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Arachnoiditis has long been considered a "rare" condition, so hardly any research has been done on it or on a patient's life expectancy. Just recently, some research on hormone therapy has been done by Dr. Forest Tennant in California. Unfortunately, he is no longer taking new patients. I have also heard about stem cell research, but the progression is still in its infancy.
As of today, there is no real effective treatment or cure for Arachnoiditis. Life expectancy, as far as I'm aware, is unknown.