonders for me.  Give it a try and let my know if it helps.