Research is now showing that when you suffer from rheumatoid arthritis, your brain is affected as is your quality of life in general.  Just taking a shower can become exhausting in general, and fatigue is constant.  They call it the blood brain barrier, which is the idea that cells in the brain capillaries function to protect the brain from surges of chemicals and molecules in general. 

 Scientists believe that this barrier keeps the brain and immune system from responding to distant and small events such as inflammation in the knee or liver.  When the immune cells reach the brain, basically you start exhibiting signs of sickness in general.  It turns into fatigue, listlessness, loss of social interest, and malaise all connected to arthritis.  They are still studying this, mostly on rats thus far. 

It can result in a brain fog in general, and the inflammatory events provoke effects in general that they really should not.  So if you have this, obviously it requires treating the arthritis to get to the basic cause.  But your doctor may gain more expertise as time goes on in specific relation to this particular problem so that he can help you in the future to achieve greater results and get the results you want.